OCS Study

BOEM 2016-081 

U.S. Department of the Interior 

Bureau of Ocean Energy Management 

Gulf of Mexico OCS Region

Deep Circulation in the Gulf of Mexico: 

A Lagrangian Study 

October 2013

LC

K

OCS Study

BOEM 2016-081

Published by

U.S. Department of the Interior

Bureau of Ocean Energy Management

Gulf of Mexico OCS Region

New Orleans, LA

December 2016

Deep Circulation in the Gulf of Mexico: 

A Lagrangian Study 

Authors

Peter Hamilton 

Amy Bower 

Heather Furey 

Robert Leben 

Paula Pérez-Brunius 

Prepared under BOEM Contract 

M10PC00112 

by

Leidos, Inc. 

615 Oberlin Road, Suite 100 

Raleigh, North Carolina, 27605 

iii

DISCLAIMER

This report was prepared under contract between the Bureau of Ocean Energy Management 

(BOEM) and Leidos, Inc. This report has been technically reviewed by BOEM, and it has been 

approved for publication. Approval does not necessarily signify that the contents reflect the 

views and policies of BOEM, nor does mention of trade names or commercial products

constitute endorsement or recommendation for use.

REPORT AVAILABILITY

To download a PDF file of this Gulf of Mexico OCS Region report, go to the U.S. Department of 

the Interior, Bureau of Ocean Energy Management, Environmental Studies Program Information 

System website and search on OCS Study BOEM 2016-081.

This report can be viewed at select Federal Depository Libraries. It can also be obtained from 

the National Technical Information Service; the contact information is below.

U.S. Department of Commerce

National Technical Information Service

5301 Shawnee Rd.

Springfield, Virginia 22312

Phone: (703) 605-6000, 1(800)553-6847

Fax: (703) 605-6900

Website: http://www.ntis.gov/

CITATION

Hamilton, P., A. Bower, H. Furey, R.R. Leben, P. Pérez-Brunius. 2016. Deep Circulation in the 

Gulf of Mexico: A Lagrangian Study. U.S. Dept. of the Interior, Bureau of Ocean 

Energy Management, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study 

BOEM 2016-081. 289 pp.

ABOUT THE COVER

The cover picture shows 30-day tracks of 1500-m (blue) and 2500-m (red) RAFOS and 1500-m 

(magenta) APEX floats deployed by the study for the month of October 2013, where the black 

dot at the end of each track corresponds to 31 October 2013. Cyan squares show the locations of 

CTD and bio-optical profiles made by the APEX floats, and the brown squares are the locations 

of US Argo CTD profiles during the month of October 2013. The thick purple lines are the 17-

cm sea-surface height contour to show the position of the Loop Current (LC) and LC eddy 

Kraken (K) in the middle of the month. The thin green lines are isobaths at 200, 1000, 1500, 

2000, 2500, 3000, and 3500 m.
iv

ACKNOWLEDGEMENTS

The enthusiastic and timely support of Dr. Alexis Lugo-Fernández, the BOEM Contracting

Officer’s Technical Representative, is gratefully acknowledged. The suggestions and support 

from the Science Review Group (Dr. John Bane, University of North Carolina at Chapel Hill, Dr. 

Joe LaCasce, University of Oslo, Norway, and Dr. Susan Lozier, Duke University) were 

positive, constructive and greatly appreciated. Leidos program personnel, Paul Blankinship 

(Data Management) and James Singer (Field Operations and Logistics Manager), also played 

key roles in the deployment and recovery cruises, as cruise leaders, supporting drifter 

deployments and deck operations for sound source deployments and recoveries. Special thanks 

are also expressed to Brian Guest (Woods Hole Oceanographic Institution) for his support during 

mooring recovery and deployment operations, as well as preparation, and check out of sound 

sources and floats prior to deployment. The support of the CANEK mooring group that included 

useful conversations with Julio Sheinbaum, José Luis Ochoa, and Julio Candela (CICESE), and 

who were in charge of sound source deployment and recovery in Mexican waters, is appreciated. 

Support of John Evans (Leidos) during the R/V Acadiana deployment cruise, and Drs. Peter 

Lazarevich and Cathrine Hancock, both of Florida State University, during the R/V Weatherbird 

II sound source replacement cruise are also gratefully acknowledged. Many others were 

important to the program success, including a number of students and technicians and the 

captains and crews of the R/V Pelican, R/V WeatherBird II, R/V Bellows, R/V Acadiana, and 

B/O Justo Sierra. Special thanks are also due to Dr. J. Ledwell (WHOI) who facilitated the 

deployment of floats from the R/V Pelican on a GoMRI-funded cruise.
v 

CONTENTS

LIST	OF	FIGURES ............................................................................................................................................................ VIII

LIST	OF	TABLES ............................................................................................................................................................. XIII

ABBREVIATIONS	AND	ACRONYMS ...............................................................................................................................XIV

CHAPTER	1:	INTRODUCTION ....................................................................................................................	1

1.1 BACKGROUND .............................................................................................................................................................	1

1.2 OBJECTIVES .................................................................................................................................................................	4

1.3 PROGRAM	DESCRIPTION...........................................................................................................................................	6

1.3.1	RAFOS	Floats........................................................................................................................................................	6

1.3.2	APEX	Profiling	Floats.......................................................................................................................................	8

1.3.3	Schedule .................................................................................................................................................................	8

1.4 STUDY	PARTICIPANTS ...............................................................................................................................................	9

1.5 MAJOR	EVENTS ........................................................................................................................................................10

1.5.1	Loop	Current	and	Loop	Current	Eddies ................................................................................................10

1.5.2	Hurricanes	and	Tropical	Storms..............................................................................................................13

1.6 ANALYSIS	FOCUS .....................................................................................................................................................13

1.7 REPORT	ORGANIZATION........................................................................................................................................16

CHAPTER	2:	METHODOLOGY................................................................................................................. 17

2.1 DRIFTING	SUBSURFACE	FLOATS...........................................................................................................................17

2.1.1	RAFOS	Float	Description..............................................................................................................................19

2.1.2	Deployment	and	Recovery	Cruises...........................................................................................................20

2.1.3	Drifting	Float	Data	Return .........................................................................................................................20

2.1.4	APEX	Float	Description	and	Sensors......................................................................................................25

2.1.5	Profiling	Float	Data	Return........................................................................................................................25

2.2 REMOTE	SENSING....................................................................................................................................................26

2.2.1	Satellite	Altimetry...........................................................................................................................................26

2.2.2	Satellite	Radiometry......................................................................................................................................37

2.2.3	Ancillary	Satellite	Altimeter	Datasets....................................................................................................38

2.2.4	Ancillary	SST	Datasets..................................................................................................................................38

2.2.5	Ancillary	Model	Datasets.............................................................................................................................38

CHAPTER	3:	BASIC	STATISTICS............................................................................................................. 39

3.1 INTRODUCTION	AND	BASIC	DESCRIPTION...........................................................................................................39

3.2 EDDY	CENSUS...........................................................................................................................................................56

3.2.1	Overview .............................................................................................................................................................56

3.2.2	Methods...............................................................................................................................................................56

3.2.3	Results..................................................................................................................................................................57

3.3 LAGRANGIAN	INTEGRAL	TIME	SCALES................................................................................................................69

3.4 THE	LC, LC EDDY	AND	UPPER	OCEAN	MESOSCALE	VARIABILITY ................................................................70

3.4.1	Mean	Dynamic	Topography.......................................................................................................................70

3.4.2	Loop	Current	and	Loop	Current	Eddies ................................................................................................73

3.4.3	Mesoscale	Eddies	in	the	Western	Gulf....................................................................................................81

3.4.4	Conclusions........................................................................................................................................................89

3.5 PSEUDO-EULERIAN	STATISTICS	AND	DOMINANT	CIRCULATION	PATTERNS...............................................89

3.5.1	Methodology	to	Calculate	the	Gridded	Fields.....................................................................................91

3.5.2	Western	Basin	Boundary	Current............................................................................................................94

3.5.3	Central	Cyclonic	Gyre .................................................................................................................................	103

3.5.4	Eastern	Gulf	of	Mexico ............................................................................................................................... 106
vi 

CHAPTER	4:	PROFILING	FLOATS ........................................................................................................107

4.1 APEX AND	ARGO	CTD PROFILES ..................................................................................................................... 107

4.2 CTD PROFILE	CHARACTERISTICS ..................................................................................................................... 112

4.2.1	Parameters	and	Basic	Statistics............................................................................................................ 112

4.2.2	Parameter	Relationships.......................................................................................................................... 114

4.2.3	Parameter	Maps........................................................................................................................................... 117

4.3 BIO-OPTICAL	PARAMETERS............................................................................................................................... 126

4.3.1	Chlorophyll	Measurements...................................................................................................................... 126

4.3.2	Density	and	Mixed-Layer	Depth	Computation................................................................................ 126

4.3.3	Chlorophyll	Time	Series	Analysis.......................................................................................................... 128

CHAPTER	5:	FLOAT	ANALYSIS .............................................................................................................131

5.1 CONNECTIVITY	OF	THE	DEEP	WATERS	IN	THE	GULF...................................................................................... 131

5.1.1	Introduction ................................................................................................................................................... 131

5.1.2	Construction of Connectivity Maps and Matrices ........................................................................... 132

5.1.3	Connectivity, Retention and Exchange Pathways............................................................................ 138

5.1.4	Discussion........................................................................................................................................................ 147

5.2 DEEP	FLOWS	UNDER	THE	LOOP	CURRENT	AND	IN	THE	EASTERN	GULF.................................................... 150

5.2.1	Introduction ................................................................................................................................................... 150

5.2.2	Lagrangian	Observations......................................................................................................................... 151

5.2.3	Statistics........................................................................................................................................................... 158

5.2.4	Topographic	Rossby	Waves..................................................................................................................... 163

5.2.5	Deep	Baroclinic	Instability	Eddies........................................................................................................ 166

5.2.6	Anticyclone	Under	the	LC	Tip ................................................................................................................. 173

5.3 DEEP	LAGRANGIAN	FLOWS	IN	THE	WESTERN	GULF ..................................................................................... 176

5.3.1	Float	Data	Summary .................................................................................................................................. 176

5.3.2	Interaction	of	LC	Eddies	and	Deep	Floats......................................................................................... 176

5.3.3	Persistence	of	Deep	Boundary	Current	in	the	Western	Gulf...................................................... 192

5.4 GENERAL	CIRCULATION	IN	A	DATA-ASSIMILATIVE	MODEL ........................................................................ 194

5.4.1	Gulf	of	Mexico	HYbrid	Coordinate	Ocean	Model	(Gulf	HYCOM).............................................. 195

5.4.2	Upper	Ocean	Circulation........................................................................................................................... 195

5.4.3	Preliminary	Statistical	Comparisons	of	Velocities	at	Depth ..................................................... 200

5.4.4	HYCOM	Mean	Flows	and	Interannual	Circulation	Variability	at	Depth.............................. 204

5.4.5	Summary.......................................................................................................................................................... 209

CHAPTER	6:	DISCUSSION	AND	RECOMMENDATIONS ..................................................................211

6.1 INTRODUCTION	AND	OVERVIEW........................................................................................................................ 211

6.1.1	Circulation	Features................................................................................................................................... 211

6.2 MAJOR	RESULTS ................................................................................................................................................... 212

6.2.1	Eastern	Basin................................................................................................................................................. 212

6.2.2	Western	Basin................................................................................................................................................ 213

6.2.3	Whole	Gulf	Water	Mass	Analysis........................................................................................................... 215

6.2.4	Numerical	Modeling ................................................................................................................................... 216

6.3 SIGNIFICANCE	FOR	GULF	OCEANOGRAPHY...................................................................................................... 216

6.4 FURTHER	ANALYSIS............................................................................................................................................. 217

6.4.1	Relative	Dispersion...................................................................................................................................... 217

6.4.2	Circulation	Processes.................................................................................................................................. 218

6.4.3	Bio-Optical	Water	Column	Properties................................................................................................ 219

6.5 FUTURE	FIELD	STUDIES...................................................................................................................................... 219

CHAPTER	7:	REFERENCES .....................................................................................................................221
vii 

APPENDIX	A	.................................................................................................................................................................. 235

APPENDIX	B	.................................................................................................................................................................. 255
viii 

LIST OF FIGURES

Figure	1-1.	Physiographic	and	geomorphic	features	of	the	Gulf	of	Mexico......................................................	2

Figure	1-2.	SST	and	SSH	maps	for	3	February	2012...................................................................................................	5

Figure	1-3.	Location	of	sound	source	moorings. ..........................................................................................................	7

Figure	1-4.	Schedule	for	data	collection...........................................................................................................................	9

Figure	1-5.	Time	line	of	major	LC	eddy	events	in	the	Gulf	of	Mexico...............................................................12

Figure	1-6.	Tropical	storm	and	hurricane	tracks......................................................................................................14

Figure	2-1.	Launch	locations	of	floats. ...........................................................................................................................18

Figure	2-2.	Timelines	of	observations	made	by	RAFOS and	RAFOS-equipped	APEX	floats...................21

Figure	2-3.	Plot	of	all	trajectory	data..............................................................................................................................24

Figure	2-4.	Map	showing	profile	positions	for	the	APEX	floats. .........................................................................27

Figure	2-5.	Timelines	of	APEX	profile	observations................................................................................................28

Figure	2-6.	Satellite	altimeter	exact-repeat	ground	track	coverage	in	study	region.................................	30

Figure	2-7. Plot	showing	“actual”,	“available”,	“windowed”,	and	“used”	altimeter	data	for	each	

day ......................................................................................................................................................................35

Figure	3-1.	Histogram	of	number	of	floats	(a)	by	month,	and	(b)	by	trajectory	length. ..........................40

Figure	3-2.	Scatter	plot	of	all	float	trajectory	data....................................................................................................41

Figure	3-3.	Displacement	vectors	for	floats	with 630-day	(full	length)	missions ......................................43

Figure	3-4.	Histogram	of	loopiness	ratio,	see	text	for	further	explanation ...................................................45

Figure	3-5.	Overview	map	showing	seven	case	studies	A	through	G...............................................................47

Figure	3-6.	Trajectory	for	case	study	A,	RAFOS	1081.............................................................................................48

Figure	3-7.	Trajectory	for	case	study	B,	RAFOS	1067.............................................................................................49

Figure	3-8.	Trajectory	for	case	study	C,	RAFOS	1137 .............................................................................................50

Figure	3-9.	Trajectory	for	case	study	D,	RAFOS	1110.............................................................................................51

Figure	3-10.	Trajectory	for	case	study	E,	RAFOS 1060...........................................................................................53

Figure	3-11.	Trajectory	for	case	study	F,	RAFOS	1232...........................................................................................54

Figure	3-12.	Trajectory	for	case	study	G,	RAFOS	1134 ..........................................................................................55

Figure	3-13.	Trajectory	segments	containing	eddies..............................................................................................58

Figure	3-14.	Trajectory	of	example	interior	eddy	float	1223	at	1500	dbar. .................................................60

Figure	3-15.	Eddy	characteristics	as	calculated	using	the	wavelet	transform	method............................61

Figure	3-16.	Trajectory	of	example	interior	eddy	float	1094	at	1500	dbar. .................................................63

Figure	3-17.	Eddy	characteristics	of	float	1094	as	calculated	using	the	wavelet	transform	

method..............................................................................................................................................................64

Figure	3-18.	Trajectory	of	example	interior	eddy	float	1101	at	1500	dbar. .................................................65
ix

Figure	3-19.	Eddy	characteristics	of	float	1101	as	calculated	using	the	wavelet	transform	

method..............................................................................................................................................................67

Figure	3-20.	Azimuthal	velocity	plotted	against radius	for	three	eddy	case	studies. ...............................68

Figure	3-21.	Sub-regions	for	calculation	of	integral	time	scales ........................................................................71

Figure	3-22.	Lagged	autocorrelations	and	integral	time	scales	for	the	sub-regions .................................	72

Figure	3-23.	Gulf	Mean	Dynamic	Topography	(MDT)	over	study	interval....................................................74

Figure	3-24.	Gulf	Mean	Dynamic	Topography	(MDT)	over	the	20-year	record..........................................75

Figure	3-25.	Difference	of	mean	AVISO	SSH	by	study year	relative	the	20-year	mean............................76

Figure	3-26.	Time	series	of	LC	length	during	the	study	program......................................................................78

Figure	3-27. Daily	LC	eddy	center positions ...............................................................................................................80

Figure	3-28.	Areal	coverage	of	LC	eddies	by	year.....................................................................................................82

Figure	3-29.	Eddy	center	tracks	of	the	three	most	energetic	and	long-lived	LC	eddies. .........................83

Figure	3-30.	Tracks	of	all	of	the	industry	named	LC	eddies.................................................................................85

Figure	3-31.	Time	series	of	the	daily	sums	of	the	number,	area	and	circulation	of	cyclones................86

Figure	3-32.	Time	series	of	the	daily	sums	of	the	number,	area	and	circulation	of	anticyclones........87

Figure	3-33.	Histograms	of	number	of	floats	by	month.........................................................................................92

Figure	3-34.	a)	Number	of	observations	b)	Degrees	of	freedom	for	each	bin..............................................93

Figure	3-35.	Mean	velocity	vectors. ................................................................................................................................95

Figure	3-36.	Mean	kinetic	energy	(MKE)	in	cm2s-2...................................................................................................96

Figure	3-37.	Standard	deviation	ellipses ......................................................................................................................97

Figure	3-38.	Eddy	kinetic	energy	(EKE)	in	cm2s-2.....................................................................................................98

Figure	3-39.	MKE/EKE	ratio ..............................................................................................................................................99

Figure	3-40.	Boundary	current	separation	shown	by	the	trajectories	of	ten	1500 m	floats. ............. 101

Figure	3-41.	Trajectories	of	floats	showing	boundary	currents...................................................................... 102

Figure	3-42.	Selected	trajectories	along	the	southern	part	of	the	boundary	current............................ 104

Figure	3-43.	Lower-layer	potential vorticity	from	mean	parameters .......................................................... 105

Figure	4-1.	CTD	profile	locations	from	BOEM	APEX	floats. ............................................................................... 108

Figure	4-2.	T/S	Diagram	for	APEX	CTD	profiles..................................................................................................... 109

Figure	4-3.	CTD	profile	locations	from	US	Argo	floats......................................................................................... 110

Figure	4-4.	T/S	Diagram	for	US	Argo	CTD	profiles................................................................................................ 111

Figure	4-6.	Scatter	plots	of	the	depth	of	the 6	°C	isotherm,	with	SSH	and	dynamic	height................. 116

Figure	4-7.	Scatter	plot	of	the	depth	of	the	6	°C	isotherm	against temperature	at	1490	m ................ 118

Figure	4-10.	a)	Dynamic	Height	(cm),	and	(b) Depth	(m)	of	the	6	°C	isotherm........................................ 122

Figure	4-11.	Maps	of	AAIW	mean	parameters. ....................................................................................................... 123
x 

Figure	4-12.	Maps	of	SUW	mean	parameters. ......................................................................................................... 124

Figure	4-14.	Surface	tracks	of	the	six	APEX	floats .................................................................................................	127

Figure	4-15.	Chlorophyll	transects	from	APEX	profiles ...................................................................................... 129

Figure	4-16.	Mean	chlorophyll	concentration	in	the	5–30	m	layer................................................................ 130

Figure	5-1.	Sub-regions	of	the	Gulf............................................................................................................................... 134

Figure	5-2.	Initial	position	of particles	released	in	region	D............................................................................. 135

Figure	5-3.	Histogram	of	the	initial	number	of particles	for	each	Gulf	region .......................................... 136

Figure	5-4.	Number	of	active	particles	as	function	of	time	and	source	region. ........................................ 137

Figure	5-5.	Initial	position	of particles	released	in	region	A............................................................................. 139

Figure	5-6.	Connectivity	and	dispersion	maps	for	region	A.............................................................................. 140

Figure	5-7.	Connectivity	matrix	as	a	function	of	time	for	source	region	A................................................. 141

Figure	5-8.	Initial	position	of particles	released	in	region	G............................................................................. 142

Figure	5-9.	Connectivity	and	dispersion	maps	for particles	released	in	region	G................................... 143

Figure	5-10.	Connectivity	matrix	as	a	function	of	time	for	source	region	G. ............................................. 144

Figure	5-11.	Connectivity	matrix	for	various	times	T	past	release................................................................ 145

Figure	5-12.	Conceptual	map	of	preferred	particle	retention	sites	and	exchange	routes ................... 149

Figure	5-13.	Monthly	mean	numbers	of	floats	found	east	of	90°W. .............................................................. 152

Figure	5-14.	16-day	float	tracks	centered	on	the	panel	date ............................................................................ 153

Figure	5-15.	Analysis	area	boxes	for	sequential	16-day	float	velocities...................................................... 159

Figure	5-16.	Time	series	of	LC	area,	area-mean	velocities,	mean	KE	and	EKE. ........................................ 160

Figure	5-17.	Mean	and	standard	deviation	ellipses	for	floats	and	current	meters.................................	162

Figure	5-18.	Trajectories	of	2500	m	floats south	of	the	Mississippi	delta.................................................. 164

Figure	5-19.	Consecutive	10-day	sequences	of	trajectories	of	2500-m	floats .......................................... 165

Figure	5-20.	TRW	velocity	time	series........................................................................................................................ 167

Figure	5-21.	Trajectories	of	floats	south	of	the	Mississippi	delta................................................................... 168

Figure	5-22.	TRW	velocity	time	series	for	1500	and	2500	m	floats. ............................................................. 169

Figure	5-23.	Sequence	of	10-day	trajectories	at	10-day	intervals.................................................................. 170

Figure	5-24.	Analysis	of	a	deep	LC	cyclone............................................................................................................... 172

Figure	5-25.	Float trajectories	in	an	anticyclone.................................................................................................... 174

Figure	5-26.	Mean	velocity	vectors	in	the	anticyclone	for	two	intervals..................................................... 175

Figure	5-27.	Histogram	of	floats	present	in	the	western	Gulf. ......................................................................... 177

Figure	5-28.	Two-month	composites	of	float	trajectories	and	LC	eddy	positions................................... 178

Figure	5-29.	Ten-day	snapshots	of	float	tracks	interacting	with	Icarus. ..................................................... 183

Figure	5-30.	Time	series	along	the	trajectories	of	floats	(a)	1081	and	(b)	1074..................................... 184
xi 

Figure	5-31.	Fifteen-day	snapshots	of	float-track	segments	and	SSH........................................................... 187

Figure	5-32.	Time	series	along	the	trajectories	of	floats	(a)	1098	and	(b)	1099..................................... 188

Figure	5-33.	Ten-day	snapshots	of	float	track	segments	and	17-cm	SSH	contour	for	Kraken........... 189

Figure	5-34.	Time	series	along	the	trajectories	of	float	1121........................................................................... 191

Figure	5-35.	Cyclonic	velocity	observations	along	the	boundary................................................................... 193

Figure	5-36.	HYCOM	(A)	mean	SSH	and	(B)	average	depth	of	the	6	°C	isotherm.................................... 197

Figure	5-37.	HYCOM	average	depth	of	the	6	°C	isotherm	during	each	year	of	the	study..................... 198

Figure	5-38.	Binned	2-D	histogram	scatterplots	of	the	float	velocities	(uF,	vF)	against HYCOM ..... 201

Figure	5-39.	Scatterplot	of	the float	velocities	(uF,	vF)	against HYCOM	for	bathymetry	>	2500	m 202

Figure	5-40.	Scatterplot	of	the float	velocities	(uF,	vF)	against HYCOM	for	bathymetry	<	2500	m 203

Figure	5-41.	HYCOM	average	velocity	field	at	1500	m........................................................................................ 205

Figure	5-42.	HYCOM	average	velocity	field	at	2500	m........................................................................................ 206

Figure	5-43.	HYCOM	average	velocity	fields	at	1500	m	during	each	year	of	the	study	program. .... 207

Figure	5-44.	HYCOM	average	velocity	fields	at	2500	m	during	each	year	of	the	study	program. .... 208

Figure	A-1.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1018	through	1032......... 243

Figure	A-2.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1036	through	1050......... 244

Figure	A-3.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1052	through	1070......... 245

Figure	A-4.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1071	through	1093......... 246

Figure	A-5.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1094	through	1108......... 247

Figure	A-6.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1109	through	1125......... 248

Figure	A-7.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1126	through	1142......... 249

Figure	A-8.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1143	through	1157......... 250

Figure	A-9.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1158	through	1233......... 251

Figure	A-10.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1234	through	1248. ..... 252

Figure	A-11.	Postage	stamp	size	plots	of	each	RAFOS	trajectory	for	floats	1249	through	9999. ..... 253

Figure	B-1.	Initial	position	of particles	released	in	region	A............................................................................. 256

Figure	B-2.	Connectivity	matrix	as	a	function	of	time	for	source	region	A................................................. 256

Figure	B-3.	Connectivity	(upper	panels)	and	dispersion	(lower	panels)	maps	for	region	A.............. 257

Figure	B-4.	Initial	position	of particles	released	in	region	B............................................................................. 258

Figure	B-5.	Connectivity	matrix	over	time	for particles	released	in	region	B........................................... 258

Figure	B-6.	Connectivity	and	dispersion	map	for particles	released	in	region	B..................................... 259

Figure	B-7.	Initial	position	of particles	released	in	region	C............................................................................. 260

Figure	B-8.	Connectivity	matrix	over	time	for	particles	released	in	region	C. .......................................... 260

Figure	B-9.	Connectivity	and	dispersion	map	for	particles	released	in	region	C..................................... 261
xii 

Figure	B-10.	Initial	position	of particles	released	in	region	D.......................................................................... 262

Figure	B-11.	Connectivity	matrix	over	time	for particles	released	in	region	D........................................ 262

Figure	B-12.	Connectivity	and	dispersion	map	for particles	released	in	region	D.................................. 263

Figure	B-13.	Initial	position	of	particles	released	in	region	E	.......................................................................... 264

Figure	B-14.	Connectivity	matrix	over	time	for particles	released	in	region	E ........................................ 264

Figure	B-15.	Connectivity	and	dispersion	map	for	particles	released	in	region	E. .................................	265

Figure	B-16.	Initial	position	of	particles	released	in	region	F .......................................................................... 266

Figure	B-17.	Connectivity	matrix	over	time	for particles	released	in	region	F......................................... 266

Figure	B-18.	Connectivity	and	dispersion	map	for particles	released	in	region	F .................................. 267

Figure	B-19.	Initial	position	of	particles	released in	region	G.......................................................................... 268

Figure	B-20.	Connectivity	matrix	over	time	for	particles	released	in	region	G. ....................................... 268

Figure	B-21.	Connectivity	and	dispersion	map	for particles	released	in	region	G.................................. 269

Figure	B-22.	Initial	position	of	particles	released	in	region	H.......................................................................... 270

Figure	B-23.	Connectivity	matrix	over	time	for particles	released	in	region	H........................................ 270

Figure	B-24.	Connectivity	and	dispersion	map	for particles	released	in	region	H.................................. 271

Figure	B-25.	Initial	position	of particles	released	in	region	I ........................................................................... 272

Figure	B-26.	Connectivity	matrix	over	time	for particles	released	in	region	I.......................................... 272

Figure	B-27.	Connectivity	and	dispersion	map	for particles	released	in	region	I ................................... 273
xiii

LIST OF TABLES

Table	1-1. LC	Eddy	Dates	from	Altimetry.....................................................................................................................10

Table	2-1.	RAFOS	Sound	Source	Information.............................................................................................................20

Table	2-2.	Instrument	Deployment	Cruises	during	the	Deep	Lagrangian	Study ........................................22

Table	2-3.	APEX	Profile	Time	Periods	and	Properties	Collected........................................................................26

Table	2-4.	Satellite	Altimeter	Missions	Active	during	the	Deep	Lagrangian	Study....................................29

Table	3-1.	Eddy	Distribution	by	Sense	of	Rotation	and	Depth............................................................................57

Table	3-2.	Eddy	Distribution	by	Category	and	Sense	of	Rotation......................................................................59

Table	3-3.	Eddy	Mean	Statistics	by	Category. .............................................................................................................69

Table	3-4.	LC	Eddy	Names	and	Separation	Dates	during	the	Study .................................................................	77

Table	3-5.	Crossing	of	90°W	by	LC	Eddy	Centers	during	the	Study..................................................................78

Table	3-6.	LC	Eddy	Census	Statistics	at	90°W ............................................................................................................88

Table	3-7.	Mean	Daily	Eddy	Statistics. ...........................................................................................................................89

Table	4-1. Statistics from APEX/ARGO CTD Profiles........................................................................................... 115

Table	5-1.	Names	of	the	Regions	used	for	the	Connectivity	Analysis ........................................................... 133

Table	5-2.	Comparison	of	Separation	Dates	during	the	Study	Program	Derived	from	AVISO,	

CCAR	and	HYCOM	SSH	Fields............................................................................................................... 199

Table	5-3.	Comparison	of	Crossing	of	90°W	by	LC	Eddy	Centers	during	the	Study	Program	

Derived	from	AVISO,	CCAR	and	HYCOM	SSH	Fields................................................................... 199

Table	5-4.	Comparison	of	6	°C	Isotherm	Depth	Statistics	Measured	by	APEX	Profiles	(N=285)	

Coincident	with	HYCOM	Blended	Dataset	during	the	Study	Program............................... 199

Table	5-5.	Correlation	of	Observed	Float	Velocity	Components	(at	all	Depths)	with	Estimated	

HYCOM	Velocity	Components	at	1500	m	and	2500	m	with	and	without	Bathymetric	

Editing............................................................................................................................................................ 200

Table	A-1.	APEX	Float	Metadata	(S/Ns	7036–8512)	and	RAFOS	Float	Metadata	(S/Ns	1016–

1042)..............................................................................................................................................................	237	

Table	A-2.	RAFOS	Float	Metadata	(S/Ns	1043–1081).........................................................................................	238

Table	A-3.	RAFOS	Float	Metadata	(S/Ns 1082–1117).........................................................................................	239	 	

Table	A-4.	RAFOS	Float	Metadata	(S/Ns	1118–1153)......................................................................................... 240	

Table	A-5.	RAFOS	Float	Metadata	(S/Ns	1154–1245)......................................................................................... 241	 	

Table	A-6.	RAFOS	Float	Metadata	(S/Ns	1246–9999)......................................................................................... 242	
xiv 

ABBREVIATIONS AND ACRONYMS

AAIW: Antarctic Intermediate Water 

ADT: Absolute dynamic topography 

APE: Available potential energy 

APEX: Autonomous Profiling Explorer 

ARGO: Advanced Research Global Observation (no longer an acronym) 

AVISO: Archiving, Validation and Interpretation of Satellite Oceanography 

BBP: Optical backscattering coefficient 

BOEM: Bureau of Ocean Energy Management 

CCAR: Colorado Center for Astrodynamics Research 

CDOM: Colored Dissolved Organic Matter 

CHL: Chlorophyll 

CICESE: Centro de Investigacion Cientifica y de Educacion Superior de Ensenada 

CLS: Carrere and Lyard (2003) sea-state bias

CNES: Centre National d’Etudes Spatiales, France 

CTD: Conductivity/Temperature/Depth 

DBAR: Decibar 

DCM: Deep chlorophyll maximum 

DOF: Degrees of freedom 

DT: Delayed time

DWBC: Deep Western Boundary Current

EKE: Eddy kinetic energy 

ERS: Earth Resources Satellite

ESA: European Space Agency 

FIO: Florida Institute of Oceanography 

FSLE: Finite scale Lyapunov exponents 

FSU: Florida State University 

GEOSAT: Geodetic Satellite 

GFO: Geosat Follow-On

GHRSST: Group for High Resolution Sea-Surface Temperature 

GLAD: Grand Lagrangian deployment 

GMT: Greenwich Mean Time or UTC 

GOES: Geostationary Operational Environmental Satellite 

GOMRI: Gulf of Mexico Research Initiative 

GSFC: Goddard Space Flight Center 

HDF: Hierarchical data format 

HPLC: High performance liquid chromatography 

HYCOM: Hybrid Coordinate Ocean Model 

ISOP: Improved subsurface ocean profile 

ISRO: Indian Space Research Organization

JPL: Jet Propulsion Laboratory 

KE: Kinetic energy 

LC: Loop Current 

LCE: Loop Current eddy 
xv 

LCFE: Loop Current frontal eddy 

LH: Left hand 

LUMCON: Louisiana Universities Marine Consortium 

MAB: Meters above bottom 

MADT: Maps of absolute dynamic topography 

MDT: Mean dynamic topography 

MEaSUREs: Making earth system data records for use in research 

MKE: Mean kinetic energy 

MLD: Mixed layer depth 

MMS: Minerals Management Service 

MODAS: Modular ocean data analysis system 

MODIS: Moderate resolution imaging spectroradiometer 

MSLA: Merged sea-level anomaly 

MUR: Multi-scale ultra-high resolution 

MVOI: Multivariate optimum interpolation 

NADW: North Atlantic Deep Water 

NAS: Network archive system 

NASA: National Aeronautics and Space Administration 

NAVOCEANO: Naval Oceanographic Office 

NCODA: Navy coupled ocean data assimilation 

NEPA: National Environmental Policy Act 

NESDIS: National environmental satellite, data, information 

NetCDF: Network common data form 

NOAA: National Oceanic and Atmospheric Administration 

NRT: Near real time

OA: Objective analysis 

OBPG: Ocean biology processing group 

PALACE: Profiling autonomous Lagrangian current explorer

PIES: Inverted echo sounder with pressure 

PO.DAAC: Physical oceanography distributed active archive center 

PSU: Practical salinity unit 

PV: Potential vorticity 

RADS: Radar altimeter database system 

RAFOS: Ranging and fixing of sound (SOFAR spelled backwards) 

RH: Right hand 

S: Salinity 

SLA: Sea-level anomaly

SSH: Sea surface height 

SSHA: Sea surface height anomaly 

SST: Sea surface temperature 

SUW: Subtropical Underwater 

T: Temperature 

TEOS-10: Thermodynamic equation of seawater – 2010 

TOP/POS: TOPEX/Poseidon 

TRW: Topographic Rossby wave 

T/S: Temperature/Salinity 
xvi 

TWR: Teledyne Webb Research 

UTC: Coordinated universal time or GMT 

WHOI: Woods Hole Oceanographic Institution 

XBT: Expendable bathythermograph 

YC: Yucatan Channel 

3DVAR: Three-dimensional variation 
1 

CHAPTER 1: INTRODUCTION 

1.1 BACKGROUND

The deep circulation of the Gulf of Mexico (Gulf) basins is not very well known because there 

have been no basin-wide observational studies in recent decades. By their very nature mooringbased measurements survey only limited regions. The Bureau of Ocean Energy Management 

(BOEM) and its predecessor agency, the Minerals Management Service (MMS), of the 

Department of the Interior conducted major physical oceanographic observational programs in 

various sub-regions of the Gulf 1. These programs advanced the state of knowledge of the deep 

Gulf, and included the first use of pressure-equipped-inverted echo sounders (PIES) in the Gulf, 

and deep Lagrangian Ranging and Fixing of Sound (RAFOS) floats. 

The deep waters of the Gulf, including the lower continental slope and abyssal depths, can be 

characterized as being in two layers. Velocity variability in the upper layer, from the surface to 

800–1200 m is dominated by cyclonic and anticyclonic mesoscale eddies (diameters ~50–300 

km). Upper-layer eddy flows are usually surface intensified with the strongest currents occurring 

in the top 100 m. The lower-layer flows from ~1000 m to the bottom are either slightly bottom 

intensified or nearly depth independent. In the eastern basin, both surface and bottom layers are 

dominated by the Loop Current (LC), a segment of the Gulf Stream system that enters the Gulf 

through the Yucatan Channel (YC), and extends northwards as a quasi-stationary clockwise 

turning meander that exits through the Florida Straits, which has a sill depth of 800 m. At 

irregular intervals of between 4 and 18 months (Sturges and Leben 2000), the LC sheds large 

anticyclonic warm rings or eddies that typically translate westward or southwestward across the 

western basin, where they dominate the upper-layer circulation. In this report, bathymetric and 

geographic features in the Gulf are identified by the names in Figure 1-1. 

The LC was the subject of an intensive observational study using moorings in both the US and 

Mexican sectors, and a PIES and bottom current meter array (Hamilton et al. 2014; Hamilton et 

al. 2016). The arrays were deployed for 2 1/2 years and were designed to obtain information on 

LC variability, meanders, frontal eddies, and LC eddy separations. Important new results on the 

dynamics of the LC were obtained: principally that large increases in eddy kinetic energy (EKE), 

in the lower water column, were caused by baroclinic instability of large-scale (~300 km 

wavelength, and 40–100 day periodicities) southward propagating meanders of the east side of 

an extended LC. These LC meanders were over deep water (~3000 m), and their associated deep 

eddies were barotropic (depth-independent velocities) with anticyclones and cyclones being 

displaced downstream or southward of the meander crests and troughs, respectively. 

 

1 Programs include the “De Soto Canyon Eddy Intrusion Study” (Hamilton et al. 2000), “Deepwater Observations in 

the Northern Gulf of Mexico from In-Situ Current Meters and PIES” (Hamilton et al. 2003), “Exploratory Study of 

Deepwater Currents in the Gulf of Mexico” (Donohue et al. 2006), “Survey of Deepwater Currents in the 

Northwestern Gulf of Mexico” (Donohue et al. 2008), “Full-Water Column Current Observations in the Central Gulf 

of Mexico” (Sheinbaum et al. 2007), “Study of Deepwater Currents in the Eastern Gulf of Mexico” (Cox et al. 

2010), “Full-Water Column Current Observations in the Western Gulf of Mexico” (Sheinbaum et al. 2010), and 

“Observations and Dynamics of the Loop Current” (Athié et al. 2014; Hamilton et al. 2014). 
Figure 1-1. Physiographic and geomorphic features of the Gulf of Mexico

(Courtesy of BOEM)

2 
3 

Donohue et al. (2016a) analyzed the baroclinic instability mechanism and showed that the EKE 

was extracted from the large pool of available potential energy (APE) in the uplifted density 

surfaces of the LC jet. Developing baroclinic instability over deep water causes a deepening 

trough to extend westward from the west Florida slope side to the Campeche escarpment that 

causes a detachment of a LC eddy. The eddy may re-attach to the LC, but eventually if the 

detachment point is far enough north, the LC eddy will separate and move into the western basin. 

The development of a large meander trough on the LC front followed by a detachment is often 

called necking down (Schmitz 2005). The fate of the deep eddies after they propagate westward 

through the neck was not clear from the observations. Hamilton (2009) suggests they may 

dissipate into westward propagating topographic Rossby waves (TRWs). 

The LC and LC eddy fronts are well approximated by the 17-cm sea-surface-height (SSH) 

anomaly that is mapped from satellite altimeter data for the Gulf by Colorado Center for 

Astrodynamics Research (CCAR) and Archiving Validation and Interpretation of Satellite 

Oceanography (AVISO) (Leben 2005). The sea surface temperature (SST) front, which is very 

weak in summer, is usually about 30 to 50 km outside of the 17-cm SSH contour (Leben 2005). 

Short period ~7 to 15 day cyclonic frontal perturbations with wavelengths of ~50 to 100 km, 

observed in satellite SST images of the LC and LC eddies, are not resolved by the altimeter SSH 

maps. They are known as frontal eddies and are similar to frontal eddies observed along the Gulf 

Stream along the South Atlantic Bight slope (Bane et al. 1981; Gula et al. 2015), and can be 

considered distinct from large-scale meanders. Frontal eddies are often observed along the 

Campeche slope when the LC extends in to the Gulf. Another result from the LC study is that 

once these eddies propagate in to deep water, after leaving the Campeche slope, they tend to 

decay and apparently have little influence on the eddy detachment process (Donohue et al. 

2016b). This is contrary to earlier descriptions of LC eddy separations (Schmitz 2005). Also 

these short-period frontal eddies do not have a deep expression in the lower layer (Hamilton et 

al. 2016), as was suggested by some numerical modeling studies (Oey 2008). 

LC eddies propagating in to the western basin are of direct importance to the circulation and flux 

of heat and salt in the upper layer (Elliott 1982). Once west of the Mississippi delta, LC eddies 

generally settle down and become nearly circular, and thus a stable baroclinic vortex (LaCasce 

1998), and appear to have minimal energy loss as measured by SSH anomalies, until they 

interact with the western slope off Mexico. The latter interactions can be complex with 

generation of subsidiary cyclones and migrations both to the south and to the north where the 

eddies often dissipate (Brooks 1984; Vukovich and Waddell 1991). Several authors show 

evidence that interactions of LC eddies with the eddy field in the western basin may lead to loss 

of coherence followed by water mass exchange, which may lead to break up of LC eddies before 

they reach the boundary (Lipphardt Jr. et al. 2008). There were a number of surveys of LC 

eddies as they moved through the central Gulf (Cooper et al. 1990) and many studies using 

surface drifters to track the western or south western trajectories (Kirwan et al. 1984a; Glenn and 

Ebbesmeyer 1993; Biggs et al. 1996). With the possible exception of the Bay of Campeche, 

where a strong wind-stress curl may drive a cyclonic gyre (Vazquez de la Cerda et al. 2005; 

Perez-Brunius et al. 2013), the upper-layer flows in the western basin are dominated by eddies. 

The lower-layer circulation in the western Gulf has had few observations. The Exploratory study 

for the north-central slope (Donohue et al. 2006; Hamilton 2009) established the existence of a 
4 

westward flowing boundary current along the Sigsbee escarpment. The six to nine months of 

RAFOS Lagrangian float data at a number of depths below 1000 m showed that this boundary 

current continued into the northwest corner and then proceeded southward along the Perdido 

escarpment. The floats did not penetrate south of ~23°N on the Mexican slope (Hamilton 2009) 

because of the short mission length. High speed currents ~90 cm s-1 were observed south of the 

Mississippi delta and seaward of the Sigsbee escarpment, at 100 m above bottom (mab) in 2000 

m (Hamilton and Lugo-Fernandez 2001). These flows were attributed to broadband TRWs 

radiating northwestward from the LC impinging on the escarpment (Hamilton 2007a), though the 

generation mechanisms and ray paths were unclear. Indeed, DeHaan and Sturges (2005) suggest 

that forcing could come from rectification of TRWs, and along with lower-layer vortex 

stretching, produce a cyclonic boundary flow around the western basin. The theoretical study by 

Mizuta and Hogg (2004) shows that TRWs propagating northwards over shoaling topography 

will generate a mean westward flow. Numerical studies and ray-tracing analysis (Oey and Lee 

2002) also indicate that TRW energy generated both by the LC and transiting LC eddies is 

concentrated along the Sigsbee escarpment from the Mississippi delta to the northwest corner. In 

the interior of the western basin, most results come from numerical studies that indicate that deep 

lower layer eddies accompany the transit of surface layer LC eddies across the Gulf (Welsh and 

Inoue 2000). However, a very limited number of moored current meter observations show 

almost depth-independent or slightly bottom intensified coherent flows below 1000 m that have 

no significant statistical relation with simultaneous upper layer currents (Hamilton 1990; 

Hamilton 2007a; Hamilton 2009). 

Figure 1-2 shows a SST image and the equivalent SSH from altimeter data for the recently 

detached eddy Icarus in the eastern Gulf, and the previous eddy Hadal in the western Gulf. Note 

the cold cyclone in the Bay of Campeche, and the interactions of Hadal with the adjacent 

cyclones (negative SSH values) that are not always visible in the SST. Hadal is in the process of 

being split into two by the cyclones; a not uncommon event in the western Gulf (Biggs et al. 

1996). In the east, the SST image clearly shows meander crests and troughs along the north and 

east sides of Icarus, and the “necking down” of the trough, with its cyclonic flows, between 

Icarus and the LC proper that effects a detachment. 

1.2 OBJECTIVES

The goal of this study is to increase our knowledge of the dynamics of the deep circulation of the 

Gulf through analysis of observations of subsurface tracked drifters. Specific objectives are: 

1. To deploy subsurface and profiling drifters over three years inside the Gulf; 

2. To analyze these data to produce maps of currents in the deep waters of the Gulf; 

3. To estimate Lagrangian statistics of these current fields such as spatial and temporal 

scales;

4. To make estimates of particle horizontal dispersion and explore the feasibility of 

estimating vertical dispersion of pollutants and biological material; and 

5. To provide information for BOEM to fulfill its regulatory mission and comply with 

National Environmental Policy Act (NEPA) requirements. 
Figure 1-2. SST and SSH maps for 3 February 2012 

The eddy detaching from the LC in the east is Icarus, and the western two anticyclones 

(positive SSH) are eddy Hadal after being split into two by interactions with cyclones (negative 

SSH).

5
6 

1.3 PROGRAM DESCRIPTION

The field program had two main elements: 1) a deep drifter study using RAFOS floats and sound 

sources (Rossby et al. 1986), and 2) a profiling float study using Autonomous Profiling Explorer 

(APEX) floats, similar to the US Argo program (Argo_Science_Team 1998), where 

conductivity, temperature, depth (CTD) profiles are obtained at regularly-spaced time intervals 

from freely drifting floats. Satellite remote sensing that collected SST and ocean color images, 

and continued the development of SSH maps using satellite altimeter data supported the field 

programs. The latter are used to resolve mesoscale eddy structures in the Gulf on a daily basis. 

1.3.1 RAFOS Floats 

The program deployed four moorings with sound sources at ~1000-m depth, approximately in 

the center of the sound channel in the Gulf (Figure 1-3). Placement of the sound sources was 

designed to have a degree of redundancy such that if a sound source failed, Gulf-wide coverage 

would still be reasonable. A fifth sound source was deployed at 600 m in the De Soto canyon by 

another study (the Gulf of Mexico Research Initiative (GoMRI) funded Deep-C study [Hamilton 

et al. 2015]) during one year of the study. The sound sources were set to pong three times a day, 

each at slightly different time. A RAFOS float listens and records the arrival times of the sound 

waves that are then post-processed into locations (see Chapter 2). Arrival time data are 

transmitted by Iridium Satellite communications when the RAFOS float surfaces at the end of its 

mission. Floats were ballasted for 1500 and 2500-m depths, with the majority at 1500 m. These 

depths are below the maximum depth of LC and LC eddy flows (~1200 m) in the upper layer, 

with the main difference being that at 1500 m, there was the possibility that a float could move 

southwards into the Caribbean across the 2000 m YC sill (none did), and at 2500 m the 

bathymetric contours are closed. Otherwise, because of the almost depth independent nature of 

currents in the lower layer, it is expected that 1500 and 2500-m floats would behave very 

similarly. 

