DeSoto Canyon

Eddy Intrusion Study

Final Report

Volume H: Technical Report

RJ5U.S. Derartrnent of thQ Interior

iHnerIs kinagerient Servic Gust ot Mecco OCS Recicri

S

OCS Stuth

h1I1S 2JJOO6C'
DeSoto Canyon

Eddy Intrusion Study

Final Report

Volume II: Technical Report

Authors

Thomas J. Berger

Peter Hamilton

James J. Singer

Evans Waddell

Science Applications International Corporation

James H. Churchill

Woods Hole Oceanographic Institute

Robert R. Leben

University of Colorado

Thomas N. Lee

University of Miami

Wilton Sturges

Florida State University

Prepared under MMS Contract

1435-01 -96-CT-30825

by

Science Applications International Corporation

615 Oberlin Rd., Suite 300

Raleigh, North Carolina 27605

Published by

U.S. Department of the Interior Minerals Management Service Gulf of Mexico OCS Region

OCS Study

MMS 2000-080

New Orleans

November 2000
DISCLAIMER

This report was prepared under contract between the Minerals Management

Service (S) and Science Applications International Corporation (SAIC)

This report has been technically reviewed by the MMS and approved for

publication. Approval does not signify that the contents necessarily

reflect the views and policies of the Service, nor does mention of trade

names or commercial products constitute endorsement or recommendation

for use. It is, however, exempt from review and. compliance with the N1IS

editorial standards -

REPORT AVAILABILITY

Extra copies of the report may be obtained from the Public InfoLnation

Office (Mail Stop 5034) at the following address:

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394

Telephone Number: 1-800-200-GULF or

504-736-2519

CITATION

Suggested Citation:

Hamilton, P., T. J. Berger, J.J. Singer, B. Waddell, J.H. Churchill,

R.R. Leben, T.N. Lee, and W. Sturges. Desoto Canyon Eddy

IntruSion Study, Final Report, Volume II: Technical Report. OCS

Study MMS 2000-080. U.S. Dept. of the Interior, Minerals

Management Service, Gulf of Mexico OCS Region, New Orleans, LA.

275 pp.

ABOTJT THE COVER

The map of the northeastern Gulf of Mexico on the cover of this report

shows the locations of moorings (red dots) and hydrographic stations

(white squares) occupied during the DeSoto Canyon Eddy Intrusion Study.

These locations are overlaid on a color-coded map of sea surface height

(SSH) as determined from satellite altimetry. The warmer colors

(yellow/orange) indicate a higher ESH. This grades through green to

blue in going to lower SSH. Note the horizontal scale of the

yellow/orange colored feature that was centered among the moorings.

Such a relative high is associated with anticyclonic (clockwise

rotating) surface currents.

iii
ACENOWLEDGENTS

The Program Manager wishes to extend thanks to the captain and crew of

the R/V Pelican for their continued support and conscientious effort

during the many cruises conducted during the study. During data

processing, Paul lankinship provided excellent data processing and

analysis support to the program principal investigators. Carol }arris

did an outstanding job in the general production of this and many other

project reports. For the duration of the program, Sharon Goodiiart

provided ac5xninistrative and project support that has been key to the

successful and timely completion of the study.

The continuing, enthusiastic and timely support of Dr. Alexis LugoFernandez, the MI'4S Contract Officer's Technical Representative, during

all phases of this project is gratefully acknowledged.

V
TABLE O CONTENTS

VOLTJ1X II

PAGE

ACKNOWLEDGMENTS v

LIST OF FIGURES xi

LIST OF TABLES xxiii

ABBREVIATIONS AND ACRONYMS xxvii

INTRODUCTION 1

1.1 Introduction 1

1.2 Project Objectives 1

1.3 Report Organization 5

DATA ACQUISITION AND PRESENTATION 7 2.1 Introduction 7 2.2 CTD Data 7 2.2.1 Introduction 7 2.2.2 CTD Data Acquisition System 7 2.2.3 CTD Performance and Data Return 9

2.3 XBT Data g 2.3.1 Introduction 9

2.3.2 XBT Data Acquisition System 10

2.3.3 XBT Performance and Data Return 10

2.4 Vessel-Mounted ADCP Data 10

2.4.1 Introduction 10

2.4.2 Vessel-Mounted ADCP Data Acquisition System 10

2.4.3 Vessel-Mounted AJDCP Performance and Data Return 12

2.5 Moored Instrument Data 12

2.5.1 Introduction 12

2.5.2 Moored Instrumentation 13

2.5.3 Moored Instrument Performance and Data Return 13

2.6 GeoSat Altimeter Data 15

2.6.1 TOPEX/POSEIDEN 15

2.6.2 ERS-1&2 18

2.6.3 Data Sources 20

2.7 Ancillary Data 22

2.7.1 Introduction 22

2.7.2 Ancillary Data - SCIJLP Drifters 24

2.7.2.1 Argos-Tracked Drifting Buoys 25

2.7.3 GulfCet II Study 25

2.7.4 Northeastern Gulf of Mexico Chemical

Oceanography and Hydrography Study 27

2.75 NEGOM Marine Ecosystem Program: MANES III 27

2.7.6 Meterological Data 29

2.7.6.1 C-MAN 29

2.7.6.2 3-Meter Buoy 30

2.7.7 Water Level Data 30

2.7.8 River Runoff Data 31. 2.7.9 Satellite Imagery 31

2.7.10 Hurricanes 31

DATA QUALITY ASSURANCE AND PROCESSING 33

3.1 Introduction 33

3.2 CTD Systems 33

3.2.1 CTD Sensor Calibration 33

322 CTD Data Processing 36

vii
TABLE O CONTENTS (continued)