The RAFOS field study was originally planned to last three years with about half the floats 

having two-year missions. However, a series of manufacturing problems, including leaks and 

internal data storage faults, resulted in relatively low data return in the first year. Delays in the 

manufacturer replacing the faulty floats (at no cost to the study) resulted in the RAFOS field 

study being extended to four years. The rather complex set of deployment cruises that resulted 

from the changes are discussed in Chapter 2. A total of 121, 1500-m, and 31, 2500-m depth 

useable float missions were obtained over a period of four years. Deployment strategies for the 

RAFOS floats were designed to provide coverage of all regions of the Gulf, so deployment 

cruises were made in US waters in the east, north central region off Louisiana, and in Mexican 

waters in the deep western basin, the Bay of Campeche and along the Campeche Escarpment. A 

variety of vessels were used including a ship-of-opportunity cruise that was independent of this 

study. Staging ports around the Gulf are given in Figure 1-3. 
3500 m

3000 m

1000 m

200 m

2000 m

Tuxpan

Brownsville

Cocodrie

EEZ

St. Petersburg

2 Sources

Shadow

Zones

Coverage

3 Sources

SS-2

SS-5

SS-1

SS-3

SS-4

Figure 1-3. Location of sound source moorings 

Shown are BOEM (SS-1 to SS-4) and the Deep-C sound source (SS-5). Reduction of deep 

water coverage, if only 3 or 2 sources are operating, is shown by the shading.

7
8 

1.3.2 APEX Profiling Floats 

APEX floats are used extensively in the Argo program that observes the density, temperature and 

salinity of the world ocean using autonomous profiling floats (Argo_Science_Team 1998). Most 

commonly, Argo floats profile temperature and salinity from 2000 m to the surface about every 

10 days. Between profiles, the float rests at 1000 m and is carried along by the prevailing 

currents at that depth. On reaching the surface, location and CTD data are transmitted to 

communications satellites. The result, over time, is a random sampling of major water bodies, 

which can be used for surveying water mass characteristics and climate studies. In June 2016, 

there were 3,759 operational Argo floats in the world ocean; however, very few of these were in 

the Gulf (< 10). 

This study’s profiling float program differs from standard Argo, though the basic platform 

(APEX) is one type of float used in the Argo program. Apart from the CTD, two additional 

instruments were added to the package. They were a Wetlabs optical sensor suite that measures 

fluorescence (chlorophyl-a), particle backscatter (turbidity) and colored dissolved organic matter 

(CDOM). The resulting bio-optical profiles are the first made in the Gulf using autonomous 

floats (Green et al. 2014). The second instrument was a hydrophone so that the path of the float 

between profiles could be observed using RAFOS acoustic tracking. Adding RAFOS to the 

APEX floats served two purposes: 1) to monitor the sound sources; because the profiling floats 

report frequently, the health of the four sound sources could be accessed as the Lagrangian study 

progressed, and over the four years, two sound sources were replaced, one because of weak 

pongs, and the other for failure; and 2) to add to database of deep Lagrangian float trajectories. 

The APEX floats had a rest depth of 1500 m, so that they corresponded to the 1500 m RAFOS 

floats, and profiles (CTD and bio-optical) were made every 14 days for most of the study. A 

profile from 1500 m to the surface and then the return to 1500 m takes ~12 hours. This is short 

compared to the cycle interval, which exceeds the maximum estimated maximum Lagrangian 

decorrelation time scale of ≲ 10 days, and thus, the APEX track data may be treated as another 

RAFOS float. 

Eight APEX floats were deployed by the study that produced a total of 573 profiles over the four 

years of the field program. Two of the earliest deployed floats had failures: one float had a 

buoyancy fault, and was lost shortly after deployment, and the other had a non-operational 

RAFOS unit and so deep trajectory data were not recorded. 

1.3.3 Schedule 

The general schedule of field measurements is shown in Figure 1-4. The contract was awarded 

in September 2010 with mobilization taking place over the first six months. Deployment of the 

sound sources and the first batch of RAFOS and two APEX floats took place in US waters in 

July 2011. The rapid discovery of faulty RAFOS floats from premature surfacing because they 

had sunk below the bailout depth caused the planned Mexican float deployments in August 2011 

to be cancelled; however deployment of SS-4 in Mexican waters proceeded as planned. Figure 

1-4 shows the float deployment cruises, which are detailed in Chapter 2. Vessels used for 

deployments included R/V Pelican and R/V Acadiana (Louisiana Universities Marine 

Consortium (LUMCON), Cocodrie, Louisiana), R/V Bellows and R/V Weatherbird II (Florida 

Institute of Oceanography (FIO), St. Petersburg, Florida), and B/O Justo Sierra (Tuxpan, 
9 

Mexico). For the B/O Justo Sierra cruises, equipment loading was done at Brownsville, Texas, 

making use of staging and custom facilities there. 

Figure 1-4. Schedule for data collection

1.4 STUDY PARTICIPANTS

Participants in the study included the science team of principal investigators (PIs), support 

personnel, both at Leidos and the home institutions of the science team. The science team and 

their primary areas of responsibility are listed in the following table. 

PI Affiliation Responsibility 

Dr. Amy Bower Woods Hole Oceanographic 

Institution (WHOI) 

RAFOS and APEX floats, 

deep circulation, whole and 

western Gulf. 

Dr. Paula Pérez-Brunius Centro de Investigación 

Científica y Educación 

Superior de Ensenada 

(CICESE), Mexico 

RAFOS floats in Mexican 

waters, deep circulation in the 

western Gulf. 

Dr. Robert Leben University of Colorado Remote sensing and altimetry. 

Dr. Peter Hamilton Leidos, Inc. Deep circulation under the LC 

and water mass analysis. 

Ms. Heather Furey (WHOI) supported Dr. Bower in all aspects of float preparation, data QA/QC, 

and analysis products, and Mr. James Valdez and Mr. Brian Guest (WHOI) provided laboratory 

and at-sea preparation and activation of sound sources and floats for deployment, respectively. 

Ms. Terry McKee (WHOI) was responsible for APEX data acquisition and monitoring. At 

CICESE, Javier Rodríguez Outerelo and Paula García assisted Dr. Perez-Brunius in the analysis, 

and Dr. Orens de Fommervault analyzed the chlorophyll data. Leidos Management and 

Logistics personnel supported the science team as follows: 

Dr. Peter Hamilton, Program Manager 

Mr. James Singer, Logistics Manager and US Cruise Chief Scientist 

Mr. Paul Blankinship, Data Manager and US Cruise Deck Operations Leader 

Sound Source

Deployment

Float 

Deployments

SS-4 SS-2 Replacement SS-1 Replacement Recovery

Remote

Sensing

US Cruises Mexican Cruises Ship-of-Opportunity Cruises
10 

Sound source moorings and float deployments in US waters were the responsibility of Leidos. 

RAFOS and APEX float preparation was the responsibility of WHOI. On one ship-ofopportunity cruise, WHOI personnel working on a GoMRI program deployed floats. 

Deployment and recovery of a sound source mooring, and the deployment of floats in Mexican 

waters was the responsibility of the CANEK mooring group (CICESE). 

1.5 MAJOR EVENTS

1.5.1 Loop Current and Loop Current Eddies 

Major events in the deep Gulf are associated with the LC and the shedding of LC eddies and 

their subsequent translation across the western basin, followed by interaction with the western 

slope topography and eventual dissipation. Both the LC and LC eddy behavior are highly 

variable, and the four years of the field study show a range of times between shedding, eddy size, 

western basin transit times, and differing fates. LC eddies are named by an oil industry group 

(Eddy Watch), and these names are adopted here. Major eddies Hadal, Icarus, Jumbo, Kraken 

and Nautilus were shed between July 2011 and July 2015. The missing eddies Lazarus and 

Michael were small and dissipated shortly after being shed. Detachments are determined when 

the 17-cm SSH contour around the LC breaks (Leben 2005), and as LC eddies may detach and 

reattach to the LC multiple times before breaking free, only the last detachment is known as a 

separation (Figure 1-2). Usually, if an eddy center crosses the 90°W meridian, it is separated 

from the LC and moves into the western basin. In this study, both the CCAR and AVISO SSH 

map products are used, and because they use different mean geopotential surfaces, and are tuned 

to resolve slightly different mesoscales, the numbers and times of LC 17-cm contour breaks and 

arrival times at 90°W do not always agree. Table 1-1 gives the separation and 90°W arrival 

dates as determined from the CCAR and AVISO maps. 

Table 1-1. LC Eddy Dates from Altimetry

LCE Separation Dates Crossing of 90°W by LCE Centers

AVISO CCAR AVISO CCAR

Hadal 23 Jul 2011 14 Aug 2011 22 Aug 2011 24 Aug 2011

Icarus 20 Dec 2012 3 Feb 2012 10 Feb 2012 12 Feb 2012

Jumbo 11 Jun 2012 4 Aug 2012 30 Oct 2012 9 Nov 2012

Kraken 19 May 2013 9 Aug 2013 30 Jul 2013 9 Aug 2013*

Lazarus 5 Nov 2014 11 Nov 2014 5 Nov 2014* 11 Nov 2014*

Michael **   ** 

Nautilus 17 May 2015 18 May 2015 17 May 2015* 18 May 2015

** not detected at the 17 cm level

* separation occurred when eddy center was west of 90°W

Separation date determined by breaking of 17-cm tracking contour
11 

Another view of LC eddy separations, using CCAR SSH, is given in Figure 1-5, where the area 

of the LC, and the area of the LC plus the LC eddy when detached, are plotted over the four 

years. The LC eddy area is only counted if it intersects a line from the Mississippi delta to the 

northeast corner of the Yucatan peninsula. When the two lines separate, a LC eddy is detached. 

Moreover, the degree of retreat of the LC, after a detachment or separation, is clear from the 

relative reduction in LC area, and the subsequent northward extension by the growth in area. 

Figure 1-5 also has time lines of each eddy’s history: after separation but before reaching 90°W; 

transit time from 90°W to the western boundary; and time interacting with the western slope 

until the last observed location (i.e., the 17-cm contour disappears). This is usually near or 

adjacent to the western boundary, but for Lazarus it occurred prematurely, and for Hadal, 

interactions with other cyclones moved it eastward off the slope, before dissipating. Figure 1-5 

shows a brief chronology.

Hadal: The separation of Hadal was observed by the LC mapping array (Hamilton et al. 

2014; Hamilton et al. 2016). After separation, the eddy moved fairly rapidly 

westward taking about 3 1⁄2 months to reach the western boundary. Just before 

reaching the western slope, the eddy elongated in an east-west direction at the end of 

November 2011, and divides into two. The two parts rotated around each other and 

eventually merge around the end of December 2011 ~24°N, 94°W. The merged eddy 

began to decay and also move rapidly southward towards the Bay of Campeche. 

Icarus: Icarus eventually shed, after several reattachments to the LC, in February 2012, and 

took about six months to cross the western basin in a southwestward direction. On 

encountering the western slope, the eddy elongated and moved offshore and 

northwards before fading away. 

Jumbo: The retreat of the LC after Hadal and Icarus was moderate to ~26°N, and the 

subsequent extensions of the LC to form Icarus and Jumbo were fairly quick. Jumbo 

lived up to its name and was initially a very large eddy, > 300 km in diameter, with a 

distorted front. Large eddies seem to have difficulty moving past the delta, and this 

was the case for Jumbo as it spent three months to reach 90°W, during which time it 

split into two and then the two fragments merged into a single average sized eddy 

later at ~91°W. It moved southwestward and included an interaction that elongated 

the eddy in January 2013 and moved it rapidly to the western slope at ~22°N, where it 

rapidly dissipated. 

Kraken: The retreat of the LC after Jumbo was to 24°N, and thus the extension of the LC 

northwards and the formation of Kraken took about nine months. Until the 

appearance of Kraken at 90°W, there was an interval of ~7 months from the death of 

Jumbo in February 2013 during which there was no LC eddy presence in the western 

Gulf. Kraken was initially also a large eddy, but reduced in size with several 

interactions with the LC, and made it to 90°W as a fairly circular eddy of ~300 km in 

diameter. It transited the western Gulf on a more westward than southwestward path 

in about five months. On reaching the western slope, it elongated towards the 

northeast and moved offshore and northward along the slope until dissipating in 

March 2014. This latter slope interaction phase is reminiscent of that described by 

Vukovich and Waddell (1991). 
10

3 km2

LC Area & Area + Eddy

Kraken

Kraken

Laz

Lazarus

Icarus

Icarus

Jumbo

Jumbo

Hadal

Hadal

N

25°N

22°N 94°W

23°N

24.5°N

22°N

22°N

25°N

25.5°N

27

Splits @ 26°N 89°W

Merge @ 91°W

Separation to 90°W

25°N

90°W to Western Slope

Encounter Latitude

24°N

Dissipation inWestern Gulfwith last location

Figure 1-5. Time line of major LC eddy events in the Gulf of Mexico

Bottom panel shows the area of the LC (green line) and the area of the LC plus detached eddy (blue line) from tSSH contour before it completely crosses a line from the delta to the northeast Yucatan. Legend gives the key tdifferent phases of the named eddy’s life history. If the longitude is not given for the last location, then the locawestern slope. In the western basin, AVISO 17-cm SSH contour is used. 

12 
13 

Lazarus: The growth of Lazarus was unusual. The LC remained south of 25°N after the 

shedding of Kraken for about four months, and then began a long period of 7–8 

months of a slow extension to the north. Lazarus detached in July 2014 for about one 

month, but was reabsorbed into the LC, which extended along the northern slope, 

south of the Mississippi delta. Eventually the LC shed a very small anticyclone in 

October 2014, which rapidly dissipated. The LC did not retreat much after this, and a 

similar initial eddy (Michael) was detached in January 2015 only to be reabsorbed a 

month later. On the third try, Nautilus was shed in May 2015, but the LC remained 

well extended north of its more usual retreat latitudes of 25–26°N. 

Nautilus: Nautilus was shed far enough west to be able to move out of the influence of the LC, 

just before the final set of floats surfaced and reported their data. The result of the 

long period (~15 months) of LC extension, between the death of Kraken and the 

appearance of Nautilus, is that the western basin had again no LC eddy presence.

The four years of the field study covered a wide variety of conditions, including long intervals of 

no LC presence in the northeast Gulf and LC eddy absence in the western Gulf. The first three 

eddy sheddings (Hadal, Icarus and Jumbo) followed established scenarios in the historical 

altimeter record. The shedding of Kraken took place ~12 months after Jumbo and was still 

within historical norms. The very long interval of LC extension towards the Mississippi delta 

between Kraken and Nautilus seems outside the historical norms (Lazarus and Michael were 

really non-events, and only affected the eastern basin). 

1.5.2 Hurricanes and Tropical Storms 

Hurricanes and tropical storms are frequent in the summers in the Gulf. The expected response 

to hurricanes is through an inertial-internal wave wake that may penetrate below the thermocline 

(Brooks 1983; Shay et al. 1989). The inertial period is about one day, and this should be 

resolved by the eight-hour sampling of the RAFOS floats. The four years of the field program 

were not particularly active for tropical storms in the summers; indeed for the summer of 2014 

there was only one minor tropical depression in the Bay of Campeche. In most years, the Bay of 

Campeche is subject to a few westward moving tropical depressions in the summer. Significant 

hurricane and tropical storm tracks that crossed the deep Gulf, and occurred during the field 

study are given in Figure 1-6, where all the storms move from south to north. Strong mixing was 

observed at the shelf break in the De Soto canyon for hurricane Isaac and tropical storm Debby 

by Hamilton et al. (2015). Significant inertial responses were not observed for any of these 

storms in the 1500 and 2500-m RAFOS tracks. 

1.6 ANALYSIS FOCUS

Lagrangian floats and Eulerian mooring array observations offer two differing perspectives on 

circulation. Mooring arrays give very detailed measurements in time, but are limited in spatial 

extent, and so are better designed to resolve features and events that propagate through a limited 

area of the ocean. Float trajectories can range far and wide, with no limitations as far as national 

boundaries are concerned, but often floats only sample circulation features (e.g., eddies) for a 

short interval before dispersing. LaCasce (2008) reviews methods for analyzing Lagrangian 

float statistics. They are a natural analog of how pollutants disperse in the deep Gulf. 
H. Isaac

26 - 30 Aug 2012

T.S. Debby

23 - 27 June 2012

T.S. Andrea

5 - 7 June 2013

H. Ingrid

13 - 17 Sep 2013

T.S. Karen

3 - 6 Oct 2013

Figure 1-6. Tropical storm and hurricane tracks

Major storm tracks across deep-water regions of the Gulf during the period of the field study. 

Isobaths at 100, 1000, 2000, and 3000 m.

14 
15 

The Gulf can be considered as a two-layer system: the eddy-dominated upper layer is strongly 

stratified with vertically sheared flows, and the lower layer with weak stratifications and (nearly) 

depth-independent flows. The interface is approximated by the depth of the 6 °C isotherm, 

which is in the lower thermocline and has a mean depth of ~800 m. In this type of system, it is 

expected that the thickness of the lower layer from the 6 °C isotherm to the sea floor (defined as 

h6) is an important constraint on the flows through potential vorticity conservation (f / h6), where 

f is the Coriolis parameter. The Gulf also has differing dynamics in the eastern and western 

basins. In the east, the LC and LC eddy shedding variability and processes dominate, whereas in 

the west, the westward transits of LC eddies, their interactions with smaller scale cyclones and 

anticyclones, and slope topography dominate the upper layer. Their influences on the lower 

layer in the western basin are largely unknown. This is one focus of the analysis in this report. 

Recent results on the LC (Donohue et al. 2016a; Donohue et al. 2016b; Hamilton et al. 2016) 

have changed ideas on the dynamics. In the eastern basin, the analysis focus is on confirming 

that similar baroclinic instability mechanisms that transfer EKE to the lower layer, apply for a 

larger selection of eddy shedding events. Because the floats simultaneously measure flows over 

a large section of the eastern Gulf, analysis of radiation by TRWs and possible westward 

translation of deep eddies generated by LC instabilities are aspects of the circulation that could 

not be investigated by the moored array in the LC dynamics study (Hamilton et al. 2014). 

Steep escarpments of order 500 to 1000 m vertical extent, largely surround the deep basins of the 

Gulf (Figure 1-1). A boundary current along the northern Sigsbee escarpment has been observed 

by moorings and floats (Hamilton and Lugo-Fernandez 2001; Donohue et al. 2006; Hamilton 

2009). With this more extensive float database, investigations of the current’s continuity around 

the western basin, variability, and influences on events in the central western basin, are topics to 

be investigated. Another important topic that can be addressed by analysis of float trajectories is 

the connectivity of various regions of the deep Gulf basins. For example, “What is the 

probability that floats in the eastern basin are transported in to the western basin, and floats in the 

western basin are transported into the eastern basin?” and “Do the boundary currents play 

important roles?” 

Readers of this report will note many new results on deep circulation processes in the Gulf. This 

will be a challenge for numerical modeling of Gulf circulation, to both reproduce the 

phenomena, and provide dynamical explanations for the results. Predictive and hindcast 

modeling that assimilate SSH and SST, has been quite successful for upper-layer eddy 

circulations (see Oey et al. (2005) for a review of Gulf modeling studies). How well these 

models perform for the lower layer has not been fully investigated. In this report, some 

preliminary investigations of the Hybrid Coordinate Ocean Model (HYCOM) results vis-à-vis 

the float trajectories are performed. 
16 

1.7 REPORT ORGANIZATION

The following chapters of the report are organized as follows: 

Chapter 2: A description of the experiment and experimental methodology. 

Chapter 3: A description of the characteristics of the float trajectories and upper-layer 

eddy events, followed by statistical analysis of the observed flows including a census of 

deep eddies, and an analysis of LC variability and LC eddy transits across the western 

Gulf from altimetry. 

Chapter 4: Primarily water mass analysis using the profiling float CTD data. A short 

section on Chlorophyll-A profiles is included. 

Chapter 5: Analyses of the connectivity of sub-regions of the Gulf, the deep LC, western 

basin boundary currents and gyres, upper-lower layer interactions in the western basin, 

and model performance studies. 

Chapter 6: A summary discussion of major results from the study, followed by 

recommendations to further exploit this database and for further studies to fill knowledge 

gaps for the Gulf.
17 

CHAPTER 2: METHODOLOGY 

2.1 DRIFTING SUBSURFACE FLOATS

The drifter component of the Deep Lagrangian Study involved deployment of two types of 

subsurface drifting floats. These consisted of a Gulf-wide deployment of deep drifting RAFOS 

floats (Rossby et al. 1986), as well as the deployment of profiling RAFOS-equipped APEX 

floats. 

The primary objective of the program was to definitively map the subsurface circulation at one 

pressure, 1500 dbar, which is the level of mid-depth circulation below the direct influence of the 

Loop Current. Secondary objectives were to investigate 2500-dbar circulation and to provide 

Gulf-wide coupled bio-optical and CTD profile data. At 1500-dbar, the floats were often 

deployed in pairs, to accommodate dispersion studies at this pressure level. Also, 2500-dbar 

RAFOS were often deployed simultaneously with 1500-dbar RAFOS to provide information on 

water column connectivity. 

One hundred eighty-six RAFOS floats, programmed to record temperature, pressure, and 

position data every eight hours, were deployed with mission lengths of up to two years. Eight 

RAFOS-equipped APEX floats were also deployed. These latter floats were programmed to 

profile from their residence depth at 1500 dbars to the surface every 14 days, recording biooptical data as well as temperature, salinity and pressure (There were some variations from this 

profiling frequency in the first year of the study). A RAFOS float provides underwater position 

data by recording the travel time of acoustic signals emitted from an array of fixed-position 

sound sources. To support the RAFOS and APEX floats, an array of four moored sound sources 

was deployed across the Gulf. Each source emitted a 260 kHz swept signal every eight hours, 

and the RAFOS and APEX floats were programmed to listen for these signals at the same time 

as the transmissions. The floats recorded the strongest of these signals, and position was 

calculated using a least squares algorithm on the identified source signals. 

The original program was designed to utilize 120 RAFOS and six APEX floats, but these 

numbers increased as the program was extended with warranty replacement of malfunctioning 

instruments. The initial meeting with the Science Review Group (SRG) concluded with the 

recommendation that the shallow and deep floats be split 80–20, so that the mid-depth mean 

circulation could be determined with some statistical confidence. The 2500-dbar floats were to 

be exploratory in nature. Figure 2-1 shows the deployment locations of the APEX and RAFOS 

floats, and the sound source moorings. The staggered RAFOS float deployment plan began in 

2011, and the last floats were deployed in 2014. The initial RAFOS float mission length was set 

at two years, and APEX floats were deployed with the intent that they would continue profiling 

until their batteries were exhausted. Most were still profiling at program end in June 2015. 
3500 m

3000 m

1000 m

200 m

2000 m

Tuxpan

Brownsville

Cocodrie

SS-1

SS-3 SS-2

SS-4

R/V Pelican16-22 July 2011R/V Pelican14-17 Jan 2012

B/O Justo Sierra23 Jun - 10 July 2012

B/O Justo Sierra10-24 Sept 2012

R/V Bellows15-19 Sept 2012

R/V Acadiana25-26 Sept 2012

B/O Justo Sierra15 Jun - 1 July 2013R/V Pelican (SOOP)1-24 Aug 2013B/O Justo Sierra23 Sept - 5 Oct 2013

SS-5

Figure 2-1. Launch locations of floatsRAFOS and APEX (shown with a black outline) deployment locations color coded by cruise. RAFOS locations multiple (up to 3) floats deployed. Sound source locations are given by the red diamonds.

18
19 

2.1.1 RAFOS Float Description 

Seascan, Inc. at Falmouth, Massachusetts, manufactured the RAFOS floats. The model type was 

DLD2, incorporating Iridium data transmission, and built with an Ultra-High 

Density Plastic (UHDP) threaded spacer between the glass float body and the steel drop-weight. 

The build for the two sampled depths of the floats required different glass lengths and diameters, 

and therefore different thermal expansion coefficients were used for calculating the ballast 

weight of the two types of RAFOS floats. All of the RAFOS floats were ballasted in the highpressure tank at the WHOI. 

In the early days of the RAFOS observing program, there were three modes of failure, apparently 

due to errors in manufacturing. The manufacturer promptly replaced all the affected floats under 

warranty. Hence, in total, 186 RAFOS floats were deployed, 66 more than the 120 floats that 

were originally planned. In addition, several scientists from the United States and Mexico, 

leading other research programs in the Gulf, generously contributed ship time to assist with 

deployment of replacement floats. 

The first reported failure mode was due to an error in a minor design modification whereby the 

pressure sensor through-hull fitting was seeded to touch at the corner with the end cap body. 

Under pressure, this unstable contact point caused water to leak inside the float: hence these 

floats were dubbed “leakers”. These floats would sink past their target pressures and reach the 

‘bail-out’ pressure within about 10–30 days, and then automatically jettison the ballast weight, 

returning to the surface to transmit whatever data had been collected to that point. 

A second failure mode occurred in 20 shallow (i.e., 1500-dbar) floats deployed in January 2012. 

These floats quickly sank (within a day) and did not re-surface, presumably because the sinking 

happened too quickly for the microprocessor to realize that the float had gone too deep. These 

floats were called “sinkers”. The cause of this failure mode was not determined. 

The final failure mode, “overwrites”, was due to a firmware modification that limited the data 

buffer to a 630-day length instead of the expected two years (730 days). In these floats, after 630 

days, new data began writing over the first 100 days of data collected at the beginning of the 

float mission. When the float surfaced, the data transmitted were from the start of the reset 

buffer file (from day 631) to the last new data recorded. All other data (originally days 101-630) 

were saved on the instrument, but could not be transmitted, and so were lost, as the floats were 

not designed to be recovered. This problem was identified in late August through early 

September 2012, when RAFOS float colleague Dr. Henrik Søiland’s floats (for a different 

project) surfaced and returned only the last 100 days of data. Subsequently, all remaining floats 

were re-programmed to 630-day mission lengths. 

Sound sources were a mix between Teledyne Webb Research (TWR) sources, and a newer 

design from URI-WHOI, equipped with a spherical transducer. In total, two newly built TWR 

sources, one newly built URI-WHOI-design source, and one refurbished TWR source were 

deployed in July-August 2011. The sound sources were moored at 1000 meters depth and 

ponged every eight hours to match the RAFOS listening schedule. Table 2-1 presents the sound 

source mooring locations, transmission schedules, clock drifts, and notes on ownership, build, 

and recovery. The original URI-WHOI source did not operate properly and the refurbished 
20 

TWR source failed on 18 August 2012. These two sources (noted as SS-2 and SS-1, 

respectively) were replaced with borrowed TWR sources from Dr. Kevin Speer (Florida State 

University - FSU) at the SS-2(b) and SS-1(b) sites in January 2012 and February 2013, 

respectively (see Table 2-1). In exchange, the URI-WHOI source was repaired and deployed by 

Dr. Speer at site SS-5 for use in the Deep-C RAFOS Float program near De Soto Canyon in the 

northeastern Gulf in May 2012. 

Table 2-1. RAFOS Sound Source Information

Name 

(Serial 

Number)

Build / Owner Moored 

Location

Operational 

Period

(mm-dd-yyyy)

Pong 

Schedule

(UTC)

Drift

(seconds/ 

day)

Comments

SS-1 

(SN 50)

Webb 

Research / 

WHOI

25.59°N 

85.00°W 

07-19-2011 

through 

08-18-2012

02:15, 10:15, 

18:15

0.007, 

estimated

This source failed early, 

and was replaced on 

02-19-2013

SS-1(b) 

(SN 54)

Webb 

Research / 

FSU, Speer

25.60°N 

85.00°W 

02-19-2013 

through 

05-10-2015

02:15, 10:15, 

18:15

0.0093, 

measured

Refurbished source lent 

by K. Speer at FSU.

SS-2 

(SN 99)

URI-WHOI / 

BOEM

27.00°N 

87.50°W 

07-16-2011 

through 

09-01-2011

02:45, 10:45, 

18:45

0.000, 

estimated

Source signal 

weakened, source 

recovered 01-12-2012

SS-2(b) 

(SN 55)

Webb 

Research / 

FSU, Speer

27.00°N 

87.50°W 

01-12-2012;

Not recovered

02:45, 10:45, 

18:45

-0.001, 

estimated

New TWR source. Tried 

to recover source June 

2015, but release 

battery dead. Still 

moored as of July 2016.

SS-3 

(SN 60)

Webb / BOEM 26.25°N 

93.00°W 

07-21-2011 

through 

05-07-2015

02:02, 10:02, 

18:02

0.000,

measured

Source recovered.

SS-4 

(SN 59)

Webb / BOEM 23.68°N 

96.53°W 

08-23-2011 

through 

01-18-2015

02:30, 10:30, 

18:30

0.004, 

estimated

Source recovered, but 

dead, no final clock 

measurement possible.

SS-5 

(SN 99)

URI-WHOI / 

BOEM 

29.40°N 

86.98°W 

05-11-2012 

through 

05-17-2013 

03:00, 11:00, 

19:00

0.000, 

estimated

BOEM source from site 

SS-2 used in this 

position for FSU Deep-C 

program in exchange for 

this program using FSU 

source at site SS-1(b).

2.1.2 Deployment and Recovery Cruises 

In total, 15 cruises were completed: nine for float and sound source deployments, and six for 

sound source deployments or recoveries only. Six of the cruises were funded by outside projects 

as ship-of-opportunity efforts following the replacement of defective floats. Table 2-2 details 

each of the cruises, the respective instruments deployed, and other relevant information. 

2.1.3 Drifting Float Data Return 

Data return for RAFOS floats and RAFOS-equipped APEX floats that completed missions was 

76%. This includes the batch failure data. The data return was 96% when not including the 

floats that were replaced. Total float-days of data collected were 70,789 days, or 194 float-years. 

A graphic presentation of actual float data return and maximum possible float data return is 

shown in Figure 2-2. Appendix A provides launch and surfacing information, mission 

parameters, and float clock drifts for individual floats. Small plots of each float trajectory are 

displayed as well. 
 

RAFOS and APEX RAFOS Float Duration Chart

2012 2013 2014 

1109

1110

1111

1122

1123

1151

1155

1156

1157

1160

1161

1108

1114

1115

1120

1121

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1154

1112

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1126

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1125

1158

1162

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1128

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 2015 2012 2013 2014 2015

 

 

 

 

 

 

 

 

 

 

7573

7544

7658

7590

8405

7671

7036

8512 1040

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1024

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1100

1101

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1066

1096

1097

1098

1099

Figure 2-2. Timelines of observations made by RAFOS and RAFOS-equipped APEX floats 

APEX floats are presented in the top-left panel. Gray and blue lines represent mission length 

and data return, respectively.

21
22 

Table 2-2. Instrument Deployment Cruises during the Deep Lagrangian Study

Cruise 

No.

Dates Port / Vessel No. 1500 

dbar 

RAFOS

No. 2500 

dbar 

RAFOS

No. 

APEX

No. 

Sound 

Source

Comments

1 15–23 July 

2011

Cocodrie, LA to 

Cocodrie, / R/V 

PELICAN

20 8 2 3 

(SS-1, 

SS-2, 

SS-3)

Eastern and Central Gulf; Deploy 

Sound Sources SS-1, SS-2, SS-3. 

All RAFOS deployed later replaced 

due to pressure leak; one APEX 

replaced due to malfunction of 

inflation bladder. One partially 

functioning APEX left and partial 

credit awarded. All were overwrite 

floats.

2 16–27 Aug. 

2011

Brownsville, TX 

to Tuxpan, 

Mexico / B/O 

JUSTO 

SIERRA

0 0 0 1 

(SS-4)

Western Gulf and Bay of 

Campeche, Sound Source SS-4; 

Was supposed to deploy 12 1500-

m RAFOS and 2 APEX but held 

back due to pressure leak 

discovered on Cruise 1.

3 11–17 Jan. 

2012

Cocodrie, LA to 

Cocodrie / R/V 

PELICAN

20 repeat 8 repeat 2 1 

(SS-2(b))

Eastern Gulf, sound source 

SS-2(b) replacement. Eastern Gulf 

re-deployment of pressure-leak 

floats. All shallow floats had batch 

failure (deemed ‘sinkers’) and 12 

that were no-shows were later 

replaced. All were overwrite floats.

4 10–15 May 

2012

Cocodrie, LA to 

Cocodrie / R/V 

PELICAN

0 0 0 1 

(SS-5)

DEEP-C consortium: FSU 

dispersion study in De Soto 

Canyon; Sound source SS-5 is 

Rossby-WHOI retune; 30 FSU 

RAFOS deployed, ballasted for 

300 decibars. 

5 20 June– 

14 July 

2012

Brownsville, TX

to Tuxpan, 

Mexico / B/O 

JUSTO 

SIERRA

12 0 2 0 CICESE Yucatan channel and 

Caribbean mooring turn around 

cruise. Make-up for August 2011 

non-deployment; one APEX was 

replacement for failed float from 

July 2011 deployment. Overwrite 

floats, were replaced.

6 6–26 Sept.

2012

Brownsville, TX

to Tuxpan, 

Mexico / B/O 

JUSTO 

SIERRA

24

(22 deployed)

0 1 0 Western Gulf and Bay of 

Campeche. Two RAFOS shipped 

back because could not turn on at 

dock. Reasons: One IMEI typo at 

JouBeh; one malfunctioning board 

Seascan.

7 14–20 Sept. 

2012

St Petersburg, 

FL to St. 

Petersburg / 

R/V BELLOWS

20 16 1 0 Eastern Gulf

8 24–29 Sept.

2012

Cocodrie, LA to 

Cocodrie / R/V 

ACADIANA

20 0 0 0 Central Gulf

9 18–21 Feb. 

2013

St. Petersburg, 

FL to St. 

Petersburg / 

R/V 

WEATHERBIRD II

0 0 0 1 

(SS-1(b))

Southeastern Gulf. Replacement 

source SS-1(b) – due to source 

failure. Originally scheduled 29-31 

January cruise delayed due to gale 

in Gulf.

10 16–19 May 

2013

Cocodrie, LA to 

Cocodrie / R/V 

PELICAN

0 0 0 1 

(SS-5) 

Deep C Consortium De Soto 

Canyon. FSU sound source 

recovery. 

11 11 June–1 

July 2013

Brownsville, TX

to Tuxpan, 

Mexico / B/O 

JUSTO 

SIERRA

8rep 0 0 0 CICESE Yucatan channel and 

Caribbean mooring turn-around 

cruise. Data storage replacement 

floats. 
23 

Cruise 

No.

Dates Port / Vessel No. 1500 

dbar 

RAFOS

No. 2500 

dbar 

RAFOS

No. 

APEX

No. 

Sound 

Source

Comments

12 1–30 

August 

2013

Cocodrie, LA to 

Cocodrie / R/V 

PELICAN

16rep

(15 deployed)

8rep 0 0 Central Gulf. Data storage 

replacement floats. One float not 

deployed due to failure to send 

start-up message. J. Ledwell 

WHOI/TAMU tracer experiment.

13 10 Sept–6 

Oct. 2013

Brownsville, TX

to Tuxpan / B/O 

JUSTO 

SIERRA

9rep 0 0 0 Western Gulf and Bay of 

Campeche. Data storage 

replacement floats. 

14 20–25 May 

2015

Tuxpan to 

Tuxpan / B/O 

JUSTO 

SIERRA

0 0 0 Recovery 

of sound 

source 

SS-4.

Southwestern Gulf. Sound source 

stopped transmitting a few weeks 

before recovery. It was found to 

have corroded, and final clock 

offset check was not possible.

15 6–2 May 

2015

Cocodrie, LA 

to Cocodrie / 

R/V PELICAN

0 0 0 Recovery 

of sound 

sources 

SS-1(b) 

and SS3.

Eastern and Central Gulf. Release 

battery died on sound source SS2(b), and this source remains 

moored in the northeastern Gulf as 

of July 2016. 

For most RAFOS floats, the initial clock offset was 0 seconds, and final clock offset after two 

years was at maximum ±3 seconds, and average offset of all RAFOS floats other than serial 

numbers 1226-1248 was less than ±1 second. All clock corrections were applied as linear drifts. 

Floats with serial numbers 1226-1248 had much larger final clock offset; maximum was -87

seconds over the two-year period, and the mean was -40 seconds. This larger drift was expected 

and is attributable to a different build of the RAFOS float, where the high-accuracy clock was 

not installed. 

RAFOS-equipped APEX clock drifts were also applied linearly. Clock adjustments occurred at 

varied intervals through individual APEX missions and the magnitude of each clock adjustment 

was recorded and returned in the engineering data. Each APEX instrument had approximately 

10 clock corrections over the course of deployment, at irregular intervals. 

Plotted in Figure 2-2 is a bar plot of RAFOS and RAFOS-equipped APEX instrument mission 

periods. Gray bars indicate the time when the instrument was in the water. Blue bars show the 

time for which trajectory data exist. If a gray dot is after the float number, the float did not 

return data at the end of mission, and the float was considered a “no-show”. If there is a gray bar 

but no blue bar overlay (e.g., 1120), the instrument reported at time of expected surface, but 

failed to transmit data. Seen in the left-hand panel are the float batches, which had very short 

missions (e.g., 1040 and 1052) due to pressure leaking and early mission failure. Also present 

are the batches of floats with the “overwrite” problem (e.g., 1041-1043) previously described, 

and the batch failure, which were mostly no-shows (e.g., 1017 and 1030) due to the fast sinking 

and presumable destruction of the instruments due to hydrostatic pressure. 

Data distributions at the two different pressure surfaces are in Figure 2-3 for 1500-dbar and 

2500-dbar floats. The data distributions are different, reflecting the SRG’s recommendation to 

focus more floats at the shallower level and to concentrate the smaller number of deeper float 

deployments in the eastern basin. 
1000

2000

3000

 18oN 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

1500-dbar RAFOS and APEX

Launch Location

1000

2000

3000

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Launch Location

2500-dbar RAFOS

Figure 2-3. Plot of all trajectory data

Top panel: 1500-dbar trajectory data collected from both RAFOS and RAFOS-equipped 

APEX floats, and bottom panel: 2500-dbar trajectory data collected from deep RAFOS floats. 

Green dots represent deployment locations.

24 
25 

At 1500 dbar, float deployments and trajectories cover the entire region. Approximately equal 

numbers of RAFOS floats were deployed east and west of the 90°W meridian. At 2500 dbar, 

float deployments are almost all in the eastern Gulf, east of 90°W. 

2.1.4 APEX Float Description and Sensors

The APEX floats were equipped to provide profiles of both physical (pressure, temperature, 

salinity) and bio-optical measurements. The profiling floats were built by Teledyne Webb 

Research, Inc. and interfaced with a CTD (SBE41-CP, SeaBird), bio-optical sensors (ECO 

FLbbCD-AP2, WET Labs, Inc.), and two-way Iridium communications, which allowed for both 

real-time data transmission and sampling plan adjustments when the floats were at the surface. 

The floats profiled the water column from the surface to a maximum depth of 1500 m, or 

shallower depending on bottom depth. The frequency of profiling was nominally set to 14 or 15 

days. In one case, for APEX S/N 7544, the frequency of profiling was varied during the mission. 

Though profiling was initially set to every 14 days (19 July 2011–17 August 2011), it was 

decreased to every five days to facilitate quick data return (17 August 2011–9 April 2012), and 

finally sampling was increased again to every 14 days in the Florida Straits to delay its leaving 

the Gulf (9 April 2012–18 December 2012). The vertical resolution of float bio-optical sampling 

was set to provide increasing resolution towards the surface, as follows: sampling every 5 m 

from 0–200 m water depth, every 10 m from 200–500 m water depth, every 25 m from 500–

1000 m water depth, and every 50 m below 1000 m water depth. 

The bio-optical sensor suite on the floats measure proxies of phytoplankton abundance 

(chlorophyll fluorescence), total particle concentration (optical backscattering), and colored 

dissolved organic matter (CDOM fluorescence). Calibration of the sensors was performed by the 

manufacturer (WET Labs, Inc.) prior to shipping for installation on the floats. Data from each of 

the sensors, output in counts, was converted to engineering units using the laboratory 

calibrations, resulting in chlorophyll concentration (Chl; μg L-1), the volume scattering function 

at a centroid angle of 140° and a wavelength of 700nm (β(140°, 700 nm); m-1sr

-1), and CDOM 

concentration (ppb). 

2.1.5 Profiling Float Data Return 

The deep exploratory component of the program was originally designed to have six profiling 

APEX floats. A summary table of APEX instruments is provided in Table 2-3. Of the original 

six APEX floats deployed, one (hull-ID 5762: board-ID 7573) failed immediately after 

deployment, and was replaced. A second (hull-ID 5763: board-ID 7544) provided profile data 

and surface position data only, and was not able to be tracked underwater. A third instrument 

(hull-ID 5764: board-ID 7036) returned faulty FLbb measurements. After running in-house 

tests, Wetlabs Inc. (manufacturers of the EcoPuck FLbb sensor) concluded that either (a) the 

rubber sensor cover was only partially removed at deployment or (b) a plastic bag covered the 

sensor suite. The data from this instrument may be recoverable to show relative changes, but not 

absolute value. One additional APEX float was bought and deployed after the start of the 

program to gather additional profile data. Table 2-3 summarizes the APEX float launch dates, 

final profile dates, number of profiles collected, and relevant notes on each instrument. Figure 

2-4 shows positions of APEX profiles color-coded by instrument and Figure 2-5 shows the 

timeline of profile data collected.
26 

Table 2-3. APEX Profile Time Periods and Properties Collected

Hull 

ID

Board 

ID 

Launch 

Date

Final 

Profile 

Date

Profile 

Interval

(Drifting

Days)

No. CTD 

Profiles 

Collected

No. BioOptical 

Profiles 

Collected

Comments

5762 7573 15 Jul.

2011

n/a 14-day 0 0 Weak inflation 

bladder prevented 

float from reporting to 

satellite.

5763 7544 19 July 

2011

18 Aug. 

2013

14-day, 

5-day

14-day

81 81 Profile data good, 

RAFOS board deaf, 

no trajectory 

information.

5760 7658 12 Jan. 

2012

11 Nov. 

2015

14-day 82 83

5761 7590 15 Jan. 

012

18 Nov. 

2015

14-day 99 99

5919 7671 10 July 

2012

21 Nov. 

2015

15-day 87 87

5764 7036 18 Sept. 

2012

1 Nov.

2015

15-day 79 80 FLbb measurements 

for this float faulty. 

6214 8405 23 June 

2012

19 Nov. 

2015

15-day 88 88

6279 8512 24 Sept. 

2012

12 Nov. 

2015

15-day 81 81

2.2 REMOTE SENSING

The remote sensing component of the Deep Lagrangian Study acquired remotely sensed satellite 

data in the Gulf to aid in the interpretation of in-situ float data collected at depth in the deep 

basins. A combination of satellite observing systems was used. To carry out this task, CCAR 

collected and processed a complementary suite of satellite observations from satellite altimeter 

and radiometer remote-sensing systems. This suite incorporates SSH data with high-resolution 

SST data and ocean color imagery. Satellite altimetry provides the all-weather multi-satellite 

monitoring capability required to map mesoscale circulation variability of the Loop Current in 

the eastern Gulf and to monitor mesoscale eddies in the western Gulf. During cloud-free 

conditions, multi-channel radiometry is used to supplement the altimetric sampling by providing 

high-resolution synoptic SST and ocean-color imagery for monitoring the rapidly evolving Loop 

Current and its associated eddies. 

2.2.1 Satellite Altimetry 

Satellite-altimeter data used are the 23-year record of near-real-time (NRT) and archival 

altimeter data streams available from the ERS-1, TOPEX/Poseidon (TOP/POS), ERS-2, Geosat 

Follow-on (GFO), Jason-1, OSTM/Jason-2, Envisat, Cryosat-2 and SARAL/AltiKa satellite 

missions spanning the time period from 1 January 1993 through 31 December 2015. Processing 

of the SSH data is based on NRT mesoscale analysis techniques designed to exploit the multisatellite altimetric sampling (Leben et al. 2002). This method has been used to operationally 

monitor the Gulf since November 1995. 
3000

2000

1000

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

7544

7590

7671

7036

7658

8405

8512

Figure 2-4. Map showing profile positions for the APEX floats

27
Jul2011 Jan2012 Jul2012 Jan2013 Jul2013 Jan2014 Jul2014 Jan2015 Jul2015 Jan2016

7544

7590

7671

7036

7658

8405

8512

Figure 2-5. Timelines of APEX profile observations

28
29 

Altimeter data from a total of six operational satellites were available during the 4-year time 

period from July 2011 through June 2015. Basic information on each of the missions is given in 

Table 2-4. The Gulf exact-repeat ground-track coverage provided by each of the satellites is

shown in Figure 2-6 except for Cryosat-2, which is in a 369-day repeat mission. The 17-day 

repeat ground track for GFO is also included in the plot to show the spatial sampling afforded by 

operational satellites in all four exact-repeat orbits historically occupied for mapping of ocean 

SSH variability. 

Table 2-4. Satellite Altimeter Missions Active during the Deep Lagrangian Study

Satellite Mission

Mission/Phase Launch Date Agency

Repeat

Period

(days)

Cross track Spacing

Degrees of 

Longitude km**

Jason-1 18 Dec1999 NASA & CNES 10 2.83 278

Jason-1/Geodetic 11 May 2012 to 21 Jun 2013* 406

Envisat 1 Mar 2002 ESA 35 0.72 71

Envisat/Extended 2 Nov 2010 to 8 Apr 2012*

OSTM/Jason-2 20 Jun 2008 NASA & CNES 10 2.83 278

Cryosat-2 8 Apr 2010 ESA 369

SARAL/AltiKa 25 Feb 2013 ISRO & CNES 35 0.72 71

*end of mission

**at 28°N

OSTM/Jason-2 and Jason-1 Tandem/Interleaved Mission: Jan 2009 to 12 Apr 2012

Tandem Mission Interleaved 10-day Repeat Orbits 10 1.42 140

During the study, the OSTM/Jason-2 and Jason-1 satellites were in tandem orbits with 

interleaved ground tracks through March 2012. NASA and CNES scientists selected this 

configuration to improve the sampling of mesoscale ocean circulation by precision altimeters 

(Dibarboure et al. 2011), as was done during the original tandem altimeter mission flown by 

Jason-1 and TOP/POS (Fu et al. 2003). The space/time sampling provided from a single satellite 

in the original 10-day repeat orbit of the TOP/POS satellite, which was selected to map the ocean 

topography associated with large-scale variations in SSH, is not sufficient for monitoring 

mesoscale variability. This is because of the large distance between neighboring ascending or 

descending tracks on the 10-day repeat ground track. In the Gulf, this spacing is 2.83° of 

longitude or about 278 km at 28°N, which is also the distance between crossover points between 

ascending and descending tracks. Moving a second satellite onto a parallel ground track that is 

midway between two adjacent ground tracks of the original TOP/POS orbit, reduced this 

distance by half to a cross-track spacing of 140 km at 28°N. At latitudes midway between intraand inter-satellite crossover points, the ascending/descending ground-track sampling improves 

by another factor of two to a cross-track spacing of just 71 km. Thus, the average cross-track 

sampling from the tandem mission data alone is 70 to 140 km within the study region. 
98 ̊W 96 ̊W 94 ̊W 92 ̊W 90 ̊W 88 ̊W 86 ̊W 84 ̊W 82 ̊W 80 ̊W

18 ̊N

20 ̊N

22 ̊N

24 ̊N

26 ̊N

28 ̊N

30 ̊N

200

200

200

200

200

200

200

1000

1000

1000

1000

1000

1000

2000

2000

2000

2000

3000

3000

3000

3000

Figure 2-6. Satellite altimeter exact-repeat ground track coverage in study region 

The 10-day repeat (thick red), 10-day repeat interleaved (thin red), 17-day repeat (green) 

and 35-day repeat (blue) ground tracks are shown.

30
31 

Tandem mission sampling from the OSTM/Jason-2 and Jason-1 orbits was more optimal than the 

TOP/POS and Jason-1 tandem mission since the orbits were interleaved in both space and time. 

For the OSTM/Jason-2 and Jason-1 interleaved tandem mission, advantage was taken of Jason1's still healthy propulsion system to put it into a much different position than TOP/POS was 

during the original tandem mission. In that mission TOP/POS remained so close to Jason-1 that 

the two spacecraft were flying almost simultaneously over the same region, even though the 

ground tracks were offset by 1.42 degrees of longitude. In the OSTM/Jason-2 and Jason-1 

tandem mission, Jason-1 was in the interleaved orbit, but much further away from OSTM/Jason2, about five days behind and on the other side of Earth. Nevertheless, its ground tracks still fall 

mid-way between those of OSTM/Jason-2 in the interleaved ground-track sampling pattern, with 

the interleaved Jason-1 track located next to Jason-2 with an apparent 5-day shift. There are 

three advantages to this configuration: 1) it is slightly better in terms of global aliasing, but is not 

the optimal 3.5-day offset tandem mission configuration described by Tai (2009), 2) it is slightly 

westwards propagating, and 3) it is slightly more efficient in terms of near-real-time observation 

as measurements from the same dates are as far away as possible (Dibarboure et al. 2011). 

After the OSTM/Jason-2 and Jason-1 tandem mission was completed, NASA and CNES jointly 

agreed to a partial passivation of Jason-1 because of the limited hardware redundancy remaining 

on the satellite. In April and May 2012, Jason-1 was moved into a “graveyard” orbit and extra 

fuel was depleted. The orbit was reconfigured into a 406-day repeat orbit for geodetic mapping, 

but was still capable of meeting the original oceanographic requirements of the mission. An 

entire 406-day repeat cycle was completed on 17 June 2013. Contact with the Jason-1 satellite 

was lost on 21 June 2013 because of the failure of the last remaining operational downlink 

transmitter on the satellite. The satellite was decommissioned on 1 July 2013. 

The addition of altimeter data from a 35-day repeat orbit augments the Jason-1 and 

OSTM/Jason-2 10-day repeat sampling, but at irregular sampling times. The 35-day exactrepeat-orbit (501 orbits per cycle) cross-track spacing is 0.72° of longitude or about 71 km at 

28°N. During the four-year study (1 July 2011 through 30 June 2015) the only satellite in this 

orbit was SARAL/AltiKa, which was launched on 25 February 2013. SARAL/AltiKa is a 

follow-on mission to Envisat, and is a collaboration between the Indian Space Research 

Organization (ISRO) and CNES (Space Agency of France). Envisat was in the nominal 35-day 

repeat orbit until 22 October 2010, when maneuvers were initiated to place the satellite in the 

drifting 30-day repeat cycle Envisat extended-mission orbit. A data gap of 35 days occurred 

between the last cycle of the nominal mission and first cycle of the extended mission. Envisat 

remained operational in this orbit until 8 April 2012. 

Satellite-altimeter data from the Cryosat-2 369-day repeat orbit were also used to augment the 

sampling available from Jason-1, OSTM/Jason-2, Envisat and SARAL/AltiKa. The 369-day 

repeat orbit includes a 29-day sub-repeat cycle with a spacing of 0.85° of longitude or about 84 

km at 28°N. GFO was not available during the study program, but historically provided crosstrack sampling along its ground track at a spacing of 1.47° of longitude or about 144 km at 28°N. 

 

Intuitively, mapping of SSH should improve if along-track altimeter data from multiple satellites 

are combined using objective analysis. SSH fields produced by combining multi-mission 

altimetry, however, may not be better than those constructed from Jason-1 or Jason-2 data alone, 
32 

if uniform errors and wavelength-frequency resolution, satisfying the Nyquist criteria, are 

required of the space-time gridded product (Tai 2009), as has been proposed by Greenslade et al. 

(1997). While these constraints may be reasonable for large-scale eddy sampling studies 

(Chelton et al. 2011) or mission design, the constraints are too limiting for mesoscale mapping in 

general. Operational multiple satellite objective mapping of the mesoscale circulation must 

therefore rely on sub-optimal smoothing to resolve eddy-scale wavelength, albeit with the 

commensurate errors caused by non-uniform sampling and aliasing. This is true of both formal 

“optimal” interpolation and sub-optimal objective analysis schemes. Nevertheless, the efficacy 

of the interpolated fields can be evaluated by comparing the interpolated altimetry with 

coincident in-situ data to quantitatively assess the processing and gridding strategies. Pressure 

Inverted Echo Sounder (PIES) data are an ideal in situ measurement for these type comparisons. 

Detailed comparisons of PIES and altimeter-derived SSH can be found in Hamilton et al. (2014). 

2.2.1.1 Along-Track Altimeter Data 

Along-track altimeter data were collected from the Radar Altimeter Database System (RADS) 

hosted by the Delft Institute of Earth Observation and Space Systems at the Delft University of 

Technology in the Netherlands. RADS (Naeije et al. 2000; Naeije et al. 2008) is an online 

database that contains validated and verified altimeter data and correction data products for most 

of the historical and operational satellite altimeter missions. Standard corrections from RADS 

were applied to the along-track data, including inverted barometer (MOG2D; (Carrère and Lyard 

2003), sea-state bias (CLS), ionosphere (smoothed dual-frequency correction for Jason-1 and 

OSTM/Jason-2; JPL Global Ionosphere Maps (GIM) for Envisat, SARAL/AltiKa and Cryosat2), wet troposphere (on-board radiometers) and dry troposphere (ECMWF) corrections. The data 

were further corrected for pole and solid-earth tides and the GOT 4.8 tide model, which is an 

update to Ray (1999), was used to remove ocean and load tides. Finally, the CNES CLS 2011 

Global Mean Sea-Surface height (Schaeffer et al. 2012) at the location of each once-per-second 

sub-satellite point was subtracted from the corrected range measurement to produce along-track 

corrected SSH anomaly (SSHA). 

 

To simplify further processing and analysis, collinear along-track SSHA datasets were created 

for each satellite when they were in an exact-repeat orbit by interpolating the sub-satellite alongtrack data to reference ground tracks. Each 10-day exact-repeat cycle of OSTM/Jason-2 and 

Jason-1 along-track SSHA data, and each 35-day repeat cycle of SARAL/AltiKa SSHA data 

were linearly interpolated to reference ground tracks based on precision orbit determination 

ephemeredes for each satellite, at once-per-second along-track spacing. The OSTM/Jason-2 

reference track used the computed ground track for TOP/POS cycle 18. The Jason-1 interleaved 

mission reference ground track was the same ground track with an offset in longitude 

corresponding to the nominal interleaved orbit. The SARAL/AltiKa 35-day reference ground 

track is based on repeat cycle 6 of the ERS-1 Multidisciplinary 1 Mission. Along-track data 

from the 30-day Envisat extended-mission orbit was stored and processed as non-repeat data as 

described for ERS-1 Geodetic Mission data in Leben et al. (2002). The along-track data from the 

369-day repeat Cryosat-2 mission and 406-day repeat Jason-1 Geodetic mission phase were 

processed in a similar manner. 
33 

2.2.1.2 Mesoscale Analysis 

The processing of the altimeter data is designed to retain mesoscale signals while filtering out 

longer wavelength altimetric errors. This filtering, however, also removes long-wavelength 

oceanographic signals. A detailed description of this processing and its implementation and 

validation in the Gulf can be found in Leben et al. (2002). The procedure incorporates data from 

all of the available satellites, treating each data set in a consistent fashion as follows: 

1. All sub-satellite data are referenced to an independent gridded mean sea surface by 

subtracting the mean sea-surface value at the sub-satellite point from each observation. 

2. Along-track loess filtering is used to remove residual orbit and environmental correction 

errors. The loess filter removes a running least-squares fit of a tilt plus bias within a 

sliding window from the along-track data. The window width is approximately 15° of 

latitude (200 once-per-second along-track data points). 

3. A multigrid preconditioned Cressman analysis with temporal weighing is used to 

objectively interpolate the along-track data to a 1/4° grid. 

4. A model mean-SSH field is added to the mapped-SSHA field to provide an estimate of 

the total SSH in the Gulf.

2.2.1.3 Mean-Reference Surface and Model-Mean SSH 

All along-track data are referenced to an existing altimetric mean sea surface. The data are 

treated as non-repeating ground tracks and are referenced directly to the mean sea surface by 

interpolating the mean sea-surface value to the sub-satellite point and subtracting it from the seasurface height. This applies an implicit cross-track geoid-gradient correction to the along-track 

data before interpolation to the reference ground tracks. 

The CNES CLS 2011 Global Mean Sea Surface (MMS_CNES_CLS11) was used as the 

reference surface for the altimetry. The MMS_CNES_CLS11 was calculated from seven 

satellite missions: TOP/POS, ERS-1 GM, ERS-2, Jason-1, TOP/POS interleaved mission, GFO, 

and Envisat. Jason-2 data were not included because the altimetry data set used to determine this 

mean sea surface was based on work done in 2009 using data validated up to and through 2008 

(Schaeffer et al. 2012). Although the data spans the time period from 1993 to late 2008, the data 

were processed to obtain a mean sea surface referenced to the time period from 1993-1999. The 

altimetry was preprocessed so that the data would be homogeneous in terms of corrections and 

less contaminated by the oceanic seasonal variability. The inter-annual signals were then 

removed to obtain a mean sea surface referenced to the seven-year time period, 1993-1999. 

To calculate the synthetic SSH estimates, the model mean SSH was computed for the time period 

1993-1999 from a data assimilation hindcast experiment performed by Drs. Lakshmi Kantha and 

Jei Choi for the MMS Deepwater Physical Oceanography Reanalysis and Synthesis Program 

(Nowlin et al. 2001). The data assimilation experiment used the University of ColoradoPrinceton Ocean Model (CUPOM) and assimilated along-track TOPEX and ERS-1&2 SSH 

anomalies (SSHA) into CUPOM on a track-by-track basis as subsurface temperature anomalies 

(Kantha et al. 2005). Before adding the model mean to the gridded SSHA fields, the 1993-1999 
34 

SSHA fields were averaged and the residual anomalous altimetric mean over the time period was 

removed. This references the SSHA fields to a mean spanning the same time period as 

determined from the CUPOM hindcast data assimilation experiment. The anomalous altimetric 

mean reflects the difference between the mean circulation contained in the CLS11 mean sea 

surface and the 1993-1999 data assimilation mean. More discussion of these differences is found 

in Section 2.4 of Hamilton et al. (2014). 

2.2.1.4 Objective Mapping 

During the study, altimeter data were available from five satellites: Jason-1, OSTM/Jason-2, 

Envisat, Cryosat-2 and SARAL/AltiKa. Figure 2-7 shows the availability and usage of data 

from each satellite over the time period from July 2011 through June 2015. In the plot, “actual” 

shows the satellites contributing data to the gridded analysis map for any given day. The 

“available” plot shows the days when there are valid data for a particular satellite. The 

“windowed” plot shows when there is no valid data on the actual day, but there are data within 

the window used for gridding. The “used” plot shows the days from which satellites could be 

chosen to be in the data product, but does not indicate if valid data are available in a specific 

region. “Actual” is computed by forming a logical AND operation between “used” and the 

union of “available” and “windowed”. 

Daily analysis maps of height anomaly relative to the mean sea surface were estimated using an 

objective analysis procedure (Cressman 1959), to interpolate the along-track data from the 

available satellite sampling on to a 0.25° spatial grid. The method uses an iterative differencecorrection scheme to update an initial guess field and converge to a final gridded map. A 

multigrid procedure provides the initial guess. Five iterations were used with radii of influences 

of 200, 175, 150, 125, and 100 km while employing a 100 km spatial decorrelation length scale 

in the isotropic Cressman weighting function. The data were weighted in time using a 12-day 

decorrelation time scale relative to the analysis date using a ±10 day window for the TOP/POS, 

Jason-1 and OSTM/Jason-2 data, and a ±17 day window for the ERS-1&2, Envisat, and GFO 

data. The details of the space and time-weighted version of the multigrid preconditioned 

Cressman analysis is described next and is based on the space-weighting only technique 

described in Hendricks et al. (1996). 

2.2.1.5 Objective-Analysis Procedure 

An objective-analysis (OA) procedure is used to interpolate the along-track SSHAs onto a 

regularly spaced 1/4° global grid. The OA algorithm is based on the iterative differencecorrection scheme of Cressman (1959). The initial guess field for the Cressman algorithm is 

supplied by an efficient multigrid procedure. 

A rough estimate of the 0.25° field is created by collecting the along-track SSHA data into 0.25°

grid cells. In grid cells where at least one SSH measurement is available, the average of all 

measurements within the cell is computed. Some of the grid cells may not contain data, 

depending on the spacing of ground tracks. The OA procedure is designed to fill in these data 

gaps by creating a SSHA field that is consistent with the along-track measurements. 
11 12 13 14 15 16

topex

saral

jsn2

jsn1

gfo

geosat

ers2

ers1

envisat

cryosat2

Actual

11 12 13 14 15 16

topex

saral

jsn2

jsn1

gfo

geosat

ers2

ers1

envisat

cryosat2

Available

11 12 13 14 15 16

topex

saral

jsn2

jsn1

gfo

geosat

ers2

ers1

envisat

cryosat2

Windowed

11 12 13 14 15 16

topex

saral

jsn2

jsn1

gfo

geosat

ers2

ers1

envisat

cryosat2

year

Used

Figure 2-7. Plot showing “actual”, “available”, “windowed”, and “used” altimeter data for each day

For operational altimeter satellites during the study program.

35
36 

The 0.25°-binned data can be used as an initial guess in the Cressman algorithm; however, 

having initial values in the empty grid cells can enhance the efficiency of the iteration procedure. 

A simple multigrid procedure is used to estimate values in cells where no altimeter 

measurements are available. Multigrid methods (Briggs 1987), rapidly solve a set of equations 

by working at several grid resolutions. In this case, if the along-track data were binned into 1° or 

2° grid cells, there would be fewer or even no empty ocean grid cells. Using a multigrid 

interpolation strategy to efficiently compute the means, a set of progressively coarser grids (0.5°, 

1°, 2°, ...) are created from the global 0.25° grid, and the average SSH is computed at all coarser 

grid resolutions in each cell containing data. The mean values are transferred back to the 

original 0.25° grid from the finest-scale grid containing a mean value coincident with that 

location. Finally, a fast red-black smoothing operator (Press et al. 1992) is used on the 0.25°

initial guess field to smooth high-frequency noise introduced by the multigrid interpolation. 

Cressman objective analysis uses an iterative-difference corrections scheme in which a new 

estimate of the SSH value for a given grid cell is equal to the sum of the previously estimated 

SSH at that location and a correction term. The correction term is forced by the difference 

between the estimated heights and the original data values over all grid cells within a specified 

radius of influence. A weight based on the number of original measurements within a grid cell is 

included in the correction term, as is a weight based on the distance of a grid cell from the point 

being updated. 

The nth iteration for the SSH at grid cell i is computed using: 

 (2-1) 

where the sums are taken over all m grid cells within the specified radius of influence R from the 

grid cell i being updated. The variables in Equation 2-1 are defined as: 

€

hi

n

 n

th iteration of SSH at grid cell i; 

hi

n−1

 (n-1)th iteration of SSH at grid cell i; 

 

€

hm

*

 average height at grid cell m based on the original data;

 

€

hm

n−1

 (n-1)th iteration of SSH at grid cell;

€

nm

* number of original measurements within grid cell m.

The weights in the correction term are defined by: 

 for ; (2-2) 

 for ; 

where 

€

rm is the distance between grid cell m and the grid cell being updated and R is the 

maximum radius of influence. The parameter, a, is an adjustable weighting factor that scales the 

€

hi

n = hin−1 +

wmnm

* hm* − hmn−1

∑ ( )

wmnm

*

∑

€

wm = exp(−arm

2 /R2

)

€

r ≤ R

€

wm = 0

€

r > R
37 

exponential spatial weighting of the data. 

The empirical weighting parameters, a and b, are selected to map the mesoscale structure within 

the limitation of the scales resolvable by the cross-track altimeter sampling. The mesoscale 

analysis uses a=4 and b=2, which correspond to decorrelation time scales of 100 km and 12 

days, respectively, for R = 200 km and T = 17 days. The maximum radius of influence, R, is 

decreased between the Cressman iterations to allow smaller scales to converge more quickly and 

to increase resolution when along-track sampling is available. For this study, R is decreased 

from 200 to 100 km over five iterations giving a decorrelation length scale of 50 km on the final 

Cressman iteration. 

2.2.2 Satellite Radiometry 

Multi-channel satellite radiometry was used to complement altimeter sampling to provide highresolution synoptic images for monitoring the LC and its associated eddies during cloud-free 

observing conditions. Radiometry from the National Aeronautics and Space Administration’s 

(NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the 

Aqua satellite was the primary radiometry data set acquired and processed by CCAR for use by 

the project team. Several ancillary SST data sets were also acquired for use during the study 

program and in preparation of this report. 

MODIS Ocean Color and SST 

All CCAR processing of MODIS ocean color and SST for this study program used MODIS level 

2 (L2) swath data. MODIS ocean-color and SST data were downloaded from the Ocean Biology 

Processing Group [OBPG (c2012)] web and ftp sites at the NASA Goddard Space Flight Center 

(GSFC). MODIS SST and ocean-color data are based on L2GEN Science Processing Algorithm 

versions 6.5.1-6.7.0 and data versions V2011.3-V2013.3. 

Ocean-color data were processed by OBPG into chlorophyll-a (CHL-a) concentration using the 

empirical chlorophyll algorithm OC3 (O’Reilly et al., 2000), which uses measured radiances at 

443, 488 and 551 nm to calculate the individual chlorophyll fields from each satellite image. 

The only editing flags applied were the processing flags and masks used by OBPG (2010). The 

OBPG generates two SST products from MODIS: short-wave (4μm) and long-wave (11μm). 

The L2 long-wave SST product generated from bands 31 and 32, located at 11 and 12 μm 

respectively, was used. The L2 SST HDF files are generated by the Multi-Sensor L1 to L2 

processing code (MSL12). For SST, the OBPG processing generates a quality level flag from a 

series of quality tests. The level flag ranges from 0 (best) to 4 (failure). It was determined, by 

visual inspection, that a quality flag of 0 or 1 produced images of sufficient quality in the study 

region and applied that editing criteria to all the L2 SST data downloaded. This editing of the 

data is less stringent than that used by the OBPG in their production of L3 image products, where 

all values where the quality flag is nonzero are flagged. The OBPG editing might be preferred 

for high-accuracy quantitative analysis of SST data; however, less stringent editing provided 

better coverage and accurate enough data for qualitative monitoring in the study region. 

Both the MODIS L2 SST and ocean-color data come as individual Hierachical Data Format 

(HDF) files per swath. These files contain the derived SST and CHL-a data at 1-km resolution. 

The geophysical values for each pixel are derived from the L1A raw radiance counts by applying 
38 

the sensor calibration, atmospheric corrections, and various retrieval algorithms. Typically, the 

data for one day in the Gulf are split up across multiple swath files, some of which may overlap. 

The processing scripts read in all of the swath files covering the Gulf for one day, apply any 

editing masks (none for the CHL and only the quality flag for the SST), bin the data into a grid 

and write the gridded data out to a single netCDF file for a given day. The binning process 

averages data where there is an overlap (rare) and forms a grid of approximately 4-km resolution 

cells. In addition to the individual daily images, a moving average of the daily values is used to 

form multi-day composite daily images by averaging all of the valid data points within each 4-

km cell. 

2.2.3 Ancillary Satellite Altimeter Datasets 

A dataset of Delayed Time (DT) mapped satellite altimeter analyses spanning the time period 

from 1 January 1993 through 11 September 2015 was downloaded from the Centre National 

d'Etudes Spatiales (CNES) Archiving, Validation, and Interpretation of Satellite Oceanographic 

(AVISO) data ftp site. This altimeter product was produced by SSALTO/DUACS and 

distributed by AVISO with support from the French Space Agency CNES. The multi-mission 

mapping of sea-level anomaly (SLA), which is another name for SSHA, is based on an optimal 

interpolation derived from (Le Traon et al. 1998), with various improvements (Ducet et al. 2000; 

Le Traon et al. 2003; Dibarboure et al. 2011). Daily Maps of Absolute Dynamic Topography 

(MADT) on a 0.25° Cartesian grid were downloaded based on the sum of the interpolated maps 

of SLA and the Mean Dynamic Topography CNES/CLS 2013. 

2.2.4 Ancillary SST Datasets 

Two fully processed ancillary SST datasets were downloaded from the Physical Oceanography 

Distributed Active Archive Center (PO.DAAC) to use in the study program. These datasets were 

the 24-hour gridded 6-km Geostationary Operational Environmental Satellite (GOES) NRT L3 

SST (GOES3 c2009) and the daily 0.011° Group for High Resolution Sea Surface Temperature 

(GHRSST, [unknown date]) L4 Multi-scale Ultra-high Resolution (MUR) SST. The GOES 

NRT L3 dataset is provided to PO.DAAC by NOAA’s National Environmental Satellite 

Information Service (NESDIS). The GHRSST MUR SST is produced by the NASA Jet 

Propulsion Laboratory (JPL) MUR, Making Earth System Data Records for Use in Research 

(MEaSURE) Project (Chin, [unknown date]) and hosted by PO.DAAC. 

2.2.5 Ancillary Model Datasets 

The 1/25° data-assimilating Gulf Hybrid Coordinate Ocean Model (Gulf HYCOM) is one of the 

highest resolution data-assimilating model simulations of the Gulf region currently in operation. 

Gulf HYCOM assimilates satellites measurements of SSH and SST, as well as temperature and 

salinity profiles, but does not incorporate observations at depths greater than 2000 m (Rosburg 

et al., 2016). Native HYCOM data are converted to Network Common Data Format (NetCDF) 

on the native Mercator-curvilinear HYCOM horizontal grid and interpolated to 40 z-levels for 

sharing on the HYCOM consortium data server (HYCOM, [unknown date]). 

Datasets of SSH, temperature, and velocity from Gulf HYCOM Analysis Experiments 31.0 and 

32.5 were downloaded from the HYCOM data server and stored locally on the CCAR Network 

Archive System (NAS) for analysis. Blended datasets spanning the program time period were 

produced from the two archived experiments. 
39 

CHAPTER 3: BASIC STATISTICS 

3.1 INTRODUCTION AND BASIC DESCRIPTION

This study has generated the most comprehensive observations of circulation in the deep (> 1000 

m) Gulf to date. Between 2011 and 2015, 186 neutrally buoyant, acoustically tracked floats 

(including APEX profiling floats), released in multiple settings in the eastern, central and 

western Gulf, collected simultaneous position, temperature and pressure data every eight hours. 

Floats had mission lengths between two and 986 days with drifting depths of ~1500 m and 

~2500 m. Despite a slow start due to some technical difficulties, ~194 float-years of data were 

collected by 162 RAFOS floats and 6 RAFOS-equipped profiling APEX floats. Float 

concentration reached a peak of 62 instruments in late 2013 (Figure 3-1a). About 57% of the 

data came from the 41% of the floats with a 630-day mission (maximum for the RAFOS floats), 

and about 7% of the data resulted from the six RAFOS-equipped profiling APEX floats, which 

collected trajectory and profile data for 704–986 days (Figure 3-1b). 

The acoustically tracked RAFOS and APEX floats spread out over the entire deep Gulf, 

providing current velocity measurements over a large area. Interestingly, none of the 1500-m 

floats left the Gulf (except for one that surfaced right at the sill), even though they were drifting 

well above the ~2000-m sill in the Yucatan Channel. In Section 3.5, the float velocity data are 

spatially binned and averaged to produce maps of quasi-Eulerian statistics. By way of 

introduction to the exceptional data coverage and spatial pattern of velocity, Figure 3-2a shows 

all the float positions (1500-m and 2500-m data combined) color-coded by float speed, and 

Figure 3-2b shows just where speeds of 15 cm s-1 or higher were observed. Figure 3-2a and b 

shows that the most energetic areas of the Gulf were in deep water (>3000 m) in the eastern 

Gulf, along the 3000-m isobath following the Sigsbee escarpment, at the west corner of the 

Yucatan shelf, and extending into the interior from the western corner of the Campeche 

escarpment. Regions of slow velocities are concentrated in the De Soto canyon, inshore of the 

2000-m isobath along the Sigsbee escarpment, along the western Gulf boundary in water 

shallower than ~2500 m and over the Campeche Knolls (for locations, see Figure 1-1). 

Velocities are also relatively low along the Campeche escarpment, east of the west corner.

It will be shown in Section 5.1 that there is limited exchange between the eastern and western 

regions of the Gulf. This idea is introduced here in Figure 3-3, showing the vector displacements 

of the 65 RAFOS floats with maximum 630-day missions. Twenty-eight (43%) of these floats 

were deployed in the eastern Gulf, and 37 (59%) in the western Gulf. Exactly seven in each set 

ended up on the other side after 630 days (colored vectors in Figure 3-3); that is, 25% of the 

floats deployed in the east ended up in the west, and 19% of those released in the west ended up 

in the east. As described in more detail later, most of this inter-basin exchange occurred along 

the boundaries. 
May12 Sep13 Feb15

0

10

20

30

40

50

60

70

Number of Floats by Month

date

0 500 1000

0

20

40

60

80

days of data

Number of Floats by Trajectory Length

a)

b)

Figure 3-1. Histogram of number of floats (a) by month, and (b) by trajectory length

40
Figure 3-2. Scatter plot of all float trajectory data

(a) color-coded by speed. Isobaths (thin black lines) at 1000, 2000, and 3000 m.

41

cm s-1

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 20 o

N 

 22 o

N 

 24 o

N 

 26 o

N 

 28 o

N 

 30 o

N 

0

5

10

15
 96 o

W 92 oW 88 oW 84 oW 80 oW 18 o

N 

 20 o

N 

 22 o

N 

 24 o

N 

 26 o

N 

 28 o

N 

 30 o

N 

Figure 3-2. Scatter plot of all float trajectory data

(b), as (a) showing only points with velocities greater than 15 cm s-1.

42
 96o

W 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure 3-3. Displacement vectors for floats with 630-day (full length) missions

Arrows are color coded: blue, float first-tracked point west of 90°W and last-tracked 

point east of 90°W; purple, float first-tracked point east of 90°W and last-tracked 

point west of 90°W; black, float trajectory both began and ended either west or east of 

90°W.

43
44 

Visual inspection of float trajectories (see Section 2.1) leaves the impression that tracks in the 

eastern Gulf are more “loopy”, while in the western Gulf floats appear to travel longer in one 

direction. This impression is confirmed quantitatively with a “loopiness” ratio, which is the ratio 

of the along-track (Lagrangian) distance traveled by a float to the straight-line distance between 

first and last positions. The loopiness ratio, a dimensionless number, has a minimum value of 

one, which results from the float travelling in a straight line so that along trajectory distance is 

exactly the same as displacement. It follows that the higher the ratio, the more the trajectory 

looped and meandered, and is indicative of energetic meanders and eddies in a relatively weak 

mean flow (Fratantoni and Richardson 1999). The higher loopiness ratio in the eastern Gulf 

results from more energetic eddies in the deep Gulf there compared to the western Gulf, where 

eddy kinetic energy is less and mean flows tend to dominate. The computation of this ratio is 

confined to RAFOS floats with a maximum mission length of 630 days that were launched and 

surfaced in the same half of the Gulf (east as opposed to west, divided by the 90°W meridian). 

There were 65 such floats with 630-day missions, with 28 released in the eastern, and 37 in the 

western Gulf. If the seven floats in each set that surfaced in the opposite basin to where they 

were released (purple and blue vectors in Figure 3-3) are subtracted, that leaves 21 floats that 

were both launched and surfaced in the east, and 30 that were launched and surfaced in the west 

(black vectors in Figure 3-3). 

Figure 3-4 shows the distribution of loopiness ratio for these two groups, where the number of 

floats in each bin is expressed as a fraction of the total in that group. It is clear that the floats 

confined to the eastern basin (blue) have a higher loopiness ratio in general. The median ratios 

for eastern and western float groups are 22 and 10 days, respectively. The mean ratio for the east 

group is 24 with a standard deviation of 13, and the mean for the west group is 17 with a 

standard deviation of 19 (the means are much different from the medians, and the standard 

deviations are relatively large due to several high outliers in both groups). The highest loopiness 

ratio is for float 1067, illustrated in the gallery of examples (Figure 3-5: Case B and Figure 3-7), 

which made long excursions up and down the western boundary of the western Gulf. Float 1081, 

also highlighted in the examples (Figure 3-5: Case A and Figure 3-6), is the trajectory of a float 

with a very low loopiness ratio (4), although it is not included in this analysis because it was in a 

mission of only 426 days. 

In sum, the ratio of total distance traveled to net displacement over two years is almost twice as 

large in the eastern Gulf compared to the west. This could be due, in part, to the smaller basin 

size in the east. However, the difference appears also to be related to the presence of more 

energetic eddying motion in the deep eastern Gulf, and more along-isobath motion in the western 

Gulf. The significant difference in loopiness between the eastern and western Gulf is consistent 

with results discussed in Chapter 5. For example, gridded fields of eddy kinetic energy from the 

float velocity data show higher values in the eastern Gulf, where the loopiness ratio is also 

higher. 
5 15 25 35 45 55 65 75 85 95

0

0.1

0.2

0.3

0.4

0.5

loopiness ratio

east

west

Figure 3-4. Histogram of loopiness ratio, see text for further explanation

45
46 

The Lagrangian data set has revealed a rich variety of flow patterns in the deep Gulf, some of 

which have not been observed previously. The following set of figures illustrates seven 

particularly vivid examples of current motions that were observed by multiple floats. Color along 

the trajectory is a measure of time (from blue at the beginning of the track to red at the end) and 

circles are plotted at 10-day intervals. The large green and red dots mark the beginning and end 

of the trajectory, respectively. Figure 3-5 shows all seven examples on one map, labeled A 

through G. 

Case A, Float 1081; Figure 3-6 

This 2500-m float trajectory is a fine example of continuous cyclonic boundary-following flow 

in the western Gulf. It was deployed over the northern slope in January 2012. After a short (~10-

day) eastward drift, it accelerated through an anticyclonic loop and reversed direction (this 

particular event is associated with a LC eddy and is detailed in Section 5.3). It then flowed 

parallel to the isobaths continuously around the western basin, more or less following the 3000-

m isobath, until the end of its 426-day mission, surfacing near the Campeche Knolls. Fastest 

speeds were observed at the beginning (up to 28 cm s-1), but they were more modest 

(5–10 cm s-1) on the western boundary. 

Case B, Float 1067; Figure 3-7 

This trajectory contrasts sharply with Case A, as it exhibits persistent oscillatory motion parallel 

to the slope of the western Gulf, demonstrating that a cyclonic boundary current is not 

necessarily a persistent feature in the western Gulf. Released in September 2012, this 1500-m 

float executed numerous direction reversals over the 3000-m isobath, with speeds of 5–15 cm s-1, 

for most of its 630-day mission. Toward the latter part of the trajectory, it moved farther 

offshore and drifted more rapidly northward (speeds up to 20 cm s-1) before turning westward 

along the northern slope. It surfaced only 37 km from its launch position, and is the “loopiest” 

float in the data set (highest ratio of Lagrangian distance traveled, ~3300 km, to net 

displacement). 

Case C, Float 1137; Figure 3-8 

This float, released in the deep central Gulf in September 2012, reveals the relatively highfrequency oscillatory motion (between 90°W and 89°W) that was ubiquitous in both the western 

and eastern basins. Preliminary results suggest that these motions are indicative of topographic 

Rossby waves (TRW). Speeds in the oscillatory motion, near the beginning of the track, were on 

the order of 20 cm s-1 for this 1500-m float, but slowed when it broke off of this behavior and 

moiled around in the eastern basin. This eddying motion was also very common in the eastern 

basin (see next example). 

Case D, Float 1110; Figure 3-9 

This 1500-m float was deployed in water deeper than 3000 m just west of the Florida 

escarpment, and moiled around in the eastern basin for 630 days before surfacing just east of the 

Campeche escarpment. Maximum speeds reached 25 cm s-1, and there is evidence (looping 

track) of the float being temporarily trapped in a coherent eddy (see Section 3.2 for a census of 

coherent eddy observations). 
1000

2000

3000

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Case A

Case B

Case C

Case D

Case E

Case F

Case G

Figure 3-5. Overview map showing seven case studies A-G

These case studies show particularly vivid examples of current motions that were 

observed by multiple floats. Detailed trajectories are given in Figures 3-6 through 

3-12. 

47
1000

2000

3000

 98oW 96oW 94oW 92oW 90oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

Figure 3-6. Trajectory for case study A, RAFOS 1081 

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

48
1000

2000

3000

 98oW 97oW 96oW 95oW 94oW 93oW 22o

N 

 23o

N 

 24o

N 

 25o

N 

 26o

N 

Figure 3-7. Trajectory for case study B, RAFOS 1067

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

49
1000

2000

3000

 91oW 90oW 89oW 88oW 87oW 86oW 85oW 24o

N 

 25o

N 

 26o

N 

 27o

N 

 28o

N 

Figure 3-8. Trajectory for case study C, RAFOS 1137

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

50
1000

2000

1000

 88oW 87oW 86oW 85oW 84oW 23o

N 

 24o

N 

 25o

N 

 26o

N 

 27o

N 

 28o

N 

Figure 3-9. Trajectory for case study D, RAFOS 1110

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

51
52 

Case E, 1060; Figure 3-10

This float trajectory is typical of several that revealed, for the first time, a strong (~30 cm s-1) jet 

over the Campeche escarpment and downstream formation of anticyclonic eddies. So far, no 

evidence of these features in surface observations has been found, suggesting that they are a 

completely subsurface phenomena. Float 1060 was deployed over the 2000-m isobath of the 

western boundary in September 2012. It drifted around the Bay of Campeche at speeds of 5–10 

cm s

-1 (with some evidence of oscillatory motion along the western boundary), drifted around 

Campeche Knolls and across the mouth of Campeche canyon before accelerating to ~30 cm s-1

over the Campeche Escarpment. The float (and presumably the jet) separated from the slope 

where it turns abruptly eastward and started looping anticyclonically in a coherent eddy. It 

looped in the eddy for about five months, before surfacing while still looping, meaning that the 

eddy likely lived on for some unknown time. Several independent eddy formation events were 

observed at this location (see another example in Section 3.2). The eddy formation process 

appears to be very similar to that of other coherent vortices (Bower et al. 1997) such as 

Mediterranean salt lenses (Meddies). 

Case F, Float 1232; Figure 3-11

Several floats released over the western slope of Campeche Bank revealed a flow connection 

between the southern and northern boundaries of the central Gulf, albeit not necessarily a direct 

connection. This 1500-m float was released in September 2012 over the 2000-m isobath and 

crawled along the Campeche escarpment (note that it was inshore of the path followed by float 

1060 above, and typical of the most inshore floats along this stretch, its along-isobath speed was 

slower). It separated from the boundary at the northeastern corner of Campeche Bank, looped 

briefly into the eastern basin, partially retraced its path back toward the west, then turned 

northward and surfaced over the 2600-m isobath 630 days after it was released. Still being 

investigated is the evidence for a semi-closed cyclonic circulation in the western Gulf, of which 

this float track might have detected the eastern edge. 

Case G, Float 1134; Figure 3-12

In Section 5.1, it is shown how the floats appear to be somewhat limited in their ability to cross 

between the eastern and western basins of the Gulf. This float represents one of several 

exceptions, illustrating a connection along the northern slope. It was released in September 2012 

east of 90°W and offshore of the 3000-m isobath. It eddied around for a while before shooting 

off to the west along the Sigsbee escarpment. It followed the slope westward all the way past 

94°W, then moved back slightly eastward and got caught in some local eddy motions. In Section 

5, these deep flow patterns and their relationships to the upper-layer circulation in the Gulf are 

explored in more detail. 
1000

2000

3000

 98oW 96oW 94oW 92oW 90oW 18o

N 

 19o

N 

 20o

N 

 21o

N 

 22o

N 

 23o

N 

 24o

N 

 25o

N 

Figure 3-10. Trajectory for case study E, RAFOS 1060

Color along the trajectory is a measure of time (from blue at the beginning of the track 

to red at the end) and circles are plotted at 10-day intervals. Green and red filled 

circles denote the start and end locations, respectively.

53
1000

2000

3000

 92oW 90oW 88oW 86oW 21o

N 

 22o

N 

 23o

N 

 24o

N 

 25oN 

 26o

N 

 27o

N 

 28o

N 

Figure 3-11. Trajectory for case study F, RAFOS 1232 

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

54
1000

2000

3000

 96oW 94oW 92oW 90oW 88oW 86oW 24o

N 

 25o

N 

 26o

N 

 27o

N 

 28o

N 

 29o

N 

 30o

N 

Figure 3-12. Trajectory for case study G, RAFOS 1134 

Color along the trajectory is a measure of time (from blue at the beginning of the 

track to red at the end) and circles are plotted at 10-day intervals. Green and red 

filled circles denote the start and end locations, respectively.

55
56 

3.2 EDDY CENSUS

3.2.1 Overview

The RAFOS and RAFOS-equipped APEX floats were at times embedded in flow that exhibited 

looping motion, indicative of coherent eddies capable of trapping and transporting water (and 

floats). The tracking of upper-layer LC eddies using surface drifters has an extensive history in 

the Gulf (Kirwan et al. 1984a; Glenn and Ebbesmeyer 1993; Hamilton et al. 1999). Kinematic 

model analysis of swirl and translation velocities, and estimation of rotational ellipse parameters 

was employed by Kirwan et al. (1984b) for a LC eddy, and subsequently Glenn and Ebbesmeyer 

(1993) applied a least-square fit to a translating-ellipse model of drifter paths to estimate the 

kinematics of a LC eddy. Hamilton et al. (1999) used a similar model to investigate the paths of 

ten LC eddies across the western Gulf. These kinematic models have also been used to develop 

statistics for northern Gulf slope cyclones and anticyclones (Hamilton 2007b) that were observed 

by the SCULP surface drifters (Ohlmann et al. 2001) that were exported from multiple 

deployments on the northern Gulf shelves. 

For this study, a different technique than least-square fit of trajectories to a kinematic model was 

employed. It has the advantage that it will automatically find cyclones and anticyclones in the 

trajectories without identifying them prior to application. To quantify float trajectories, and 

survey what kind of eddies were present in these data, Matlab-based freeware “jLab” (Lilly 

2016) was used, which includes programs written for wavelet analysis. The methodology has 

been documented in a series of papers (Lilly and Olhede 2009a; Lilly and Olhede 2009b; Lilly 

and Olhede 2010a; Lilly and Olhede 2010b; Lilly and Olhede 2012), which were based on a 

prototype study by Lilly and Gascard (2006). Using this analytical software, the analyst can 

objectively quantify the number of cyclonic and anticyclonic coherent eddies in the data set, 

along with statistics on their kinematic properties such as rotation period, diameter, velocity, and 

Rossby number. The resultant eddies were divided into three categories based on formation 

region and characteristics. 

3.2.2 Methods 

The jLab ellipse detection software allows the user to modify input parameters of maximum and 

minimum ratios of rotation frequency to Coriolis frequency [FMIN and FMAX], wavelet 

duration [P], ridge point cutoff for minimum log10 ratio of rotary spectra [C], and ridge length 

cutoff for minimum number of complete cycles [L]. Each parameter was tuned to this data set, 

so that the algorithm successfully identified the trajectory segments that visually looked to be in 

or near an eddy. For FMIN and FMAX, 1/2 and 1/32 were chosen. The inertial period ranges 

from 38.7 hours at 19°N (five, eight-hourly trajectory points) to 25.5 hours at 28°N (three, 8-

hourly trajectory points). By choosing FMIN to be 1⁄2, eddy rotation periods are required to be at 

least twice the inertial period. Wavelet duration P was set to three, which allowed for a bit more 

translation (or stretch) in the eddy trajectory. Ridge point cutoff C was set to log10(3), which 

eliminates trajectory segments with minimal eccentricity. The minimum number of complete 

cycles L was set to three, so that only ellipses with at least three complete rotations were 

included in the analysis. 
57 

Algorithm output parameters include the segment of track identified as positively being in 

rotation, residual path of trajectory, once rotation is removed, and a variety of ellipse parameters, 

of which geometric mean radius, mean velocity, and Rossby number, Ro = U/Lf, the ratio of the 

vertical component of relative to planetary vorticity, are reported. Because of edge effects, 

output was discarded from the start and end of each segment equal to two rotation periods of the 

eddy, as described in Bower et al. (2013). Also, similar to Bower et al. (2013), the full length of 

the segment, identified with this wavelet ridge analysis is used to define eddy duration. 

3.2.3 Results 

Out of a total of 168 float trajectories, 45 were identified to contain eddies; some with multiple 

independent eddies. In total, 2,620 float-days were measured in eddies, or 10% of the total 

trajectory data. This is a conservative estimate, as the criterion for positive identification was set 

to a potential eddy having at least three complete rotations. Table 3-1 lists the count of eddies 

sorted by float depth and sense of rotation. A nearly equal number of cyclones and anticyclones 

were found in this data set, and four times more eddies found in the shallow data than the deep 

data, probably reflecting the skewed distribution of float deployments at the two depths. 

Table 3-1. Eddy Distribution by Sense of Rotation and Depth

Total # eddies # anticyclones # cyclones

All trajectories 51 26 25

Shallow (1500 dbar) 40 18 22

Deep (2500 dbar) 11 8 3 

Deep and shallow float trajectory segments that indicated a coherent eddy are plotted in Figure 

3-13. The segments are color-coded by sense of rotation, with blue indicating cyclonic rotation 

and red indicating anticyclonic rotation. The upper panel shows all eddies, and the lower panels 

separate eddies by target pressure, 1500-dbar (lower-left) or 2500-dbar (lower-right). A 

qualitative comparison between the complete trajectory data sets and the eddy segments indicate 

that more eddies are found in the eastern Gulf than the western Gulf due to data distribution, 

because the deep floats were launched only in the eastern Gulf. There are more cyclonic eddy 

trajectory segments found in the eastern Gulf than in the western Gulf, and this is particularly 

true for the shallower floats. This is possibly due to the presence (and observation) of cyclonic 

Loop Current Frontal Eddies in the eastern Gulf. 

The coherent eddies fall into three categories based on region of formation (if the float trajectory 

appears to capture the formation event of an eddy) or existence (if a float was launched in an 

eddy or appears to be entrained in an existing eddy), summarized in Table 3-2. First are interior 

eddies located in water depths greater than 3000 m, but precluding the anticyclones in deep water 

west of about 90°W, which are in a different category. These interior eddies are predominantly 

cyclonic (24 out of 28 sampled), although the balance is shifted from nearly 100% cyclones (21 

out of 22 sampled) at the 1500-dbar level to an equal number of cyclones and anticyclones (3 

and 3, respectively) at the deeper 2500-dbar level. 
3000

1000

2000

 96oW 92 oW 88 oW 84 oW 80 oW 18o

N

 21o

N

 24o

N

 27o

N

 30o

N

3000

1000 2000

 18oN

 21o

N

 24o

N

 27o

N

 30o

N

1000

3000

2000

 96oW 92 oW 88 oW 84 oW 80 o 96 W oW 92 oW 88 oW 84 oW 80 oW

 18oN

 21o

N

 24o

N

 27o

N

 30o

N

All Eddies 

Shallow Eddies Deep Eddies

Cyclonic Rotation

Anticyclonic Rotation

Figure 3-13. Trajectory segments containing eddies

Trajectories are color coded by sense of rotation, and plotted separately by pressure 

level.

58
59 

The second category includes the small scale eddies that are present along the boundary around 

the perimeter of the Gulf. These eddies are predominantly anticyclones (19 out of 20 sampled), 

and are found particularly along the western boundary of the Gulf, along the Campeche 

escarpment, in the Catoche Tongue, and along the Florida Escarpment. Last are the 

anticyclones that appear to be generated offshore of the Campeche escarpment, at about 24.0°N, 

90.5°W. These eddies are only present in the 1500-dbar data set. Because there were no 2500-

dbar floats released in the western Gulf (and few migrated there from the east), it is unknown if 

these anticyclones exist at the deeper depth. 

One example from each of the three categories of eddies is presented below. In the first 

example, an interior eddy, the full trajectory of the float path is presented in Figure 3-14. This 

float (1223, 1500-dbar) was launched on 22 August 2013 (green marker) and spent 630 days 

traveling around the eastern Gulf. It completed its mission and surfaced on 14 May 2015 (red 

marker). During that time, it became entrained in a cyclonic eddy that was detected by the 

wavelet transform algorithm, shown by the segment highlighted in blue. This eddy is a typical 

example of the cyclonic eddies found at 1500-dbar in the eastern basin.

Table 3-2. Eddy Distribution by Category and Sense of Rotation 

AC stands for anticyclonic and C stands for cyclonic.