VOLT.JNE II

PAGE

3.3 XBT System 36

3.3.1 XBT Calibration 36

3.3.2 XBT Data Processing 36

3.4 Altimetry 37

3.4.1 Data Processing 37

3.4.2 Blended T/P and ERS l&2 Climatology 38

3.5 Moored Instrumentation 38

3.5.1 Moored Instrumentation and Calibration 40

3.5.1.1 Aanderaa Current Meters 40

3.5.l2 Genera]. Oceanics Current Meters 40

3.5.1.3 Hugrn Temperature Recorders 42

3.5.1.4 InterOcean Current Meters 42

3.5.1.5 RDt.JADCP5 - 42

3.5.1.6 SeaBird CT Recorders 42

3.5.2 Moored Instrumentation Data Processing 43

3.6 Drifting-Buoy Data 43

LOOP CURRENT MiD ITS INTRUSIONS 45

4.1 Introduction 45

4.2 Loop Current/Loop Current Intrusions from Altimetry 45

4.2.1 Introduction 45

4.2.2 Loop Current Monitoring 45

4.2.3 Loop Current Metrics and Statistics 47

4.2.4 Loop Current Ring Shedding Cycle 53

4.2.5 Geostrophic vs. SCULP Drifter Velocities 56

4.2.6 Loop Current Characteristics during Field

Measurements 57

43 Loop Current and LC Rings - Energetics and Dynamics 64

4.3.1 Introduction 64

4.3.2 Background and Procedures 64

4.3.2.1 Historical Analysis 64

4.3.2.2 New Data and Processing 66

4.3.2.3 Methods 67

4.3.3 Discussions of Ring Shedding Frequency 69

43.4 Forcing of LC Variability by the Wind Curl 72

4.3.4.1 Background 72

4.3.4.2 Energetics of the Forcing 74

43.43 Comparison of LC Variability and Wind Curl

Forcing 76

4.3.5 Discussions and Conclusions 81

SLOPE CIRCULATION PATTERNS 83

5l Introduction 83

5.2 Event Description 83

5.3 Loop Current Frontal Eddies 120

5.3.1 Background 120

5.3.2 Influence of Loop Current Frontal Eddies on the

Study Area 125

5.4 Characteristic Slope Circulation Patterns 126

5.4.1 Geostrophic Current Patterns 126

5.4.2 Long Period Events 137

viii
TABLE OF CONTENTS (continued)

VOLtJNE II

PAGE

5.4.3 Two-Year Statistical Properties 147

5.4.4 Long Period Variability 147

5.5 Slope Dynamics and Fluxes 161

5.5.1 Divergence and Relative Vorticity 161

5.5.2 Across Margin Transport of Momentum at the

Canyon Edge 165

5.5.3 Across Margin Heat and Salt Fluxes at the

Canyon Edge 172

5.6 Deep Currents 178

SLOPE RESPONSE TO ATMOSPHERIC FORCING 187

6.1 Introduction 187

6.2 Slope Response to Synoptic Wind Forcing 187

6.2.1 Time Domain EOF's ITEOF) ...... 190

6.2.2 Frequency Domain EOF's (FEOP) 199

6.2.3 Discussion 204

6.3 Seasonal Wind Response on the Slope 209

6.4 Canyon Response to Storms 214

6.4.1 Overview 214

6.4.2 The Canyon Edge Response 220

6.4.3 Cross-marginal Volume Transports at the Canyon

Edge 227

6.4.4 The Inertial Wake 228

6.4.5 Storms vs. Canyon Edge Eddies during the winter of

1997-1998 236

6.5 Inertial Currents on the Slope 242

CONCEPTUAL MODELS OF CIRCULATION PATTERNS 251

7.1 Introduction 251

7.2 Modes of LC and LC Rings 251

7.3 Regional Scale Patterns 259

References 263

APPENDIX A 271

ix
LIST OF FIGURES

icrure No. Caption Page

Image showing contours of sea surface height

with examples of the types and scales of eddy -

like features that can occur in the eastern and

northeastern Gulf of Mexico. 2 1.2-1. Base map of the study area with color coded image

of the height of the sea surface (warmer colors are

higher/taller) as determined by satellite altimeter 3

2.2-1. Nap showing standard hydrographic grid for the

DeSoto Canyon Eddy Intrusion Study 8

2.3-1. Map showing the two AJDCP/XBT Feature Surveys conducted

in November 1997 and December 1998 11

2.5-la. Timeline of data return by instrumenb level far the

indicated DeSoto Canyon Eddy Intrusion Study moorings .. 16

2.5-lb. Timeline of data return by instrument level for the

indicated DeSoto Canyon Eddy Intrusion Study moorings .. 17

2.6-1. TOPEX/POSEIDON ground track over the Gulf of Mexico

from the operational 10-day exact repeat orbit 19

2.6-2. ERS-1 and ERS-2 ground track over the Gulf of Mexico

from the 35-day exact repeat orbit 21

2.6-3. ERS-1 ground track showing data coverage for the

first 37-day near repeat from the 168-day exact

repeat orbit 23

2.7-1. Composite track plot of ARGOS drifters 26

2.7-2. Map showing CTD stations for the MMS-funded NEGOM

Chemical Oceanography and Hydrography Study being

done by Texas A&M University 28

3.2-1. Final Temp./Salinity (T/S) plots of CTD data from

the Sea-Bird SeaCat 19 (left panel) and 911 Plus CTD

(right panel) . 35

3.4-1. Sea surface height map for January 12, 1998 39

42-1. 1993-1998 mean SSH from a hindcast data assimilation

experiment 46

4.2-2. The 17-cm SSH contour from the altimetry map for

January 12, 1998 overlaid on the composite

SST imagery 48

xi
LIST OF FIGURES (continued)

Fiqure No. Caption Paqo

4.2-3. SST pixel values from coincident imagery are

shown for 3° north and 2° south of the

northernmost point on the 17-cm sea surface

height contour 49

4.2-4. Time series of LC area, volume, and circulation

for 1/1/1993 through 4/30/1999 51

4.2-5. Time series of LO northward and westward penetration,

and length for 1/1/1993 through 4/30/1999 52

4.2-6. Percent occurrence of 17-cm contour defining high

velocity core of LC 54

4.2-7. Percent occurrence of LC waters for the interval

from 1/1/1993 through 4/30/1999 55

4.2-8. Map of the vector correlation coefficient between

daily averaged drifter velocities and coincident

geostrophic velocities 58

4.2-9.