The residual trajectory, with estimated eddy ellipses drawn to scale every seven days, and eddy 

parameters of radius, azimuthal velocity, rotation period, and Rossby number are shown in 

Figure 3-15. The large diameter cyclone moved from the northeast to the southwest across the 

eastern basin and an extended LC that was in the process of detaching LC eddy Lazarus. This 

type of deep eddy path has been observed for earlier LC eddy detachments by a mooring-PIES 

array (Donohue et al. 2016a). In the bottom three panels, the tails of the calculated eddy 

parameters that fall outside a two-period window are colored green. These values are not 

included in estimations of eddy statistics, as described in Section 3.5. The radius varied between 

40 and 55 km, azimuthal velocity between 25 and 30 cm s-1, and the rotation period was about 

13–14 days. The ratio of the vertical components of relative to planetary vorticity, the Rossby 

number, Ro, was about 0.08, indicating this was a relatively weak eddy. This eddy persisted for 

51 days. 

Interior

Total (AC/C)

Boundary

Total (AC/C)

Campeche Escarpment

Total (AC/C)

All trajectories 28 (4/24) 20 (19/1) 4 (4/0)

Shallow (1500 dbar) 22 (1/21) 15 (14/1) 4 (4/0)

Deep (2500 dbar) 6 (3/3) 5 (5/0) 0 (0/0)
2000

1000

3000

 90oW 89 oW 88 oW 87 oW 86 oW 85 oW 84 oW 24o

N

 25o

N

 26o

N

 27o

N

 28o

N

Figure 3-14. Trajectory of example interior eddy float 1223 at 1500 dbar

The green and red dots denote the start and end of the trajectory.

60
 89o

W 88 oW 87 oW 86 oW 85 oW 84 oW 24o

N

 25o

N

 26o

N

 27o

N

 28o

N

20

40

60

Radius

km

0

20

40

V azimuthal

cm/s

10

15

20

Rotation Period

days

28/09 08/10 18/10 28/10 07/11 17/11 27/11

0.05

0.1

Ro

Figure 3-15. Eddy characteristics as calculated using the wavelet transform method

In the top panel, the thick light gray lines show the location of the 17-cm SSH contour: 

long dashes for 8 October 2014 and short dashes for 7 November 2014.

61
62 

The second eddy category found in this data set included small anticyclones along the slope 

boundaries of the Gulf. These eddies were nearly all anticyclones, and their placement suggests 

interaction between a cyclonic boundary current and bathymetry, because these eddies are all 

inshore of the mean counter-clockwise boundary current presented in this report. An example is 

presented in Figure 3-16 for float 1094 at 1500 dbar. This eddy was launched midway along the 

steep slope of the Campeche escarpment (green marker) on 10 July 2012, and travelled along the 

slope until reaching a small canyon in the slope wall. At that point, the float enters the canyon, 

and begins looping anticyclonically. The float, embedded in the eddy, continues along the 

boundary around the eastern corner of the Campeche escarpment. As it turns from travelling 

northeast to southeast, the float moves upslope and grounds. It breaks free just before surfacing 

on 18 March 2014, completing its 630-day mission.

Eddy parameters for this boundary anticyclone are presented in Figure 3-17. The eddy residual 

trajectory follows a path from the head of the small canyon due north, out of the canyon, and 

ends after it turns east. Eddy ellipses, drawn every four days, are small, only 2–8 kilometers in 

diameter. This eddy has a radius, which varies from 1–4 kilometers, has azimuthal speeds of -5 

cm s

-1, and rotation period of about 4–5 days. Ro is comparatively larger for this eddy, about 0.3 

on average, indicating the newly formed eddy’s relative vorticity is comparatively larger than the 

interior eddy previously described. The eddy persists for 83 days. 

The final eddy category includes features that appear to be generated at the northeast extent of a 

jet-like boundary current that has separated from the steep western Campeche escarpment wall. 

There are only four of these eddies identified in this data set, all anticyclones. It is worth 

mentioning here that a few more of this category of eddy probably exists in the data set, but they 

did not complete three full rotations, and so were not identified by the automatic detection 

algorithm. In several instances, the reason for the shorter duration of these identified eddies, and 

also their apparent terminations, were that the floats completed their missions at this point and 

surfaced, not that the eddies necessarily dissipated. 

Float 1101, at 1500 dbar, serves as an example of this type of eddy, shown in Figure 3-18. This 

float was launched along the 2000-meter isobath in the southwestern Bay of Campeche on 15 

September 2012. The float travelled with the cyclonic boundary current past the Campeche 

Knolls, around Campeche Canyon, and then rounded the sharp corner at the west end of the 

Yucatan shelf. After following the bank wall for about 50 kilometers, the flow appears to 

separate from the wall, and form an offshore current, or jet. At its northeastern most extent, the 

float’s trajectory changes from linear to looping. It appears that anticyclones are formed 

preferentially at this location, and once formed, as shown by the float path, travel westward. The 

eddy identified in this trajectory is color coded blue. The eddy stops being measured when the 

float mission ends, and the float surfaces. 
1000

3000

 91oW 90 oW 89 oW 88 oW 87 oW 86 oW

 30’ 

 23oN

 30’ 

 24oN

 30’ 

 25oN

 30’ 

Figure 3-16. Trajectory of example interior eddy float 1094 at 1500 dbar

The green and red dots denote the start and end of the trajectory.

63
 89o

W 48’ 36’ 24’ 12’ 88oW 24o

N

 15’ 

 30’ 

 45’ 

 25oN

 15’ 

0

2

4

Radius

km

−10

−5

0

V azimuthal

cm/s

0

5

10

Rotation Period

days

08/10 18/10 28/10 07/11 17/11 27/11 07/12 17/12 27/12 06/01

−0.4

−0.2

0

Ro

Figure 3-17. Eddy characteristics of float 1094 as calculated using the wavelet transform method

64
1000

2000

3000

 98oW 96 oW 94 oW 92 oW 90 oW 18o

N

 19o

N

 20o

N

 21o

N

 22o

N

 23o

N

 24oN

 25o

N

Figure 3-18. Trajectory of example interior eddy float 1101 at 1500 dbar

The green and red dots denote the start and end of the trajectory.

65
66 

Eddy parameters are presented in Figure 3-19, as done for the other two case-study eddies. The 

top panel shows the eddy residual trajectory, which begins in the east and continues westward 

after a half-circle shaped a dip to the south and then north. Eddy ellipses are drawn to scale 

every three days, and are on the order 20 km in diameter. The eddy radius varies from 4–12 

kilometers, and azimuthal velocity ranges from -5 to -15 cm s-1. The rotation period for this 

eddy is about five days. This eddy was measured from formation until the float surfaced, for 129 

days. With Ro of about 0.2 for the four month lifetime, this eddy is not as vigorous as the 

smaller boundary eddy, but stronger than the cyclone in the eastern Gulf.

The relationship of azimuthal velocity against radius over the course of the eddy lifespan 

indicates the extent to which each eddy is in solid body rotation. Figure 3-20 illustrates this 

relationship for the interior (float 1223, left-hand panel), boundary (float 1094, middle panel), 

and Campeche escarpment (float 1101, right-hand panel) eddies. Only the data plotted in black 

are viable for this estimation, as the beginning and end segments are mathematically affected by 

edge effects. For the three cases, all have some portions of the eddy segment that conform to a 

linear fit. All three eddies’ (V)elocity against (R)adius relationships are more linear than not. 

However, the interior eddy is not of long duration. The boundary eddy has portions that exhibit 

a linear relationship and also portions that do not. The Campeche escarpment eddy has the 

strongest linear relationship between velocity and radius, indicating that it is in solid body 

rotation for the course of its life and as long as the float was able to measure it.

Eddy mean statistics, including radius, azimuthal velocity, period, duration and Rossby number, 

are presented next. In calculating mean statistics, only eddies that persisted for longer than four 

rotation periods were included, so as to eliminate edge effects described earlier. This strict 

criteria reduced the number of eddies used to calculate the statistics by 60%, from 51 to 21 

examples. In each case, statistics are presented as median, mean, and standard deviation from 

the mean. For all eddies, the absolute value of velocity and Ro were used in the calculations of 

median, mean, and standard deviation. This was done to get accurate statistics on categories that 

included both cyclonic and anticyclonic rotating eddies. 

The statistics presented in Table 3-3 indicate that overall, the median deep eddy in the Gulf has 

period of ~7 days, velocity of ~8–9 cm s-1, radius of 8–9 km, and a Ro value of 0.2. The 

Campeche Bank eddies have the largest median radii (16.5 km), followed by the interior eddies 

(9.6 km), and lastly the small boundary eddies (3.6 km). Median velocities and median period 

length follow the same pattern: Campeche escarpment eddies are the fastest (10.8 cm s-1) and 

have the longest period (11.0 days), followed by interior eddies (8.4 cm s-1, 9.7 days), and finally 

the boundary eddies (6.5 cm s-1, 4.0 days). The measure of relative to planetary vorticity (Ro) 

follows a different pattern. In this case, the boundary eddies are strongest with a median Ro 

value of 0.31, then the Campeche escarpment eddies (median Ro = 0.15), and finally the interior 

eddies (median Ro = 0.12). 
 94o

W 93 oW 92 oW 91 oW 90 oW 23o

N

 20’ 

 40’ 

 24oN

 20’ 

 40’ 

 25oN

0

10

20

Radius

km

−20

−10

0

V azimuthal

cm/s

0

5

10

Rotation Period

days

21/01 10/02 02/03 22/03 11/04 01/05 21/05 10/06

−0.4

−0.2

0

Ro

Figure 3-19. Eddy characteristics of float 1101 as calculated using the wavelet transform method

67
20 40 60

0

1020304050

1223

V (cm/s)

R (km)

−2 0 2 4 6 8

−10

−8−6−4−2

0

1094

V (cm/s)

R (km)

0 5 

−20−15−10

−5

0

1101

V (cm/s)

R (km

Figure 3-20. Azimuthal velocity plotted against radius for three eddy case studies

68


SE

(LC)

NE

North

Central NW

SW

South

Central

Figure 3-21. Sub-regions for calculation of integral time scales

71
Autocorrelations <U(t).U(t+Τ)> and <V(t).V(t+Τ)>

Average U integral timescale : 3.3 days

Average V integral timescale : 3.0 days

North Central Rectangle

Average U integral timescale : 1.9 days

Average V integral timescale : 1.2 days

Northeast Rectangle

Average U integral timescale : 2.0 days

Average V integral timescale : 2.3 days

Southeast Rectangle

(Under LC)

Autocorrelations <U(t).U(t+Τ)> and <V(t).V(t+Τ)>

Average U integral timescale : 4.8 days

Average V integral timescale : 3.0 days

Northwest Rectangle

Average U integral timescale : 5.7 days

Average V integral timescale : 5.6 days

Southwest Rectangle

Average U integral timescale : 4.9 days

Average V integral timescale : 6.1 days

South Central Rectangle

Figure 3-22. Lagged autocorrelations and integral time scales for the sub-regions.

Thin lines from individual floats, with averages shown as thick lines. Solid lines and dashed 

lines from east (U) and north (V) velocity components, respectively. Sub-regions defined in 

Figure 3-21.

72
73 

The MDT during the four-year program period from 1 July 2011 through 30 June 2015 are 

shown in Figure 3-23, computed from the CCAR (Leben et al. 2002) and AVISO DUACS 

DT2014 (Pujol et al. 2016) SSH data products. Significant differences exist in the two surfaces, 

primarily due to inaccuracies in the MDT surfaces used to estimate the ADT height fields. In 

general, CCAR SSH captures LC intrusions and the timing of LC eddy separation well when 

compared to coincident SST and color imagery, which indicates a more accurate MDT in the 

eastern Gulf in the CCAR product. Nevertheless, the CCAR MDT likely suffers from errors due 

to the fixed specification of the inflow and outflows into and out of the basin in the data 

assimilative model used to estimate the MDT. The AVISO MDT exhibits a mean anticyclonic 

gyre in the western Gulf and mean cyclonic circulation in the Bay of Campeche consistent with 

wind-stress curl forcing (Sturges 1993; Perez-Brunius et al. 2013). This is in contrast to an eddydominated circulation in the western Gulf in the CCAR MDT without a well-defined western 

wind-forced anticyclonic gyre. 

Differences of the MDT over the program time period relative to the 20-year altimeter record 

from 1 January 1993 through 31 December 2012 show more consistency between the two 

datasets (Figure 3-24). This is expected because the differencing removes systematic errors in 

the MDT and the SSH anomaly fields are based on the same satellite altimeter data records and 

differ only in the methods used to produce the gridded data from the along-track altimetry. From 

this long-term perspective, the LC was weaker, but extended further northward into the eastern 

basin, and the cyclonic circulation in the western Bay of Campeche was stronger during the fouryear study program. These differences relative to the long-term mean, however, are relatively 

small in comparison to the year-to-year differences during the four-year study. 

Interannual variations were significantly stronger throughout the basin (Figure 3-25; only 

AVISO data shown) and were primarily caused by LC and LC eddy variability. The most 

significant interannual variability occurred during the fourth year of the study program (July 

2014 through June 2015) when the LC was deeply intruded into the eastern basin and cyclonic 

circulation dominated the western basin. 

3.4.2 Loop Current and Loop Current Eddies 

LC and LC eddy fronts during the study program were tracked using the 17-cm contour in 

demeaned SSH fields. Demeaned fields were calculated by subtracting a spatial mean from each 

daily SSH field, which removes biases in the fields associated with differences in the MDT 

reference surfaces and seasonal height variations associated with upper-ocean warming and 

cooling. The spatial means were calculated over the deepwater basin within the Gulf where 

bottom depths exceed 200 m. Following Leben (2005), the objective technique tracks the 17-cm 

contour entering the Gulf from the Yucatan Channel and exiting through the Florida Straits. 

Breaking of this contour is used to identify LC eddies detaching from the LC. Each eddy is 

tracked until the eddy signature ultimately dissipates below the 17-cm level or the detached 

contour re-attaches to the LC 17-cm tracking contour. LC eddies that detach and ultimately reattach to the LC are called detachment events. Eddies that detach and ultimately dissipate while 

separated from the LC are called separation events (Leben, 2005). 
Figure 3-23. Gulf Mean Dynamic Topography (MDT) 

Program time period is from 1 July 2011 through 30 June 2015: (A) CCAR, (B) AVISO. 

The 17-cm LC tracking contour is shown by the thick black line.

74
Figure 3-24. Gulf Mean Dynamic Topography (MDT) over the 20-year record

The altimetric record is from 1 January 1993 through 31 December 2012: (A) CCAR, (B) 

AVISO. Difference of the MDT over the 4-year study program (1 July-2011 through 30 June 

2015) relative to the 20-year means: (C) CCAR, (D) AVISO.

75
Figure 3-25. Difference of mean AVISO SSH by study year relative the 20-year mean

The 20-year mean is from 1993 to 2012: (A) 1 July 2011 through 30 June 2012, (B) 1 July 

2012 through 30 June 2013, (C) 1 July 2013 through 30 June 2014, and (D) 1 July 2014 

through 30 June 2015.

76
77 

A variety of statistics can be computed based on objective tracking of LC and LC eddy SSH 

contours (Leben, 2005). Sensitivity of these statistics to satellite sampling and differences in the 

MDT used to derive SSH fields were explored in an idealized setting using a multidecadal 

numerical ocean-model simulation of the Gulf using HYCOM (Dukhovskoy et al. 2015). The 

primary conclusion of the study is that LC metrics derived from satellite altimeter SSH fields are 

less sensitive to the reference MDT field than to satellite sampling patterns. Nevertheless, 

differences in MDT do impact the timing and identification of LC separation events for a given 

satellite sampling. For this reason, both the CCAR and AVISO SSH datasets have been used to 

derive LC and LC eddy statistics during the study program. These statistics are based on a 

MATLAB® toolbox developed at CCAR with support from BOEM (Dukhovskoy et al. 2015; 

Hall and Leben 2016). Statistics have been derived also from Gulf HYCOM Analysis 

Experiments 31.0 and 32.5. LC length time series during the study program derived from these 

datasets are shown in Figure 3-26. The deep intrusion of the LC during the fourth year of the 

study program is unique to the altimeter record and has been the focus of recent industry 

monitoring and research (Sharma et al. 2016). 

The toolbox also automatically tracks (at the 17-cm level) detached LC eddies and eddies that 

split off from the detached eddies in order to identify LC eddy re-attachment and separation 

events. Detachment and re-attachment events can significantly affect the intensity of the 

circulation within the LC eddy and the timing of when the LC eddy ultimately separates. 

Detachment and re-attachment events are more frequently observed in the CCAR SSH than in 

the AVISO SSH, which is primarily an artifact of the differences in MDT fields used to estimate 

the ADT. As a result, LC eddy separations dates objectively derived from the CCAR SSH 

dataset are typically later than those determined using the AVISO dataset. Often the two 

estimates differ significantly. Industry names for each of the separation events that occurred 

during the study program are listed in Table 3-4 along with the corresponding separation dates 

determined from the AVISO and CCAR datasets. 

Table 3-4. LC Eddy Names and Separation Dates during the Study 

LC Eddy Name AVISO CCAR

Hadal 23 Jul 2011 14 Aug 2011

Icarus 20 Dec 2012 3 Feb 2012

Jumbo 11 Jun 2012 4 Aug 2012

Kraken 19 May 2013 9 Aug 2013

Lazarus 5 Nov 2014 11 Nov 2014

Michael ** **

Nautilus 17 May 2015 18 May 2015

** not detected at the 17-cm level
Figure 3-26. Time series of LC length during the study program

LC Length determined from automated tracking of the 17-cm LC contour in SSH fields from (A) CCAR, AVIHYCOM datasets and (B) the individual HYCOM datasets used to produce the blended product. The average crossing of 90°W determined from CCAR and AVISO are shown as vertical lines with labels correspond to indVertical dashed lines delineate the individual years spanning the four-year study program.

78
79 

The agreement between the AVISO and CCAR datasets, however, is much better for the timing 

of LC eddies propagating into the western Gulf. The centroid of the area enclosed by the 17-cm 

LC eddy tracking contours (on a Mercator projection) was used to track the eddy centers as the 

LC eddies propagated westward after separation. Crossing of 90°W by the eddy centers of LC 

eddies tracked during the study program, using the AVISO and CCAR datasets, are listed in 

Table 3-5. The average dates of LC eddies crossing of 90°W determined from the two altimetric 

analyses are shown as vertical lines in Figure 3-26 with labels corresponding to the named eddy 

event. 

Table 3-5. Crossing of 90°W by LC Eddy Centers during the Study

LC Eddy Name AVISO CCAR

Hadal 22 Aug 2011 24 Aug 2011

Icarus 10 Feb 2012 12 Feb 2012

Jumbo 30 Oct 2012 9 Nov 2012

Kraken 30 Jul 2013 9 Aug 2013*

Lazarus 5 Nov 2014* 11 Nov 2014*

Michael ** **

Nautilus 17 May 2015* 18 May 2015

* separation from the LC occurred when the eddy center was west of 90°W

** not detected at the 17-cm level

Tracking of LC eddies after separation continues until all tracking contours no longer encircle 

any of the eddies that coincide with or split from the original eddy that separated from the LC. 

Tracking was performed using both AVISO and CCAR datasets, but results will only be shown 

for tracking with the AVISO dataset. Typically LC eddies can be tracked at the 17-cm level over 

longer time periods in AVISO SSH fields because AVISO does not filter the along-track data as 

aggressively as is done in the CCAR data processing. AVISO also uses longer time and space 

correlation scales in their objective analysis, which further increases eddy persistence at the 17-

cm level in the analyzed SSH fields. 

Daily eddy-center positions computed from all LC eddy-tracking contours detected in the 

AVISO SSH fields during the study program are shown in Figure 3-27 with color coding to 

identify the tracked LC eddy. Eddy Michael was not detected at the 17-cm level, but all of the 

other industry named LC eddies were identified by the tracking toolbox. Filled circles mark the 

locations of the eddy centers at the time of separation. Filled triangles mark the last center 

location tracked. Scattered points of the same color are a result of splitting, merging and 

dissipation events during the lifetime of the tracked LC eddy. 
Figure 3-27. Daily LC eddy center positions

Centriods computed from the LC eddy-tracking contours detected in the AVISO SSH 

fields at the 17-cm level during the study program. Filled circles mark the locations of 

the eddy centers at the time of separation. Filled triangles mark the last eddy center 

location tracked. Color-coding identifies the industry names corresponding to the 

tracked eddy.

80
81 

The extent of the areal coverage of LC eddies over the course of the four-year study is visualized 

by plotting the LC tracking contours and eddy centers during each of the program years at 30-

day intervals starting with 1 July in each year (Figure 3-28). These composite maps are useful 

for identifying LC eddy events that contributed to the SSH interannual variability in the western 

Gulf observed during each of the program years shown in Figure 3-25. During the first year of 

the program, LC eddies Hadal and Icarus contributed to high SSH anomalies observed in the 

western Gulf. Icarus also contributed to high SSH anomalies in the southwestern Gulf during the 

second year; however, the contribution of Jumbo was minimal. Although Jumbo was a large 

energetic eddy near the time of separation, it underwent a series of splitting and merging events 

as the eddy separated from the LC and propagated westward that dissipated much of the eddy 

circulation, before it entered the western Gulf. In contrast, during the third year of the study 

Kraken propagated westward as a single coherent feature before finally dissipating on 26 March 

2014 in the far western Gulf. The impact of Kraken on SSH anomalies, however, was not as 

large as would be expected, which indicates that cyclonic circulation in the western Gulf may 

have cancelled out some of the LC eddy signature over the course of the third year. The absence 

of LC eddies in the western Gulf from March 2014 and throughout nearly the entirety of the 

fourth year of the study likely contributed to the large anomalous cyclonic low in the central 

western Gulf SSH anomaly field during the fourth year.

The tracks of the three most energetic and long-lived LC eddies during the study (Hadal, Icarus, 

and Kraken) are shown in Figure 3-29. Eddy splitting and merging events are identified by 

dashed lines marked with Xs connecting eddy-center locations before and after the events. 

According to eddy tracking at the 17-cm level in the demeaned AVISO SSH fields, a small eddy 

split off from the eastern flank of Hadal on 18 August 2011 and dissipated two days later. Three 

months later Hadal split almost perfectly in half and the two eddy centers co-rotated 

anticyclonically as an eddy pair before merging back together again on 24 December 2011. The 

fused portion of Hadal then propagated southward until it dissipated on 15 February 2012 in the 

southwestern Gulf. At about the same time as Hadal dissipated, Icarus propagated into the 

western Gulf and remained a coherent feature for over eight months until splitting off a small 

piece just as it dissipated in the far western Gulf on 15 October 2012. Kraken did not split over 

its entire eight-month lifespan in the western Gulf, and dissipated on 26 March 2014 very near to 

the location where Icarus dissipated. 

3.4.3 Mesoscale Eddies in the Western Gulf 

In addition to the LC and LC eddies, the general circulation of the upper layer of the Gulf also 

contains strong mesoscale circulation associated with large long-lived cyclonic and anticyclonic 

mesoscale eddies, especially in the western Gulf. An altimeter-derived mesoscale eddy census is 

used to quantify this eddy variability over the modern altimeter record in order to place the 

mesoscale activity during the study program in perspective. This is the fourth update of a global 

mesoscale eddy census first described in Chelton et al. (2011). 
Figure 3-28. Areal coverage of LC eddies by year

Panels span the four-year study program and areas are visualized by plotting the LC 

eddy tracking contours and eddy centers at 30-day intervals starting with July 1 in each 

year: (A) 1 July 2011 through 30 June 2012, (B) 1 July 2012 through 30 June 2013, (C) 

1 July 2013 through 30 June 2014, and (D) 1 July 2014 through 30 June 2015.

82
Figure 3-29. Eddy center tracks of the three most energetic and long-lived LC eddies

During the study, they were Hadal, Icarus, and Kraken. Filled circles mark the 

locations of the eddy centers at the time of separation. Filled triangles mark the last 

eddy center location tracked. Eddy splitting and merging events are identified by 

dashed lines marked with Xs connecting eddy center locations before and after each 

splitting or merging event.

83
84 

The most recent analysis is based on the AVISO DT-2014 daily two-satellite merged sea-level 

anomaly (MSLA) dataset for the time period from January 1993 through April 2015. This twosatellite merged product is based on, at most, two satellite altimeter missions throughout the 22-

year record (Topex/Poseidon/ERS, Jason-1/Envisat, Jason-2/Envisat, Jason-2/AltiKa, or Jason2/Cryosat-2) with sampling comparable to the nominal 10-day and 35-day repeat sampling from 

the two reference repeat track orbits. The two-satellite product is not as accurate as the allsatellite AVISO product used elsewhere in this report, but does provide a consistent benchmark 

to evaluate the mesoscale activity during four-year program record relative to the long-term 22-

year record. 

An eddy has to be identified and tracked for at least 28 days to be included in the database. An 

initial check found that all of the industry named eddies identified during the program time 

period were identified and tracked in the global database (Figure 3-30). Because the tracking 

algorithm uses AVISO MSLA and not ADT height fields, the LC eddy tracks begin in the far 

eastern Gulf even though the recirculation anomalies are likely still trapped within the LC. The 

tracking method is based on a modified version of the algorithm described in Williams et al. 

(2011). This is a significant modification of the eddy and tracking procedure used in the original 

census by Chelton et al. (2011). Rather than defining eddies by an outermost closed SSH 

anomaly contour as in the previous three censuses, each eddy was defined on the basis of 

connected pixels around extremum in the AVISO MSLA dataset that satisfy a specified sequence 

of selection threshold criteria as described in Schlax and Chelton (2016). The five criteria used 

were chosen to yield eddies that are statistically similar to those found in Chelton et al. (2011). 

Table 3-6 lists summary statistics derived from the database for each of the named LC eddies as 

they cross 90°W. Longitude and latitude values refer to the centroid of the eddy pixels. 

Amplitude is defined as the magnitude of the height difference between the SSH extremum 

within the eddy and the average of SSH over the edge pixels that define the outer perimeter of 

the eddy. Radius is the eddy radius scale, defined as the radius of a circle with area equal to that 

enclosed by the contour of SSH within the eddy around which the circum-average speed is 

maximum. The radial speed is the average speed around the SSH contour that was used to 

determine the eddy radius scale. The circulation is twice the eddy radius times the radial speed. 

For plotting purposes and for comparison of eddy statistics, circulation will be plotted and 

quoted as an unsigned magnitude for both cyclones and anticyclones in lieu of the conventional 

positive and negative signed circulation values, respectively. 

Another search was performed on the eddy database to identify all Gulf cyclones and 

anticyclones with eddy centers west of 90°W in the daily AVISO two-satellite merged MSLA 

fields spanning the 22-year historical record. Time series of the daily sums of the number, area 

and circulation of the identified cyclonic and anticyclonic eddies are shown in Figure 3-31 and 

Figure 3-32, respectively. The colored time series plotted in the figures correspond to the 

program time period and show several noteworthy time periods of mesoscale eddy activity in the 

western Gulf during the study. 
Figure 3-30. Tracks of all of the industry named LC eddies

Eddies were identified during the program time period and tracked in the global eddy 

census database. To be included in the database, an eddy had to be identified and 

tracked for at least 28 days. 

85
Figure 3-31. Time series of the daily sums of the number, area and circulation of cyclones

The cyclones were identified in the global eddy census database that had eddy centers in the 

Gulf, west of 90°W in the daily two-satellite AVISO MSLA fields. Colored time series 

correspond to the program time period.

86
Figure 3-32. Time series of the daily sums of the number, area and circulation of anticyclones

The anticyclones were identified in the global eddy census database that had eddy centers in 

the Gulf, west of 90°W in the daily two-satellite AVISO MSLA fields. Colored time series 

correspond to the program time period. Verical lines are times that LC eddy centers crossed 

90°W, during the study program.

87
88 

Table 3-6. LC Eddy Census Statistics at 90°W

Eddy Name

Census ID#

Date

(m/d/y) Latitude Amplitude(cm)

Radius

(km)

Radial 

Speed

(cm s-1) 

Circulation

(105 m s-1) 

Hadal

207312 8/19/11 26.2°N 43 113 97 6.88

Icarus

219886 2/12/12 25.3°N 45 120 75 5.69

Jumbo

216797 11/7/12 24.6°N 9 93 31 1.84

Kraken

239315 7/29/13 25.5°N 44 114 65 4.70

Lazarus

242228 10/27/14 27.5°N 14 75 64 3.03

Michael

243765* 3/16/15 27.0°N 5 55 34 1.17

Nautilus

243413** 4/24/15 26.6°N 41 127 75 6.09

 * Michael was initially detected at 27.0°N and 93.1°W on 16 March 2015.

** Nautilus was located at 26.6°N and 89.1°W on the last date in the database (24 April 2015) and was 

still attached to the LC.

LC eddies Hadal, Icarus, and Kraken contributed significantly to the anticyclonic circulation in 

the western Gulf during in the first and third years of the study. The lack of any contribution by 

Jumbo coincided with weak anticyclonic eddy circulation in the western Gulf throughout the 

second year of the study. Cyclonic eddy circulation was more uniform than anticyclonic eddy 

circulation in the western Gulf over the program record, however, there was a notable increase in 

cyclonic eddy circulation in the last year and a half of the program.

Mean statistics computed from the daily summed values of eddy number, area, and circulation 

over the program time period and the historical record are listed in Table 3-7. All of the mean 

statistics were remarkably similar over the two records. The mean number of cyclones and 

anticyclones detected in the western Gulf during the study were somewhat less than detected on 

average over the historical record, but the summed areal extent and eddy circulation values were 

virtually identical. There were, however, differences in the mean statistics between anticyclones 

and cyclones, which also are consistent over the two time periods. On average a greater number 

of cyclones were observed than anticyclones, but the anticyclones covered a greater areal extent 

and were more energetic. Because LC eddies are included in the eddy census the difference 

between the anticyclone and cyclone mean statistics in the western Gulf are not particularly 

surprising or unreasonable. From the standpoint of overall mesoscale eddy activity in the 

western Gulf, the program time period was comparable to the 22-year historical record. 
89 

Table 3-7. Mean Daily Eddy Statistics. 

Intervals are the four-year study and the 20-year historical record*

Mean Number Mean Area

(104 km2

) 

Mean Circulation

(105 m2

 s-1) 

4-year 22-year 4-year 22-year 4-year 22-year

Anticyclones 2.78 2.95 1.70 1.74 4.96 4.96

Cyclones 2.93 3.11 1.68 1.67 4.60 4.66

* The four-year record spans 1 July 2011 through 16 April 2015 and the 22-year record spans 15 

January 1993 through 16 April 2015. Two weeks were trimmed from the census analyses to account 

for the four-week minimum lifetime of tracked eddies included in the updated Chelton et al. (2011) 

eddy census database.

3.4.4 Conclusions

From a remote sensing perspective, the upper-ocean conditions in the Gulf during the four-year 

study program were comparable to long-term average conditions over the 22-year satellite 

altimeter record. Significant LC, LC eddy, and mesoscale eddy variability, however, contributed 

to strong interannual variability in each of the individual years within the program time period. 

Given that the sampling of the deep flows by floats at depth was much greater in years two and 

three of the study care should be exercised when interpreting conditions at depth as 

representative of long-term mean conditions. 

3.5 PSEUDO-EULERIAN STATISTICS AND DOMINANT CIRCULATION PATTERNS 

The main patterns of circulation in the deep Gulf remain uncertain given the lack of velocity 

measurements at depth, particularly in the western and southern basins. Past studies, based on 

available data and numerical models, show indications of a mean cyclonic circulation along the 

boundary that extends to the abyssal plain (Hamilton 1990; Oey and Lee 2002; DeHaan and 

Sturges 2005; Bracco et al. 2016). The forcing for this circulation has been typically attributed 

to rectification of topographic Rossby waves (Mizuta and Hogg 2004; DeHaan and Sturges 

2005; Hamilton 2009). 

The TRWs observed over the northern slope are generated by the complex interaction between 

the upper and lower layers resulting from the dynamics of the Loop Current and the detachment 

of Loop Current Eddies in the northeastern Gulf (Hamilton 1990; Oey and Lee 2002; Lee and 

Mellor 2003; Hamilton et al. 2016). In the northwestern Gulf, LC rings that interact with the 

slope may also generate TRWs (Oey and Lee 2002), although moorings on the northern part of 

the western slope (Rio Grande Slope) show no evidence of intensified bottom flow in the 

observed velocity fluctuations in the deep layer. This suggests that Kelvin waves are produced 

instead (Sheinbaum et al. 2010). In addition, one-year records from moorings in the Bay of 

Campeche show presence of TRWs only on the western slope; while on the eastern slope the 

fluctuations have higher frequencies and no bottom intensification (Kolodziejczyk et al. 2011). 

It is thought that the TRWs dissipate as they travel into the eastern side of the Bay of Campeche 

due to the rough topography of the Campeche Knolls, and that edge waves are responsible for 

the fluctuations observed there. No velocity observations exist in the deep layer adjacent to the 

northern Yucatan-Campeche Slope to confirm the existence of TRWs propagating along that 

portion of the southern boundary. 
90 

On the other hand, DeHaan and Sturges (2005) suggest that the boundary current is fed by the 

inflow of cold, dense North Atlantic Deep Water (NADW) through the Yucatan Channel, which 

would tend to flow cyclonically against the slope as it mixes with the warmer water of the 

interior. 

Another mechanism suggested for the cyclonic circulation is vortex stretching driven by 

upwelling of the main thermocline, as has been suggested by the Gulf of Mexico Oscillator 

theory suggested by (Oey and Chang 2011). Their argument is that the expansion and retraction 

of the LC in the northeastern basin, on average, results in a deepening thermocline in the eastern 

Gulf, compensated by shoaling in the western basin, leading to vortex stretching and hence 

cyclonic vorticity in the Sigsbee Plain. This argument is based on potential vorticity 

conservation, which in turn is due to conservation of angular momentum (Cushman-Roisin and 

Beckers 2011). In the case of the homogeneous bottom layer of the Gulf, where no external 

forcing acts and assuming dissipation is small, the water columns below the thermocline are 

expected to conserve their potential vorticity (PV), where PV= (f+ζ)/h, with f the local Coriolis 

frequency, ζ, the relative vorticity of the water column, and h the thickness of the bottom layer. 

Hence, if the water columns on the western basin on average are being stretched, as suggested by 

Oey and Chang (2011), they will acquire cyclonic vorticity to conserve PV. 

In the Bay of Campeche, the cyclonic Campeche gyre observed at the surface has been proposed 

to be equivalent barotropic, suggesting its influence reaches down to the bottom layer (PerezBrunius et al. 2013). The gyre was observed in the intermediate layer in the PALACE floats 

study by Weatherly et al. (2005), but the lack of observations below 1000m has not made it 

possible to confirm if it extends to the bottom. 

The dynamics of the lower layer in the eastern Gulf has been explored more, thanks to several 

observational programs and modeling studies (Oey and Lee 2002; Lee and Mellor 2003; 

Donohue et al. 2006; Donohue et al. 2008; Oey 2008; Cox et al. 2010; Athié et al. 2014; 

Hamilton et al. 2014; Hamilton et al. 2016). These studies have shown that the main source of 

energy comes from the expansion and retraction of the Loop Current, which induces movement 

in the lower layer, both in the interior and on the slope, resulting in baroclinic instabilities, deep 

eddies and topographic Rossby waves that can transfer energy towards the western basin, both to 

the interior and along the boundary. 

A valuable aspect of the dataset is that the floats were deployed in several regions of the Gulf, 

and their trajectories covered the entire basin as the floats spread (Figure 3-2a). It is a unique 

asset of these Lagrangian data that explores the main patterns of circulation, and the local 

variability of the flow in the entire Gulf. This is particularly relevant in the western and southern 

Gulf where flow statistics are most uncertain, given the scarcity of velocity measurements at 

depth. For this purpose, the floats can be used as roving current-meters to estimate the statistics 

of the Eulerian velocity field, by averaging their data into geographical bins. This approach has 

often been used to map the statistics of the velocity field both from surface drifters and 

subsurface floats in various regions of the world ́s oceans: e.g., global ocean (Lumpkin and 

Johnson 2013), North Atlantic (Fratantoni 2001; Bower et al. 2002a; Bower et al. 2002b; 

Lavender et al. 2005), Nordic Seas (Poulain et al. 1996; Rossby et al. 2009; Anderson et al. 

2011), South Atlantic (Rodrígues et al. 2010), Pacific and Indian Oceans (Davis 1998; Davis 
91 

2005). In the case of the Gulf, this methodology has been applied to surface drifters for the 

entire Gulf (DiMarco et al. 2005), and in the Bay of Campeche (Vazquez de la Cerda et al. 2005; 

Perez-Brunius et al. 2013). At depth, similar analysis produced estimates of the statistics of the 

velocity field at 900 m for the entire Gulf from PALACE floats (Weatherly et al. 2005), and 

(Hamilton et al. 2015) used this approach to compare the statistics of the surface and 400 m 

velocity field in the De Soto Canyon region using drifters and RAFOS floats. 

Three patterns stand out in the binned statistics of the float dataset from the present study: a 

cyclonic boundary current, a mean cyclonic gyre in the deep western Gulf, and the very high 

eddy kinetic energy observed in the eastern Gulf. The discussion will focus mostly on these 

three features, but individual trajectories will also be employed to further the analysis. Before 

the results are presented, the methodology for the binned averages is described. Previously, the 

names of the Gulf physiographic provinces were identified (Figure 1-1).

3.5.1 Methodology to Calculate the Gridded Fields 

Velocities were calculated along the trajectories of the 155 RAFOS (both 1500 and 2500 m) and 

the six APEX floats. Each float registered its position every eight hours with the good data 

record lengths varying between a few days to 730 days, from July 2011 through June 2015 (see 

details on data return in Chapter 2 and Appendix A). The trajectories of the entire data set are 

shown in Figure 3-2a. Figure 3-33 shows the number of floats present in the western and eastern 

Gulf at any given month during the period of the experiment. The first year has little coverage, 

but from July 2012 on, both basins had between 20 and 50 floats recording data, with the eastern 

Gulf having the largest number of floats. Therefore, the statistics presented are representative of 

the period between September 2012 and June 2015, because much less data are available in the 

first year, particularly in the western Gulf. The 1500 and 2500 m floats were considered 

together, because not much difference is expected between those two layers given that, at least in 

the eastern basin, the currents are quite uniform with depth below 1000 m (Hamilton et al. 2016).

The study area was divided into a regular grid of 0.5°x0.5° overlapping boxes, centered on a 

0.25°x0.25° grid. The overlap provides spatial smoothing of the final fields. To establish 

statistical confidence, the degrees of freedom for each bin were estimated based on the integral 

time scale (T), which averages between one and six days depending on the region of the Gulf, 

although for any given float the decorrelation time scale varies between 0 and 15 days (see 

Section 3.3). Five days were used as the time scale at which data points were considered 

independent, and the degrees of freedom (DOF) were based on unique days sampled in each bin, 

(i.e., the number N of unique days that have observations), and the degrees of freedom are 

defined as DOF = N/T. Hence, if a bin has three floats that were recording data during the same 

five day period, that counts as five unique days, and hence DOF = 1. If by contrast, each of the 

three floats visited the area in different months and recorded data for 5 continuous days, N = 15 

and the corresponding DOF = 3. Figure 3-34 shows the number of data points and number of 

DOF for each bin. Only bins with more than five degrees of freedom are considered in the 

results. The eastern Gulf was the most sampled region, with more than 115 DOF in most bins, 

followed by the boundary of the western Gulf (> 65 DOF). The abyssal plain and the deep Bay 

of Campeche were the least sampled regions.
Oct11 May12 Nov12 Jun13 Jan14 Jul14 Feb15

0

10

20

30

40

50

60

7

0

0

0

0

0

5

0

70

0

10

20

30

40

50

60

7

0

0

0

0

0

5

0

70

a) Number of Floats East of 90°W

Oct11 May12 Nov12 Jun13 Jan14 Jul14 Feb15

b) Number of Floats West of 90°W

Figure 3-33. Histograms of number of floats by month

(a) East of 90°W, and (b) West of 90°W.

92
Figure 3-34. a) Number of observations b) Degrees of freedom for each bin 

Geographical bins are 0.5°x0.5° in size, centered on a 0.25° grid. Only bins with at 

least 5 degrees of freedom are plotted. Isobaths are shown as black contours at 1000, 

2000, and 3000 m. Note that the number scales for the two plots are different.

93

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Number of observations 

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

a)

b)

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

5

15

25

35

45

55

65

75

85

95

105

115

Degrees of freedom
94 

The mean velocity is shown in Figure 3-35, with black vectors representing averages 

significantly different from zero at the 95% confidence level. Mean kinetic energy, calculated as 

MKE = 1⁄2 (U2+V2), where U and V are the mean zonal and meridional velocities, is shown in 

Figure 3-36. The standard deviation ellipses are shown in Figure 3-37, and the eddy kinetic 

energy (EKE), calculated as half the sum of the variances along the major and minor axes, is 

shown in Figure 3-38. To look at the relative importance of mean as opposed to eddy flow, the 

ratio of MKE/EKE is shown in Figure 3-39. As argued previously, potential vorticity is 

expected to play a significant role in the dynamics of the lower layer. Therefore, planetary 

potential vorticity contours f/H are plotted on all maps, where f is the local Coriolis frequency 

and H is the bottom depth. In a barotropic ocean, float displacements and dispersion are 

expected to be along contours of constant f/H, and so it is important to determine if floats have a 

tendency to be steered by f/H (LaCasce 2000). This topic will be explored more in Chapter 5, 

where evidence for using the lower-layer thickness for H, rather than the full water column depth 

is discussed.

The main features that stand out are the cyclonic boundary current, the cyclonic gyre in the deep 

western Gulf, and the very high EKE observed in the eastern basin. Also noticeable is the rapid 

decay of the EKE towards the western basin, with lower values observed in the Bay of 

Campeche and the central part of the western boundary. 

Note that standard deviation ellipses are rather circular and are only slightly aligned with 

topography in some cases (Figure 3-37). The places where ellipses are most elongated and along 

the slope coincide with regions of steep slopes along the boundary: the Florida escarpment, the 

northwestern Gulf, the Campeche escarpment, and the northwestern Cuban shelf. It remains to 

be assessed if the apparent isotropy in the variability of the flow is real or an artifact given the 

uneven distribution in time and space of the floats in each bin. 

3.5.2 Western Basin Boundary Current 

Evidence of a deep cyclonic boundary current is observed everywhere except in the northeastern 

Gulf (Mississippi through and De Soto canyons, east of 91°W). Regions where the boundary 

current shows more clearly coincide with high gradients of f/H, such as in the northwest 

boundary (Perdido escarpment), in the Bay of Campeche, and along the Campeche escarpment. 

By contrast, regions where these contours diverge correspond to where the boundary current 

shows weak mean flow or where it is not present, such as on the central western boundary (East 

Mexico slope) and northeastern Gulf (Mississippi Fan). The boundary current has speeds ~5 cm 

s

-1. The highest mean velocities, MKE and MKE/EKE ratios of the boundary current are 

observed in the northwestern Gulf (between 24° and 25°N), in the Bay of Campeche (particularly 

where the flow turns north at ~95°W), and most notably on the west corner of the Campeche 

escarpment. 
Figure 3-35. Mean velocity vectors 

Black (red) arrows indicate averages significantly (not) different from zero at the 95% 

confidence level. Planetary potential vorticity (f/H) contours in gray ([1.52:0.1:2.5]*10-8

m-1s-1) with f the local Coriolis parameter, H the bottom depth. Isobaths are shown as black 

contours at 1000, 2000, and 3000 m.

95

10 cm/s

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Mean velocity RAFOS and APEX BUOYS 

10 cm s-1
Figure 3-36. Mean kinetic energy (MKE) in cm2s-2

Isobaths and f/H contours as in Figure 3-35.

96

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27oN 

 30o

N 

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

MKE (cm2 s-2) ALL RAFOS
Figure 3-37. Standard deviation ellipses 

Isobaths and f/H contours as in Figure 3-35.

97

10 cm/s

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Standard deviation ellipses RAFOS and APEX

10 cm s-1
Figure 3-38. Eddy kinetic energy (EKE) in in cm2s-2

Isobaths and f/H contours as in Figure 3-35.

98

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

EKE (cm2 s-2) ALL RAFOS
Figure 3-39. MKE/EKE ratio

Isobaths and f/H contours as in Figure 3-35. 

99

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

MKE/EKE ALL RAFOS

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
100 

The boundary flow converges as it exits the Bay of Campeche and continues east along the steep 

Yucatan-Campeche slope. Part of the boundary flow separates at ~92°W, turning westwards as 

it enters the abyssal plain, feeding into the deep cyclonic gyre. The high MKE observed along 

the western portion of the Campeche escarpment coincides with the region before the point of 

separation. Figure 3-40 shows the process of separation observed by the trajectories of floats 

released in September 2012 that flowed from the Bay of Campeche towards the Campeche 

escarpment. Floats that entered at different times between March 2013 and May 2014, observed 

the strong jet, suggesting it is a persistent phenomenon, not an isolated event. All ten floats 

converged at the west corner of the Campeche escarpment and continued in a strong boundary 

jet. The speeds in some (not all) floats embedded in this current increase from 5 cm s-1 to 35 cm 

s

-1 as the corner is navigated. Two floats continue east along the boundary, but the other eight 

exit the boundary at ~92°W, coinciding with the location of a topographic bump. As they enter 

the abyssal plain, most end up captured in westward moving anticyclones (see also Section 3.2). 

This subsurface oceanographic feature is not evident in the altimetry SSH field, so the separation 

in this region does not seem to be connected to the upper layer dynamics. The separation process 

and its apparent relation to the deep eddies is a remarkable discovery of this program, and merits 

further study. 

Although the boundary current appears in the mean velocity field, it is not necessarily a coherent 

structure all the time. The analysis in Section 5.3 shows that the boundary flow is episodic in 

character, so floats travel along this current for a given period of time and either slow down and 

stall, or reverse and show back and forth motion, including movement across isobaths. Floats 

may also be trapped in small eddies when encountering topographic features before or after 

entering the boundary flow (see Section 3.2). Nevertheless, there are periods when this 

boundary current is continuous, because floats could been seen to travel far and without looping 

along this flow (Figure 3-41). For example, float 1081 (2500 m) was trapped in a coherent 

boundary flow all around the western Gulf along the 3000m isobath (Figure 3-41). It joined the 

boundary current in March 2012 at the central northern slope and continued all the way to the 

Bay of Campeche. Unfortunately, it got stuck to the bottom in October 2012 after going around 

the Bay of Campeche, and remained in the same spot for five months until its mission ended in 

March 2013.

Central Western Boundary 

Weak mean flow and low EKE is observed in the central region of the East Mexican slope, 

between 22° and 24°N and west of the 3000m isobath. This region of sluggish flow coincides 

with broadening of the isobaths and f/H contours, so the intensification of the flow upstream and 

downstream of this shadow zone is consistent with stronger gradients in bottom depth and hence 

potential vorticity. It is not clear what happens to the boundary current as it reaches this region, 

whether it broadens or breaks apart, to reform as it enters the Bay of Campeche. 
 94 o

W 93 oW 92 oW 91 oW 90 oW 

 20 oN 

 21 o

N 

 22 oN 

 23 o

N 

 24 o

N 

3500

Campeche-Yucatan Shelf

Figure 3-40. Boundary current separation shown by the trajectories of ten 1500 m floats 

101
−1000−2000

−3000

 96oW 92oW 88oW 84oW 80oW 18oN 

 21oN  24oN  27oN  30oN 

0212 0312 

0412 

0512 0612 

0712 

0812 

0912 

1012 0313 

rafos 1081 rafos 1021

 96oW 93oW 90oW 87oW 84oW 

0212 0312 0412 

0512 0612 0712 

0812 

rafos 1107

 96oW 93oW 90o

1012 

1112 

0113 

0413 0513 0713 0913 

1013 

1113 1213 

0114 0214 0314 0414 

0614 0514 

rafos 1060

 96oW 93oW 90oW 87oW 84oW 18oN 

 21oN  24oN  27oN  30oN 

1012 

0713 0813 

0913 

1113 

1213 0114 

0214 0314 0414 0514 

0614 

rafos 1227

 96oW 93oW 90oW 87oW 84oW 18oN 

 21oN  24oN  27oN  30oN 

0813 1013 1113 1213 0114 0214 0314 0414 0714 

0914 

1014 

1114 

1214 0115 

0215 

0315 

rafos 1224

 96oW 93oW 90o

Figure 3-41. Trajectories of floats showing boundary currents Deployment shown by green dot and final location by the red triangle. Monthly date tags correspond to the w

102
103 

Campeche Gyre 

Floats show no clear evidence of the presence of a semi-permanent cyclonic gyre in the bottom 

layer in the Bay of Campeche, as was suggested by Perez-Brunius et al. (2013). Instead, a 

relatively strong and persistent boundary current is observed, its MKE representing 20–30% of 

the EKE, with even higher values in the narrow f/H region where the current turns north near 

95°W. As shown in Figure 3-40 and Figure 3-42, it is common for floats to flow along the 

boundary, and then turn east along the Yucatan-Campeche slope. Some floats go around the 

narrow Campeche Canyon in the eastern Bay of Campeche, before making a very sharp eastward 

turn on the west corner of the Campeche escarpment (Figure 3-40 and Figure 3-42). It was only 

during the last months of the study that a cyclonic eddy located in the deepest part of the basin 

trapped three floats. This circulation may or not be the lower layer expression of the gyre in the 

Bay of Campeche. 

Nevertheless, the trajectories of floats flowing along the boundary current show that the water 

cools as it enters the Bay of Campeche, and warms again as it leaves the region (Figure 3-42). 

This is consistent with floats travelling underneath a cyclonic structure, which creates a dome of 

cooler water through which the isobaric floats travel. Whether this is indeed a signature of the 

upper layer Campeche gyre remains to be tested. 

3.5.3 Central Cyclonic Gyre 

The cyclonic gyre is observed in the deep abyssal plain of the western Gulf (Sigsbee Plain, > 

3500 m), although it is much more clearly defined in a cell centered at 22.5°N and 94°W and 

located between 22 and 24°N. This feature is the one that has the highest MKE and MKE/EKE 

ratio; hence it appears as the most consistent pattern of circulation of the deep Gulf. More 

careful analysis needs to be carried out to see how many floats were sampling this feature, as 

well as the period sampled, to assess the persistence of this flow. It is noted that this region in 

the abyssal plain had bins with relatively low numbers of degrees of freedom (DOF < 55). 

The gyre is located in a region that is rather flat, with low gradients of f/H, except on the part 

near the Campeche escarpment. This contrasts with the boundary flow, which clearly lies on top 

of regions with strong gradients of f/H. Therefore, these two circulations seem distinct from 

each other, and may be driven by different mechanisms. Because of the strong stratification of 

the Gulf and given that the upper and lower layer behave quite differently, it makes more sense 

to use the thickness of the bottom layer in the calculation of its planetary potential vorticity 

(Figure 3-43). 

The potential vorticity is calculated this way for the lower layer (Figure 3-43), and it is striking 

that the same pattern emerges when total water depth is used, which also results in a closed 

contour with the lowest potential vorticity in the region of the gyre, closed contours west of 

88°W, and strong gradients over the slope where the boundary current flow is observed. More 

details of the calculation of the lower layer f/H from profiling float CTD data are given in 

Chapter 4. 
 96 o

W 93 oW 90 oW 87 oW 84 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 

4.2 4.22 4.24 4.26 4.28 4.3 4.32 4.34 4.36

°C

Figure 3-42. Selected trajectories along the southern part of the boundary current 

Trajectories are color-coded by temperature, showing cooling within the Bay of 

Campeche.

104
f / h6 (x109 m-1 s-1) where h6 is grid averaged water depth - depth of 6 °C isotherm

Figure 3-43. Lower-layer potential vorticity from mean parameters

Bottom depths, latitude and depth of the 6 °C isotherm are grid-averaged using the locations 

of the profiles. Bathymetry contours are 100 m and every 500 m from 1000 m. See more 

details in Section 4.2.

105
106 

The mean velocity field also suggests that the flow that separates from the boundary feeds into 

the gyre. It will be interesting to study this further, because many of the floats that separate from 

the slope are trapped in anticyclones that move west, so it is not clear if they contribute to the 

mean flow, or if they are imbedded in it. Note also that the highest EKE in the gyre is observed 

at the point of separation, and that relatively high EKE is observed all along the Campeche gyre, 

except in its southern limb where its flow converges with the boundary current. 

Numerical models have reproduced the cyclonic circulation in the abyssal plain (Chang and Oey 

2011; Bracco et al. 2016), and it is quite striking how the mean circulation, observed with the 

high resolution model used in Bracco et al. (2016), is very similar to what the float statistics 

show; that the deep Campeche gyre and boundary current are distinct features. The high mean 

velocities along the Campeche escarpment, and separation of the boundary flow is also suggested 

by the model, although the separation occurs a little further east than what is observed by the 

floats. Therefore, these features appear to be real and persistent, and not a result biased by the 

events observed by the relatively small number of float observations in this region. 

The fact that the gyre lies in the region of lowest f/H suggests that potential vorticity 

conservation is important. The question remains as to what feeds the cyclonic vorticity. 

Because it appears distinct from the boundary current flow, and is located in a region of small 

slopes and f/H gradients, it is unlikely that it is driven by rectification of topographic Rossby 

waves. Some other possible mechanisms are eddy-vorticity fluxes (Dewar 1998), or eddytopography interactions (Holloway 1987), which may be likely given the relatively high EKE 

along the northern gyre, and the presence of the eddies at the point of separation of the boundary 

flow on the Campeche escarpment. Vortex stretching driven by upwelling of the main 

thermocline may be a more effective way to drive this circulation, as has been suggested by the 

Gulf of Mexico Oscillator theory suggested by (Oey and Chang 2011). 

3.5.4 Eastern Gulf of Mexico 

A brief summary of highlights of the eastern basin circulation is presented here, since a more 

detailed analysis is presented in Section 5.2. The mean flow shows three recirculation cells: two 

cyclonic to the southeast and northeast, and an anticyclonic to the northwest. The convergence 

due to the dipole in the northern basin results in relatively high and significant mean flow 

towards the south. The northern dipole appears to be a persistent feature, since it has been 

observed previously with moorings (Hamilton et al. 2014; Hamilton et al. 2016). These authors 

associate its existence to potential vorticity conservation due to stretching or compression of the 

deep layer as the LC expands towards the northern slope. The very high EKE observed is 

generated by baroclinic instabilities produced by interaction of the upper and lower layer due to 

the dynamics of LC expansion (Donohue et al. 2016a). This process results in deep eddies that 

trap energy from the LC and translate it towards to the Sigsbee escarpment, where they 

contribute to the generation of the TRWs that are observed along the northern slope in the 

western Gulf (Oey and Lee 2002). The float data also indicate that the deep LC eddies may 

contribute to the radiation of TRWs towards the Sigsbee escarpment (see Section 5.2) 
107 

CHAPTER 4: PROFILING FLOATS 

4.1 APEX AND ARGO CTD PROFILES

Seven APEX profiling floats with CTD, optical and RAFOS sensors were deployed during the 

study. In general, the floats profiled every 14 days, from 1500 m to the surface, over the four 

years of the field program, and generated CTD data for all regions of the deep Gulf. The 

locations of the profiles are given in Figure 4-1, and it can be seen that, for the most part, the 

floats remained in the same relative sub-regions over their lifetimes. Exceptions are when the 

floats, at their rest depths of 1500 m, were entrained into the cyclonic boundary currents of the 

western Gulf. For example, 4902286 was deployed in the eastern Gulf, but was transported to 

the western boundary by the deep Sigsbee and Perdido escarpment boundary currents. Float 

4902284 was transported into the Straits of Florida, where it slowly moved eastward with the 

Florida Current, as it was on the seabed at ~800 m between profiles. Profiles from this float 

were analyzed by Green et al. (2014). The CTD profile data were QA/QC’d by Service Argo, 

and as a further check, the T/S diagram (Figure 4-2) for all profiles was evaluated and shows the 

tight T/S curve for temperatures below ~16 °C in the deep Gulf. The salinity minimum on the 

27.4 σt surface is caused by the northward spreading of Antarctic Intermediate Water (AAIW; 

Wust (1963)), which is usually found ~800–1000 m in the Gulf. In the upper water column, a 

characteristic of the LC and LC eddies is the occurrence of Subtropical Underwater (SUW) with 

salinities > 36.6 (traditionally [Wust 1963; Nowlin 1972], 36.5 psu, but 36.6 psu seems a better 

divider in Figure 4-2 between SUW and less saline Gulf Common water). Surface-layer waters 

can have a wide range of temperatures and salinities because of the annual heating cycle, 

evaporation, and river flows. The Mississippi plume is often transported offshore into deep 

water (Schiller et al. 2011).

To increase the CTD database, profiles were obtained for the Gulf from US Argo for the same 

interval as the study profiles. A total of seven Argo floats were found in the database. These 

CTDs were filtered such that the minimum depth was 1000 m, the depth record was complete, 

and there was at least seven days between profiles. The latter was imposed because some of the 

floats profiled on a daily basis and thus were sampling for short periods the same upper layer 

eddies, whereas a more random sampling is required for statistics. Seven days seems adequate in 

most cases to decorrelate the profiles. The resulting 217 locations add to the 489 APEX profiles 

in Figure 4-1, are given Figure 4-3, along with the T/S diagram in Figure 4-4. The additional 

profiles provide increased coverage in the LC, along the western boundary, and in the Bay of 

Campeche. The T/S diagram (Figure 4-4) overlays the curves in Figure 4-2 and has the same 

characteristics, identifying AAIW, SUW and Gulf Common water masses. 
4902284

4902285

4902286

4902287

4902288

4902289

4902290

BOEM APEX CTD Profiles 2011-07-20 to 2015-08-17

Average Interval Between Profiles = 14 days

Minimum Profile Depth = 1450 m (except 4902284 in the Florida Straits)

Number of Profiles = 489

Figure 4-1. CTD profile locations from BOEM APEX floats

Locations color-coded by float number.

108
, , , , , ,

BOEM Gulf of Mexico Apex CTD

Number of Points = 247,114

Figure 4-2. T/S Diagram for APEX CTD profiles

Locations given in Figure 4-1; color-coded by float number.

109
US ARGO CTD Profiles 2011-07-07 to 2014-11-12

4901043

4901044

4901272

4901526

4901528

4901529

4901646

Minimum Interval Between Stations = 7 days

Minimum Profile Depth = 1000 dbar

Number of Profiles = 217

Figure 4-3. CTD profile locations from US Argo floats

Locations color-coded by float number.

110
, , , , , ,

Number of Points = 49,288

US Argo Gulf of Mexico CTD

Figure 4-4. T/S Diagram for US Argo CTD profiles

Locations given in Figure 4-3; color-coded by float.

111
112 

As an aid to identifying water masses, for each profile location, the SSH was interpolated from 

the CCAR daily SSH maps. The 17-cm SSH contour is used to mark the position of the LC and 

LC eddy fronts (Leben 2005). Thus profile locations that have SSH > 17 cm are identified with 

the LC or LC eddies, and would be expected to show occurrences of SUW. Similarly, profile 

locations with SSH < -10 cm are associated with cyclones that have Gulf Common water 

characteristics. The average temperature and salinity profiles with standard deviations and 

extremes are given in Figure 4-5 for all locations in Figure 4-1, locations with SSH > 17 cm, and 

locations with SSH < -10 cm. Compared to the mean, LC and LC eddy profiles are warmer and 

more saline in the upper 500 m, and cyclone profiles are cooler and less saline. 

Note there are about twice as many profiles in cyclones as in the LC and LC eddies, and upperlayer variability is also much less in cyclones with SSH < -10 cm than for the LC and LC eddy 

locations. There is also little variability below 1000 m, which implies that the RAFOS floats at 

1500 and 2500 m should measure only small temperature changes along their trajectories. 

Spatial analysis of profile variability is given in the next section. 

4.2 CTD PROFILE CHARACTERISTICS

4.2.1 Parameters and Basic Statistics 

For each temperature (T), salinity (S), and depth profile from the BOEM APEX and US Argo 

database discussed above, a set of characteristic parameters was derived as follows: 

1. SSH: Interpolated to the location and time of the profile from daily CCAR 0.25° altimeter 

maps. 

2. Depth of the 6 °C Isotherm: Interpolated using the profile depth and temperature data. 

The 6 °C isotherm, derived from temperature-depth profiles, is a good indicator of the 

depth extent of the LC and LC eddies because it is also the depth of the Straits of Florida 

sill (Bunge et al. 2002). It may also be used as the division between upper and lower 

layers in the Gulf. 

3. 1000–50 m dynamic height: A measure of the upper-layer geostrophic circulation that 

can be directly compared with SSH. The lower integration limit corresponds to the 

common deepest depth of the profiles and the approximate depth extent of the upper 

layer. The 50 m upper limit avoids seasonal heating and surface mixed layer effects. 

4. Depth of the AAIW salinity minimum: The salinity minimum is searched for in the 

salinity-depth profile where σt

 > 27.0. 

5. AAIW properties: Salinity, temperature and σt at the depth of the salinity minimum. 

6. SUW depth and thickness: The salinity maximum is searched for the part of the σt

 profile 

between 24.5 and 26.5. If Smax > 36.6 psu, the depth is computed, and the thickness is 

defined for the adjacent parts of the S profile where S ≥ 36.6 psu. 

7. SUW properties: Salinity, temperature and σt

 at the salinity maximum if it exists. 
5 July 2013 24°50.6’N 91°28.9’W

20 July 2012 24°51.3’N 86°54.0’W

12 July 2013 25°03.0’N 92°06.8’W

All Gulf APEX/ARGO Profiles: 1 July 2011 to 30 June 2015

SSH > 17 cm

SSH < -10 cm

Figure 4-5. Statistics from APEX/Argo CTD profiles

Average profiles of temperature, salinity and sigma-t (LH panels). The RH panels show the 

mean, standard deviations (thin solid lines) and extrema (thin dashed lines) for the temperature 

and salinity profiles. The number of values at each depth is given by the long-dashed lines in the 

center panel. All available profiles (top row), profiles filtered for CCAR SSH > 17 cm (middle 

row), and SSH < -10 cm (bottom row).

113


600 650 700 750 800 850 900 950 1000 1050

Depth of 6°C Isotherm (m)

0

20406080

100120140

Number of CTD Profiles

-40 -20 0 20 40 60 80

CCAR SSH (cm)

0

20406080

100120140160

Number of CTD Profiles

80 90 100 110 120 13

1000 - 50 dbar Dyn

0

20406080

100120140

-40 -20 0 20 

CCAR SS

8090

100110120130140150160170180

1000 - 50 dbar Dynamic Height (cm)

y = 0.0016*x2 + 0.84*x + 1.1e+02

80 90 100 110 120 130 140 150 160 170 180

1000 - 50 db Dynamic Height (cm)

600650700750800850900950

10001050

Depth of 6°C Isotherm Depth (m)

y = - 0.029*x2 + 11*x - 16

data1 quadratic

Figure 4-6. Scatter plots of the depth of the 6 °C isotherm, with SSH and dynamic height

Parameters are derived from APEX and Argo CTD profiles for isotherm depth with dynamic height (LH columCCAR SSH anomalies (center column), and for SSH anomalies and dynamic height (RH column) for the Gulisotherm depth, CCAR SSH, and dynamic height at the location and time of the profiles on the second row.

N = 704

-40 -20 0 20 40 60 80

CCAR SSH (cm)

600650700750800850900950

10001050

Depth of 6°C Isotherm (m)

y = - 0.012*x2 + 3.2*x + 8.1e+02

data1 quadratic

y = 4.5x+8.1e2y = 2.8x+8.0e2

116
117 

However, it was recognized that temperature measured by an isobaric RAFOS float has a strong 

relation with the upper-layer thermocline fluctuations, even though the magnitude of the 

temperature fluctuations at 1500 dbar is typically only a few tenths of a degree Celsius. The 

relationship is evident in the APEX profiles, where the depth of the 6 °C isotherm was correlated 

against the temperature at 1490 m for each profile (Figure 4-7). The 1490-m depth was the 

deepest available on the majority of profiles, and because floats are not precisely at the same 

depth, some adjustments to the float temperatures would be needed to apply the linear fit. Even 

though there is scatter, the linear fit has a R2 of 0.61, and therefore RAFOS float temperature 

variations may be used as a proxy for the depth of the 6 °C isotherm (top of the lower layer). A 

similar relationship between temperature along the tracks of RAFOS floats at 3000 m in the 

Deep Western boundary Current (DWBC) and thermocline depth was found where the DWBC 

crosses under the Gulf Stream near Cape Hatteras (Bower and Hunt, 2000). This relationship 

was exploited to show that the deep floats drifted offshore into deeper water to conserve f/h as 

the floats crossed under the deepening thermocline of the Gulf Stream. Qualitative use is made 

of this relation in the western Gulf in the following Chapter.

4.2.3 Parameter Maps 

The spatial distribution of parameters is estimated by averaging the parameters over grid boxes 

arranged to cover most of the deep Gulf. This is generally preferable to averaging the T/S 

profiles in a grid box and then calculating the parameters from the averaged T/S profile (Lozier 

et al. 1994) because water mass characteristics (e.g., AAIW) may be diminished by the 

smoothing. The adopted grid, with approximately square elements with width ~2° longitude are 

given in Figure 4-8. The density of stations within each grid square varies considerably with the 

lowest numbers of profiles in boxes 9, 12 and 22. As a check on how well major features are 

represented by the space and time averaging in Figure 4-9, the SSH from the profile locations 

and times, averaged over grid boxes, is compared with the “true” four-year average of the daily 

SSH maps that are on a 0.25° grid. The comparison is given in Figure 4-9, and despite the 

sparseness of profile grid data, and thus the subsampling of the SSH data, the major features of 

the mean SSH field are fairly well reproduced. These include the LC in the east, though much 

less intense, the low-high-low triplet between 26°N and the Bay of Campeche along the western 

slope, and the lows and highs in the center of the western basin. The latter are weaker and more 

broken up in the altimeter SSH map than on the profile grid, again a function of resolution. 

Therefore, reasonable comparison and agreement of features in the “true” and subsampled 

averaged SSH fields does indicate that tracing patterns for the subsurface parameters will have 

some validity. 
4.2 4.25 4.3 4.35 4.4 4.45 4.5

Temperature at 1490 m (°C)

600

650

700

750

800

850

900

950

1000

1050

Depth of 6 °C Isotherm (m)

BOEM APEX CTD Profile Data

y = 1.3e+03*x - 4.9e+03

R2 = 0.61

N = 335

Figure 4-7. Scatter plot of the depth of the 6 °C isotherm against temperature at 1490 m 

Parameters from APEX CTD profiles in the Gulf.

118
US ARGO & BOEM APEX CTD Profiles 2011-07-07 to 2015-08-17

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Figure 4-8. Map of analysis grid squares

CTD profile locations from BOEM APEX and US Argo profiling floats are given by 

black squares. Orange numbered squares give grid averaging boxes with the 

averaged locations within each box given by the red squares. The grid is used for 

contour maps of the box area averages of analysis parameters derived from individual 

T/S profiles.

119
a)

b)

Figure 4-9. Four-year mean SSH maps

a) Mean SSH from daily 0.25° gridded CCAR altimeter SSH maps. b) Mean SSH 

from APEX and Argo profile locations in the grid given in Figure 4-8.

120
121 

Mean maps of 1000 to 50 dbar dynamic height and the depth of the 6 °C isotherm (Figure 4-10) 

have similar features and the expected inverse relation. Besides the northward shoaling in the 

LC, there is a prominent high in the western basin and corresponding depression in the interface 

surface. This is a reflection of LC eddies stalling and slowly decaying against the western 

boundary (see the discussion of Icarus, Kraken and Lazarus in the western basin in Chapter 3). 

The AAIW mean parameter maps are given in Figure 4-11 and, as expected, the depth of the 

AAIW salinity minimum has the same pattern as the depth of the 6 °C isotherm, but about 50 to 

60 m shallower under the LC that reduces to 20 to 30 m shallower in the west and in the Bay of 

Campeche. The salinity minimum shows a slow increase in value from east to west as the water 

mass becomes more distant from its input source in the Yucatan Channel (Figure 4-11b). The 

density (στ) of the salinity minimum also increases from east to southwest corresponding to the 

increase in salinity and ~0.4 °C decrease in temperature. Colder deep temperatures in the 

southwest are influenced by the dynamic height low and the cyclonic gyre in the Bay of 

Campeche (Vazquez de la Cerda et al. 2005). 

The SUW mean parameter maps are given in Figure 4-12. The percentage of locations in each 

grid box that show presence of SUW ranges from >70% in the LC to 30 to 40% across a swath 

of the central and western Gulf, to less than 10% in the northwest and southwest corners (Figure 

4-12d). The source of SUW is the LC with the salinity maximum of the 70 to 90 m thick core (S 

> 36.6 psu) being between 180 and 150 m (Figure 4-12a and b). The SUW core thickness is at a 

maximum in the northeast from northward extensions of the LC, and in the central western basin 

from the mean WSW trajectories of LC eddies (Elliott 1982; Vukovich and Crissman 1986; 

Hamilton et al. 1999). However, the SUW maximum salinity decreases from >36.8 psu in the 

LC to 36.65 psu on the western boundary (Figure 4-12c), indicating a mixing of the core with 

surrounding Gulf water. 

Interestingly, the depth of this core deepens from where the SUW is thickest in the western basin 

(~92 to 94°W) from ~130 m to ~170 m on the western boundary ~23°N (Figure 4-12a and b). 

This western maximum depth region roughly corresponds the maximum depths of the AAIW 

and the 6 °C isotherm (Figure 4-11a and Figure 4-10b). This implies that SUW water sinks in 

the region where LC eddies stall and/or interact with the western boundary, and may imply a 

return flow at depth in the thermocline as compensation for the westward drift of LC eddies and 

possibly surface mean flows as suggested by Sturges and Bozec (2013).

The mean PV of the lower layer, f / h6, where h6 is the grid-averaged depth of the lower layer 

derived from grid-averaged water depth, depth of the 6 °C isotherm and latitude, is shown in 

Figure 4-13. There are connections along the northern boundary with the western boundary, and 

given the low resolution of the map, there is a suggestion that the 30 x10-9 m

-1

s

-1 contour 

connects through the Bay of Campeche and along the Campeche slope back to the LC. Over 

steeper slopes, the total water depth controls the f /H gradients. One feature of the map is the 

closed PV contour in the western basin. This coincides with the deep cyclonic gyre in the 

western Gulf (see Section 3.5). Thus, the deepening of the 6 °C isotherm depth over the abyssal 

plain in the western Gulf produces a closed f /h6 circuit, which may be a strong constraint of the 

mean deep flows. 

 
Figure 4-10. a) Dynamic Height (cm), and (b) Depth (m) of the 6 °C isotherm

Maps of grid-averaged APEX and Argo CTD profile data.

a)

b)

122
Figure 4-11. Maps of AAIW mean parameters

a) Depth (m) of the salinity minimum, b) salinity minimum (psu), c) density (σt) and d) temperatur

(°C) of the salinity minimum.

a) b)

c) d)

123
a) b)

c) d)

Figure 4-12. Maps of SUW mean parametersa) Depth (m) of the salinity maximum, b) thickness for S > 36.6 psu, c) salinity maximum, and d) percentage of locations in each grid box with SUW present.

124
f / h6 (x109 m-1 s-1) where h6 is grid averaged water depth - depth of 6 °C isotherm

Figure 4-13. Lower-layer potential vorticity from mean parameters

Bottom depths, latitude and depth of the 6 °C isotherm, used in the calculation of f / h6, are 

grid-averaged using the locations of the profiles. Bathymetry contours are 100 m and every 

500 m from 1000 m.

125
126 

4.3 BIO-OPTICAL PARAMETERS

Seven Bio-Argo floats were deployed in the Gulf in 2011 and 2012. Among these floats, one 

float experienced technical issues on optical sensors (4902288_G4901605) and was discarded 

from the dataset. Each of the floats was equipped with chlorophyll (Chl) and colored dissolved 

organic matter (CDOM) fluorometers, and optical backscattering coefficient (BBP) sensors, at 

700 nm (see Green et al. (2014) for more details). 

Bio-optical measurements were performed from 0 to 1500 m depth (about 5 m resolution in the 

0–200 m layer, 10 m resolution in the 200–500 m layer, 25 m resolution in the 500–1000 m 

layer, and 50 m below 1000 m. Temperature and salinity were also collected and interpolated 

from the CTD profiles to the same depths as the optical measurements. Profiles were generally 

taken at 14-day intervals on each APEX float. Floats collected a total of 537 profiles over the 

study period. Figure 4-14 shows the tracks of the six APEX floats based on surface fixes each 

time they made a profile. 

4.3.1 Chlorophyll Measurements

Chlorophyll concentration is the main proxy of phytoplankton biomass and represents a key 

oceanic biogeochemical variable (Cullen 1982). The lack of knowledge on the vertical 

chlorophyll distribution is mainly due to the cost and difficultly of making in-situ chlorophyll 

measurements. A way to overcome the scarcity of these data, is to use fluorescence observations 

(chlorophyll absorbs blue light and re-emits it, as fluorescence, in the red, (Lorenzen 1967)). 

Fluorescence, however, is only a proxy for chlorophyll and a calibration of the fluorescence 

signal is generally required (change in fluorescence/chlorophyll ratio depending on taxonomic 

assemblages, environmental conditions, etc.). 

Fluorescence data were converted to chlorophyll using the laboratory calibrations (Green et al. 

2014). After the deployment, chlorophyll profiles were also corrected from an offset resulting in 

some variation from the dark counts determined by the manufacturer, by assuming that at depth 

fluorescence of chlorophyll should be zero (Schmechtig et al. 2014). 

4.3.2 Density and Mixed-Layer Depth Computation 

Density profiles were calculated from temperature and salinity profiles, using the 

Thermodynamic Equation of Seawater - 2010 (TEOS-10). The intensity of mixing was assessed 

through the variation of the Mixed-Layer Depth (MLD). The MLD was calculated as the depth 

where the difference of density from the surface reference, fixed at 10 m-depth, is 0.03 kg m-3

(de Boyer Montégut et al. 2004), and considering density profiles linearly interpolated at 1 mdepth intervals. If no observation was available at 10 m, the shallowest measurement was used 

as the surface reference, if it was above 20 m. Otherwise, MLD was not computed. 
A

B C

D

E F

 100oW 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure 4-14. Surface tracks of the six APEX floats

Tracks are based on the surface fixes recorded each time a profile is made. 

127
128 

4.3.3 Chlorophyll Time Series Analysis

Float time-series acquired in the Gulf show that the majority of the variability in chlorophyll 

occurs above 200 m (Figure 4-15). A Deep Chlorophyll Maximum (DCM), characteristic of 

oligotrophic environments, is observed generally around 100 m-depth, whatever the sampled 

area. Despite a noticeable inter-annual variability, float time-series also reveal a clear seasonal 

pattern in MLD dynamics. In summer, MLD values are minimal, but in winter a slight 

deepening of the MLD is observed, and values greater than 50 m are not uncommon. Maximum 

MLD are measured from the time-series A in winter 2012–2013, and time-series D in winter 

2013–2014 (Figure 4-15). During these periods, MLD could reach more than 150 m.

The winter MLD generally reaches the DCM and a subsequent seasonal increase in surface 

chlorophyll is thus observed (Figure 4-15 and Figure 4-16). However, a strong inter-annual and 

spatial variability is observed in the magnitude of this increase. Mean winter surface chlorophyll 

concentration ranges from around 0 to 0.7 mg m-3, and values are consistent with previous 

studies based on High Performance Liquid Chromatography (HPLC) analysis (Biggs and Ressler 

2001; Qian et al. 2003). Data measured by the float that drifted along the coast (time-series E), 

also suggest a seasonal variability in the depth of the DCM, likely due to the seasonal upward 

and downward shift of isolumes. This is less obvious for the others floats.

Superimposed on this general feature, float transects also show a variability that occurs at shorter 

time-scales. The subsurface temporal changes in both depth and intensity of the DCM seem 

closely related to short time-scale isopycnal displacements (black lines, Figure 4-15). Such a 

variability, already highlighted by Green et al. (2014) from the analysis of the float time-series F 

(Figure 4-15), was supposed to be due to mesoscale structures. The present dataset provides a 

step forward since it demonstrates that this high-frequency variability is observed everywhere in 

the basin. 

This preliminary analysis clearly demonstrates the potential of bio-optical APEX data to improve 

our understanding of the ecosystem functioning in the Gulf. The mean seasonal cycle of DCM is 

given in more detail than is available through conventional ship-based measurements. Further 

analysis could also identify the impact of small spatiotemporal scale perturbations over this 

seasonal cycle. 
−350

−300−250−200−150−100

−50

0 a) Float A b) Float B

−350−300−250−200−150−100

−50

0 c) Float C d) Float D

−350−300−250−200−150−100

−50

0 f) Float F e) Float E

Jul−11 Nov−11 Mar−12 Aug−12 Dec−12 May−13 Sep−13 Jan−14 Jun−14 Oct−14 Mar−

Jul−11 Nov−11 Mar−12 Aug−12 Dec−12 May−13 Sep−13 Jan−14 Jun−14 Oct−14 Mar−15 Jul−15 Dec−15

Figure 4-15. Chlorophyll transects from APEX profilesDensity (1023, 1024, 1025, 1026, black), and mixed layer depth (red) contours are superimposed. Each panecorresponds to transect along path of float shown in Figure 4-14.

129
01−Jul−11 12−Nov−11 26−Mar−12 07−Aug−12 20−Dec−12 03−May−13 15−Sep−13 27−Jan−14 11−Jun−14 23−Oct−14 07−Mar−

0

0.10.20.30.40.50.60.70.80.9

1

Chl [mg m−3]

Time

Figure 4-16. Mean chlorophyll concentration in the 5–30 m layer Data are smoothed using a seasonal filter. The legend corresponds to the different panels shown in Figure 

130 
131 

CHAPTER 5: FLOAT ANALYSIS 

The following sections have a primary focus on the relationships between different regions of the 

Gulf, as well as between upper- and lower-layer circulation, as described by deep Lagrangian 

trajectories. The first section looks at the probabilities of exchange between different subregions of the deep basins, and the role of escarpment boundary currents. The next section 

studies the connections between the LC and deep flows, including baroclinic instabilities and 

radiation by TRWs. The third section looks at circulation processes in the central and western 

Gulf, including the effects of transiting LC eddies on deep float trajectories, and the final section 

describes how well the HYCOM model can reproduce the essentials of the observations, and 

provide guidance on the dynamics of the deep Gulf. 

5.1 CONNECTIVITY OF THE DEEP WATERS IN THE GULF

5.1.1 Introduction 

Lagrangian observations are very useful in providing information on how particles (water parcels 

or passive tracers) spread in the ocean. From float trajectories, one can learn which regions 

exchange water and at what time scales, or distinguish the regions that have limited or slow 

exchange with their surroundings. For example, drifter card landings on the US coast of the Gulf 

showed that cards released in the western basin rarely landed on the eastern coast, while the 

western coast received cards released in both the western and eastern Gulf (Lugo-Fernández et 

al. 2001). Drifters from the GLAD and SCULP-II experiments show that the exchange of 

surface water from the continental slope towards the shelf is limited, while it is more common 

for shelf waters to cross towards the slope (Hamilton et al. 2015). At depth, the analysis of 

PALACE floats in Weatherly et al. (2005) showed limited exchange between the eastern and 

western basins in the intermediate layer (900m) at times-scales of ~5 years. Modeled particles 

released below 1500 m over the entire Gulf suggest that only ~10% of particles released west 

(east) of 88°W are exchanged between the eastern and western basins after 15 years (Welsh and 

Inoue 2002). Another approach to measure the spread of a tracer diluted in water is via dye 

release experiments, in which a known concentration of dye is released in a small area, and 

tracked over a time interval. Ledwell et al. (2016) made such a study, releasing dye at 1100 m 

on the continental slope where the 2010 Deepwater Horizon rig oil spill occurred. The tracer 

patch moved both westwards along the slope as well as into the interior. After 12 months, the 

highest concentrations were found over the northwestern slope, as well as in the interior of the 

central and eastern basins of the Gulf. Numerical experiments releasing both particles and 

tracers in the same region as the Ledwell et al. (2016) study suggests a similar evolution (Bracco 

et al. 2016). This pattern seems to be persistent, because several releases under different times 

repeated the spreading pattern of the first four months. By contrast, the release of particles and 

tracers on the central northern slope shows a faster westward spread, although intermittency of 

the boundary currents result in different scenarios depending on the time of the release (Bracco 

et al. 2016). 

One approach to quantify the exchange of particles between fixed regions at various time scales 

is via a connectivity matrix M(i,j), which represents the probability that a particle released in 

region j at time t=0 is found in region i after a time t=T has passed. Therefore, the connectivity 

matrix represents the most likely spatial distribution of particles after a set time has passed 

following their release. Once constructed, the connectivity matrix can be used to estimate the 
132 

evolution of the concentration of a tracer (Mitarai et al. 2008). The data generally used to 

estimate the connectivity matrices are particle trajectories obtained from numerical model 

simulations (Mitarai et al. 2008; Feng et al. 2010; Marinone 2012). Connectivity matrices can 

also be estimated from Lagrangian observations, where each float or drifter trajectory is 

interpreted as the path followed by a passive particle (Manning et al. 2009; Rodríguez-Outerelo 

2015). Because large numbers of particles, released repeatedly under different flow conditions, 

are needed to get statistical confidence, these studies are much less common in the literature. 

Connectivity matrices have been predominantly used to address population dynamics of marine 

species, estimating the connectivity between populations via larval dispersal (Marinone et al. 

2008; Cowen and Sponaugle 2009; Kim and Barth 2011). Nevertheless, the approach can also 

be used to address the possible spread of pollutants or other passive tracers in the water (Paris 

and Chérubin 2008; Mitarai et al. 2009; Shan and Sheng 2012). 

In this case, the main purpose is to find how different regions of the deep Gulf are connected at 

the time-scales addressed by this study (< 18 months). In particular, we are interested in 

quantifying the connectivity between the Eastern and Western Gulf, the retention capability of 

the mean cyclonic gyre present in the deepest part of the basin, and the preferred pathways of 

exchange of particles in the Gulf. 

5.1.2 Construction of Connectivity Maps and Matrices 

In order to estimate the connectivity matrix, the area of study is divided into different regions. 

This subdivision is rather arbitrary and the choice largely depends on the purpose of the study, 

and the size of the database available for the analysis. One example is to subdivide in regions 

where distinct populations exist for which one wants to quantify connectivity via larval dispersal 

(Sanvicente-Añorve et al. 2014). Other studies establish the regions based on distinct dynamical 

characteristics, such as permanent gyres, persistent currents, or enhanced eddy activity 

(Marinone 2012). The study area can also be divided on a latitude-longitude grid, and the matrix 

would represent the connectivity between all geographical bins (Manning et al. 2009). The size 

of these regions is limited to the database of trajectories available; i.e., they need to be large 

enough to have statistical significance when estimating the probability matrix, and small enough 

to provide relevant information for the purpose of the study. 

Given the relatively small database of trajectories for the float data in this study, the Gulf was 

divided into nine rather large regions, based on the main features of the mean flows given in 

Section 3.5. The eastern Gulf was divided into two regions: the area where the exchange with 

the Caribbean occurs (A), and the region where Loop Current Eddies are formed (B). The 

western Gulf was divided into five regions: four along the boundary (D, E, F, and G), while H 

encloses the mean cyclonic gyre in the central western Gulf, as well as the narrow current along 

the Campeche shelf. Finally, C and I are the regions connecting the eastern and western Gulf 

(Table 5-1, Figure 5-1). 
133 

Table 5-1. Names of the Regions used for the Connectivity Analysis

Identifier Name Location

A Southeastern region Eastern Gulf 

B Northwestern region 

C Northern exchange region

I Southern exchange region

D North-central region 

E Northwestern region

F Western region Western Gulf 

G Southwestern region

H Central region 

The data from all floats were used to estimate the connectivity matrices (155 RAFOS + 7 

APEX). Calculations were made as follows. For each region X, N trajectories of floats are 

found that either were deployed there or entered the region at a later time, with the first data 

point within the region representing the start of the trajectory. As an example, Figure 5-2 shows 

the initial position of each trajectory in region D as an example. Data points along the boundaries 

of the region represent the start points of floats that were not released there but entered region D 

at a later time, while the start points away from the boundary represent floats that were originally 

deployed within region D, or whose first data were recorded there (this would be the case for the 

floats with the memory override problem, which were deployed at a very different position than 

that of the first data in their record). The initial positions of the trajectories for all other regions 

are shown in Appendix B.

In order to construct the connectivity matrix, it is assumed that all trajectories start at the same 

time (t=0), even though in reality, most trajectories start at different times, even on very different 

dates. The assumption is made that the dispersion statistics do not change in the interval 

analyzed. In that sense, each region has N “floats” that are used to represent how water parcels 

released in the source region X spread over the rest of the regions of the Gulf as time passes by. 

These are referred to as “particles” that were originally “released” in region X. Note that the 

initial distribution of particles is not homogeneous: regions B and C have more than 90 particles 

within them at time t=0, while regions F and G have less than 40 (Figure 5-3). Another 

important thing to notice is that the duration of the trajectories varies. For particles that were 

originally deployed in region X, positions are available over 1.5 years (except for the floats with 

the memory override problem or that surfaced early). The rest of the particles within region X 

will have shorter records, because they result mostly from floats deployed elsewhere. Therefore, 

as time increases, the number of available particles declines. Figure 5-4 shows the number of 

particles as a function of their source region and time after release. All regions had at least 18 (9) 

active particles after 12 (18) months.
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Figure 5-1. Sub-regions of the Gulf

Upper panel: Regions named cyclonically from A to I. Lower panel: Mean velocity 

field for a 0.25 grid (see Section 3.5), using RAFOS floats at 1500 and 2500 m 

depth, and APEX floats at 1500 m. Isobaths are shown in gray contours at 50 and 

1500 m.

134
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Figure 5-2. Initial position of particles released in region D

135
Gulf Areas

A B C D E F G H I

Initial number of particles

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Figure 5-3. Histogram of the initial number of particles foreach Gulf region

136
Elapsed time since the release (months)

0 3 6 9 12 15 18

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# of particles

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Figure 5-4. Number of active particles as function of time and source region

137
138 

In order to estimate connectivity between a particular region X and the rest of the Gulf after a 

certain time T has passed, the percentage of particles released in X that are present in each of the 

regions of the Gulf at time T is calculated. Because the particles have different life times, the 

percentage is estimated with the total number of active particles at time t=T (Figure 5-4). An 

example of this for Region A is shown in Figure 5-5 and Figure 5-6. Particles start within the 

region at time t=0, and as time passes by, they start spreading as shown in the dispersion maps 

(bottom panels of Figure 5-6). The top panel of Figure 5-6 shows the corresponding 

“connectivity maps”, in which the active particles present in any given region, at times T= 6, 12 

and 18 months, are represented by a percentage of the total of all active particles that started in 

Region A. Hence it can be estimated that after six months, ~30% of the particles stayed in 

Region A, ~50% moved to region B, ~20% to region C and ~10% to region I. The connectivity 

map information for particles released in A is summarized in Figure 5-7, showing the percentage 

of particles present at any region as a function of time t past their release (Mt(j,t): called a 

“connectivity matrix as a function of time” for source region A). Note that percentages less than 

5% are considered not different than zero, and that each column should add to100%. Also note 

that this matrix is not the same as the classical connectivity matrix M(i,j) mentioned in the 

introduction, because Mt(j,t) shows the distribution of particles for a given source region as a 

function of time t, while M(i,j) shows the distribution of particles from all source regions for a 

set time t=T. The classical connectivity matrix M(i,j) is constructed from the corresponding 

column t=T of the “connectivity matrix as a function of time” Mt(j,t) of each source region i, as 

will be seen in Section 5.1.3. 

From Figure 5-7, it is clear that particles released in the southeastern region (A) did not visit the 

western Gulf regions during the 18 months for which active particles existed, and that most of 

them ended up in region B (at least 50% of region A particles, regardless of the time elapsed). 

Similarly, particles released in the southwestern most region (G: Figure 5-8, Figure 5-9, and 

Figure 5-10) spread mostly in the western Gulf, and into the exchange regions (I and C), but 

none enter the eastern Gulf regions during the 18 months analyzed (although only 14 particles 

remained active after 15 months, so the statistics are not very robust in this case). These results 

are an indication that exchange between the eastern and western Gulf is limited. This will 

become more evident in the next section.

The original distribution of particles, connectivity and dispersion maps, and connectivity 

matrices as a function of time for all source regions, are shown in Appendix A. These are useful 

when the focus is the evolution of particles from one particular source region. 

5.1.3 Connectivity, Retention and Exchange Pathways 

From the information of each source region, the connectivity matrices are constructed, and are 

shown in Figure 5-11. These matrices show how particles from all source regions are distributed 

in the entire basin at set times after they were released, the rows showing the proportion of 

particles from a given source region present on each region of the Gulf at that time. Each row is 

the corresponding column of the “connectivity matrix as a function of time” Mt(j,t=T) for the 

source region and time elapsed T in question. For example, the second column of the 

“connectivity matrix as a function of time” for region A (Figure 5-7), corresponds to the bottom 

row of the connectivity matrix for T=3 months in Figure 5-11. In this case, each row of Figure 

5-7 should add to 100%. 
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Figure 5-5. Initial position of particles released in region A

139
Figure 5-6. Connectivity and dispersion maps for region A

Connectivity (upper panels) and dispersion (lower panels) maps for particles that start in region A at 6, 1months after their release. Colors represent percentages of particles originally released in A that are preregion at each elapsed time. The total number of active particles at each elapsed time is used to calculatpercentages. Black circular markers show the location of those particles at 6, 12, and 18 months after th

140 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in A % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

Figure 5-7. Connectivity matrix as a function of time for source region A 

Colors represent percentages of particles originally released in A that are present in each 

region of the Gulf past a certain time. The total number of active particles at each 

elapsed time is used to calculate the percentages.

141
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Figure 5-8. Initial position of particles released in region G

142
Figure 5-9. Connectivity and dispersion maps for particles released in region G 

See Figure 5-6 for details.

143 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

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Particles released in G % particles

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Figure 5-10. Connectivity matrix as a function of time for source region G 

See details in Figure 5-7.

144
Final region

A B C D E F G H I

Source region

ABCD

E

F

G

H

I

Elapsed time: 3 months

Final region

A B C D E F G H I

Source region

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Elapsed time: 6 months

Final region

A B C D E F G 

Source region

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Elapsed time: 9 mo

Final region

A B C D E F G H I

Source region

ABCD

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F

G

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I

Elapsed time: 12 months

Final region

A B C D E F G H I

Source region

ABCD

E

F

G

H

I

Elapsed time: 15 months

Final region

A B C D E F G 

Source region

ABCD

E

F

G

H

I

Elapsed time: 18 mo

Figure 5-11. Connectivity matrix for various times T past release Colors represent the percentages of active particles that start at a source region (y axis) and arrive to a fina(x axis). Diagonal black line shows the retention capability of each region. The red boxes enclose the regithe Eastern and Western Gulf basins.

145 
146 

Three aspects can be analyzed from the connectivity matrices: the retention capability of each 

source region; the regions whose particles spread the most, that is, that export particles to most of 

the Gulf; and the regions that act as importers or sinks. 

The diagonal of the connectivity matrix shows the retention capabilities of each source region. 

The least retentive are the northwestern (E) and southern exchange (I) regions. After six months, 

less than 30% of the particles released are still present there, while all other regions still contain 

30–60% of their original particles. On the other hand, the western (F) and northeastern (B) 

regions, and to a lesser extent the central region (H), retain most particles during the first nine 

months, with ~50% of their particles remaining there. After that, particles from the central (H) 

and northeastern (B) regions exit and recirculate back to their source regions at a later time. 

After 18 months, a consequence is that those two regions have 30–50% of their original particles, 

although not all remained there the entire time. 

The regions that export particles to most of the Gulf can be inferred from the rows of the 

connectivity matrices. For the first nine months, the southern exchange region (I) exported 

particles to all except the westernmost regions (E, F and G). After that, many source regions 

have particles that visited most of the rest of the Gulf at some point in time. There are some 

notable exceptions: 1) no particles from the northwestern and western regions (E and F) ever 

visited the eastern Gulf regions (A and B); 2) particles from the southeastern Gulf region (A) 

never made it to any of the western regions (D through H), while very few particles from the 

northeastern region B reached the north-central and northwestern regions (D and E), and 3) 

particles released in the southern exchange region (I) never visited the western and southwestern 

regions (F and G). These results suggest that there is virtually no connection between the 

easternmost and western-most regions of the Gulf at the time scales analyzed (< 18 months). 