4 . 2-10 -

4.2-11 -

Time series of LC northward and westward penetration,

and length during the field program 59

Time series of LC northward and westward penetration,

and length during the field program 60

Mean SSH for interval from April 1997 to April 1999. - . - 61

4.2-12. Mean SSH for the interval from April 1997 to

April 1998. 62

4.2-13. Mean SSH for the interval from April 1998 to

April 1999. 63

43-1. Periodicity of ring separations from the Loop Current - - 70

43-2. Periodicity of ring separations from the Loop Current;

similar to Figure 4.3-1. 71

4.3-3. Power spectra of the north-south motion of the Loop

Current and of wind stress curl over the full

Gulf of Mexico 77

43-4. Cross spectra between the wind curl and Loop Current

variability 78

43-5. The mean annual cycle of wind curl over the Gulf

of Mexico from the NCEP data 80

xii
LIST O FIGURES (c:!ontinued)

figure No. Caption Paqe

5.1-1. Contours of sea surface salinity as measured by

Drennen (1968) during a survey in the general area

of this project. 84

5.2-la. Subtidal current vectors from depths of 8 to 40m

for all mooring sites from March to September 1997. 85

5.2-lb. Subtidal current vectors from depths of 8 to 40m

for all mooring sites from September 1997 to

March 1998. 86

5.2-ic. Subtidal current vectors from depths of 8 to 40m

for all mooring Sites from March to September 1998. 87

5.2-id.

5. 2-2a.

5 .2-2b.

5 .2-2c.

Subtidal current vectors from depths of 8 to 40m

for all mooring sites from September 1998 to

March 1999. 88

Subtidal current vectors from 300m depths at

mid-slope moorings (A2, B2, C2 and D2) from

March to September 1997. 89

Subtidal Current vectors from 30Cm depth at

mid-slope moorings (A2, B2, C2 and D2) from

September 1997 to March 1998. 90

Subtidal current vectors from 300m depth at

mid-slope moorings (A2, B2, C2 and D2) from

March to September 1998. 91

5.2-2d. Subtidal current vectors from 300m depths at

mid-slope moorings from September 1998 to

March 1999. 92

5.2-3a. SST image for April 30, 1997 (1355 CST) . . . 93

5.2-3b. SSH for April 30, 1997 derived from TOPEX/ERS-2 plus

model mean. . 95

5.2-4a. SSH for July 1, 1997 derived from TOPEX/ERS-2 plus

model mean.. 96

5.2-4b,c. Shipboard geostrophic currents for b) 6m depth and c)

500rrt depth, based on a level of no motion at 1000 m for

July 8-16, 1997.. 97
IXST OF FIGURES (continued)

Fiqure No. Caption Page

5.2-5a,b. Shipboard geostrophic currents for a) 6m depth and b)

300m depth, based on a level of no motion at l000m

for November 11-19, 1997. 98

5.2-Sc. SSH for October 1, 1997 derived from TOPEX/ERS-2

plus model mean. 100

5.2-6. SSH for December 15, 1997 derived from TOPEX/ERS-2

plus model mean. 101

5.2-7. SST image for December 6, 1997 (0238 CST) . 102

5.2-8. SST image for February 5, 1998 (0310 CST) . 103

5.2-9. SST image from March 15, 1998 (0253 CST) . . 104

5.2-lOa. SST image for April 12, 1998 (0247 CST) . 105

5.2-l0b,c. Shipboard geostrophic currents for b) 6m depth and C)

300m depth for March 31 - April 8, 1998. 107

5.2-11. SSH for April 12, 1998 derived from T0PEX/ERS-2 plus

model mean. 108

5.2-12a. SSH for August 1, 1998 derived from TOPEX/ERS-2 plus

model mean. 109

5.2-l2b,c. Shipboard geostrophic currents for b) Gm depth and c)

500m depth for August 4-12, 1998. 110

5.2-12d. Trajectories of satellite tracked near surface

drifters 11887, 11896 and 11897 from August 5

- September 4, 1998. 111

5.2-13. SST image for December 30, 1998 (1416 CST). 113

5.2-14a. SSH for December 30, 1998 derived from TOPEX/ERS-2

plus model mean. 114

5.2-14b,c. Shipboard geostrophic currents for b) Gm depth

and c) 500m depth for December 1-9, 1998. 115

5.2-lL4d. Trajectories of satellite tracked near surface

drifters 12061 from November 21, 1998 April 1, 1999. 116

5.2-15. SST for January 25, 1999 at O3O8CST derived from

satellite AVHRR together with upper level currents

from the moored array at the time of the image 117

xiv
LIST OF FIGURES (continued)

Figure No. Caption Page

5.2-16a. Sea Surface Temperature image for March 2, 1999

(0309 CST) 118

5.2-16b,c. Shipboard geostrophic currents for b) 6m depth

and C) 500m depth, for March 28 - April 6, 1999 119

5.2-17. SST-I for January 25, 1999 derived from TOPEX/ERS-2

plus model mean. 121

5.2-18. SSH for March 2, 1999 derived from TOPEX/ERS-2 plus

model mean 122

5.3-1. Bathyrnetry of the eastern Gulf of Mexico and southern

Straits of Florida 123

5.4-la-d. Geostrophic velocities at 6rn depth from the

hydrographic surveys 128

5.4-le-g. Geostrophic velocities at 6m from the hydrographic

surveys and 5-day average velocity vectors 129

5.4-2a-d. Geostrophic velocities at 500m from the hydrographic

surveys 130

5.4-2e-g. Geostrophic velocities at 500m from the hydrographic

surveys 131

5.4-3a-d. Geostrophic velocities on Section B for the indicated

surveys 133

5.4-3e-g. Geostrophic velocities on Section B for the indicated

surveys 134

5.4-4. (a) surface salinity from July 1997 (PE-9803), and

(b) temperature at lOOm from March 1997

(Cruise PE-9722) 136

5.4-5. Seven-DLP current vectors and isotherm depths for

the upper 300m of C2 138

5.4-6a,b. Mean currents and standard deviation ellipses for

summer 1997 (5/10 to 8/16/97) for: (a) 15m, (b) 70m

(c) 200m (solid) and 300m (dashed), and (d) 500m

(solid) and l290m (dashed) depths 139

5.4-6c,d. Means and standard deviation ellipses at indicated

depth 140
5.4-10.