The low connectivity between the eastern and western basin was also reported by Weatherly et 

al. (2005), using four years of Profiling Autonomous Lagrangian Current Explorer (PALACE) 

float drift data at 900 m. The lifetimes of the PALACE floats varied between 1 and 3.5 years, so 

the timescales analyzed are comparable. This result may seem puzzling, since the high 

concentration of oxygen in the lower layer of the western Gulf can only be maintained by the 

supply of North Atlantic Deep Water (NADW) from the Caribbean (Nowlin et al. 1969; Jochens 

and DiMarco 2008). The apparent contradiction may be resolved by considering that the 

ventilation of the deep waters occurs at long timescales: estimates of the residence times of the 

bottom layer are of the order of 100 years (Rivas et al. 2005). 

The degree of connectivity between the eastern and central Gulf can be inferred by analyzing the 

exchange regions (C and I). Over 60% of the particles from the northern exchange region (C) 

either remain there or travel to the eastern Gulf, regardless of the time elapsed past their release. 

After 12 months, ~30% have crossed into the western basin, along the northern regions (D and 

E). Very few enter the southern exchange region (I) at any time. In the case of the southern 

exchange region (I), particles released there preferentially end up in the northeastern Gulf (B) or 

in the northern exchange region (C), with less than 30% entering the western Gulf at any given 

time past their release. 
147 

The results suggest preferred pathways of exchange between the eastern and the central basins. 

Particles from the eastern basin (B) preferentially exit via the northern exchange region (C) and 

are only found in the north-central and northwestern regions (D and E). This suggests that the 

east to west exchange occurs preferentially along the northern Gulf. The west to east exchange 

pathway is not so clear, although there is some indication that it preferably occurs along the 

southern boundary: particles from central region H show slightly more presence in the eastern 

basin than the particles from the north central region D, and the particles are found in higher 

concentrations in the southern exchange region (I) at the initial time scales (T < 9 months). 

From the columns of the connectivity matrices one can infer the largest importers of particles. 

The central regions (D and H) are the ones that receive particles from most Gulf regions. The 

difference is that only the north-central region (D) receives particles from the eastern Gulf, while 

the central region (H) tends to have higher concentrations of particles than D, acting as a sink for 

the western Gulf regions. The northeastern region (B) is a sink of the southeastern region (A), 

since it contains at least 50% of its particles for any time T > 3 months. This result is biased, 

however, because the initial positions of the particles in A are located near the boundary with B. 

What is clear is that most of the particles released in the eastern Gulf (A and B), remain there. 

The connectivity matrices allow us to look at the preferential exchange pathways between the 

regions of the western Gulf. This can be inferred by analyzing the box containing the 

corresponding source and final regions (D through H) for the initial times (T < 9 months). The 

largest concentrations are found below the diagonal, suggesting that particles preferentially move 

between the western regions following a cyclonic pattern. That is, particles are preferentially 

exchanged from the northeastern region E to the western region F; from F to the southwesternmost region G; and from G to the central region H. 

It is interesting to note that no floats left the Gulf. The Straits of Florida are too shallow, but the 

Yucatan Channel is deep enough to make it possible for the 1500 m floats to cross towards the 

Caribbean Sea. 

5.1.4 Discussion

The connectivity analysis presented in this section provides a coarse approximation of the 

exchange routes of particles in the deep lower layer of the Gulf, and of the time scales involved 

in the exchange. It is noted that the results apply only to particles released at 1500 and 2500 m, 

so very different results should be expected in the upper layer and surface. 

The major results can be summarized as follows: 

• Very low connectivity between the eastern and western Gulf at the ~1 year timescales. 

• The exchange of passive tracers from the eastern to the western Gulf seems to 

preferentially occur along the northern boundary. The exchange pathway from the 

western to eastern Gulf is less clear, but there is some indication that it preferentially 

occurs along the southern boundary. 
148 

• There are two regions that rapidly export particles to other regions: the northwestern (E) 

and southern exchange (I) regions. Because more than half of their particles are exported 

in the first three months, they are expected to be regions where tracers are more rapidly 

transported away from their source. 

• The regions where tracers are expected to stay the longest are the westernmost region (F) 

and the northeastern Gulf (B). 

• The preferred exchange direction of particles released in the western Gulf follows a 

cyclonic pattern, particularly along the western boundary. The deep cyclonic gyre 

entrains particles from all western regions. 

Figure 5-12 shows a cartoon that summarizes the main exchange pathways and retention areas 

inferred from the dispersion and connectivity maps for each source region, and connectivity 

matrices for time scales T < 9 months, which is the time scale at which most regions have 

exported more than half of their particles. 

It is important to bear in mind the limitations of this analysis, and be cautious on the 

interpretations and implications of the results. The most important limitation is the relatively 

low number of “particles” used, which are distributed unevenly in space and time. In addition, 

large regions are used to get reasonable statistics, and the result is a coarse analysis of the 

exchange of particles in the Gulf. The bias towards the boundaries of the source regions of the 

initial positions of the particles may affect the estimated time scales at which particles are 

retained or exchanged, as well as the estimated percentage of particles exchanged between 

regions. In addition, the assumption that the statistics do not change over the interval analyzed 

may also have its drawbacks. Regions with high variability are expected to show different 

spreading scenarios for tracers released at different times. Numerical simulations of particles 

and tracers released from point sources over the northern continental slope suggest different 

spreading scenarios depending on initial conditions in regions characterized by strong and 

intermittent boundary current flows (Bracco et al. 2016). Finally, the connectivity matrices are 

only representing the statistics for the period of the experiment (2011–2015). A much larger 

number of particles, released evenly in space and in time over a much longer period, would be 

required to have robust statistics of the most likely connectivity scenarios. 
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Figure 5-12. Conceptual map of preferred particle retention sites and exchange routes

Circles represent regions with high retention, where particles remain relatively 

long times.

149
150 

Another concern is the variable lifetimes of the particles, so that the total number used to 

calculate the percentages changes with the time elapsed past their release. Therefore, the 

connectivity matrices for the larger time scales (T > 12 months) will have less trustworthy 

statistics and should be interpreted or used with even more caution. This is particularly true for 

T =18 months, when most source regions had less than 20 active particles available for the 

statistics. In order to estimate the importance of the lifetimes of particles in the connectivity 

analysis for time scales T < 9 months, the connectivity matrices were recalculated with two 

subsets of floats whose lifetimes (mission lengths) were equal to or greater than nine and 12 

months. The percentages of the connectivity matrices estimated with the both subsets and those 

with all active drifters shown in Figure 5-4 differed by less than 5%. Therefore, there is no effect 

on the analysis due to the variable lifetimes of the particles for times scales less than a year. 

Despite the limitations of the analysis, the main results provide a basis for the formulation of 

specific hypotheses and the design of studies to further understand the exchange processes and 

timescales involved in the spread of passive particles in the deep waters of the Gulf. 

5.2 DEEP FLOWS UNDER THE LOOP CURRENT AND IN THE EASTERN GULF

5.2.1 Introduction 

The LC dominates the circulation of the eastern Gulf through the irregular shedding of rings or 

large warm anticyclones, usually called LC eddies, that translate into the western basin, the 

generation of secondary cyclones and anticyclones in the upper layer, and the generation of 

energetic deep currents. The LC enters the Gulf through the Yucatan Channel and takes various 

paths to its exit through the Florida Straits, including a direct port-to-port mode, and a fully 

extended mode where the current has a clockwise loop that often reaches towards the Mississippi 

Delta. It is during the northward extensions that LC eddies detach and separate from the LC 

proper, though the sequence can be variable with often many detachments and reattachments 

before the eddy separates and moves into the western basin. 

LC flows are large (~1.5 m s-1 near surface) with strong vertical shears that penetrate to ~800 to 

1200 m. The sill depth at the Florida Straits is ~800 m, approximately the mean depth of the 6 

°C isotherm, which can be used to divide the upper-layer LC and eddy flows from the deep 

circulation (Bunge et al. 2002). In the eastern Gulf, the lower-layer currents under the LC are 

essentially depth independent from ~1000 m to near bottom (≳ 3000 m in the eastern basin), and 

are highly variable with large increases in variance during detachment events (Hamilton et al. 

2016). The origin of these deep events is baroclinic instability of the upper-layer southward 

flowing arm that exhibits large-scale (~300 km wavelengths and 40–100 day periods) southward 

propagating meanders on the east side of the LC over deep water. It has been speculated that 

these deep, under the LC events, are a source of topographic Rossby waves (TRWs) that radiate 

towards the northwest and the Sigsbee escarpment where high-speed currents have been 

observed (Hamilton 1990; Oey and Lee 2002; Hamilton 2009). Furthermore, rectification of 

TRWs by shoaling topography may be the source of the deep westward-flowing boundary 

current found at the base of the Sigsbee escarpment in the north-central and northwestern Gulf 

(Mizuta and Hogg 2004; DeHaan and Sturges 2005). 

Previous observations in deep waters have largely been from moorings, and include BOEM 

funded arrays in the central, northwestern and eastern Gulf (Donohue et al. 2006; Donohue et al. 
151 

2008; Cox et al. 2010; Hamilton et al. 2014; Hamilton et al. 2016). The north-central Gulf study 

(called the Exploratory study; Donohue et al. (2006)) also included the first deployments of 

RAFOS drifters in the Gulf and about six to nine months of track data were obtained that showed 

the existence of the deep northern boundary current along the Sigsbee escarpment. Moored 

current measurements showed generally that the lower-layer flows were depth independent or 

slightly bottom intensified, and thus highly coherent through the lower water column, and 

usually were uncorrelated with simultaneous upper-layer eddy flows. However, in the LC 

dynamics study array, Donohue et al. (2016a) showed that the barotropic bottom flows interacted 

with upper layer LC meanders through baroclinic instabilities that produced deep anticyclones 

and cyclones displaced downstream from their corresponding meander crests and troughs. The 

baroclinic instabilities extracted energy from the enormous available potential energy (APE) of 

the LC. Deepening meander troughs, combined with their associated lower layer cyclones, cause 

LC eddy detachments (Donohue et al. 2016a; Donohue et al. 2016b). The deep cyclones 

translate westward across the “neck” (Schmitz 2005) of the detached eddy and are presumed to 

move into the central Gulf where modeling studies suggest they generate TRWs either indirectly 

(Oey 2008) or through direct dissipation (LaCasce 1998). 

5.2.2 Lagrangian Observations 

Deep float tracks are available from 1500 and 2500 m RAFOS deployments, and the RAFOS 

units on the APEX profiling floats. The latter had rest depths of ~1500 m and produced 14-day 

track segments between CTD/optical profiles. As the profiling floats only spend a brief time at 

the surface (~15 minutes) before descending to 1500 m, the APEX tracks are treated as 

continuous. Similarly, the 1500 and 2500 m float tracks are considered to measure the same 

deep circulations because of the highly coherent flows below 1000-1200 m observed by moored 

velocity measurements that are depth independent in the regions of the LC (Hamilton et al. 

2016), and only slightly bottom intensified south of the Mississippi delta (Hamilton 2007a; 

Hamilton 2009). Therefore, all the RAFOS and APEX tracks in the eastern Gulf are used to 

investigate the horizontal and time variability of the deep eastern basin circulation. The interval 

with a consistently large number of available float tracks is between 10 December 2012 and 3 

July 2014, with a maximum of 77 floats within the region east of 90°W that cover some portion 

of this period, resulting in 23,938 days of track data or an average of 42 floats/day. A histogram 

of monthly float numbers east of 90°W shows relatively uniform coverage with high numbers 

during this interval (Figure 5-13). 

The trajectories are plotted as 16-day segments at 15-day intervals for almost the complete 

period (December 2012 through July 2014) in Figure 5-14. The LC front position is given by the 

17-cm contour from the CCAR 0.25° x 0.25° SSH daily maps. The 17-cm contour is the 

approximate position of the highest surface velocities for the LC and LC eddies (Leben 2005). 

The altimeter SSH maps only resolve the large-scale (~50–100 km) perturbations of the LC and 

LC eddy fronts. In the following discussion, the individual maps will be referred to by date, 

which is the center point of the 16-day trajectories. Moreover, identifiable deep cyclones (C - 

anticlockwise rotations) and anticyclones (A – clockwise rotations) are noted. The complete 

December 2012 to July 2014 interval begins with the northward extension of the LC from a 

relatively retreated position, the formation, detachment, reattachment and eventual separation of 

eddy Kraken. 
Oct11 May12 Nov12 Jun13 Jan14 Jul14 Feb15

0

10

20

30

40

50

60

70

Number of Floats East of 90°W

Figure 5-13. Monthly mean numbers of floats found east of 90°W

152
10 December 2012 25 December 2012 9 January 2013

24 January 2013 8 February 2013 23 February 2013

10 March 2013 25 March 2013 9 April 2013

30°N

28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W

200 m

200 m

1000 m

2000 m 3000 m

3500 m

A A

A

A

A

A A

A

A

C

C

C C C

C

C

C

Kraken

TRW

TRW

Figure 5-14a. 16-day float tracks centered on the panel date 

Magenta, Blue and Red tracks correspond to Apex 1500 m, RAFOS 1500 and 2500 m, 

respectively. The black dot denotes the end of each trajectory. The 17-cm SSH contour for 

the LC on the map date are given as a thick gray lines. (A)nticyclonic and (C)yclonic deep 

circulations are indicated.

153
24 April 2013 9 May 2013 24 May 2013

8 June 2013 23 June 2013 8 July 2013

23 July 2013 7 August 2013 22 August 2013

1000 m

30°N

28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W

200 m

200 m 2000 m

3000 m

3500 m

C

C

C C C

C

C

C

C

C

C

C

C

C

A

A

A A

A

A

A

A

Figure 5-14b. 16-day float tracks centered on the panel date

Magenta, Blue and Red tracks correspond to Apex 1500 m, RAFOS 1500 and 2500 m, 

respectively. The black dot denotes the end of each trajectory. The 17-cm SSH contour for 

the LC on the map date are given as a thick gray lines. (A)nticyclonic and (C)yclonic deep 

circulations are indicated.

154
6 September 2013 21 September 2013 6 October 2013

21 October 2013 5 November 2013 20 November 2013

5 December 2013 20 December 2013 4 January 2014

30°N

28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W

200 m

200 m

2000 m 3000 m

3500 m

1000 m

C

A A

TRW

TRW

TRW

A

C

A

Figure 5-14c. 16-day float tracks centered on the panel date

Magenta, Blue and Red tracks correspond to Apex 1500 m, RAFOS 1500 and 2500 m, 

respectively. The black dot denotes the end of each trajectory. The 17-cm SSH contour for 

the LC on the map date are given as a thick gray lines. (A)nticyclonic and (C)yclonic deep 

circulations are indicated.

155
19 January 2014 3 February 2014 18 February 2014

5 March 2014 20 March 2014 4 April 2014

19 April 2014 4 May 2014 19 May 2014

3000 m

30°N

28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W

200 m

200 m

2000 m

3500 m

1000 m

C C

C

C

C

C

A

A

A

A

A

A

C

C C C

C

C

C

C

A A

Lazarus

Figure 5-14d. 16-day float tracks centered on the panel date

Magenta, Blue and Red tracks correspond to Apex 1500 m, RAFOS 1500 and 2500 m, 

respectively. The black dot denotes the end of each trajectory. The 17-cm SSH contour for 

the LC on the map date are given as a thick gray lines. (A)nticyclonic and (C)yclonic deep 

circulations are indicated.

156
157 

After Kraken drifts off into the western Gulf, the LC retreats far to the south where it remains for 

several months until the beginning of 2014 when it begins to extend to the north again and form 

eddy Lazarus (Figure 5-14).

The first three maps (10 and 25 December 2012, and 9 January 2013; Figure 5-14a) show an 

intensifying anticyclone under the northern tip of a slowly extending LC. This may be 

explained, through PV conservation, as a result of the deep layer being squeezed by the 

extending LC. Low-amplitude fluctuations are observed north and west of the LC. The 

anticyclone appears to weaken through early March (24 January, 8 and 23 February, and 10 

March, 2013), however fluctuations to the west become more energetic showing fairly linear 

trajectories that remain roughly in the same area (88 to 90°W). These could be attributed to 

rectilinear TRW motions at differing frequencies formed as deep energy is radiated away from 

the growing LC or decaying tip anticyclone. In the same sequence, a cyclone appears to the 

northeast of the LC and translates and intensifies southwards with evidence of a trailing 

anticyclone. In the 25 March 2013 map, the north-south anticyclone-cyclone pair are displaced 

southwards from their respective LC meander crest and trough. The lower- to upper-layer phase 

leads are approximately a quarter wavelength, and correspond exactly with Donohue et al.’s 

(2016a) baroclinic instability analysis where intensifying deep eddies lead growing LC 

meanders, and draw their energy from the APE of the LC jet. The intensifying deep cyclone 

causes a detachment of Kraken from the LC in early April 2013.

During May through August 2013 (Figure 5-14b), Kraken dominates the northern half of the 

eastern basin, and cyclones and anticyclones develop under the northern front of the eddy as it 

slowly separates, with a few reattachments, from the LC. Between 9 and 24 May, an 

intensifying anticyclone-cyclone pair develops along the leading edges of a developing crest and 

trough on the northern front of the eddy. Though the sequence is not entirely clear, the strong 

cyclone between the LC and the eddy seen in the 23 June map may have resulted from the 

previously observed anticyclone-cyclone pair, left behind as the eddy moved off to the west. 

This cyclone is not in solid body rotation, as indicated by the faster rotation of floats in the center 

than on the periphery. The trough cyclone on 25 March 2013 (Figure 5-14a) shows similar 

characteristics.

By the beginning of September 2013, Kraken is west of 90°W, and there begins a relatively long 

period through February 2014 when the LC is south of ~25°N (Figure 5-14c). As the floats in 

the northern part of the eastern basin are quite numerous, the circulation characteristics when the 

LC or a LC eddy are not present, may be examined. The floats remain in small localities in that 

they oscillate largely in place and do not show much evidence of any mean transport. There is 

also a lack of identifiable eddy circulation, either cyclonic or anticyclonic. Compare the 

November 2013 maps with the July and August 2013 maps in Figure 5-14b. Some of the 

oscillations are quite rectilinear with long periods of ~one month. For example, the three colocated 2500 m floats oscillating approximately along the 3000 m isobath south of the delta 

between 21 September 2013 and 4 January 2014 (Figure 5-14c) might be considered as 

characteristic of a train of TRW waves of perhaps differing frequencies passing through this 

region. Long and short period TRW motions are at a slight and more acute angle to the general 

trend of the isobaths direction, respectively, and these three floats show large amplitude 

fluctuations along the isobaths between 21 September and 21 October 2013, and then move into 
158 

a more cross-isobath mode in November. TRW group speeds decrease with increasing 

frequency (Oey and Lee 2002), and for broadband sources at similar distance, say for a decaying 

deep eddy, the long period TRWs would precede the shorter periodicities.

The tip of the LC makes a brief northward excursion at the end of December 2013, and again 

generates an anticyclone under the tip as it did previously, prior to Kraken in December 2012 

(Figure 5-14a). A more persistent anticyclone under the tip forms in the middle of February 

2014 as Lazarus begins to form with the northward growth of the LC (Figure 5-14d). Vigorous 

deep cyclones and anticyclones develop on the deep-water sides of the LC front through May 

2014 in a similar manner to Kraken, though the deep activity with respect to the LC front 

configurations is not so clearly delineated. The southern cyclone on 19 May 2014 is not 

displaced from the center of the trough and the trough does not amplify. Subsequent maps (not 

shown) show that the anticyclone-cyclone pair on the northern LC front intensifies by 3 June 

2014; however, the LC grows to the northwest during June and detaches a small eddy in early 

July 2014. At this time, the number of active floats in the eastern basin rapidly reduces, thus 

detailed analysis becomes more speculative. The formation and eventual separations of Lazarus 

and Michael are particularly complex.

5.2.3 Statistics 

The eastern basin is divided into three equal area rectangles: under the mean LC or southeast, 

northeast, and an extension to the north-central and the Sigsbee escarpment (Figure 5-15). 16-

day windows are advanced by one day to cover the 18-month interval described above. For each 

16-day window, velocities from all floats in the rectangle are accumulated and averaged to 

generate a mean velocity vector and corresponding mean kinetic energy (KE), as well as a total 

eddy kinetic energy (EKE). The average numbers of velocity observations per 16-day interval 

for each rectangle are given in Figure 5-15. There are three observations per day, and typical 

Lagrangian integral timescales are ~2-3 days in the eastern Gulf (see Section 3.3). The time 

series, including the area of the LC with the area of the LC plus the area of the LC eddy, as 

determined by the 17-cm SSH contour, are given in Figure 5-16. A separated eddy area is 

counted if it intersects a line from the tip of the Mississippi delta to the NE corner of the Yucatan 

peninsula. Thus, detachments are indicated where the two quantities separate. After the last 

detachment of Kraken, the LC stays in a retreated port to port mode between the latter half of 

August and November 2013.

The center panels show the area-averaged velocities of all the float observations that occur 

within each of the rectangles for each sliding16-day window, centered on the time series date. 

The velocity magnitudes under the LC (black vectors) show large fluctuations just prior to the 

first detachment of Kraken and then later in May 2014 when the eddy was moving to the west 

(see individual maps above). The magnitudes also increase just before the detachment of 

Lazarus (June 2014). In the northeast area (blue vectors), magnitudes are less except when the 

eddy or the northern front of an extended LC begin to show signs of a developing baroclinic 

instability (e.g., May 2013 for Kraken, and May 2014 for Lazarus). In the north-central region 

(red vectors), there are substantial westward area-averaged flows in July and August 2013 that 

could be contributions to the Sigsbee deep boundary current. The next-to-the-top panel shows 

these magnitudes in terms of total mean KE, calculated from the area averaged velocities. 
Mean LC for 10 December 2012 to 3 July 2014

30°N

28°N

26°N

24°N

22°N

92°W 90°W 88°W 86°W 84°W

<N> = 791 ± 247

<N> = 873 ± 133

<N> = Average 

Number of 

Observations in 

Box per 16 day 

Interval

Northeast

200 m

200 m

1000 m

2000 m

3500 m

North-Central 3000 m

Southeast

<N> = 641 ± 132

Figure 5-15. Analysis area boxes for sequential 16-day float velocities

The average number <N> of observations in the 16-day intervals are given, and the average 

center location of all the observation in each box are given by the solid squares. The mean 

LC front location for the interval is given by the thick purple line. The small orange squares 

show the locations of moorings from the LC dynamics experiment with deep current meters 

100 mab.

159
km

2 cm s-1 cm2 

s-2 cm2 

s-2

LC Area & Area + Eddy

Area Averaged Velocities

1/2 (<u>2

 + <v>2)

1/2 (<u’ 2

>+ <v’ 2>)

Kraken Lazarus

NW of LC

North of LC

Under LC

Figure 5-16. Time series of LC area, area-mean velocities, mean KE and EKE

Series are color coded by rectangular regions (see Figure 5-15).

160
161 

The top panel shows the EKE calculated from the variances of velocities that occur in each 

rectangle for each 16-day interval. The high EKE during the eddy detachments is noted, and is 

similar to that found for LC eddy detachments between 2009 and 2011 from the LC mooring 

array, situated mostly within the Under LC (southeast) rectangle (see Hamilton et al. (2016); 

their Figure 2). When Kraken separated and began to move rapidly westward, the northeast 

rectangle EKE increased (July 2013). After August 2013, the EKE showed an exponential decay, 

particularly in the northeast rectangle when the LC is far to the south. This decrease in northeast 

rectangle variances lasted through February 2014, when a growing Lazarus begins to influence 

the region. It could be speculated that this decay, when the LC is not present, is caused by 

frictional dissipation, but it could also be due to westward radiation out of the region by TRWs. 

The latter explanation is supported by the large peaks of EKE in June and July 2013 in the northcentral region that lag the post-Kraken peaks in the two eastern areas. Moreover, the northcentral area July peak precedes the strong westward mean-flow event in August and September 

2013 that implies transfer of TRW energy into the mean flow as discussed in DeHaan and 

Sturges (2005). It was noted in the the discusion of Figure 5-14 that when deep eddies were not 

present north of the LC, the floats tended to oscillate in a more rectilinear fashion, largely in 

place consistent with wave trains that transport momentum but not mass. Because baroclinic 

instability is the major source of deep EKE, the implication is that this deep EKE is dissipated 

through radiating TRWs that remain after the LC eddy has departed. (Hamilton 2009).

The deep mean flows and standard deviation ellipses for the 18-month interval, when the highest 

number of floats were available in the eastern Gulf are given in Figure 5-17a. Float statistics are 

calculated for 3⁄4°x 3⁄4° squares at 1⁄2° latitude and longitude intervals. Thus, the averaging squares 

overlap by 1/8° on each side. Only squares with number of observations, N > 30, are shown, 

with <N> = 1515 ± 81, and an average of 54 floats occuppying each square. In Figure 5-17a, the 

location of the mean vector is the average of all the positions in the square rather than the center 

of the square. For comparison, two-year mean statistics for current meters 100 mab from the LC 

mapping array are given in Figure 5-17b. The mean deep circulation from moorings is discussed 

in more detail in Hamilton et al. (2014; 2016). 

Both plots show a west-to-east anticyclone-cyclone mean-flow pair beneath the mean LC. This 

is a result of lower-layer PV conservation under a LC extending towards the NW that will 

squeeze the lower water column in that direction and increase it on the opposite side, 

consequently decreasing and increasing relative vorticity, respectively. The float means also 

show an eastward boundary current around the steep escarpment at the north end of the 

Campeche slope that merges with, and possibly enhances, the cyclone along the west Florida 

slope escarpment. The Lagrangian results have lower magnitudes than the Eulerian, however, 

this is to be expected because of area and time averaging against time averaging at a point. A 

mean anticyclonic-cyclonic paired circulation under the LC seems to be a robust feature because 

it occurs in both Eulerian and Lagrangian velocity data for two different periods with differing 

LC eddy-shedding events. 
30°N

28°N26°N24°N22°N

90°W 88°W 86°W 84°W

200 m

200 m

1000 m

2000 m

3000 m

3500 m

10 December 2012 - 3 July 2014

3 cm/s

30 cm/s

Deep Mean Currents from RAFOS Floats

7 June 2009 - 2 May 2011

Near-Bottom Moored Current 

W 

Campêche

Bank

a) b)

Figure 5-17. Mean velocities and standard deviation ellipses for floats and current meters(a) - LH panel: floats , and (b) - RH panel: moored current meter subtidal records 100 mab, for the indicated inorange (anticyclonic) and blue (cyclonic) arrows give a sketch of the sense of the deep mean circulations.

162 

200 m

200 m

1000 m

2000 m

3500 m

3000 m

28 August 2013 - 25 January 2014

10 cm/s

∇H

Figure 5-18. Trajectories of 2500 m floats south of the Mississippi delta

For the indicated interval, arrow heads are every 20 days. Initial positions on 28 August 

2013 marked by solid squares. The standard deviation velocity ellipse, centered on the 

average locations of floats 1151, 1250 and 1254 is given for the indicated period. The 

direction of the mean slope is given by the ∇H vector.

164
27°N

26°N25°N

91°W 90°W 89°W 88°W

28 Aug to 7 Sep 2013

2000 m

3000 m

7 Sep to 17 Sep 2013 

27 Sep to 7 Oct 2013 7 Oct to 17 Oct 2013 

27 Oct to 6 Nov 2013 6 Nov to 16 Nov 2013 

Figure 5-19. Consecutive 10-day sequences of trajectories of 2500-m floatsFor floats 1151, 1250 and 1254, the end points are indicated by black dots.

165 
166 

The velocity records from the the three floats for this TRW interval show both longer and shorter 

period fluctuations (Figure 5-20). The eye tends to integrate Lagrangian trajectories and thus 

there is a perceived bias towards the longer periods (Figure 5-19). The longer ~two-month 

periods are evident in the component plots, but are not resolved by the EKE spectra (Figure 

5-20b), where the spectral peak is ~25 days, similar to spectral peaks found from current meters 

in this region (Hamilton 2007a; Hamilton 2009). There is a burst of short-period fluctuations ~3-

5 days towards the end of December for float 1151 that occur when the float is adjacent to the 

steep slope of the Sigsbee escarpment, in the northwest part of the trajectories. Short period 

TRW’s have been observed in this region previously (Hamilton 2007a) and may have differing 

origins to the longer-period waves. The presence of a fairly broad band of energetic frequencies 

favors an interpretation of wave generation by deep eddy decay rather than a resonance with 

upper-layer eddies or meanders (e.g., Pickart (1995)). The trajectories of two other 2500 m 

floats (1152 and 1153) that were a little further west during this same interval, show them being 

much more influenced by westward mean flows that result in much stronger dispersion (Figure 

5-18). It is not clear if these trajectories are being influenced by the TRWs observed in the 

trajectories further east.

The earlier interval, from 24 January to 30 March 2013, is another example of floats (four at 

1500 m and two at 2500 m) clustering around the 3000 m isobath south of the delta (Figure 

5-21). The discussion above refers to an interval after Kraken had separated and departed, and 

the overall EKE in the region north of the LC was decreasing. This earlier period is just prior to 

the first detachment of Kraken, and thus may be influenced by circulations associated with the 

northwestern front of the LC. The results are only suggestive of TRWs as the clustering is not as 

strong as later, and the interval is much shorter. The velocities from these six floats are given in 

Figure 5-22, and it can be seen that the velocity fluctuations are much less coherent than in 

Figure 5-20, though again the records show a range of periodicities. 

5.2.5 Deep Baroclinic Instability Eddies 

The development of deep eddies through baroclinic instability of the east-side jet of the LC was 

discussed above, and evidence from the float trajectories corresponds to the scenarios analyzed 

by Donohue et al. (2016a) using PIES and current meters. A good example is the intensification 

of the cyclone below a meander trough in the southeast, between February and April 2013 

(Figure 5-14a) that leads to the first detachment of Kraken. The LC array did not have the 

resolution (~50 km) to capture details of deep eddy structures. However, about five floats 

tracked simultaneously the deep cyclone in question while Kraken was detaching, and allow 

investigation of its circulation. A sequence of 10-day trajectories are shown at large scale in 

Figure 5-23 where from 3 March 2013, the floats circle at ~120 km diameter, then spiral in 

towards the center (floats 1108, 1110 & 1123: 13 March–2 April; Figure 5-23) with a diameter 

about half that of 3–13 March, and then spiral out again as Kraken detaches, dispersing to the 

west after ~20 April 2013. The center of the cyclone is clearly displaced southeastward from the 

mid point of the LC trough, as determined by the SSH contours.
1151

12501254

cm/s cm/s cm/s North component

East component a) b)

Figure 5-20. TRW velocity time seriesa) Velocity vector and component time series from the trajectories of three 2500-m floats, located south of the Mfor the indicated period. For the stick vectors up is north. b) EKE spectra of the time series in (a) in energy pre

167 
200 m

200 m

1000 m

2000 m

3500 m

3000 m

24 January 2013 - 30 March 2013

Figure 5-21. Trajectories of floats south of the Mississippi delta

Blue and red-orange shades for 1500 and 2500 m floats, respectively. For the indicated 

interval, arrow heads are every 10 days. Initial positions on 24 January 2013 marked by solid 

squares. The location of the 17-cm SSH contour is shown for 24 January (dashed) and 30 

March 2013 (solid thick line).

168
RF1161

RF1153

RF1147

RF1146

RF1138

RF1135

cm s-1

1500 m

2500 m

Figure 5-22. TRW velocity time series for 1500 and 2500-m floats

For the stick vectors up is north.

169
2000 m

2000 m

3000 m

1000 m

1000 m

1500 m

200 m

1145

1149

1123

1108

1110

21 Feb - 3 Mar 2013

25°N

24°N

86°W 85°W

1145

1149

1123

1108

1110

3 Mar - 13 Mar 2013

1145

1149

1123 1108

1110

13 Mar - 23 Mar 2013

1145

1149

1123

1108

1110

23 Mar - 2 Apr 2013

1145

1149

1123

1108

1110

2 Apr - 12 Apr 2013

1145

1149

1123

1108

1110

12 Apr - 22 Apr 2013

1145

1149

1123

1108

1110

22 Apr - 2 May 2013

1145

1149

1123

2 May - 12 May 2013

SSH Contours

 -10 cm

 +17 cm

 +30 cm

1108

1110

1123 1145

°C

Figure 5-23. Sequence of 10-day trajectories at 10-day intervals

Trajectories for the indicated dates showing the devlopment of a cyclone beneath a LC 

meander trough. Blue and red tracks are at 1500 and 2500 m, respectively. The end point of 

each 10-day trjaectory is marked with a black dot. Overlaid are CCAR SSH contours for the 

center day of each interval. Bottom panel shows the time series of 1500-m float temperatures 

for the interval of the sequence plots.

170

5 cm/s

3 March - 20 April 2013

2000 m 2000 m

3000 m 1000 m

1000 m

1500 m

200 m

2000 m 

1000 m

1500 m

Fraction of f 10 cm/s 5 cm/s

a) b)

Figure 5-24. Analysis of a deep LC cyclonea) Composite of float tracks in the deep LC cyclone for 3 March to 20 April, 2013. Standard deviation ellvelocities for the interval is overlaid (blue scale arrow), and sub-areas of the ellipse for velocity mean calcindicated. b) Mean velocities for the sub-areas are centered on the average locations of the floats (dark grscale). Contour map of relative vorticity is overlaid with ageostrophic vectors (red arrows and scale) calcvelocity potential.

172 
173 

5.2.6 Anticyclone Under the LC Tip 

The first four panels of Figure 5-14a show a prominent anticyclone under the leading tip of a 

slowly extending LC that is probably a consequence of lower layer PV conservation rather than 

baroclinic instability. The speculation above was that the decay of this anticyclone during 

February 2013 could be the source of apparent TRW motions further west during March 2013 

(Figure 5-21). A more detailed view of float tracks in this anticyclone is given in Figure 5-25. 

Because the anticyclone made a move to the east with a decrease in the diameter of the orbits 

between 2 December 2012 and 10 February 2013, the float trajectories are divided into two 35-

day segments. In the first segment, six floats make clockwise circuits of the elliptical eddy, 

which has a diameter of ~120–150 km (Figure 5-25a), and rotational periodicities of ~25 to 30 

days. After early January, three of the floats spiral towards the center and move eastward 

(Figure 5-25b). There is a change in velocity scale between Figure 5-25a and b, however, the 

EKE (=1/2(a2

 + b2) where a and b are the ellipse semi-major and minor axes, respectively) are 

almost the same (70.4 and 83 cm2 s

-2). The periodicities in Figure 5-25b are ~15–20 days. 

Therefore there is an increase in eddy vorticity, ζ, between the first and second intervals of about 

a factor two. ζ is ~-0.15f in Figure 5-25b. This could be explained as being a result of 

compression of the lower water column as the smaller later anticyclone is more within the LC 

front than the larger earlier eddy. The anticyclone rotational periods are longer and the relative 

vorticity is smaller than for the similar sized baroclinic instability cyclone discussed previously.

The northward advance of the LC front between the beginning and end of the trajectories (see 

Figure 5-25), accounts for the compression of the lower layer, resulting in the generation of 

anticyclonic vorticity. There was one 1500 m float (1108) in the constellation in Figure 5-25, 

and its temperature again reflects the location of the trajectory relative to the cold and warm 

sides of the LC front. Note that the front slopes down to the south from its surface expression 

and so the first 50 km of the northward path before 4 January 2013 are relatively cold, until it 

enters the anticyclone and the advancing front catches up. The 1500 m temperature reflects the 

overlying upper-layer circulation and the lower thermocline, rather than the water mass 

characteristics of the deep eddy, per se. 

In Figure 5-26, mean velocity vectors are calculated for the subsectors of the ellipses as shown in 

Figure 5-25. The location of the vectors is the center of all the locations in the sub-area. In the 

first interval, velocities are stronger on the north side, which may explain the movement of the 

eddy to the east. In the second interval, the velocities are generally larger in the more compact 

anticyclone, and similar to the cyclone discussed above, gradients of azimuthal velocities are 

weak and far from linear, as would be expected for solid body rotation. Maximum speeds are 

17.1 and 19.2 cm s-1 for the first and second interval, respectively. Moored current meter 

measurements under the LC (Hamilton et al. 2016) indicate depth-independent flows below 1000 

m, and therefore, with water depths of ~3000 m, these eddy circulations contain considerable 

kinetic energy. Similar “tip” anticyclones are observed in September and December 2013 

(Figure 5-14c), and February-March 2014 (Figure 5-14d).
2 December 2012 - 5 January 2013

2000 m

3000 m

1000 m

1500 m

500 m

3000 m

1108

1150

1161

1151

1153

1154

5 cm/s

1108

6 Jan

2000 m

3000 m

1000 m

1500 m

500 m

3000 m

1150 1161

a) b)

°C

1108 (1500 m)

M

c)

Figure 5-25. Float trajectories in an anticyclone Composite tracks for an anticyclone a) the first 35 days, b) the second 35 days. The standard deviation ellipses solid lines (note scale change between a) and b)) and the annular sectors used to average velocities. Red/orangare 2500 and 1500 m floats, respectively. The standard deviation ellipse from a) is repeated in b) (dashed blue of scale to show the relative motion of the center over 35 days and the contraction in area. LC 17-cm SSH con2012 and 10 February 2013 are given as thick purple lines on the left and right plots, respectively. c) Time serietemperature. Mean of 628-day record is indicated.

174 
10 cm/s

2000 m

3000 m

1000 m

1500 m

500 m

3000 m

2 December 2012 - 5 January 2013

6 January - 10 February 2013

Figure 5-26. Mean velocity vectors in the anticyclone for two intervals 

LC 17-cm contour (gray lines) shown for the midpoint of each interval (19 December 2012: 

solid, and 24 January 2013: dashed).

175
176 

5.3 DEEP LAGRANGIAN FLOWS IN THE WESTERN GULF

5.3.1 Float Data Summary 

The western Gulf of Mexico differs from the eastern Gulf in several fundamental respects, i.e., 

the western basin is significantly larger, there is a much greater area of deep (> 3000 m) water in 

the center of the basin, and the vigorous upper-layer flow induced by the Loop Current (LC) in 

the east is largely absent, except (and importantly) when spun-off LC eddies translate westward 

into the region and dissipate. The large amount of new Lagrangian data collected in the western 

Gulf provides the first opportunity to systematically investigate the impact, if any, of LC eddies 

on fluid pathways below the thermocline. 

Because the 2500-m RAFOS floats were all released in the eastern Gulf, the Lagrangian 

observations in the western Gulf are almost exclusively (96%) from 1500-dbar floats and 

RAFOS-equipped APEX profiling floats, which were parked at 1500 dbar. Figure 5-27 shows 

the number of floats in the water west of 90°W by month for the four-year observational period. 

The average monthly population was 28 floats, and the peak was 62 from 15 April to 15 June 

2014. Through August 2012, only 2–6 floats populated the western Gulf. During the most 

intensive time period, with 20 or more floats in the western Gulf (September 2012–February 

2015), the monthly mean population in the western Gulf was 39 floats. 

5.3.2 Interaction of LC Eddies and Deep Floats 

One of the primary objectives of the Lagrangian field program was to investigate coupling 

between the upper and lower layers in the Gulf. Because the strongest upper-layer features in the 

western Gulf are LC eddies, the first task is to examine the impact of these eddies on float 

behavior. 

Seven LC eddies fully or partially entered the western Gulf (defined here as the region west of 

90°W) during the float field program. These eddies are named Hadal, Icarus, Jumbo, Kraken, 

Lazarus, Michael and Nautilus. By way of summary of float distribution in relation to the LC 

eddy events, Figure 5-28a-c shows 60-day float-track segments (blue for 1500-m floats, red for 

2500-m floats) superimposed on 10-day snapshots of the 17-cm SSH contour (when it exists; as 

elsewhere in this report, the 17-cm SSH contour is used to define the location and approximate 

size of LC eddies). The life histories of each LC eddy are described in detail in Section 3.4. 

Here, a brief summary is provided in preparation for the more detailed analysis of eddy-float 

interactions in the next section.
Oct11 May12 Nov12 Jun13 Jan14 Jul14 Feb15

0

10

20

30

40

50

60

70

Number of Floats West of 90°W

Figure 5-27. Histogram of floats present in the western Gulf

Numbers of floats west of 90°W, by month, during the four-year observational period.

177
01JUL11-30AUG11 30AUG11-29OCT11 29OCT11-28DEC11

28DEC11-26FEB12 26FEB12-26APR12

25JUN12-24AUG12 24AUG12-23OCT12 23OCT12-22DEC12

26APR12-25JUN12

30N

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18N

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18N98W 96W 94W 92W 90W 98W 96W 94W 92W 90W 98W 96W 94W 92W 90W

Hadal Hadal Hadal-a Hadal-b

Hadal-a

Hadal-b

Icarus

Icarus

Icarus

Icarus

Jumbo

Jumbo-a

Jumbo-b

Jumbo-a

Jumbo-b Icarus

Figure 5-28a. Two-month composites of float trajectories and LC eddy positions

During the four-year observational period, blue (red) line segments indicate 1500-m 

(2500-m) float trajectory segments, with a black dot at the leading end of the segments. 

The 17-cm SSH contour is plotted every 10 days. Isobaths (green) are drawn for 200, 

1000, and 3000 m.

178
22DEC12-20FEB13 20FEB13-21APR13 21APR13-20JUN13

20JUN13-19AUG13 19AUG13-18OCT13

17DEC13-15FEB14 15FEB14-16APR14 16APR14-15JUN14

18OCT13-17DEC13

30N

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26N

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20N

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30N

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26N

24N

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18N

98W 96W 94W 92W 90W 98W 96W 94W 92W 90W 98W 96W 94W 92W 90W

Jumbo-a

Icarus

Jumbo-a

Kraken

Kraken Kraken

Kraken

Kraken Kraken

Figure 5-28b. Two-month composites of float trajectories and LC eddy positions

During the four-year observational period, blue (red) line segments indicate 1500-m 

(2500-m) float trajectory segments, with a black dot at the leading end of the segments. 

The 17-cm SSH contour is plotted every 10 days. Isobaths (green) are drawn for 200, 

1000, and 3000 m.

179
15JUN14-14AUG14 14AUG14-13OCT14 13OCT14-12DEC14

12DEC14-10FEB15 10FEB15-11APR15 11APR15-10JUN15

30N

28N

26N

24N

22N

20N

18N

30N

28N

26N

24N

22N

20N

18N

98W 96W 94W 92W 90W 98W 96W 94W 92W 90W 98W 96W 94W 92W 90W

Lazarus Lazarus Lazarus

Michael Michael Nautilus

Figure 5-28c. Two-month composites of float trajectories and LC eddy positions

During the four-year observational period, blue (red) line segments indicate 1500-m 

(2500-m) float trajectory segments, with a black dot at the leading end of the segments. 

The 17-cm SSH contour is plotted every 10 days. Isobaths (green) are drawn for 200, 

1000, and 3000 m.

180
181 

At the beginning of the image series, LC eddy Hadal is seen entering the domain centered at 

26°N, and crossed 90°W in August 2011. It moved west-southwestward and broke into two 

eddies near the end of 2011. One eddy drifted northwestward and dissipated over the slope of the 

northwestern Gulf, while the other half moved into the southwestern Gulf. Both eddies 

“disappeared” (meaning dropped below 17 cm in SSH) in January-February 2012. Hadal crossed 

the western Gulf at a time with low float population: only two floats interacted with Hadal, 

where “interaction” is defined as more than five days under SSHs greater than 17 cm. A total of 

32 floats met this criterion during the float observational period. 

LC eddy Icarus appeared in the domain in January 2012, centered at about 25°N, and crossed 

90°W in February 2012. It propagated generally west southwestward into the southwestern Gulf. 

In the fall of 2012, its surface expression weakened temporarily such that SSH dropped below 17 

cm for two months. It finally disappeared completely in February 2013. Three floats overlapped 

with Icarus for five or more days, one in the northeastern and three in the southwestern parts of 

the western Gulf. 

LC eddy Jumbo first appeared entering the domain about seven months after Icarus, in July 

2012, centered near 26°N. By August 2012, it was elongated along topographic contours of the 

northern slope, and did not officially cross 90°W until the beginning of November 2012. The 

eddy temporarily broke into two features: a small eddy lying over the northern continental slope 

and the other portion at the eastern edge of the domain, centered south of 25°N. By the beginning 

of 2013, only one feature was apparent, drifting west-southwestward over the northern slope, 

where it disappeared around April 2013. During Jumbo’s lifetime, the population of floats was 

growing and there were seven that crossed under its core. 

At the same time that Jumbo was moving westward over the slope, another high SSH anomaly 

was entering the western Gulf, centered just south of 26°N. This feature, which actually 

separated from the Loop Current west of 90°W around the beginning of August 2013, became 

the large LC eddy Kraken. It propagated west southwestward, following topography, until 

December 2013, when it crossed the 3000-m isobath of the western continental slope. It then 

followed the slope slightly northward to the northwestern corner of the Gulf, where it remained 

until it disappeared in April 2014. Kraken was a very large LC eddy that coincided with 

maximum float population and 20 floats crossed under it at some point in their missions, some as 

members of float clusters. 

From June 2014 to the end of the float observational period, three LC eddies (Lazarus, Michael 

and Nautilus) separated from very extended configurations of the LC (see detailed description in 

Section 3.4), and only a few floats coincide with them so they are not detailed here. 

The float trajectories provide significant evidence of interaction between LC eddies and the deep 

flow field. In the following, three examples are shown of eddy deep-flow interactions that 

highlight the two most common phenomena in the entire data set: (1) “slow” cross-slope motion 

in the deep water associated with the passage of an LC eddy overhead, and (2) “fast” oscillatory 

cross-slope motion associated with TRWs under LC eddies. 
182 

Icarus-NE 

Figure 5-29 shows 10-day “movie frames” of CCAR SSH along with 10-day float track 

segments for a region in the eastern part of the western Gulf in January-March 2012. This 

sequence shows the slow westward passage of LC eddy Icarus along the northern slope. Floats 

1081 (red; 2500-m float) and 1074 (blue; 1500-m float) were released close together over the 

~3250-m isobath west of Icarus on 17 January 2012 and drifted rapidly eastward toward the 

eddy. In the second frame, Icarus has drifted slightly westward, and both floats encountered the 

eddy core, crossing under the 17-cm SSH contour. At about the same time, both floats turned 

sharply offshore into deeper water. Figure 5-30a and b show time series of SSH, float 

temperature and water depth along the tracks of both floats. Vertical dashed lines indicate the 

dates of the SSH images in each movie frame. At about the same time that the floats turned 

offshore (see water depth change in lower panels), in late January 2012, temperature measured 

by both began to increase (middle panels). Float temperature continued to increase as the floats 

moved into water that was 200-300 m deeper than the starting isobath. In the third frame, 1074 

turns right back upslope with a simultaneous decrease in temperature, while 1081 turns back 

westward traveling more along than across the isobaths with relatively constant temperature. 

Float 1074 then executed a couple of higher-frequency cross-slope oscillations that were highly 

anti-correlated with its observed temperature, all while sitting under the core of Icarus. Float 

1081 remained in deeper water measuring relatively warm temperatures until frame five, when it 

apparently escaped from the eddy core (moving westward). Float 1081 temperature decreased 

while the float moved back upslope, ending up near the same isobath where it was released.

The time series for both floats show a strong anti-correlation between float temperature and 

water depth under the float. Two periodicities are clearly evident, and they are discussed 

separately. The lowest-frequency signal in the time series, best illustrated by float 1081, but 

present in both floats, shows a single “wavelength” of anti-correlated variations. The correlation 

coefficient for 1081, for the time period when it is within the 17-cm SSH contour, is -0.85 (as 

temperature increases, water depth becomes deeper—more negative: see two lower panels in 

Figure 5-30a).

This relationship is strongly reminiscent of observations from deep RAFOS floats in the Deep 

Western Boundary Current (DWBC) where it crosses under the Gulf Stream off the US east 

coast (Bower and Hunt 2000a; Bower and Hunt 2000b) and is interpreted as a lower-layer 

response to the deep thermocline under LC eddies. In short, the lower layer “sees” a deepening 

thermocline associated with the LC eddy and is diverted into deeper water in order to conserve 

planetary potential vorticity, f/H, where H is the thickness of the lower layer, bounded on top by 

the thermocline and on the bottom by the sea floor. Hogg and Stommel (1985) first 

hypothesized this behavior for the DWBC as it crosses under the Gulf Stream. Bower and Hunt 

(2000b) were able to confirm their hypothesis with floats drifting in the DWBC because 

isopycnals (isotherms) slope steeply down under the Gulf Stream, even near the sea floor, so the 

deep isobaric floats drifting southward in the DWBC measured increasing temperature when 

they passed under the Gulf Stream, and their trajectories simultaneously turned toward deeper 

water in an attempt to conserve layer thickness in light of the deepening thermocline. 
Figure 5-29. Ten-day snapshots of float tracks interacting with Icarus 

The single representation of the eddy position is plotted for a date in the center of the 

10-day time interval for each panel. Isobaths are plotted every 250 m, beginning with 

1000 m.

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183
−0.5

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)

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−3600

−3500

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bottom depth (m)

(a)

(b)

Figure 5-30. Time series along the trajectories of floats (a) 1081 and (b) 1074

SSH (top panel), float temperature (middle panel; low-pass filtered with 40-hour cutoff) and 

water depth (lower panel) for the time span matches that of the images in Figure 5-29. 

Vertical dashed lines correspond to the date of the SSH snapshot in each image.

184
185 

There is a similar relationship between thermocline depth and deep temperature in the Gulf, 

which allows one to use deep float temperature as a proxy for lower thermocline depth. Figure 

4-7 shows the close correspondence between the depth of the 6 °C isotherm and temperature at 

1490 m from the profiling APEX floats used in this field program. As in the case of the Gulf 

Stream-DWBC crossover, deep temperature in the Gulf is strongly related to thermocline depth, 

so float temperature can be used as a proxy for thermocline depth, and the top of the lower layer, 

here as well. 

Note that these observations of low-frequency cross-slope motion associated with changing 

depth of the top of the lower layer do not necessarily confirm that f/H is conserved along the 

float tracks; only that changes in relative vorticity are not large enough to compensate for the 

squeezing or stretching of the lower layer under an eddy, and the lower layer must move into 

deeper or shallower water, respectively, to help conserve total potential vorticity. 

Returning to the Icarus example above, the offshore then onshore motion of floats 1081 and 1074 

over about two months is interpreted as a response of the lower layer to the intersection of the 

lower-layer flow with Icarus. The change in water depth along the floats, 200–300 m, is 

characteristic of the thermocline deflection associated with LC eddies, which might point to 

near-conservation of f/H and negligible changes in relative vorticity on these time scales. 

The second periodicity evident in the time series is shorter, and is interpreted as the signature of 

TRWs (see also discussion of eastern Gulf trajectories in Section 5.2.4). Float 1074 exhibits this 

behavior most clearly. In frames 2 and 3 of Figure 5-29, float 1074 turned cyclonically after it 

reached the 3500-m isobath under Icarus, and then headed back upslope in frame 3. In frame 4, it 

executed an anticyclonic loop and headed back into deeper water. These cross-slope oscillations, 

with a period of about 22 days, are clearly evident in float temperature as well as water depth 

along the float track (Figure 5-30b: middle panel). Cross-isobath oscillatory motion with this 

periodicity and a “figure 8” trajectory pattern is consistent with TRWs. The coincident float 

temperature oscillations again reflect vertical displacements of isopycnals or isotherms along the 

float track as for the lower-frequency signal, but at these frequencies, there appears to be an 

interplay between vortex stretching and generation of relative vorticity, the latter evident in the 

turning of the trajectories at the maximum and minimum water depth. Interestingly, float 1081 

(at 2500 m) did not exhibit the same higher-frequency cross-slope oscillations: it reversed 

direction in frame 3, crossed back into shallower water in frames 4 and 5 as it shot back out in 

front (west) of the eddy, and then followed the Sigsbee escarpment westward.

Icarus-SW 

LC eddy Icarus drifted into the far southwestern Gulf before dissipating. Three floats, 1098, 

1099 and 8512 (an APEX float) interacted with this eddy in the time frame October 2012 to 

January 2013 in the southwestern Gulf. Floats 1098 and 1099 are highlighted here, but 8512 

shows similar behavior. In frame 1 of Figure 5-31, these floats were drifting eastward in the 

boundary current between the 3200- and 3400-m isobaths. During November and December 

2012, Icarus drifted southward, deeper into the Bay of Campeche, and float temperatures 

increased, signaling the approach of the eddy with its depressed isotherms. Float 1098 and 1099 

followed each other very closely (they were deployed as a pair) as they both smoothly crossed 

the isobaths into deeper water, from about 3250 m to 3700 m and crossed under the core of the 
186 

eddy as defined by the 17-cm SSH contour and the increasing float temperature (Figure 5-32a, 

b). In the second half of December 2012, the two floats turned abruptly eastward and began 

drifting into shallower water as temperature decreased and the eddy moved away to the west. 

Small-amplitude, higher-frequency oscillations in water depth, and to a lesser extent in float 

temperature (especially for 1099: Figure 5-32b), as the floats drift eastward are likely signatures 

of weak TRWs, but the larger scale flow is directed upslope toward the Campeche escarpment. 

Overall, the pattern is consistent with a deep cyclonic boundary current in the lower layer that is 

temporarily deflected into deeper water to conserve f/H when the anticyclonic eddy impinges on 

the slope.

Kraken

The final example discussed here involves float 1121, which drifted around under LC eddy 

Kraken for 84 days; this was the longest float track under and a LC eddy. The time series of 

images begins well before the float intersected with Kraken to contrast float behavior before and 

during eddy interaction (Figure 5-33). Starting in June 2013 and continuing through frames 1 and 

2 (20 days), float 1121 was drifting slowly north-northwestward up the isobaths from the very 

deepest part of the basin. Float temperature (Figure 5-34: middle panel) decreased slightly 

during this time, suggestive of shoaling isotherms along the float track. After crossing the 3500-

m isobaths, float 1121 turned eastward and drifted parallel to the isobaths. In frames 3–6, the 

float drifted relatively swiftly eastward with higher-frequency cross-slope oscillations with a 

period of about 12 days superimposed. The cross-slope motion is not easy to see in the maps, 

but is clearly evident in the time series of water depth along the float track (Figure 5-34: lower 

panel), and in the now-familiar anticorrelated oscillations in float temperature. Starting in frame 

4, the oscillations were superimposed on a slow trend of increasing float temperature and SSH. 

The small cross-slope excursions and associated float temperature oscillations have similar 

features in SSH indicating that the float is oscillating under (and somewhat perpendicular to) the 

SSH gradient surrounding the eddy. In frame 6, the float slipped under Kraken’s core, and 

reversed direction at about the 3300-m isobath. In frame 7, Kraken expanded westward 

significantly, and in frames 7–10, float temperature increased and the float drifted into deeper 

water, probably reflecting the approach of the center (or deepest thermocline) of Kraken. This 

doesn’t correspond to the geometric center based on SSH, but it does match the warmest float 

temperatures, which were also observed during the time period of frame 10. For the rest of the 

time series, float temperature decreased, and the float drifted back upslope, with higherfrequency oscillations still superimposed. Still under the core of Kraken, it shoots rapidly west 

northwestward (same direction as eddy drift) until it reached the Sigsbee escarpment (frame 12), 

where it executed an anticyclonic loop (frames 13 and 14). The float then shot westward along 

the isobaths and out from under the core of Kraken as the eddy moved southwestward away from 

the escarpment. Note that there were a number of additional floats under Kraken at the same time 

as float 1121; more analysis is needed to fully describe the response of the lower layer to this LC 

eddy.
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6

Figure 5-31. Fifteen-day snapshots of float track segments and SSH

The impact of LCE Icarus is shown for the far southwestern Gulf on the deep boundary 

current. Floats 1098 and 1099 are highlighted, and time series of properties along their 

tracks are shown in Figure 5-32.

187
−0.5

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07Dec12 27Dec12 16Jan13 05Feb13

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(a)

(b)

28Oct12 17Nov12

188

Figure 5-32. Time series along the trajectories of floats (a) 1098 and (b) 1099 

SSH (top panel), float temperature (middle panel; low-pass filtered with 40-hour cutoff) 

and water depth (lower panel) for the time span matches that of the images in Figure 5-31. 

Vertical dashed lines correspond to the date of the SSH snapshot in each image.
Figure 5-33. Ten-day snapshots of float track segments and 17-cm SSH contour for Kraken 

Float 1121 drifted underneath Kraken for 84 days and time series of properties along its 

track are shown in Figure 5-34.

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189
Figure 5-33 Continued

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 94oW 93oW 92oW 91oW 90oW 

 24oN 

 30’ 

 25oN 

 30’ 

 26oN 

 30’ 

 1121

16

190
−0.5

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4.5

) 26May13 15Jun13 05Jul13 25Jul13 14Aug13 03Sep13 23Sep13 13Oct13 02Nov13

−4000

−3500

−3000

−2500

bottom depth (m)

Figure 5-34. Time series along the trajectories of float 1121

SSH (top panel), float temperature (middle panel; low-pass filtered with 40-hour 

cutoff) and water depth (lower panel) for the time span matches that of the images in 

Figure 5-33. Vertical dashed lines correspond to the date of the SSH snapshot in each 

image.

191
192 

This Kraken example has strong similarities to the first example, with float 1081 in Icarus. In 

both cases, there was initially strong eastward flow parallel to the isobaths toward the LC eddy, 

then offshore displacement under the eddy, followed by upslope flow and finally strong 

westward flow back along the isobaths. Also, in both cases, higher-frequency TRW-like 

oscillations were superimposed on the longer time scale signal, but particularly evident in the 

Kraken example, the high-frequency oscillations were observed as the float approached the eddy 

core from the west, perhaps suggesting TRW radiation out in front of the westward-propagating 

LC eddy. 

In summary, the deep float trajectories in the western Gulf at first glance, do not appear to 

exhibit any coherent motion related to the presence of LC eddies in the upper layer, but on closer 

inspection, the lower- and upper-layer circulation appear to be coupled in two ways. First, the 

depressed isopycnals (isotherms) associated with LC eddies alter the deep planetary potential 

vorticity, f/H, where H is the thickness of the lower layer. Currents in the lower layer must 

therefore cross isobaths (by several hundred meters) into deeper water as they approach a LC 

eddy and then flow back upslope as they move away from the eddy (or the eddy moves away 

from them). The time scale of this cross-isobath displacement depends on several factors 

(propagation speed of the eddy, speed of the current in the lower layer) but is on the order of 1-2 

months. Superimposed on this motion is a higher-frequency cross-isobath motion that has the 

characteristics of TRWs. These waves appear under the LC eddies and may be initiated by the 

interaction of the LC eddies with the sloping bottom. 

5.3.3 Persistence of Deep Boundary Current in the Western Gulf 

Elsewhere in this report (see Section 3.5 on Eulerian Statistics), strong evidence for the existence 

of a deep cyclonic boundary current has been presented, in the gridded mean velocity and in 

individual “long-distance” trajectories following the isobaths around the western basin. But 

there are also copious examples of oscillatory motion parallel or nearly parallel to the isobaths at 

times (see Section 5.3.2 above). This raises the question of how persistent the boundary current 

is, and whether there are times when it is interrupted by other deep circulation features. 

Evidence of intermittency of this boundary current is provided in the following sections. 

Figure 5-35a shows time series of along-isobath velocity in three regions of the western Gulf: 

the northern slope (top panel), western slope (middle panel) and western slope of Campeche 

escarpment (lower panel). A map showing the locations of the float velocity data used, which 

include only float velocities inshore of the 3000-m isobaths, is shown in Figure 5-35b. For the 

purpose of documenting the large-scale boundary current, the float velocity observations were 

first rotated into along- and across-isobath components, where the bathymetric gradient was 

estimated at each float position by fitting a plane to the bathymetric data in a ~50 km x 50 km 

box centered at the float position. The along-isobath velocity component was then smoothed 

with a third order low-pass Butterworth filter with a 30-day cutoff period. Finally, for each 30-

day time period, the numbers of observations with counter-clockwise (cyclonic) sense of flow 

were counted. Figure 5-35a shows the fraction of observations in each time step that indicated 

cyclonic boundary current circulation. The 0.5 level is highlighted with a horizontal black line: 

this is where there are an equal number of observations with clockwise and counter-clockwise 

observations. Circles indicate the points where there are more than 100 observations. 
0

0.2

0.4

0.6

0.8

1

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24Oct11 11May12 27Nov12 15Jun13 01Jan14 20Jul14 05Feb15 24Aug15 0

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(a)

(b) 28N

26N

24N

22N

20N

18N

98N 96N 94N 92N 90N

Figure 5-35. Cyclonic velocity observations along the boundary

(a) Time series of the fraction of smoothed float velocity observations along the boundary 

in the cyclonic sense in three regions of the western Gulf: (top panel) northern slope, 

(middle panel) western slope, and (lower panel) Campeche Bank slope. (b) Map showing 

locations of float velocity observations used. 

193
194 

In general, the time series are consistent with the Eulerian mean statistics in that, most of the 

time, for all three regions, there were more observations in the cyclonic boundary-current 

direction than in the anticyclonic direction. That said, there were multi-month time periods when 

there was a high fraction of boundary current-like behavior, and other extended time periods 

were characterized by more back-and-forth motion. For example, in the northern region (Figure 

5-35: top panel), most of the observations (although small in number) were in the cyclonic sense, 

but in the October–December 2012 time frame, back-and-forth motion (with periods longer than 

30 days) predominated. About a one-year time period of predominantly westward (cyclonic) 

boundary current flow was observed starting in May-June 2014 and extending to the end of the 

data record (with a one-month exception). This is more or less true at the end of the record for 

the western and Campeche escarpment boundaries as well with the period of persistent cyclonic 

flow starting later along the western boundary (with the exception of a big flow reversal on the 

Campeche escarpment boundary based on very few observations in late 2014). It is noted that 

LC eddy Kraken completed its sweep across the northern part of the western Gulf early in 2014, 

and no other LC eddies penetrated far into the western basin after that time. This is suggestive of 

a role for LC eddies in the disruption of an otherwise cyclonic boundary current around the 

western Gulf, but more analysis is needed to confirm this relationship. Further evidence to 

support this hypothesis is that the boundary current along the Campeche escarpment appears to 

be the most persistent, with a mean of 75% of observations in the cyclonic sense. The northern 

and western boundaries have mean percentages of 68% and 70%, possibly reflecting the greater 

impact of LC eddies there. 

5.4 GENERAL CIRCULATION IN A DATA-ASSIMILATIVE MODEL

Satellite altimetry is one of the essential components of operational near-real-time ocean remote 

sensing and monitoring. Satellite altimeters provide all-weather sampling that accurately 

measures SSH variability at the ocean surface. In regions like the Gulf, where the general 

circulation is dominated by the first baroclinic vertical mode, this variability largely represents 

the circulation changes in the upper layer of the ocean. Some inferences can be made about the 

deeper circulation, but they are more qualitative than quantitative. Modern data-assimilative 

ocean models, however, are capable of dynamically ingesting satellite altimeter observations to 

produce estimates of the ocean state, not just at the ocean surface, but also throughout the water 

column. Many of these systems were first developed and tested in the Gulf (Hurlburt et al. 1990; 

Kantha et al. 2005). Some of the highest resolution and most sophisticated ocean data 

assimilation nowcast/forecast systems are currently operational in the Gulf. Model-observation 

comparisons using these systems are providing valuable insight into oceanic processes in the 

region (Rosburg et al. 2016). 

In this section, state-of-the-art data-assimilation Gulf HYCOM analysis experiments are 

compared with coincident ocean observations during the Lagrangian study period to better 

understand the degree to which data assimilative models are capable of simulating the deep 

circulation in the Gulf. Skillful data-assimilative ocean models are required for oil spill pathway 

predictions in the Gulf, especially in deep water where observations are difficult to obtain. 

Ocean state estimates during the study program may also provide ancillary information and a 

dynamically consistent context for interpreting the measurements at depth. 
195 

5.4.1 Gulf of Mexico HYbrid Coordinate Ocean Model (Gulf HYCOM) 

HYCOM is a hybrid isopycnal, sigma, and z-coordinate ocean model. In the open stratified 

ocean, the model employs isopycnal coordinates, but reverts to terrain-following (sigma) 

coordinates in shallow coastal regions and to z-level coordinates in the surface mixed layer 

(Bleck 2002; Chassignet et al. 2003; Chassignet et al. 2007). The Gulf model is discretized on a 

Mercator grid with 1/25° longitudinal resolution and 1/25°cos(latitude) latitudinal resolution, 

which gives ~4 km at latitudes in the Gulf. There are 20 coordinate surfaces in the vertical. 

Data assimilation in Gulf HYCOM is performed using the Navy Coupled Ocean Data 

Assimilation (NCODA) system (Cummings 2005; Cummings and Smedstad 2013). NCODA 

uses a three-dimensional variational (3DVAR) data assimilation to assimilate along-track 

satellite altimeter measurements of sea surface height anomaly obtained from the Naval 

Oceanographic Office (NAVOCEANO) Altimeter Data Fusion Center, satellite and in-situ SST 

observations, and in-situ temperature and salinity profiles from expendable bathythermographs 

(XBTs), Argo floats, moored buoys and gliders. This system was developed as an upgrade to the 

multivariate optimum interpolation (MVOI) system employed previously (Cummings 2005). 

There are significant advantages of 3DVAR over MVOI. The 3DVAR algorithm allows all 

assimilated observations to influence all model grid points, can incorporate observed variables 

that are different from the model prognostic variables through the use of observation operators, 

and uses background error covariance formulations, which insure that observations of one model 

variable produce dynamically consistent corrections in the other model variables (Cummings and 

Smedstad 2013). Most of the ocean observations assimilated into NCODA are remotely sensed 

surface observations from which subsurface properties within the water column are inferred. 

Altimeter data provide the bulk of this subsurface information because changes in sea level are 

highly correlated with changes in the ocean thermocline depth associated with ocean mesoscale 

eddies and fronts. SSH data are provided to the data assimilation system as anomalies relative to 

the time-mean state. Synthetic temperature and salinity profiles are derived from the altimeter 

measurements using either the Modular Ocean Data Analysis System (MODAS; Fox et al. 2002) 

or the Improved Subsurface Ocean Profile (ISOP; Helber et al. 2013) system. 

Two Gulf HYCOM NCODA analysis experiments overlap the four-year study program (July 

2011- June 2015): Experiments 31.0 and 31.5. Experiment 31.0 spanned the time period from 1 

April 2009 through 31 July 2014. Experiment 32.5 spans the time period from 1 April 2014 to 

the present (June 2016). Native HYCOM gridded data from the experiments are converted to 

Network Common Data Format (NetCDF) on the native Mercator-curvilinear HYCOM 

horizontal grid, interpolated to 40 standard z-levels, and shared on the HYCOM consortium data 

server (HYCOM [Internet]). Full 3-D grids of temperature and velocity fields at six-hour 

intervals were downloaded for comparison with the coincident float observations at depth. 

Blended datasets combining the fields from the two experiments were produced using linear 

weighting in time during the overlap time period. SSH fields were also downloaded and 

processed in a similar manner. 

5.4.2 Upper Ocean Circulation 

LC and LC eddy fronts during the study program time period were tracked using the 17-cm 

contour in demeaned SSH fields. Discussion of the tracking results and specific events during the 

study program can be found in Section 3.4. In this section, the agreement between the AVISO, 
196 

CCAR and blended HYCOM SSH datasets is evaluated to assess the ability of the data 

assimilative model analyses to represent the upper-ocean state during the study program. LC 

length time series derived from these datasets are shown in Figure 3-26. The overall agreement is 

generally good between the LC metrics derived from each of the datasets; however, the timing of 

individual eddy detachment, re-attachment and separation events varies between the datasets. 

These differences in timing are primarily a result of the MDT fields used in each of the altimeter 

datasets and the MDT implicit in the ocean data system climatological mean fields (Cummings 

and Smedstad 2013). Industry names for each of the separation events that occurred during the 

study program are listed in Table 5-2 along with the corresponding separation dates determined 

from the AVISO, CCAR, and blended HYCOM SSH datasets. Crossing of 90°W by the eddy 

centers of LC eddies tracked during the study program are listed in Table 5-3. 

Two mean fields representative of the upper ocean layer in the Gulf over the four-year program 

time period are shown in Figure 5-36. The mean SSH (or MDT) is shown in the upper panel and 

the mean depth of the 6 °C isotherm is shown in the lower panel. The spatial correlation of the 

two fields within the Gulf is 0.91, which is one of reasons why the depth of the 6 °C isotherm 

has frequently been used as a proxy for the thickness of the upper layer in the Gulf (Bunge et al. 

2002). In the eastern Gulf, the 800-m isopleth of 6 °C temperature water closely approximates 

the 17-cm LC tracking contour in the HYCOM mean fields. 

The 285 APEX temperature profiles measured coincident with HYCOM blended dataset during 

the study program are used to evaluate the depth of the 6 °C isotherm estimated by the data 

assimilation system. Comparison of the in-situ and the data-assimilative model data indicates 

that the observed 6 °C isotherm depth is, in the mean, deeper in the observations than in the 

model by ~60 to 70 m (Table 5-4). The linear correlation between the depth of the 6 °C isotherm 

determined from observations and the coincident model estimate is 0.17, which is significant at 

the 99% confidence level (p = 0.01) if all the measurements are assumed independent. Although 

there is a significant but weak correlation in the vertical between the observations and the model, 

the amount of uncorrelated variance (97%) highlights the difficultly in extrapolating surface 

satellite observations to predict instantaneous conditions at depth, even with a sophisticated data 

assimilation system. Nevertheless, correspondence can be found when averaging over larger 

spatial scales and longer time periods. For example, the HYCOM blended dataset shows 

significant interannual variability in the depth of the 6 °C isotherm during the study program 

(Figure 5-37). Because the mean conditions over the four-year program were not significantly 

different than the long-term mean (see Section 3.4.1), the year-to-year anomalies observed in the 

altimeter SSH record are reflected in the changes in the mean depth of the 6 °C isotherm over 

each of the four program years. These changes resulted in a deepening of the mean isotherm 

depth in the eastern Gulf during the deep intrusions of the LC in years 1 and 4, and the shoaling 

of the mean isotherm depth during years 2 and 3. In the western Gulf, the deepening of the 

upper layer associated with the passage of LC eddies through the central western Gulf is seen in 

years 1 and 2. Shoaling of the mean isotherm depth in the western Bay of Campeche in years 1 

and 3 is consistent with the stronger cyclonic circulation anomalies found in the year-to-year 

variability in the interannual SSH (Figure 3-23). 
Figure 5-36. HYCOM (A) mean sea surface height and (B) average depth of the 6°C isotherm

Fields from blended Gulf HYCOM Analysis Experiments 31.0 and 32.5. The period is from 1 

July 2011 through 30 June 30 2015. The 17-cm LC tracking contour is shown by the thick black 

line.

197
Figure 5-37. HYCOM average depth of the 6 °C isotherm during each year of the study

(A) 1 July 2011 through 30 June 2012, (B) 1 July 2012 through 30 June 2013, (C) 1 

July 2013 through 30 June 2014, and (D) 1 July 2014 through 30 June 2015. Fields 

from blended Gulf HYCOM Analysis Experiments 31.0 and 32.5.

198
199 

More detailed comparisons are in progress to further quantify the skill of both the altimeter data 

products and the data assimilation system to accurately represent the upper-ocean circulation in 

the Gulf. These preliminary results show that the data-assimilative model experiments produce 

estimates of the ocean state that well represent the low frequency upper-layer circulation in the 

Gulf, which is dominated by the LC and LC eddies. Ocean state estimates, however, are still 

limited at the finest spatial and temporal scales by the unconstrained dynamics associated with a 

mesoscale-dominated turbulent ocean circulation such as is found in the Gulf. 

Table 5-2. Comparison of Separation Dates during the Study Program Derived from AVISO, CCAR 

and HYCOM SSH Fields

LC Eddy Name AVISO CCAR HYCOM

Hadal 23 Jul 2011 14 Aug 2011 11 Aug 2011

Icarus 20 Dec 2012 3 Feb 2012 2 March 2012

Jumbo 11 Jun 2012 4 Aug 2012 15 Jun 2012

Kraken 19 May 2013 9 Aug 2013 20 May 2013

Lazarus 5 Nov 2014 11 Nov 2014 * 

Michael * * * 

Nautilus 17 May 2015 18 May 2015 15 May 2015

* not detected at the 17-cm level 

Table 5-3. Comparison of Crossing of 90°W by LC Eddy Centers during the Study Program Derived 

from AVISO, CCAR and HYCOM SSH Fields

LC Eddy Name AVISO CCAR HYCOM

Hadal 22 Aug 2011 24 Aug 2011 27 Aug 2011

Icarus 10 Feb 2012 12 Feb 2012 02 Mar 2012*

Jumbo 30 Oct 2012 9 Nov 2012 22 Sep 2012**

Kraken 30 Jul 2013 9 Aug 2013* 22 Jul 2013

Lazarus 5 Nov 2014* 11 Nov 2014* ***

Michael *** *** ***

Nautilus 17 May 2015* 18 May 2015 15 May 2015*

* separation from the LC occurred when eddy center was west of 90°W

** at 89.5°W, not automatically tracked through large displacement of eddy centers 

*** not detected at the 17-cm level 

Table 5-4. Comparison of 6 °C Isotherm Depth Statistics Measured by APEX Profiles (N=285) 

Coincident with HYCOM Blended Dataset during the Study Program

6°C Isotherm Depth Mean Median Std Deviation

APEX 842 m 836 m 70 m

HYCOM 781 m 766 m 83 m

APEX minus HYCOM 61 m 70 m 116 m rms 
200 

5.4.3 Preliminary Statistical Comparisons of Velocities at Depth 

Before reviewing the circulation at depth, captured by the Gulf HYCOM data assimilation 

experiments, preliminary results will be shown comparing the float velocities at depth to the 

coincident HYCOM estimates during the study program. Because the flow is dominantly 

barotropic in the lower layer, i.e., independent of depth, the float velocities at all depths are 

compared with the model estimates at coincident time, latitude and longitude points at both 1500 

m and 2500 m depth. Figure 5-38 shows binned histogram scatterplots of each of the float 

velocity vector components (at all depths) against the respective blended Gulf HYCOM velocity 

components at 1500 m and 2500 m. Correlation values are also shown on the individual plots 

and are listed in Table 5-5. Better correlation of the velocity components is found between 

HYCOM velocities at 2500 m and equivalent float velocities, than are found between HYCOM 

velocities at 1500 m and float velocities, even though the majority of the floats were ballasted to 

the 1500 m depth level. The lower correlation values of vector components from HYCOM at 

1500 m can be attributed, in part, to uncorrelated velocity values in the model solution over 

bathymetric depths that range from 1500 m to 2500 m depth, which are primarily along the 

western and northern Gulf continental slope. If the points located over those ocean depths are 

removed from the comparison (Figure 5-39), the correlation coefficients at the 1500 m level 

improve, but are still below the correlation values found in the comparison to the model 

estimates at 2500 m. Velocity components at locations over ocean bathymetry, in that depth 

range, are effectively uncorrelated between the observations and the model (Figure 5-40). The 

source of this discrepancy is unknown at this time and merits further investigation. Possible 

sources include model truncation errors associated with the sigma-coordinate formulation over 

steeply sloping topography and/or the lack of vertical and horizontal resolution in the model to 

resolve bottom-intensified currents. There could also be issues with the 3DVAR assimilation 

scheme in such situations. Even with a 1/25° horizontal grid, Gulf HYCOM does not have the 

grid resolution to resolve the small cross-isobath length scales of the currents along the Sigsbee 

escarpment, and multi-model nesting and dynamical downscaling may be required to resolve 

strong currents in such regions (Morey and Dukhovskoy 2013). 

Table 5-5. Correlation of Observed Float Velocity Components (at all Depths) with Estimated HYCOM 

Velocity Components at 1500 m and 2500 m with and without Bathymetric Editing

Correlation Coefficient

Depth Level Bathymetric Depth Range of 

Float Locations

East-West

Velocity 

Component

North-South

Velocity 

Component

1500 m > 1500 m 0.15 0.12

1500 m 1500 to 2500 m 0.00 0.01

1500 m > 2500 m 0.19 0.16

2500 m > 2500 m 0.27 0.24
Figure 5-38. Binned 2-D histogram scatterplots of the float velocities (uF, vF) against HYCOM

(A) u-component at 1500 m, (B) v-component at 1500 m, (C) u-component at 2500 m, (D) 

v-component at 2500 m. Correlation coefficients between the individual vector 

components are shown in the upper right of each panel. Gulf HYCOM velocities from 

blended Analysis Experiments 31.0 and 32.5.

201
Figure 5-39. Scatterplot of the float velocities (uF, vF) against HYCOM for bathymetry > 2500 m

(A) u-component and (B) v-component at 1500 m, (C) u-component and (D) 

v-component at 2500 m. All points located over ocean bathymetry less than 2500-m 

depth have been removed. Correlation coefficients between the individual vector 

components are shown in the upper right of each panel. Gulf HYCOM velocities from 

blended Analysis Experiments 31.0 and 32.5.

202
Figure 5-40. Scatterplot of the float velocities (uF, vF) against HYCOM for bathymetry < 2500 m

(A) u-component and (B) v-component at 1500 m. All points located over ocean 

bathymetry greater than 2500-m depth have been removed. Gulf HYCOM velocities from 

blended Analysis Experiments 31.0 and 32.5. Correlation coefficients between the 

individual vector components are shown in the upper right of each panel. The correlation 

between the observed and model estimated velocity components are essentially zero.

203
204 

5.4.4 HYCOM Mean Flows and Interannual Circulation Variability at Depth 

Mean currents computed from the daily HYCOM blended dataset velocity fields at 1500 m and 

2500 m over the four-year program time period are shown in Figure 5-41 and Figure 5-42, 

respectively. The statistical analysis shown in the previous section indicates that there are some 

issues with the daily HYCOM 1500-m currents over the western and northern Gulf continental 

slope. Discrepancies are also evident in the mean currents at the 1500-m depth level. The mean 

circulation at the 1500-m level along the continental slope at depths between 1500 m and 2500 m 

is in the clockwise direction (anticyclonic) around the edge of the deep basin. This is in contrast 

to the nearly continuous strong anticlockwise (cyclonic) boundary current in the model at 2500 

m, extending from the Sigsbee escarpment all the way around the deep western Gulf basin to 

beyond 92°W, along the Campeche escarpment. Over bathymetric depths greater than 2500 m 

there is better qualitative agreement between the general circulation features in the flow fields at 

the two depth levels in the model. Both depth levels show a closed cyclonic gyre that covers 

much of the western deep basin and a separation point midway along the Campeche escarpment 

defining the easternmost extension of the gyre. There is also an anticyclonic recirculation region 

north of the escarpment at both depth levels in the model near 90°W. This anticyclonic 

recirculation region is better defined and more energetic at the 1500-m level than at the 2500-m 

level; however, the boundary current around the western basin cyclonic gyre is stronger at 2500 

m depth in the model. Similar circulation features were observed during the study program in 

the trajectories of the Lagrangian floats at depth in the western deep basin. 

In general, there appears to be little connectivity between the eastern and western deep basins 

mean flows in the model except for a broad weak westward flow along 26°N. The mean 

circulation at depth in the eastern deep basin is less coherent in the vertical than in the western 

basin, unlike the lower-layer current observations from moorings (Hamilton et al. 2016), 

presumably because of the dominant influence of the LC over the region. Nevertheless, there is 

mean cyclonic circulation at both 1500 m and 2500 m depth centered at 26°N and 86°W under 

the NE flank of the mean position of the LC during the study program. Similar circulation has 

been observed with deep moorings and in the float trajectories (Figure 5-17). 

The HYCOM mean currents at 1500 m and 2500 m during each year spanned by the four-year 

study program are shown in Figure 5-43 and Figure 5-44, respectively. Substantial interannual 

variations of the mean circulation features can be identified in the 4-year mean fields when 

viewed over one-year averaging periods, but several robust features can be identified throughout 

the record. The circulation associated with the western deep cyclonic gyre can be identified in 

all years and at both depths with the possible exception of year 1 at 1500 m. The extent of the 

deep cyclonic gyre varies from year-to-year, covering much of the western deep basin in year 1 

and becoming more compact and energetic in years 2–4 until recirculating primarily south of 

24°N in the deep Bay of Campeche. The separation point along the Campeche escarpment is 

relatively fixed at 2500 m, but is more highly variable at the 1500-m level because of changes in 

the anticyclonic circulation north of the escarpment in the central Gulf in years 3 and 4. In all 

years, unrealistic flows over steeply sloping topography are evident in the model state estimate at 

1500 m and the anomalous flows themselves exhibit significant variability year to year. The 

eastern deep circulation is more highly variable and exhibits little coherence year to year. This 

may not be too surprising given the large interannual variations in the areal extent of the LC 

observed during the study period. 
Figure 5-41. HYCOM average velocity field at 1500 m

The time period is from 1 July 2011 through 30 June 2015. The 2-D velocity flow field is 

visualized with streamlines computed using the Matplotlib streamplot function overlaid 

on a color-mapped image of the velocity magnitude. Arrow indicates direction of flow. 

Gulf HYCOM velocities from blended Analysis Experiments 31.0 and 32.5.

205
Figure 5-42. HYCOM average velocity field at 2500 m

The time period is from 1 July 2011 through 30 June 2015. The 2-D velocity flow field is 

visualized with streamlines computed using the Matplotlib streamplot function overlaid 

on a color-mapped image of the velocity magnitude. Arrow indicates direction of flow. 

Gulf HYCOM velocities from blended Analysis Experiments 31.0 and 32.5.

206
Figure 5-43. HYCOM average velocity fields at 1500 m during each year of the study program

(A) 1 July 2011 through 30 June 2012, (B) 1 July 2012 through 30 June 2013, (C) 1 July 

2013 through 30 June 2014, and (D) 1 July 2014 through 30 June 2015. Gulf HYCOM 

velocities from blended Analysis Experiments 31.0 and 32.5.

207
Figure 5-44. HYCOM average velocity fields at 2500 m during each year of the study program

(A) 1 July 2011 through 30 June 2012, (B) 1 July 2012 through 30 June 2013, (C) 1 July 

2013 through 30 June 2014, and (D) 1 July 2014 through 30 June 2015. Gulf HYCOM 

velocities from blended Analysis Experiments 31.0 and 32.5.

208
209 

5.4.5 Summary 

In Section 3.4 it was noted that the upper-ocean conditions in the Gulf during this four-year 

study were comparable to long-term average conditions over the 22-year satellite altimeter 

record; however, pronounced interannual variability was observed in each of the individual years 

within the program time period because of significant year-to-year LC, LC eddy, and mesoscale 

eddy variability. Because sampling of the deep flows by floats at depth was much greater in 

years two and three of the study program it was cautioned that care should be exercised when 

interpreting conditions at depth as representative of long-term mean conditions. The preliminary 

survey of deep flows at depth estimated by the data-assimilative Gulf HYCOM NCODA system 

mitigates those concerns. Mean circulation features mapped by the Lagrangian floats trajectories 

at depth in the western and central Gulf were persistent and robust in the ocean state estimate at 

depth derived from the coincident HYCOM data assimilation experiments. More effort is needed 

to investigate specific synoptic events sampled at depth by the floats during the study program 

and how well they are captured in the data assimilation experiments. Further investigation of the 

deficiencies in the model state estimate over steeply sloping topography is also warranted. 

211 

CHAPTER 6: DISCUSSION AND RECOMMENDATIONS 

6.1 INTRODUCTION AND OVERVIEW

The first Gulf-wide observational study of the deep circulation has resulted in an immensely rich 

data set, which it has been difficult to do justice within the time available for analysis. The 

principal instruments used were long deployments of substantial numbers of Lagrangian 

subsurface RAFOS floats, and several autonomous CTD and bio-optical profiling floats that 

were also equipped with RAFOS receivers for subsurface tracking. Used together, 

measurements of deep flows and water properties were made in all parts of the deep basins of the 

Gulf during the four years of the field program (2011–2015). The nominal float depths were 

1500 and 2500 m, with the latter primarily restricted to the eastern basin. CTD and bio-optical 

profiles were obtained approximately every 14-days during the same interval from seven APEX

profiling floats from the 1500 m park depth to the surface. The bio-optical profiles, obtained by 

six of the APEX floats, were the first made by autonomous instruments in the Gulf (Green et al. 

2014). US Argo profiling CTD floats that were in the Gulf during the years of the field study 

supplemented this study’s data. The US Argo floats generally had park depths of ~1000 m. 

An essential tool for the physical oceanography of the Gulf is remote sensing to keep track of the 

LC and eddies in the upper layers. Remote sensing data included daily SSH maps from CCAR 

and AVISO, and SST and ocean color images. One of the goals of the study was to investigate 

relations between deep flows and the surface mesoscale eddy field, which includes the LC in the 

east and the transits of LC eddies across the western basin, and their eventual interaction with 

other cyclones and anticyclones, and the topography of the northern and western slopes. 

High-resolution numerical modeling of the Gulf (and wider Caribbean and Atlantic Ocean) has 

become routine in the last few years, and simulated mesoscale circulation of the upper layers is 

now quite realistic (Dukhovskoy et al. 2015). Data assimilation systems based on these models 

(e.g., the Navy’s Gulf HYCOM NCODA system) routinely assimilate SSH and SST, and in 

some cases in situ data, for near real-time prediction and hindcasting of Gulf circulation. 

However, the ability of these numerical simulations to accurately model lower-layer circulation 

throughout the Gulf has not been rigorously tested. This study has made preliminary attempts to 

relate deep observations to model-predicted currents. 

6.1.1 Circulation Features 

In the eastern Gulf, recent advances in understanding the LC and LC eddy shedding processes 

from a major mooring-PIES based observational study (Hamilton et al. 2014; Hamilton et al. 

2016) has shown the importance of baroclinic instability mechanisms that transfer EKE to the 

lower layer (Donohue et al. 2016a). It is the mutual amplification of large scale (~300 km 

wavelengths and 40–100 day periodicities) meanders of the southward flowing branch of an 

extended LC with subsurface cyclones and anticyclones that are displaced from their respective 

meander troughs and crests, which provide the mechanism for LC eddy detachments and 

separations. It has been suggested that the LC and LC eddies radiate energy in the form of 

TRWs towards the northern slope and Sigsbee escarpment (Oey and Lee 2002; Oey 2008; 

Hamilton 2009), but observational confirmation has been lacking. 
212 

In the western Gulf, LC eddies transiting the basin are also thought to generate TRWs that 

mainly affect the lower layer (Oey and Lee 2002). Numerical modeling results sometimes 

indicate that transiting LC eddies have companion deep cyclones and anticyclones (Welsh and 

Inoue 2000; Frolov et al. 2004), however, this has not been observed. A deep cyclonic boundary 

current along the Sigsbee and Perdido escarpments was observed in mooring-based arrays and a 

more limited earlier RAFOS float study (Donohue et al. 2006; Donohue et al. 2008; Hamilton 

2009). The boundary current was narrow (~10–20 km in width), at least in a section ~91°W, but 

it was unknown whether it extended down the Mexican slope into the Bay of Campeche and 

perhaps along the Campeche escarpment. Deep circulation in the center of the western basin, 

and along the slope of the Campeche escarpment is largely unknown. In the Bay of Campeche, 

an upper-layer cyclonic gyre is observed, apparently wind-forced (Perez-Brunius et al. 2013), but 

again its deep expression, if any, is unknown. 

6.2 MAJOR RESULTS

The relative separateness of the deep circulations in the eastern and western basins is indicated 

by the float trajectories. The connectivity analysis (Section 5.1 and Appendix B) shows that 

floats originating or finding themselves in the southeast did not transit into the western basin. 

Similarly, floats originating in the northeast had only weak connections with the western basin, 

mainly dispersing along the northern boundary, over an 18-month time span. Deep circulation 

processes are also different between east and west. In the west, there is evidence for at least an 

intermittent cyclonic boundary current, an interior gyre over the deepest part of the basin, and 

eddy generation by boundary current instability. LC eddy transits also affect float paths through 

cross-slope displacements and TRW generation. In the east, the LC dominates, transferring 

energy to the lower layer through baroclinic instability. Again, TRW radiation seems to play a 

role in the dissipation of deep eddies. Major results from the study are first summarized by 

basin. 

6.2.1 Eastern Basin 

The observed float trajectories are consistent with LC meanders being involved with the 

generation of high levels of deep EKE through baroclinic instability, with deep anticyclones and 

cyclones being displaced downstream of surface meander crests and troughs, respectively, by 

approximately a quarter wavelength. These processes occur on the east or north side of an 

extended LC, and along the north side of a detached LC eddy, while the latter’s center is still east 

of 90°W. Large-scale upper-layer meanders only occur over deep water (~3000 m). The LC 

dynamics study (Hamilton et al. 2014; Hamilton et al. 2016) studied these processes for three 

eddies: Ekman, Franklin and Hadal, and part of the formation of Icarus. This study extends the 

observational database to two more separations, namely Kraken and Lazarus. The latter took an 

extraordinary long time to form, and thus there was an extended interval when there was no LC 

or LC eddy in the eastern Gulf. Under such circumstances, deep EKE levels in both the 

northeastern and southeastern parts of the Gulf rapidly decayed. There was evidence that the 

EKE resulting from the formation and detachment of Kraken propagated northwestward towards 

the northern slope in the form of TRWs. When the LC and LC eddy were not present in the 

northern part of the eastern basin, floats tended to oscillate in place, consistent with planetary 

waves that propagate momentum but not mass, rather than from eddies that trap and transport 

mass.
213 

There were a few examples of small clusters of floats with large-amplitude rectilinear 

oscillations that were consistent with TRWs radiating at slight angles to the isobaths towards the 

Sigsbee escarpment. Motions were similar to that found by Price and Rossby (1982) for SOFAR 

floats north of the Gulf Stream that were also attributed to TRWs, and were also consistent with 

periodicities observed from current meter arrays in the northern Gulf (Hamilton 2009). It is 

noted that very few coherent deep eddies were tracked by floats for more than three rotations in 

the eastern Gulf that were not associated with the LC or LC eddy meander front (see Section 

3.2). 

Not all deep eddies were directly a result of baroclinic instabilities. Observed for the first time to 

the authors’ knowledge was the generation of a “tip” anticyclone by the northward advance of 

the LC front. This anticyclone appears to be the result of lower-layer PV conservation. The 

lower layer is squeezed as the LC front advances with a deepening pycnocline over a nearly flat 

bottom, and thus generates anticyclonic vorticity. When the LC stops advancing, the tip 

anticyclone appears to decay and is also a possible source of radiating TRWs. The circulation of 

both a baroclinic instability cyclone and a tip anticyclone was examined in both cases with 

multiple simultaneous float tracks, and it was noteworthy that neither eddy was in solid body 

rotation. 

Detailed deep mean flow patterns were obtained for a two-year interval that showed cyclonic 

boundary currents around the southern part of the basin with a connection to eastward flow 

around the northern corner of the Campeche escarpment. Also a west-to-east anticyclonecyclone pair of deep circulations was found under the mean LC, almost exactly reproducing the 

mean flows found from the earlier current meter arrays of the LC dynamics experiment 

(Hamilton et al. 2014; Hamilton et al. 2016). The wider area covered by the floats indicates that 

the eastern cyclone may be fed by or interacts with the northward boundary current along the 

Florida escarpment. The anticyclone-cyclone pair under the LC is again explained by PV 

conservation and the tendency, during its growth phase, for the LC to lean over towards the 

northwest and the delta, which results in squeezing the lower water column to the northwest, and 

stretching to the east. 

To summarize: 

• Deep cyclones and anticyclones generated under the meandering LC front are consistent 

with the baroclinic instability process. 

• An extending LC generates a tip anticyclone through PV conservation. 

• There is evidence that both baroclinic instability and tip eddies decay by radiating TRWs 

towards the Sigsbee escarpment. 

• Mean flows show both the influence of the LC directly, and a cyclonic boundary current 

that connects to the eastward flows around the Campeche escarpment out of the western 

Gulf. 

6.2.2 Western Basin 

If the analysis in the eastern basin extends and confirms some of the previous conclusions from 

recent studies, the results in the western basin are almost all new. One principal result involves 

the effects on lower-layer flows of the passage of a LC eddy. There were two responses, both of 

which depend on the bottom slope. The first is the slow offshore displacement of a float 
214 

approaching an LC eddy (or vice-versa). As the eddy passes over the float (or water parcel) the 

float is displaced, first offshore (into deeper water), and then onshore. This slow cross-slope 

oscillation, related to the translation speed of the LC eddy, may be explained as the lower water 

column conserving planetary potential vorticity (f/H), and the model is the deep western 

boundary current in the North Atlantic being displaced into deeper water as it passes under the 

Gulf Stream at Cape Hatteras (Bower and Hunt 2000a). For this to occur, the bottom must have 

a steeper onshore-offshore slope than the lower thermocline of the LC eddy. Thus, this kind of 

response is more likely for LC eddies translating along the base of continental slopes than over 

relatively flat topography. If the bottom slope and slope of the thermocline are not favorable for 

maintaining a lower layer constant depth during a displacement, then relative vorticity must be 

generated to conserve PV (LaCasce 1998). The evidence, so far, is that this is likely, but the 

response is observed as higher-frequency rectilinear oscillations corresponding to TRWs, rather 

than generation of deep coherent vortices that could accompany the surface layer eddy. Note 

that TRWs can only exist and propagate if there is a bottom slope. Float tracks in response to a 

LC eddy translating through the western Gulf show both cross-slope displacement and TRW 

characteristics. Because TRWs propagate, with group speeds somewhat larger than LC eddy 

translation speeds (Hamilton 1990; Oey and Lee 2002), it is often uncertain if observed TRWlike oscillations are directly generated by the LC eddy or are propagating into the region from 

somewhere else further east, or are a mixture of remote and local waves (Section 5.3). There are 

clearly important implications in these results for numerical model simulations. 