5 .4-11.

LIST OF FIGTES (continued)

FicTure No. Caution Page

S .4-7a,b. Mean currents and standard deviation ellipses for

the cycloriic flow period (2/1 to 4/30/98) at (a) 16 in,

(b) 70m,(c) 200m(solid) and 300m(dashed).and (d) 500m

(solid) and 1290m(dashed) depths 142

5.4-7c,d. Mean currents and standard deviation ellipses for

the indicated depths 143

5 .4-8a,b. Mean currents and standard deviation ellipses for

the anticyclonic flow period (11/1/98 to 1/16/99) for

(a) 16m, (b) 70m, (c) 200xn (solid) and 300rn (dashed)

and (d) 500m (solid) and 1290m (dashed) depths. 145

5.4-8c,d. Mean currents and standard deviation ellipses for the

anticyclonic flow period (11/1/98 to 1/16/99) 146

5.4-9a,b. Mean current vectors and standard deviation ellipses

for the two-year study period at the indicated

depths 148

5.4-9c,d. Mean current vectors and standard deviation ellipses

for the two-year study period 149

Contours of mean temperatures for the two-year

study period (4/2/97 to 3/31/99) at (a) 90m and (b)

500m 150

Two-year mean TOPEX/ERS-2 SSH anomaly added to a

10-year climatological LC derived from numerical

model studies 151

5.4-12a-c. Kinetic energy spectra in variance preserving form

for two-year velocity records 152

5.4-l2d-f Figure 5.4-12 continued 153

54-13.

5 .4-14.

EOF ellipses for mode 1 of the 724-50 day period

band 156

EOF Mode 1 (a) amplitude (°C), and (b) phase

(degrees) of the 90m temperatures for the

724-50 day period band. 157

5.4-15. EOF ellipses for mode 1 of the 50 to 15-day period

band 159

5.4-16. EOF ellipses for Mode 2 of the 50 to 15-day period

band. 160

xvi
LIST OF FIGURES (continued)

Figure No Caption Paqe

55-1. Positions used for divergence and vorticity

calculated at the circled points. 162

5.5-2. Time series of divergence (normalized by f) and

40-HLP temperature 163

5 .5-3. Relative vorticity (normalized by f) and 40-HLP

current vectors for the indicated moorings and

measurement depths 164

5.5-4. (a) Spectra of the divergence and relative vorticity

at B3. 166

5.5-5. The top three panels show vertically averaged alongisobath velocity. The lower three panels show

estimated cross-margin momentum transport 168

5.5-6. For the winter of 1997-1998, computed surface wind

stress at buoy 42040, vertically-averaged

along-isobath velocity and cross-isobath momentum

transport 169

5.5-7. Characteristics of the estimated wind-driven

cross-margin fluxes of momentum at mooring Cl 171

5.5-8. Time series of cross-isobath temperature flux at

16 m depth at mooring Cl 173

5.5-9. Plot of cross-margin salinity flux against

cross-margin temperature flux. 174

5.5-10. Time series of cross-margin temperature flux at

the indicated moorings and depths together the with

surface wind stress components 177

5.6-ia. 40-HLP velocity vectors and temperatures for the

bottom current meters of the l300rn moorings. 179

5.6-lb. 40-HLP velocity vectors and temperatures for

the bottom current meters of the l300m moorings 180

5.6-ic. 40-HLP velocity vectors and temperatures for

the bottom current meters of the 1300m moorings 181

5.6-ld. 40-HLP velocity vectors and temperatures for

the bottom current meters of the 1300m moorings 182
LIST OF ExGTrns (continued)

Fiqure No. Caption Paqe

5.6-2 For bottom instruments on 1300m moorings:

(a) Kinetic energy spectra (b) temperature spectra.

In panel (C) coherence squared and phase differences

are shown. 183

5.6-3. 50-10-day frequency domain EOF analysis 185

6.1-1. Map showing which partitions the NE Gulf of Mexico and

indicates the number of tropical storms occurring

in each partition per decade. Also shown are DeSoto

Canyon moorings and present key meteorological

measurement sites 188

6.2-la.

6 - 2-lb.

6.2-2a.

6 . 2-2b.

6. 2-2c -

6 .2-2d.

6.2-3a.

Subtidal current vectors from depths of 8 to 95m

from mooring site Dl during March to September 1997

together with subtidal winds from buoy 42040 and

coastal sea level at Panama City, FL 191

Subtidal current vectors from depths of 8 to 95m

from mooring site Dl from September 1997

to March 1998 192

Vertically averaged, rotated, subtidal current

vectors from all shelf break moorings together

with subtidal winds and coastal sea level March

to September 1997 193

Vertically averaged, rotated, subtidal current

vectors from all shelf break moorings together

with subtidal winds and coastal sea level

September 1997 to March 1998 194

Vertically averaged, rotated, subtidal current

vectors from all shelfbreak moorings together with

subtidal winds and coastal sea level for the period

March to September 1998 195

Vertically averaged, rotated, subtidal current

vectors from all shelf break moorings together with

subtidal winds and coastal sealevel for the period

September 1998 to March 1999 196

Kinetic energy spectra at Cl for different depths

and seasons during the two-year study 197

6 .2-3b. Kinetic energy spectra at C3 for different depths

and seasons during the two-year study 198
Ficflire No. Caption Pacie

6.2-4a.

6 .2-4b.

6.2-7a.

6 .2-7b.

6 . 2-7c.

6. 2-7d.

LIST OF FIGURES (continued)

Cross-shelf components of TEOF modes of shelf

break currents and wind stress and coastal. sea

level for winter 1998 200

Alongshelf components of TEOF modes of shelf break

currents and wind stress and coastal sealevel for

winter 1998 201

62-5. Spectra of TEOF modes of shelf break alongshelf

currents and i and wind stress components for winter

1998 202

6.2-6. Coherence and phase spectra of TEOF modes of a)

alongshelf wind stress and currents; b) mode 1

cross-shelf wind stress and alongshelf currents;

and C) mode 2 cross-shelf wind stress and

alongshelf currents for winter 1998 203

Results of FEOF analyis for mode 1 at mooring Al

for summer and winter seasons presented as ellipses

for selected depths 205

Results of FEOF analyis for mode 1 at mooring Cl

for summer and winter seasons presented as ellipses

for selected depths 206

Results of FEOF analyis for mode 1 at mooring Dl for

summer and winter seasons presented as ellipses for

selected depths 207

Results of FEOF analyis for mode 1 at mooring El

for summer and winter seasons presented as ellipses

for selected depths 208

6.3-1. Time domain EOF eigenvector components presented

as vectors. Analysis is for the 40-HLP, 0 to lOOm

depth mean currents. Summer 1997 modes 1 (a)

and 2 (b), and summer 1998 modes 1 (c) and 2 )d) 211

6.3-2. Time domain EOF eigenvector components represented as

vectors. Analysis is for the 40-HLP, 0 to lOOm depth

mean currents. Winter 1998 modes 1 (a) and 2 (b),

and winter 1999 modes 1 (c) and 2 )d) 212

6.3-3. Time series of the noLiLalized amplitudes of mode 1

(solid) and 2 (dashed) for the two summer

and two winter seasons 213

xix
LIST OF FIGURES (continued)

Figure No. Caption Page

6.3-4. Time domain EOF eigenvector components represented

as vectors. Analysis is for the 40-HLP wind stress

time series at the indicated stations. (a)

winter 1998, and (b) winter 1999 215

6.3-5. Variance preserving spectra for EOF (a) mode 1

wind-stress, and (b) 0-lOOm depth mean current

modes 1 and 2 for the winters of 1998 and 1999. 216

6.4-1. Estimated total surface wind stress at NOAA buoys

42040 and 42039 during the DeSoto Canyon study 217

6.4-2. Wind velocities and near-surface currents (at 30

and 7Orn depth) measured during Hurricane Earl 218

6.4-3. Same as Figure 6.4-2 except showing wind velocities

and near-surface currents 219

6.4-4. Time series of wind stress (top panel), and nearbottom and near-surface along-isobath velocities

(lower panels) measured at the indicated canyon-edge

moorings during Hurricanes Earl and Georges 221

6.4-5. Same as Figure 6.4-4 except showing wind stress and

across-isobath velocities measured during the

passage of Hurricanes Earl and Georges 222

6.4-6. Same as Figure 6.4-4 except showing wind stress

and temperatures measured during Hurricanes Earl

and Georges 223

6.4-7. Contours of along-isobath (positive to the northeast)

and acx-oss-jsobath (positive onsheif) velocity

recorded at mooring Cl during the passage of

Hurricane Earl 225

6.4-8. Same as Figure 6.4-7 except showing contours of

velocity at mooring Cl during the passage of

Hurricane Georges 226

6.4-9. Surface wind stress (top panel) together with nearbottom and near-surface inertial current magnitude

observed at the indicated canyon edge moorings during

late summer-early autumn 1998 230

6 .4-10. Same as Figure 6.4-9 except showing near-bottom and

near-surface inertial current magnitude at interior

canyon moorings 231

xx
Fiqiire No. Caution Paqe

6.4-il.

6 - 4-12.

6.4-13.

6 .4-14.

6.4-15.

LIST OF FIGURES (continued)

Comparison of inertial current magnitudes seen

near the surface at moorings C3, D2 and El

during late summer-early autumn 1998 232

Contours of the magnitude and phase of inertial

currents observed at mooring El during and after the

passage of Hurricane Georges 234

Surface wind stress together with near-bottom and

near-surface inertial current magnitude observed

at the indicated moorings over the study period 235

SST on January 10, 1998 (1414 CST) with measured

near-surface current velocities overlaid on

the image. 237

SST on January 16, 1998 (1307 CST) with measured

near-surface current velocities overlaid on

the image. 238

6.4-16. Near-bottom and near-surface, along-isobath

velocities measured at the canyon edge moorings

during the winter of 1997-1998 239

6.4-17. Same as Figure 6.4-16 except showing near-bottom

and near-surface across-isobath velocities 240

6.4-18. Contours of along-isobath and cross-isobath velocity

measured at mooring Al during January and February

1998 241

6.5-i, 3-HLP time series of cross- and along-isobath

components at selected depths on the indicated

moorings during a two-month period in summer 1997. . 243

6.5-2. Clockwise rotary spectra for two depths on moorings

Cl and 03 for summer 1997 244

6.5-3. 3-HLP time series of cross- and along-isobath

(V-solid) components at selected depths on the

indicated moorings during a two-month period in

winter 1999. 246

xxi
LIST OF FIGURES (continued)

Ficrure No. Caption Pacre

6.5-4. The inertial amplitude at the 32-rn depth of

the Cl and C3 moorings and the 40-HLP temperature

records from the 20-rn and 80-rn depths at Cl. Also

shows the 40-HLP wind speed at the BURL1

C-MAN station 247

6.5-S. Mode 1 EOF amplitudes in cm/s (a) and phases in

degrees (b) calculated from the inertial band of

upper-layer clockwise spectra for the summer of

1997. 248

7.2-la. Conceptual model of direct Loop Current or warm ring

flow interaction with the slope in the study

region. 252

7.2-lb. Conceptual model of LC warm ring flow interaction

with slope west of the Mississippi River delta 253

7.2-2a. Conceptual model of a LCFE flow interaction with the

slope in the study region 255

7.2-2b. Conceptual model of flow interaction with the slope

in the study region from a LCFE on the boundary of a

warm LC ring 256

7.2-2c. Conceptual model of flow interaction with the slope

from a LCFE close to the study region 257

7.2-2d. Conceptualization of flow interaction with the slope

from a LCFE on the boundary of a warm LC ring

located west of the Mississippi delta and blocked

by the LC 258

7.3-1. Conceptual schematic of the multilayered flow

conditions in the DeSoto Canyon study area. 260
ab1e No. Caption Page

2.2-1. Summary of CTD data collected during the DeSoto

Canyon Eddy Intrusion Study. 9 2.4-1. Data acquisition setup for vessel-mounted ADCPs