The western Gulf has two cyclonic deep circulations. The first is a deep boundary current that 

tends to hug the steep escarpments, and the second is a sub-basin scale gyre over the deepest part 

of the basin. There are indications that these circulations may interact at certain points including 

at the sharp corner where the eastern Bay of Campeche slope turns into the steep slope of the 

Campeche escarpment, and on the western boundary between 22 and 24°N. 

The anticlockwise boundary current appears to be a source of limited exchange between eastern 

and western basins. There is eastward mean flow around the northern-most point of the 

Campeche escarpment, and a more diffuse westward flow into the boundary current south of the 

Mississippi delta. However, there are indications from the mean-flow maps and the connectivity 

analysis (Section 5.1) that there is some recirculation between the northeastward flow along the 

Campeche escarpment to the south with the westward flowing arm along the Sigsbee 

escarpment, suggestive of a partially closed anticlockwise circulation encompassing the whole of 

the western basin. There is evidence that this boundary current may not be continuous all the 

time, as there are times when flows reverse, and flow continuity becomes fractured. In 

particular, the region of the western slope between 22 and 24°N, where bottom slopes are 

weaker, flow continuity becomes disrupted. The cause of the along-isobaths flow reversals is not 

understood, but possible explanations may involve surface layer eddy-eddy or eddy-slope 

topography interactions, for which this region is particularly rich. Reversals also occur on the 

northern boundary along the Sigsbee escarpment. 

The boundary current also supplies energy in the form of deep eddies to the interior of the 

western basin. Boundary flow along the Campeche escarpment is observed to separate from the 

topography and form anticyclones, which drift westward in the interior of the basin. This is a 

new mechanism for deep eddy generation in the Gulf, and seems completely independent of the 
215 

surface-layer eddy field (Section 3.2). 

The deep basin cyclonic gyre, prominent in the mean-flow maps, occupies a region of low, and 

presumably homogeneous, lower-layer PV. The region is less extensive than closed contours of 

f/H, where H is the total depth, and arises because of the almost flat bottom in the deepest part of 

the basin, and an on-average deeper lower thermocline than in the central Gulf. The latter 

appears to arise because, for the time interval of the study, LC eddies tended to accumulate in 

this region next to the western slope. The dynamics of this gyre, and how and if it is sustained, 

given the highly variable nature of LC eddy tracks across the western basin, are unknown. 

To summarize: 

• LC eddies over the base of the continental slope cause slow and fast cross-slope 

displacements of deep-water parcels under their paths. The latter are probably TRWs. 

• The cyclonic deep boundary current often shows continuity from the Sigsbee escarpment 

all the way around to the northern corner of the Campeche escarpment. 

• The boundary current along the Campeche escarpment can separate and generate smallscale interior, westward propagating deep anticyclones. 

• The boundary current seems to be a mechanism for exchange between the largely isolated 

eastern and western basins. However, there is evidence of recirculation in the central part 

of the Gulf. 

• A large-scale cyclonic gyre occupies a region of low PV over the deepest part of the 

basin. 

6.2.3 Whole Gulf Water Mass Analysis 

For the first time since the 1960s and 1970s, Gulf-wide hydrography has been measured. For 

this study, this was achieved with autonomous profiling floats rather than ships. Because of the 

four-year time period, and along with some supplemental US Argo float data, reasonable 

coverage of the deep regions of the Gulf from the surface to 1500 or 1000 m was obtained. 

Because the lower-layer at 1500 m is disconnected from the upper-layer eddy field, the profiling 

floats act as random samplers of upper-layer temperature and salinity. Thus, water mass 

characteristics (SUW, AAIW, mixed layer depth, thermocline depth, etc.) could be mapped 

across the basins, and also related to upper-layer cyclones and anticyclones via simultaneous 

SSH anomalies. The upper-layer salinity maximum associated with SUW was primarily 

associated with LC and LC eddies (SSH > 17 cm), and was almost entirely absent from cyclones 

(SSH < -10 cm). Similarly, the salinity minimum of the AAIW increases from east to west with 

distance from its source in the Yucatan Current, with corresponding decreases in temperature and 

increases in density. 

It was hoped that SSH could serve as a proxy for the depth of the 6 °C isotherm, which is a good 

representation of the depth extent of the LC and the lower thermocline (Bunge et al. 2002). The 

mean depth is 800 m, which is also the limiting depth of the Straits of Florida sill. However, 

there was too much scatter in the correlations, so that remote sensing derived SSH could not be 

reliably used to estimate the lower-layer thickness. It was found that the depth of the 6 °C 

isotherm was correlated with temperature at 1500 m, raising the possibility that relative changes 

in RAFOS float temperatures can serve as a proxy for lower-layer thickness. This should be 

pursued further, and is discussed in the next section. 
216 

6.2.4 Numerical Modeling

During the period of float deployments, the assimilative Gulf HYCOM NCODA system was able 

to reproduce the broad features of the mean deep-Gulf circulation. It is unknown how well 

individual events such as generation of TRWs and cross-slope displacements by transiting LC 

eddies are reproduced. Given the results of the profile analysis that SSH is not a particularly 

good proxy for thermocline depth, which is essentially what assimilative models use for 

corrections to water column density structure, it is not surprising that there is scatter in 

reproducing measured deep float velocities. Making use of an array of regularly reporting Argo 

floats in the Gulf could be a way to provide more accuracy in the vertical temperature and 

salinity structure in the models, and therefore, more confidence in reproducing deep currents. 

6.3 SIGNIFICANCE FOR GULF OCEANOGRAPHY

A significant survey of circulation in the Gulf was the AGU Monograph, Circulation in the Gulf 

of Mexico: observations and models (Sturges and Lugo-Fernandez 2005). In 2016, a number of 

papers that resulted from the LC Dynamics Study were published in a special issue of Dynamics 

of Atmospheres and Oceans (Donohue et al. 2016a; Donohue et al. 2016b; Hamilton et al. 2016; 

Rosburg et al. 2016; Sheinbaum et al. 2016). Taken together with the results of this Lagrangian 

study, a significant revision in understanding circulation processes of the Gulf has taken place, 

though much analysis and modeling remains to be performed. A broad outline is as follows: The 

LC is the primary source of energy both for mean flows and mesoscale variability in both the 

upper and lower layers. The irregular intrusion of the LC and the shedding of large LC eddies, 

and the latter’s effect on the western basin, has been well studied with remote sensing and 

models. The recent in-situ measurements have shown that the eddy-shedding process involves 

large-scale meanders on the northern and eastern sides of an extended LC that grow by 

baroclinic instability, which extracts energy from the APE of the flow, and transfers EKE to the 

lower layer. The resulting strong deep eddy-flows associated with baroclinic instability and also 

with eddies generated by PV conservation due to an extending LC, appear to dissipate by 

transferring momentum westward through TRWs. Manifestations of deep eddies translating 

westward from the eastern to western basins were generally not observed by the floats. The eddy 

dissipation into TRWs results in rapid decreases of the amplitudes of deep flow variability when 

the LC is absent in the eastern basin. One result of this process, revealed by this study, is that 

deep water parcels only very slowly disperse from the eastern basin. Similarly, the deep meanflow patterns of a west to east anticyclone-cyclone pair, under the LC, will tend to retain 

particles in this region. There is northwards deep mean flow along the steep Florida escarpment, 

which is partially fed by boundary flows originating at the northeast corner of the Campeche 

escarpment. However, there is no connection across the Mississippi Fan to the Sigsbee 

escarpment westward flows, and so a boundary cyclonic circuit appears to be interrupted in the 

northeast Gulf. 

This study shows that the western basin is more dominated by mean currents than by high 

variability. A prominent feature of the mean flow is a cyclonic boundary current flowing along 

the base of the deep escarpments all around the basin from the Mississippi delta to the Campeche 

escarpment. EKE levels in the deep western basin are much lower than in the east. However, 

there is evidence that translating LC eddies displace deep water parcels across isobaths, and very 

probably also generate TRWs. The complex mesoscale eddy field in the upper layer of the 

western Gulf is likely to have a role, not understood yet, in influencing the variability of the 
217 

boundary current. Impingement of TRWs from the eastern basin and from translating LC eddies 

on the shoaling topography of the Sigsbee escarpment is thought to drive the deep westward 

boundary current there (DeHaan and Sturges 2005). There is some theoretical (Mizuta and Hogg 

2004) and modeling (Oey and Lee 2002) basis for this supposition. It appears that the westward 

Sigsbee escarpment boundary current is a pathway for water parcels to transfer from the eastern 

to the western basin. The boundary current has some continuity around the escarpments of the 

whole western basin, and the return flow into the eastern basin is along the Campeche 

escarpment. However, there is some evidence from float trajectories, and the connectivity 

calculations, that there is recirculation from the eastward flowing current, along the Campeche 

escarpment, across the central Gulf into the northern westward flowing branch of the boundary 

current. This also helps to explain why deep-water mass transfer between the basins, appears to 

be small. In contrast, the upper layer LC eddies transfer water mass, heat and salt westward from 

its source in the LC (Elliott 1982), and the continuity required upper-layer return transfers back 

to the east have been somewhat lacking in explanation and evidence (Sturges and Bozec 2013). 

There is eddy activity in the deep western basin, but its source seems largely to be the boundary 

current. Anticyclonic eddies spin off an apparently unstable boundary current, particularly from 

the eastward flowing branch along the very steep Campeche escarpment, and then propagate 

westward towards the Mexican slope. Their fate was not observed and is unknown. However, 

these eddies may be a mechanism for stirring the deep water of the western basin, unrelated to 

the upper-layer mesoscale eddy field. A feature of the western basin deep circulation, not 

previously observed, is a large-scale cyclonic gyre, separate from the boundary current, over the 

deepest part of the basin. It was found in a region of very weak topographic slopes and low, 

lower-layer PV. The latter may be influenced by the cumulative effect of LC eddies impinging 

on the Mexican slope, and a consequent deepening of the mean thermocline. If this is a quasipermanent feature of the deep circulation (its variability is unknown), then it may be a region for 

the accumulation of particles. 

The discussion above, on this study’s major contributions to understanding the circulation 

processes important to the Gulf, is to some extent speculative. However, it is hoped that it will 

spur further investigation, both observational and numerical, into these phenomena to confirm, 

modify or refute these hypotheses. These circulation processes will control dispersal of 

biological material and pollutants in the deep waters of the Gulf. The next two sections propose 

some approaches for further studies to examine these processes in more detail. 

6.4 FURTHER ANALYSIS

6.4.1 Relative Dispersion 

Even though progress has been made in identifying phenomena in the deep Gulf, the complexity 

and extent of the data that have been acquired for this study invite further analysis. One of the 

advantages of deploying clusters of floats is that relative dispersion statistics can be calculated 

and inferences made on the turbulent eddy regimes of the flows. LaCasce (2008) reviews the 

theory of two particle displacement statistics. The deployment strategy emphasized pairs and 

triplets, though it was somewhat compromised by the loss of early data because of the 

“overwrite” problem. However, it is estimated that there are ~100 pairings, both from 
218 

deployments and including chance pairs, and this provides an opportunity to study dispersion as 

it relates to potential spreading of contaminated water or biological material at depth. 

Future analysis should include calculations of relative dispersion and diffusivity, as well as the 

first, second and third moments of the probability density function of the relative displacements. 

These metrics provide insights on the time-scales and nature of the different dispersion regimes. 

Further, finite scale Lyapunov exponents (FSLEs) could be estimated that provide information 

on the spatial-scales of dispersion. Comparison of the dispersion statistics between the western 

and eastern Gulf should be illuminating because of the highly different levels of EKE of the two 

basins. 

6.4.2 Circulation Processes 

This study has provided preliminary looks at several important phenomena that should be further 

analyzed, using these data and data from contemporary programs. For example, there is some 

overlap with GoMRI current meter moorings in the De Soto canyon (Hamilton et al. 2015) and 

in deep water near the Mississippi delta. The LC array overlaps with the formation of Icarus 

(Hamilton et al. 2014), and there are overlaps with 300 to 400 m RAFOS drifters deployed 

between May 2012 and May 2013, and the intensive Grand Lagrangian Deployment (GLAD) 

surface drifter study in the summer of 2012 (Poje et al. 2014). Specific topics are: 

• Effects of Upper-Layer Eddy Flows: Deep eddies, cross- and along-isobath float 

displacements, and TRWs relationships to the time and space variable upper-layer eddy 

field need to be clarified, probably through layer PV dynamics. This is an overarching 

theme for the following phenomena.

• Topographic Rossby Waves: TRWs generated by the LC and LC eddies may be studied 

across the entire deep Gulf using these float data. Identifying TRWs from track data 

needs to be systemized, and higher-level analyses such as ray tracing applied. 

• Western Boundary Current: Boundary current flow needs further analysis into periods 

and locations where flow is steady and cyclonic, and oscillatory. An attempt should be 

made to relate these flows to surface-layer eddy activity. Float temperatures may provide 

evidence of fluid parcels cooling as they are advected counterclockwise around the basin 

as suggested by DeHaan and Sturges (2005). 

• Western Basin Gyre: The cyclonic gyre over the deepest part of the western basin needs 

further investigation into its formation mechanisms, persistence and time variability. 

• Deep Eddies: Generation by boundary-current separation along escarpments, propagation 

and fate are new topics that need further investigation. Though a preliminary comparison 

with surface SSH fields seems to indicate that the deep eddy formation is de-coupled 

from surface processes, a systematic comparison of simultaneous deep and surface 

observations is needed. 

• Interbasin Connections: It has been established that exchange between the eastern and 

western basins is limited, and apparently exchange with the Caribbean through the 

Yucatan Channel is also limited because no 1500-m floats left the Gulf over four years. 

The effect of the LC eddy-shedding cycle (Chang and Oey 2011) on such exchanges is 

important for the dispersal of pollutants and the renewal of deep Gulf water. 
219 

Numerical circulation models and data-assimilation experiments in the Gulf could provide a 

useful framework to explore these topics in more detail given sufficiently realistic simulations. 

All the observed phenomena would benefit from the exploratory uses of numerical simulations to 

provide and refine dynamical explanations for the observed flows. 

6.4.3 Bio-Optical Water Column Properties 

Preliminary analysis of the bio-optical profiles demonstrated the potential to improve our 

understanding of the ecosystem functioning in the Gulf. The analysis provided insights on the 

mean seasonal cycle of the subsurface chlorophyll maximum, and the data showed the impact of 

small spatio-temporal scale perturbations over the seasonal cycle. Further investigations should 

be extended to the other parameters such as CDOM and BBP. Possible extensions to the biooptical measurement analyses are: 

• Annual Variability of Chlorophyll: Comparison of float data with satellite chlorophyll or 

“historical” measurements would improve our understanding of trophic regimes in the 

Gulf. 

• Variability along Float Trajectories: The aim would be to characterize bio-optical 

variability with respect to water mass temperature and salinity. This, in turn, will help 

relate the observed biochemical variability to mesoscale eddies present in the Gulf. 

• Biogeochemical Models: Float profile data, once calibrated, could be used to improve 

and validate regional biogeochemical models. 

6.5 FUTURE FIELD STUDIES

Numerous field studies could be proposed to further investigate the circulations discussed above. 

Further work with RAFOS floats would require establishment of sound sources. As with any 

experimental study in the deep Gulf, instruments should be deployed to sample through several 

LC eddy-shedding cycles (e.g., 3–5 years), and because of dispersal of floats with one or twoyear missions, sound-source coverage should extend to the whole of the deep Gulf. Eulerian 

mooring-PIES arrays are also another experimental tool to investigate deep flows, and are 

inherently more suited to measuring relatively fixed circulations in small regions, such as 

boundary currents close to escarpments. Simultaneous deployments of floats in the upper and 

lower layers would help map velocity fields in both parts of the water column. Because of the 

first-mode baroclinic vertical structure of deep Gulf waters, it may be beneficial to use isopycnal 

RAFOS floats in the upper layer. Thus, vertical displacements of isopycnals in eddies could be 

measured. A few ideas, in no particular order, are given as recommendations for future work in 

the Gulf. Details, of course, remain to be fleshed out in the future. 

TRW Radiation from the LC 

In the dynamics of the LC report (Hamilton et al. 2014), it was recommended that a mooringPIES array be deployed across the Mississippi Fan to connect the LC and the Sigsbee escarpment 

so as to provide observations on deep eddy and TRW emissions from the LC. This study has 

identified at least two sources: baroclinic instabilities associated with eddy detachments, and 

decay of “tip” anticyclones. Details of a provisional experimental design are given in Chapter 7 

of Hamilton et al. (2014). Such a study would be enhanced with simultaneous deployments of 

RAFOS floats that would provide a Lagrangian perspective of TRW fluctuations. 
220 

Deep Western Basin Boundary Current 

A RAFOS float study of the cyclonic boundary current would provide important information on 

time variability, cross-slope velocity structure, continuity around the basin, and generation of 

spin-off eddies. A possible approach would be to deploy closely spaced clusters across the 

current at regular intervals (say 3–6 months) at say two locations (e.g., off Cocodrie, Louisiana 

and the City of Veracruz, Mexico). Dispersal of the floats over time (several years) would likely 

to provide decent coverage of the western basin deep cyclonic gyre. Judicious use of 

simultaneous mooring transects would also be an enhancement to a float experiment. There is an 

on-going Mexican study involving Amy Bower, Heather Furey, and Paula Pérez-Brunius that is 

regularly seeding the boundary current with RAFOS floats (300 and 1500 m) at a location just 

south of the US-Mexico border (Tamaulipas). Four deployments of 22 floats at intervals of four 

to eight months are planned. The sound sources will be in position until spring 2020. Clearly, 

supplementing this work with boundary current deployments in US waters would expand 

coverage of the flows and be highly beneficial for increasing understanding of this important 

circulation feature. 

Exchange Between Basins 

Regular deployments of floats between the Mississippi delta and the Campeche escarpment 

could be used to quantify exchanges between the eastern and western basins. Similarly, with 

suitably positioned sound sources, regular deployments near the Yucatan Channel sill could help 

define exchange with the Caribbean. 
221 

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FLOAT METADATA AND PLOTS OF TRAJECTORY DATA

Appendix A
236 

Appendix A includes individual RAFOS and RAFOS-equipped APEX float trajectory 

information. Tables A-1 through A-6 contain individual float metadata, including programmed 

mission length, ballast depth, launch date and position, surface date and position, and initial and 

final clock offsets. Figures A-1 through A-11 present float trajectories as postage stamp plots. 

Both the tables and figures are numerically ordered by float serial number. If a float returned no 

trajectory data, then the postage stamp plot is not presented. 
237 

Table A-1. APEX Float Metadata (S/Ns 7036–8512) and RAFOS Float Metadata (S/Ns 1016–1042)

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude

(N)

Longitude

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude

(N)

Longitude

(W)

Clock 

Offset 

(s)

		 		 		 APEX Floats 		 		 		 		

7036 999 1500 20120918 27.467 86.366 0	 20150806 41.470 65.540 0	

7573 999 1500 20110715 25.652 86.193 0	 NaN NaN NaN NaN

7544 999 1500 20110719 25.652 86.193 0	 20130818 24.670 80.250 0	

7590 999 1500 20120115 25.821 88.317 0	 20150808 21.410 96.440 0	

7658 999 1500 20120112 27.469 88.521 0	 20150916 27.070 93.360 0	

7671 999 1500 20120710 23.162 90.552 0	 20150826 27.420 87.800 0	

8405 999 1500 20120623 23.385 95.183 0	 20150810 24.100 94.430 0	

8512 999 1500 20120924 21.056 94.752 0	 20150916 24.350 88.490 0	

		 		 		 		 RAFOS Floats 		 		 		 		 		

1016 730 1500 20120114 26.531 85.755 0	 NaN NaN NaN NaN

1017 730 1500 20120113 27.250 86.249 0	 NaN NaN NaN NaN

1018 730 1500 20110720 27.293 89.786 0	 20110814 27.126 90.319 2	

1019 315 1500 20110720 27.293 89.786 0	 20110814 26.990 90.254 1	

1020 730 1500 20120115 25.835 87.407 0	 NaN NaN NaN NaN

1021 730 1500 20120115 25.835 88.408 0	 20120904 26.477 93.278 2	

1022 730 1500 20120115 25.750 87.249 0	 NaN NaN NaN NaN

1023 730 1500 20120114 26.304 85.298 0	 NaN NaN NaN NaN

1024 315 1500 20110720 26.502 89.253 0	 20120530 26.563 87.442 0	

1025 730 1500 20110720 26.502 89.253 0	 20120808 24.830 84.356 0	

1026 730 1500 20120116 27.294 89.787 0	 20120311 26.695 90.766 3	

1027 730 1500 20120116 27.294 89.787 0	 NaN NaN NaN NaN

1028 730 1500 20110722 25.924 91.134 1	 20130721 25.368 85.015 0	

1029 315 1500 20110722 25.924 91.134 2	 20120601 23.974 95.194 0	

1030 730 1500 20120113 27.901 86.833 0	 NaN NaN NaN NaN

1031 730 1500 20120113 27.901 86.833 0	 NaN NaN NaN NaN

1032 730 1500 20110722 26.133 91.834 0	 20110929 25.750 95.215 0	

1033 315 1500 20110722 26.133 91.834 0	 20120601 40.087 51.031 2	

1034 730 1500 20120116 26.503 89.253 0	 NaN NaN NaN NaN

1035 730 1500 20120116 26.503 89.253 0	 NaN NaN NaN NaN

1036 730 1500 20110719 25.653 86.193 0	 20110801 25.118 85.588 1	

1037 315 1500 20110719 25.653 86.193 0	 20120529 22.925 84.463 1	

1038 315 1500 20110720 25.836 88.408 1	 20120530 23.095 83.727 2	

1039 730 1500 20110720 25.836 88.408 0	 20130719 27.843 87.572 0	

1040 730 1500 20110716 27.004 87.498 0	 20120326 27.320 91.270 1	

1041 730 1500 20110717 27.900 86.833 0	 20130716 27.330 90.382 7	

1042 730 1500 20110717 26.530 85.756 0	 20130716 25.589 84.709 6	
238 

Table A-2. RAFOS Float Metadata (S/Ns 1043–1081)

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude

(N)

Longitude

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude

(N)

Longitude 

(W)

Clock 

Offset 

(s)

 		 RAFOS Floats 		 

1043 730 1500 20110717 27.250 86.251 0	 20130716 27.131 85.710 0	

1044 730 1500 20110719 25.750 87.249 0	 20130718 25.798 92.091 1	

1045 730 1500 20110719 25.750 87.249 0	 20130718 26.982 88.783 2	

1046 180 1500 20110718 26.304 85.299 0	 20120114 27.782 87.140 1	

1047 730 1500 20110718 26.304 85.299 0	 20130717 23.734 96.004 1	

1051 730 2500 20110718 26.304 85.299 0	 20130717 23.328 85.250 0	

1052 730 2500 20110717 26.530 85.756 0	 20110801 25.240 85.109 1	

1053 730 2500 20110717 27.250 86.251 0	 20111209 28.072 90.936 1	

1054 730 2500 20110720 26.502 89.253 0	 20130802 26.621 89.230 5	

1055 730 2500 20110720 25.836 88.408 0	 20110728 25.972 88.921 0	

1056 630 1500 20120912 23.395 95.160 0	 20140604 20.458 95.721 0	

1057 630 1500 20120912 23.395 95.161 0	 20140604 20.619 95.918 0	

1058 630 1500 20120912 23.692 96.539 0	 NaN NaN NaN NaN

1059 630 1500 20120912 23.692 96.539 0	 20140604 21.332 93.342 1	

1060 630 1500 20120913 22.147 96.633 0	 20140605 24.310 93.360 1	

1061 630 1500 20120913 22.147 96.633 0	 20140605 24.020 93.300 1	

1062 630 1500 20120910 25.362 95.442 0	 20140602 21.690 95.860 0	

1063 630 1500 20130926 20.743 95.801 0	 NaN NaN NaN NaN

1064 630 1500 20120910 25.121 95.272 0	 20140603 24.509 91.797 1	

1065 630 1500 20130926 20.743 95.801 0	 20150618 24.578 91.107 0	

1066 630 1500 20120923 20.672 93.402 0	 NaN NaN NaN NaN

1067 630 1500 20120910 25.120 95.273 0	 20140603 24.919 95.041 0	

1068 730 1500 20120116 25.924 91.133 0	 20140115 22.835 92.969 1	

1069 730 1500 20120117 26.133 91.834 0	 20140116 26.748 86.415 1	

1070 90 1500 20120114 25.303 86.083 0	 20120413 25.262 87.820 1	

1071 730 1500 20120115 25.750 87.249 0	 20140114 26.680 87.325 0	

1072 730 1500 20120114 26.304 85.298 0	 20140113 25.761 85.471 1	

1073 730 1500 20120117 26.133 91.834 0	 20140116 24.865 84.806 2	

1074 90 1500 20120116 25.924 91.133 0	 20120415 25.406 89.227 1	

1075 730 1500 20120114 25.303 86.083 0	 20140113 25.095 85.840 1	

1076 730 2500 20120114 26.304 85.298 0	 20140114 25.806 91.582 0	

1077 730 2500 20120114 25.597 85.005 0	 20140113 23.934 83.990 1	

1078 730 2500 20120113 27.250 86.249 0	 NaN NaN NaN NaN

1079 730 2500 20120114 26.531 85.755 0	 20140113 25.252 85.156 1	

1080 730 2500 20120114 25.303 86.083 0	 NaN NaN NaN NaN

1081 730 2500 20120116 25.924 91.133 0	 20130322 21.322 93.993 2	
239 

Table A-3. RAFOS Float Metadata (S/Ns 1082–1117)

		 		 		 		 		 		 		 		 		 		 		

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

		 		 		 		 RAFOS Floats 		 		 		 		 		

1082 730 2500 20120115 25.835 88.407 0	 20140115 23.760 95.771 2	

1083 730 2500 20120115 25.750 87.249 0	 20140114 27.897 86.405 0	

1084 730 1500 20120710 22.633 92.133 0	 20140711 25.071 88.885 2	

1085 730 1500 20120710 22.633 92.133 0	 20140711 25.367 94.060 0	

1086 730 1500 20120623 23.384 95.182 0	 20140626 20.529 94.926 1	

1087 730 1500 20120623 23.384 95.182 0	 20140626 23.366 91.380 0	

1088 730 1500 20120623 24.082 95.635 0	 20140626 24.280 91.100 1	

1089 730 1500 20120623 24.082 95.634 0	 20140626 21.251 93.970 1	

1090 730 1500 20120623 24.622 96.005 0	 20140627 23.751 96.925 1	

1091 730 1500 20120623 24.622 96.005 0	 20140626 20.591 94.958 1	

1092 730 1500 20120624 20.375 95.920 0	 20140626 25.170 86.496 1	

1093 730 1500 20120624 20.375 95.920 0	 20140626 23.872 93.145 1	

1094 730 1500 20120710 23.162 90.552 0	 20140318 24.375 86.607 1	

1095 730 1500 20120710 23.162 90.552 0	 20140710 25.503 91.155 1	

1096 630 1500 20120923 20.672 93.402 0	 20140615 23.745 89.333 3	

1097 630 1500 20120923 20.672 93.402 0	 20140615 23.322 91.066 1	

1098 630 1500 20120924 20.990 94.740 0	 20140616 22.665 93.782 0	

1099 630 1500 20120924 20.990 94.740 0	 20140616 24.283 92.662 0	

1100 630 1500 20120915 20.667 95.911 0	 20140607 23.189 90.364 0	

1101 630 1500 20120915 20.667 95.910 0	 20140607 24.034 93.844 0	

1102 630 1500 20120918 19.280 95.194 0	 20140607 24.699 94.746 1	

1103 630 1500 20120918 19.280 95.194 0	 20140610 22.718 93.772 1	

1104 630 1500 20120915 21.858 94.032 0	 20140607 24.307 95.472 0	

1105 630 1500 20120915 21.859 94.032 0	 20140607 22.469 93.666 4	

1106 630 1500 20120910 25.356 95.445 0	 20140602 22.900 96.870 0	

1107 630 1500 20120910 25.361 95.443 0	 20140602 24.380 93.714 1	

1108 630 1500 20120916 25.951 88.166 0	 20140609 24.265 84.831 0	

1109 630 1500 20120915 24.833 85.335 0	 20140607 24.737 84.953 0	

1110 630 1500 20120915 25.599 84.998 0	 20140607 24.261 86.184 0	

1111 630 1500 20120915 25.599 84.998 0	 20140607 21.061 86.039 0	

1112 630 1500 20120917 28.002 87.882 0	 20140610 27.156 87.394 1	

1113 630 1500 20120917 28.002 87.882 0	 20140610 26.897 86.550 0	

1114 630 1500 20120916 25.393 86.759 0	 20140608 26.420 90.518 0	

1115 630 1500 20120916 25.393 86.759 0	 20130320 26.439 87.319 1	

1116 630 1500 20120918 27.615 87.194 0	 20140610 22.871 84.717 0	

1117 630 1500 20120918 27.615 87.194 0	 20140610 27.151 88.845 0	
240 

Table A-4. RAFOS Float Metadata (S/Ns 1118–1153)

		 		 		 		 		 		 		 		 		 		 		

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

		 		 		 		 RAFOS Floats 		 		 		 		 		

1118 630 1500 20120919 27.334 85.584 0	 20140611 25.173 86.622 1	

1119 630 1500 20120919 27.334 85.584 0	 20140611 28.174 86.793 1	

1120 630 1500 20120916 25.951 88.166 0	 20140629 25.704 80.223 0	

1121 630 1500 20120916 25.951 88.166 0	 20140609 25.924 94.775 0	

1122 630 1500 20120915 24.833 85.335 0	 20140608 25.699 87.108 1	

1123 630 1500 20120915 24.833 85.335 0	 20140608 24.794 86.316 1	

1124 630 1500 20120918 27.467 86.367 0	 20140611 26.766 91.308 0	

1125 630 1500 20120918 27.467 86.367 0	 20140611 25.530 87.187 0	

1126 630 1500 20120917 28.002 87.882 0	 20140610 27.611 88.607 0	

1127 630 1500 20120919 27.334 85.584 0	 20140611 26.036 92.426 0	

1128 630 1500 20120925 27.300 89.334 0	 20140617 27.451 89.838 0	

1129 630 1500 20120925 27.300 89.334 1	 20140617 23.319 83.662 1	

1130 630 1500 20120925 27.300 89.334 0	 20140617 21.990 96.447 0	

1131 630 1500 20120925 26.582 89.559 0	 20140617 27.290 86.810 0	

1132 630 1500 20120925 26.582 89.559 0	 20140617 26.120 89.380 0	

1133 630 1500 20120925 26.582 89.559 1	 20140618 26.947 68.505 0	

1134 630 1500 20120925 25.867 89.800 1	 20140617 25.360 95.170 0	

1135 630 1500 20120925 25.867 89.800 1	 20140617 28.240 87.850 1	

1136 630 1500 20120925 25.867 89.800 1	 20140617 25.070 86.410 1	

1137 630 1500 20120925 25.917 90.802 1	 20140617 25.170 85.540 0	

1138 630 1500 20120925 25.917 90.802 0	 20140617 25.990 87.260 0	

1139 630 1500 20120925 25.917 90.802 0	 20140617 21.540 93.300 0	

1140 630 1500 20120925 25.951 91.801 0	 20140618 19.720 95.630 0	

1141 630 1500 20120925 25.951 91.801 0	 NaN NaN NaN NaN

1142 630 1500 20120926 26.167 92.801 0	 20140618 26.380 93.830 1	

1143 630 1500 20120926 26.167 92.801 0	 20140619 26.980 93.000 0	

1144 630 1500 20120926 26.167 92.801 1	 20140618 25.880 95.090 0	

1145 630 1500 20120926 26.334 92.116 1	 20140618 24.738 87.270 0	

1146 630 1500 20120926 26.334 92.116 1	 20140618 24.747 95.667 0	

1147 630 1500 20120926 26.334 92.116 1	 20140619 22.931 96.452 1	

1148 630 2500 20120916 25.393 86.759 0	 20140608 24.892 87.363 0	

1149 630 2500 20120916 25.393 86.759 0	 20140608 27.386 86.142 0	

1150 630 2500 20120916 25.951 88.167 0	 20140530 25.021 85.473 0	

1151 630 2500 20120915 24.833 85.335 0	 20140608 26.396 89.381 0	

1152 630 2500 20120919 27.334 85.584 0	 20140611 22.215 94.893 0	

1153 630 2500 20120919 27.334 85.584 0	 20140611 25.995 93.833 1	
241

Table A-5. RAFOS Float Metadata (S/Ns 1154–1245)

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

RAFOS Floats

1154 630 2500 20120916 25.951 88.167 0 20140609 25.511 90.249 1

1155 630 2500 20120915 25.951 88.167 0 20140609 26.418 89.424 1

1156 630 2500 20120915 25.599 84.997 0 20140607 24.818 88.129 0

1157 630 2500 20120915 25.599 84.997 0 20140607 27.602 86.652 0

1158 630 2500 20120918 27.467 86.366 0 20140611 27.632 86.485 0

1159 630 2500 20120919 27.334 85.584 0 20140611 26.451 90.627 0

1160 630 2500 20120915 24.833 85.335 0 20140608 26.558 85.755 1

1161 630 2500 20120915 24.833 85.335 0 20140610 26.035 91.222 0

1162 630 2500 20120918 27.467 86.366 0 20140613 26.219 90.080 1

1163 630 2500 20120918 27.467 86.366 0 20140611 24.963 85.320 2

1172 630 1500 20130923 23.635 96.484 0 20150615 23.562 92.392 0

1220 630 1500 20130923 23.635 96.483 0 20150615 21.900 92.806 1

1221 630 1500 20130819 23.837 92.835 0 NaN NaN NaN NaN

1223 630 1500 20130822 25.599 88.368 -2 20150514 27.035 86.003 -23

1224 630 1500 20130822 25.598 88.368 -2 20150514 28.511 87.709 -27

1225 630 1500 20130803 26.056 86.704 0 NaN NaN NaN NaN

1226 630 1500 20130615 23.600 90.017 0 20150307 24.936 95.435 -27

1227 630 1500 20130630 21.620 92.581 0 20150322 24.505 86.286 -24

1228 630 1500 20130701 21.245 95.432 0 20150323 24.893 92.218 -37

1229 630 1500 20130711 21.245 95.432 -2 20150323 23.277 94.228 -74

1230 630 1500 20130615 24.005 92.001 -3 20150307 23.553 94.708 -74

1231 630 1500 20130615 24.003 92.001 0 NaN NaN NaN NaN

1232 630 1500 20130630 21.620 92.582 0 20150322 26.459 89.198 -39

1233 630 1500 20130615 23.601 90.017 0 20150307 23.274 90.821 -35

1234 630 1500 20130803 26.056 86.704 0 20150424 26.751 87.787 -27

1235 630 1500 20130924 22.324 96.073 -1 20150616 21.366 94.132 -41

1236 630 1500 20130801 27.603 88.106 -2 20150423 24.595 93.956 -50

1237 630 1500 20130801 27.603 88.106 0 20150423 25.782 85.207 -2

1238 630 1500 20130824 25.524 93.444 0 20150516 25.902 88.362 -62

1239 630 1500 20130824 25.524 93.444 0 20150516 24.892 90.713 -52

1240 630 1500 20130820 25.306 91.485 0 20150512 25.374 89.045 -24

1241 630 1500 20130820 25.306 91.485 0 20150512 20.587 95.882 -20

1242 630 1500 20130806 27.399 86.749 0 20150428 28.219 87.634 -34

1243 630 1500 20130806 27.399 86.749 0 20150428 26.056 89.360 -95

1244 630 1500 20130819 23.837 92.835 0 20150512 22.805 92.222 -4

1245 630 1500 20130819 23.837 92.835 0 20150511 25.062 95.044 -16
242 

Table A-6. RAFOS Float Metadata (S/Ns 1246–9999)

		 		 		 		 		 		 		 		 		 		 		

S/N

Mission 

Length

Target 

Pressure

Launch Surface

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

Date 

YYYYMM-DDLatitude 

(N)

Longitude 

(W)

Clock 

Offset 

(s)

		 		 		 		 RAFOS Floats 		 		 		 		 		

1246 630 1500 20130924 22.265 96.073 0	 20150616 21.445 94.677 -48

1247 630 1500 20131005 21.858 94.025 0	 20150627 25.700 90.598 -13

1248 630 1500 20131005 21.859 94.024 0	 20150627 24.831 87.108 -38

1249 630 2500 20130822 25.598 88.368 2	 20150514 26.182 91.081 -54

1250 630 2500 20130822 25.598 88.368 0	 20150514 25.904 91.478 -48

1251 630 2500 20130803 26.056 86.704 0	 NaN NaN NaN NaN

1252 630 2500 20130803 26.056 86.704 0	 20150425 23.356 84.356 -44

1253 630 2500 20130806 27.399 86.749 0	 20150428 24.871 88.653 -33

1254 630 2500 20130822 25.598 88.368 0	 20150514 24.942 89.564 -21

1255 630 2500 20130806 27.399 86.749 0	 20150428 26.673 85.250 -62

1256 630 2500 20130803 26.056 86.704 0	 20150425 24.861 89.842 -87

9999 630 1500 20130923 23.635 96.483 0	 20150615 19.746 94.480 0	
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 7671 8405 8512

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1024 1025 1026

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1028

 96 oW 92 oW 88 oW 84 oW 80 oW 

1029

 96 oW 92 oW 88 oW 84 oW 80 oW 

1032

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 7036 7590 7658

APEX.

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1018 1019 1021

RAFOS.

Figure A-1. Postage stamp size plots of each RAFOS trajectory for floats 1018 through 1032 

Shown in order of deployment to match Table A. APEX floats 7036 through 8512 are plotted 

separately at the top of this figure. If a float has no plot, then there were no returned trajectory 

data. Green and red dots show the start and end locations.

243
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1036 1037 1038

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1039 1040 1041

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1042 1043 1044

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1045 1046 1047

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1048

 96 oW 92 oW 88 oW 84 oW 80 oW 

1049

 96 oW 92 oW 88 oW 84 oW 80 oW 

1050

Figure A-2. Postage stamp size plots of each RAFOS trajectory for floats 1036 through 1050 

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

244
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1052 1054 1055

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1056 1057 1059

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1060 1061 1062

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1064 1065 1067

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1068

 96 oW 92 oW 88 oW 84 oW 80 oW 

1069

 96 oW 92 oW 88 oW 84 oW 80 oW 

1070

Figure A-3. Postage stamp size plots of each RAFOS trajectory for floats 1052 through 1070 

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

245
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1071 1072 1073

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1074 1075 1079

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1081 1083 1084

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1085 1086 1087

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1088

 96 oW 92 oW 88 oW 84 oW 80 oW 

1092

 96 oW 92 oW 88 oW 84 oW 80 oW 

1093

Figure A-4. Postage stamp size plots of each RAFOS trajectory for floats 1071 through 1093

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

246
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1094 1095 1096

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1097 1098 1099

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1100 1101 1102

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1103 1104 1105

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1106

 96 oW 92 oW 88 oW 84 oW 80 oW 

1107

 96 oW 92 oW 88 oW 84 oW 80 oW 

1108

Figure A-5. Postage stamp size plots of each RAFOS trajectory for floats 1094 through 1108

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

247
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1109 1110 1111

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1112 1113 1114

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1115 1117 1118

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1119 1121 1122

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1123

 96 oW 92 oW 88 oW 84 oW 80 oW 

1124

 96 oW 92 oW 88 oW 84 oW 80 oW 

1125

Figure A-6. Postage stamp size plots of each RAFOS trajectory for floats 1109 through 1125

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

248
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1126 1127 1128

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1129 1130 1131

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1132 1134 1135

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1136 1137 1138

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1139

 96 oW 92 oW 88 oW 84 oW 80 oW 

1140

 96 oW 92 oW 88 oW 84 oW 80 oW 

1142

Figure A-7. Postage stamp size plots of each RAFOS trajectory for floats 1126 through 1142

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

249
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1143 1144 1145

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1146 1147 1148

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1149 1150 1151

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1152 1153 1154

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1155

 96 oW 92 oW 88 oW 84 oW 80 oW 

1156

 96 oW 92 oW 88 oW 84 oW 80 oW 

1157

Figure A-8. Postage stamp size plots of each RAFOS trajectory for floats 1143 through 1157

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

250
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1158 1159 1160

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1161 1163 1172

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1220 1223 1224

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1226 1227 1229

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1230

 96 oW 92 oW 88 oW 84 oW 80 oW 

1232

 96 oW 92 oW 88 oW 84 oW 80 oW 

1233

Figure A-9. Postage stamp size plots of each RAFOS trajectory for floats 1158 through 1233

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

251
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1234 1235 1236

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1237 1238 1239

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1240 1241 1242

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1243 1244 1245

 96 oW 92 oW 88 oW 84 oW 80 oW 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1246

 96 oW 92 oW 88 oW 84 oW 80 oW 

1247

 96 oW 92 oW 88 oW 84 oW 80 oW 

1248

Figure A-10. Postage stamp size plots of each RAFOS trajectory for floats 1234 through 1248

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

252
 18 o

N 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1249 1250 1252

 18 oN 

 21 oN 

 24 o

N 

 27 o

N 

 30 o

N 1253 1254 1255

 18 oN 

 21 o

N 

 24 o

N 

 27 o

N 

 30 o

N 1256 9999

 96 oW 92 oW 88 oW 84 oW 80 oW 96 oW 92 oW 88 oW 84 oW 80 oW 

 96 o

W 92 oW 88 oW 84 oW 80 oW 

Figure A-11. Postage stamp size plots of each RAFOS trajectory for floats 1249 through 9999

Shown in order of deployment to match Table A. If a float has no plot, then there were no 

returned trajectory data. Green and red dots show the start and end locations.

253
254 
INITIAL POSITIONS, DISPERSION AND CONNECTIVITY MAPS, AND CONNECTIVITY 

MATRICES OVER TIME FOR EACH SOURCE REGION ANALYZED IN SECTION 5.1

Appendix B
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in A % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-1. Initial position of particles released in region A

Figure B-2. Connectivity matrix as a function of time for source region A 

Colors represent percentages of particles originally released in A that are present in 

each region of the Gulf of Mexico past a certain time. The total number of active 

particles at each elapsed time is used to calculate the percentages.

256
Figure B-3. Connectivity (upper panels) and dispersion (lower panels) maps for region A

Particles start in region A with results after 6, 12 and 18 months of their release. Colors represent percentaoriginally released in A that are present in each region of the Gulf at each elapsed time. The total number oparticles at each elapsed time is used to calculate the percentages. Black circular markers show the locatioparticles at 6, 12 and 18 months past their release.

257 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in B % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-4. Initial position of particles released in region B

Figure B-5. Connectivity matrix as a function of time for source region B 

See Figure B-2 for details.

258
Figure B-6. Connectivity (upper panels) and dispersion (lower panels) maps for region B

See Figure B-3 for details.

259 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in C % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-7. Initial position of particles released in region C

Figure B-8. Connectivity matrix as a function of time for source region C 

See Figure B-2 for details.

260
Figure B-9. Connectivity (upper panels) and dispersion (lower panels) maps for region C

See Figure B-3 for details.

261 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in D % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-10. Initial position of particles released in region D

Figure B-11. Connectivity matrix as a function of time for source region D 

See Figure B-2 for details.

262
Figure B-12. Connectivity (upper panels) and dispersion (lower panels) maps for region D

See Figure B-3 for details.

263 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in E % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

Figure B-13. Initial position of particles released in region E

Figure B-14. Connectivity matrix as a function of time for source region E 

See Figure B-2 for details.

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

264
Figure B-15. Connectivity (upper panels) and dispersion (lower panels) maps for region E

See Figure B-3 for details.

265 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in F % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-16. Initial position of particles released in region F

Figure B-17. Connectivity matrix as a function of time for source region F

See Figure B-2 for details.

266
Figure B-18. Connectivity (upper panels) and dispersion (lower panels) maps for region F

See Figure B-3 for details.

267 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in G % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-19. Initial position of particles released in region G

Figure B-20. Connectivity matrix as a function of time for source region G 

See Figure B-2 for details.

268
Figure B-21. Connectivity (upper panels) and dispersion (lower panels) maps for region G

See Figure B-3 for details.

269 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in H % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

Figure B-22. Initial position of particles released in region H

Figure B-23. Connectivity matrix as a function of time for source region H 

See Figure B-2 for details.

270
Figure B-24. Connectivity (upper panels) and dispersion (lower panels) maps for region H

See Figure B-3 for details.

271 
Elapsed time since the release (months)

0 3 6 9 12 15 18

A

B

C

D

E

F

G

H

I

Particles released in I % particles

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100

Figure B-25. Initial position of particles released in region I

Figure B-26. Connectivity matrix as a function of time for source region I 

See Figure B-2 for details.

 96oW 92oW 88oW 84oW 80oW 18o

N 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

272
Figure B-27. Connectivity (upper panels) and dispersion (lower panels) maps for region I

See Figure B-3 for details.

273 
The Department of the Interior Mission

As the Nation’s principal conservation agency, the Department of the Interior has 

responsibility for most of our nationally owned public lands and natural 

resources. This includes fostering the sound use of our land and water 

resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; 

and providing for the enjoyment of life through outdoor recreation. The 

Department assesses our energy and mineral resources and works to ensure 

that their development is in the best interests of all our people by encouraging 

stewardship and citizen participation in their care. The Department also has a 

major responsibility for American Indian reservation communities and for people 

who live in island communities.

The Bureau of Ocean Energy Management Mission

The Bureau of Ocean Energy Management (BOEM) works to manage the 

exploration and development of the nation's offshore resources in a way that 

appropriately balances economic development, energy independence, and 

environmental protection through oil and gas leases, renewable energy 

development and environmental reviews and studies.