used during the DeSoto Canyon Eddy Intrusion

Study Feature Surveys. 12

2.5-1. Approximate mooring deployment periods for the

DeSoto Canyon Eddy Intrusion Study. 12

2.5-2. Mooring instrumentation and deployment levels

during the last deployment period (December 1998

- April 1999) of the DeSoto Canyon Eddy

Instrusion Study. 14

2.5-3. Moored instrument data return (by good record

Count) during the DeSoto Canyon Eddy Intrusion

Study. 18

2.6-1. TOPEX/POSEIDON Mission Phases 20

2.6-2. ERS-1 Mission Phases 22

2.6-3. ERS-2 Mission Phases 22

2.7-1. NMS Davis-type Argos drifting buoys deployed

during the DeSoto Canyon Eddy Intrusion Study-----25

2.7-2. MMS/tJSGS GulfCet II cruises. 27

2.7-3. N4S Chemical Oceanography and Hydrography

Cruises. 27

2.7-4. Northeastern Gulf of Mexico Coastal Marine and

Ecosystem Program: Ecosystem Monitoring

Mississippi/Alabama Shelf moorings. 29

2.7-5. C-MAN stations used during DeSoto Canyon Eddy

Intrusion Study. 29

2.7-6. Offshore meteorological buoys used during the

DeSoto Canyon Eddy Intrusion Study. 30

2.7-7. Water-level stations used during the DeSoto

Canyon Eddy Intrusion Study. 30

2.7-8 River discharge stations used during the DeSoto

Canyon Eddy Intrusion Study. 31

LIST OF TABLES
Table No.

LIST OF TABLES (contixrned)

Caption Page

2.7-9. Hurricanes that passed through the study area

during the DeSoto Canyon Eddy Intrusion Study. 32

3.2-1. Manufacturer's specifications for SeaBird

Electronics,Inc., 911 Plus CTD. 33

3.2-2. Time (in months) of most recent calibration of

911 Plus CTD sensors before and after each cruise

during the DeSoto Canyon Eddy Intrusion Study. 34

3.2-3. Mixed layer 911 Plus CTD salinity minus Bottle

salinity comparisons by cruise during the DeSoto

Canyon Eddy Intrusion Study. 34

3.5-1. Manufacturer's specifications for moored

instruments used during DeSoto Canyon Eddy

Intrusion Study. 41

4.2-1. Statistics for Loop Current Metrics (1/1/1993 -

4/30/1999) . 53

4.2-2. Correlation Matrix for Loop Current Metrics 56

4.3-1. A compilation of ring-separation events; i.e.,

times when data are available to show a ring

separating reliably from the Loop Current. 68

5.3-1. Characteristics of Loop Current frontal eddies

(LCFE) that influence the study area as

determined from satellite derived SST and SSH

fields 126

5.4-1 Frequency domain EOF analysis of two-year velocity

records. 155

5.4-2 Wave numbers from EOF Mode 1 phases for 724 to

50 day period fluctuations 158

5.5-i. Estimated cross-margin temperature fluxes at

DeSoto Canyon edge moorings (at 100 m isobath) . ... 175

5.5-2. Estimated cross-margin salinity fluxes at DeSoto

Canyon edge moorings (at 100 in isobath) - 176

xxiv
LIST OF TLES (continued)

Table No. Caption Paqe

6.4-1. Met onshore and offshore volume transport per

unit alongshelf distance measured at canyon edge

moorings during early February 1998 storm and

at mooring Al during the presumed eddy passage of

the mooring. 224

6.4-2. Net onshore and offshore volume transport per

unit alongshelf distance measured at canyon edge

moorings during Hurricanes Earl and George 228

6.5-1 EOF Mode 1 inertial oscillation analysis

vertical wavelength estimates from Mode 1 phase

differences 249
ACRONYMS AND ABBRVIAT IONS

Aanderaa

A,B,C,D,E Mooring Identifier

ACE Anticyclonic Eddy

ALTOPR Altimeter Ocean Products

AVHRR Advanced Very High Resolution Radiometry

C Centigrade (temperature scale)

CCW Counterclockwise

C-MAN Coastal-Marine Automated Metwork

cpd Cycles per day

CST Central Standard Time

CIJPOM Colorado Princeton Ocean Model

D,D Divergence, depth

df,DF Degrees of Freedom

DLP Day Low Pass

EOF Empirical Orthogonal Function

ESI Environmental Sensors, Inc.

F Flux

f Coriolis Parameter

FC Florida Current

FEOF Frequency EOF

g Gravitational acceleration

GDR Geophysical Data Records

GEOSAT

H Water depth

HLP Hour Low Pass

kHz Kilohertz (1000 cycles/sec.)

L Shelf width, alongshore length scale

LCFE Loop Current Frontal Eddy(ies)

NAB Mid-Atlantic Bight

NDBC National Data Buoy Office

NOAA National Ocean and Atmospheric Administration

NE Northeastern

NOS National Ocean Service

OEI Oregon Environmental Instruments

PODBM Physical Oceanography Data Base Management

ppt Parts per thousand

psu Practical Salinity TJnits

R Correlation Coefficient

r Bottom resistance

Average cross shelf slope

SAB South Atlantic Bight

SD Standard Deviation

SSF Southern Straits of Florida

SSH Sea Surface Height

SST Sea Surface Temperature

T Momentum Transport, temperature

t Time

TEOF Temporal BOF

TRW Topographic Rossby Wave

USGS United States Geological Survey

u,v Velocity component
v,V Velocity component

w Width of outersheif band

x,X Horizontal component direction

y,Y Horizontal component direction

Vertical direction

Vorticity
I. INTROETJCTION

1.1 Introduction

The DeSoto Canyon Eddy Intrusion Study will provide the Minerals

Management Service (NNS) with information and analyses which expands the

understanding of physical oceanographic conditions and processes in the

northeastern Gulf of Mexico. In turn, these insights will support an

enhanced basis for developing sound, rationally based environmental

assessments. The threat of a spill contacting land in the northeastern

Gulf of Mexico is of great concern to the MtS. To estimate the

potential for such a spill coming into contact with resources in the

region, a robust "climatological" circulation, which includes the means

and dominant oceanographic phenomena is needed. On the northeastern

Gulf slope, Loop Current (LC) and eddy intrusions are two important

processes to be included in any climatologica]. circulation

characterization of the area (see Figure l.l-1 as an example)

To gather the data needed to assemble the oceanographic climatological

database for oil spill trajectory analysis, the MMS funded the present

DeSoto Canyon Eddy Intrusion Study. Knowledge acquired through this

study will facilitate MMS's understanding of the outer shelf

circulation, e.g., how the LC and associated eddies exchange momentum

and mass with the shelf. The role of the ]JeSoto Canyon as a route that

facilitates these intrusions and as a conduit of mass exchange between

the deep Gulf and the shelf will be further elucidated by characterizing

processes and conditions over the adjacent slope. As described below,

results and data from this study will also be useful to other concurrent

oceanographic studies that the MMS and others are sponsoring in the

northeastern Gulf.

1.2 ?roject Objectives

The general objectives of this study are:

Use in-situ current measurements, hydrographic data, and satellite

images to document and characterize LC intrusions and interactions

with the northeastern Gulf slope (Figure 1.2-1, for illustration)

This study shall examine the frequency and horizontal and vertical

extent of these interactions and intrusions. Through the use of

dynamical principles, a conceptual model will be used to help

explain the character and evolution of LC-slope interactions

observed in the course of the study.

Document and examine the dynamical processes of momentum, mass and

vertical vorticity exchanges occurring during LC-slope

interactions. These analyses shall be based on the in-situ

current measurements and hydrographic data.

Estimate the frequency of LC, LC rings and secondary eddies'

interactions with the northeastern slope, and conduct an

assessment of the vertical and horizontal shears, exchanges of

vorticity, momentum, arid mass fields associated with the eddyslope interactions.

1
Gulf of Mexico Sea Surface Height

TOPEXIERS-2 ea surfce Fie hit rioriiaFy plus 10-year climatological model mear

Contour Increment 5 cm

k

F

LI

III 10

-7n

':TL.- -..1- Thzwig to.r3 of urLt. w. ii i L-i P.yc 3t] tcf ey-....k tiL,u Litit.

ccc.iv- i-i thc otert rd Lu t :L XxL:.
DSoto Canyon Eddy Intruson 3udy

:-]Ef".1' .cj

th4I .E t 'ZLC CQ 1ht.4 QL Lh hjJ.LL

Lbe urfc ' y :t imtrAl, 'I Ur- 1abc.e. n j;

Ott 2. riu- rj piep... te

c LXLLL m'tCr Y FW1 LL_c ccui Lfti LlL .0 L/OT Rdy 1tft t4
Elucidate the role of the DeSoto Canyon in LC and eddy processes

as a mechanism and as well as a route of mass and momentum

exchange between the shelf and deep water of the northeastern

Gulf.

The overall program was to last 4 years with two complete years of field

observations in the general study area (enclosed by solid lines in

Figure 1.1-1) . The general location and configuration of moorings were

specified in the Scope of Work provided by the MMS. The general area

for the hydrographic sampling scheme was also specified. SAIC proposed

some limited modifications that were incorporated into the overall

design that produced the observational database used for this report.

It is important to note that several concurrent and complementary

programs involving physical oceanographic measurements were conducted in

this general study area. The MIAS is funding Texas A&M University to

conduct a study titled " Northeastern Gulf of Mexico Chemical

Oceanography and Hydrography Study" (Contract No. 1435-Ol-97-CT-30851) - Additionally, selected physical oceanographic measurements are being

made in the pinnacle area near and at the shelf break offshore of

Alabama as part of an NBR/USGS-funded study being conducted by

Continental Shelf Associates.

Members of the program's scientific team (Principal Investigators - P15)

and their primary areas of investigation on this project are presented

below in alphabetical order:

James Churchill (Woods Hole Oceanographic Institute WHOI) -

Canyon processes; atmospheric forcing due to hurricanes; shelfslope exchange processes.

Peter Hamilton (Science Applications International Corporation

SAIC) - Slope eddy dynamics; scales of motion; atmospheric

forcing.

Robert Leben (University of Colorado - UCo1) - Loop Current,

dynamics and metrics, ringshedding patterns and characteristics.

Thomas Lee (University of Miami - tjNiami) - Slope eddy kinematics;

shelf-slope exchange processes and mechanisms.

Wilton Sturges (Florida State University - FSU) - LC and LC ring

dynamics.

An important support function was provided by James Singer (SAIC) who

was responsible for the planning and conduct of all field operations and

logistics and served as Program Chief Scientist on all cruises.

Throughout most of the program, Thomas J. Berger was the Program

Manager. Although recently retired, he was responsible for the

oversight and guidance of activities during all but the creation of this

final report.

4
1.3 Report Organization

The report is composed of eight chapters covering the topics listed

below:

Chapter 1.: Introduction - General overview of program rationale.

Chapter 2: Data Acquisition - Instruments, procedures and methods by

which data used in this prolect was acguired.

Chapter 3: Data Processing and Management - Procedures for data

handling, analysis and management of the various types of data used

during the study.

Chapter 4: Loop Current and its Intrusions - Characteristics of Loop

Current, cycle of ring shedding, various metrics and dynamics associated

with the intrusion and ring shedding process.

Chapter 5: Slope Circulation Patterns - Describing specific circulation

patterns measured and documented on the slope. The analytical tools

used in this description help isolate relation and patterns.

Chapter 6: Slope osponse to Atmospheric Forcing - Characterizing slope

circulation patterns as a response to atmospheric forcing mechanisms at

a wide range of periods.

Chapter 7: Conceptual Model and Characterization - Effort at

synthesizing some of the LC and slope circulation relationships and

patterns deduced from this program.

Chapter 8: References

5
II. DATA ACQUISITION AD PRESENTATION

2..1 Introduction

Data collection during the DeSoto Canyon Eddy Intrusion Study included

shipboard Conductivity/Temperature/Depth (CTD), Acoustic Doppler Current

Profiler (ADCP) and Expendable Bathytherinograph (XBT) surveys; in-situ

moored current, temperature, conductivity and pressure time series

measurements and the collection of GeoSat altimeter data. Available

ancillary data from other coincidental programs include drifting buoy

data from Davis-type Argos drifters, hydrographic data from the GulfCet

II and Northeastern Gulf of Mexico Chemical Oceanography and Hydrography

studies, and near-bottom current measurements from the Northeastern Gulf

of Mexico Coastal Marine and Ecosystem Program. Additional ancillary

data from various government or government-funded entities include

meteorological, water-level, river-runoff, satellite-imagery and

hurricane-tracking data, all of which were evaluated or used in this

study.

2.2 CTD Data

2.2.1 Introduction

CTD data were collected during each of seven mooring deployment/rotation

cruises aboard the R/V PELICAN (at approximately four-month intervals),

beginning in March 1997 and concluding in April 1999. Initially, a

standard grid with 75 stations was occupied. This was increased to 80

stations in Movember 1997 to obtain additional shelf/slope observations

along five of the seven onshore/offshore sections. Seven additional

stations were added for the December 1998 cruise, as part of a Feature

Survey. Figure 2.2-1 shows the standard grid for this latter survey

with the Feature Survey stations (Stations 81-87) and all 13 current

meter mooring locations included.

2.2.2 CTD Data Acquisition System

The primary CTD data acquisition system was a SeaBird 911 Plus CTD

System provided and operated by the technical staff at Louisiana

universities Marine Consortium (LUMCON). The CTD fish was equipped with

two sets of conductivity and temperature sensors for redundant CT data

collection, a Datasonics altimeter and a SeaBird. Carousel Water Sampler

with General Oceanics Niskin water sample bottles. A SeaBird 19 SeaCat

Profiler was also used during brief periods when the main CTD system was

not available. To eliminate or significantly reduce the possibility of

sensor mismatch (salinity spiking) when passing through a sharp

thermocline, the lowering speed of the CTD was adjusted from 15 meters

per minute for the first 90 meters of descent to 30 meters per minute

from 90 meters to 200 meters depth and then to 60 meters per minute

below 200 meters.

7
Figure 2.2-1 Map showing standard hydrographic grid for the DeSoto Canyon Eddy Intrusion Study with feature survey stations for Decemher 1998 cru

(Stations 81-87) and all 13 current meter mooring locations (solid circles) 31°N 3OaN 29°N 28°N90°W 89°W 88°W 8TW 86GW 85

/ La. rI çMiss. ------ 1,0 )/ / D) L116 ' o0 ---El '-- Ala. I_- FJ0. 'El ' \, ---- 0 N 77 '"' ]9.SEl D2mEl SSS54-- ' -" Eti- "-, El S B2 L115 --------------------- ______f4z C31 S;h S( El S 024 \54 900W 890W 88GW 87GW 86W 85
2.2.3 CTD Perfornance and Data Return

Over ninety-eight percent of the planned CTD stations were completed

during the two-year field program. In addition, on occasion, some

stations were sampled twice during a particular cruise. All but two of

the unsarnpled stations were lost due to the passage of Tropical Storm

Danny during the July 1997 cruise when Stations 66-72 along the 90 meter

isobath section were not sampled. The remaining two unsampled stations

were due to time/redundancy considerations (Station 75 in November 1997)

and termination problems (Station 6 in March 1999) . The actual useful

data return was reduced by a pump plumbing problem on the CTD fish

during the December 1998 cruise which caused the conductivity data, and

subsequently the calculated salinity data, while profiling, to be noisy.

These salinity data were discarded as it was not possible to smooth and

adjust the data with any confidence that the adjustments were valid at

shallower depths, particularly near the shelf break. Table 2.2-1

summarizes the CTD data return.

Table 2.2-1. Summary of CTD data collected during the DeSoto Canyon

Eddy Intrusion Study.

* Tropical Storm Danny prevented completion of CTD Stations 66-72 along

the 90 meter isobath.

** Problem with CTD pump plumbing resulting in noisy salinity data.

Temperature data ok. Salinity data discarded.

2.3 XBT Data

2.3.1 Introduction

XBT data were collected in conjunction with two Feature Surveys

conducted on 20-22 November 1997 and lO-ll December 1998. In November

9

Crtiise Dates, - cTZ Statons áP]&

PE 9722 03/18/97 - 03/28/97 75 75

PE 9803 07/08/97 - 07/19/97 75 69*

PE 9820 11/10/97 - 11/22/97 80 79

PE 9830 03/31/98 - 04/10/98 80 81

PE 9908 08/03/98 - 08/14/98 80 81

PE 9923 12/01/98 - 12/13/98 87 93**

PE 9932 03/29/99 - 04/06/99 80 79

TOTALS 03/18/97 - 04/06/99 557 557
1997, eighteen T-7 XBT5 were deployed along two east-west sections

offshore of and parallel to the lOOm sections (Stations 101-109 and ill119) are shown in Figure 2.3-1. In December 1998, twenty-one XBT

stations were sampled along three transits of an east-west section

across the mouth of the DeSoto Canyon arid repeated at approximately 12-

hour intervals at the same station locations as shown for CTD Stations

81-87. This section is also shown in Figure 2.3-1.

2.3.2 XBT Data Acciuisition System

Data from T-7, XBTs were collected using a Sippicari LM-3A handheld

launcher and a Sippican MK-l2 Oceanographic Data Acquisition System.

All probes were manufactured by Sippican.

2.3.3 XBP Performance and Data Return

Fifteen of eighteen XBT5 provided useful data during the November 1997

cruise, but only fifteen of twenty-six drops (from probes deployed at

twenty-one stations) provided useful data in December 1998. The poor

data return in December 1998 was due to the receipt of defective probes

from the manufacturer. The insulating film on the signal wire from the

probe to the data logger was of poor quality and did not provide a

reliable insulation against shorting when the wire came in contact with

the ocean. These probes had been received in March.

2.4 Vessel-Mounted ADCP Data

2.4.1 Introduction

Underway ADCP data were collected as part of the two Feature Surveys

(approximately 24 hours on 20-22 November 1997, and approximately 48

hours on 10-11 December 1998) . The November 1997 survey consisted of

three east-west sections crossing the A-Line and B-Line moorings, and

the December 1998 survey consisted of four transits of a triangular grid

at the mouth of the DeSoto Canyon (see Figure 23-1) . This second

survey was 48 hours long instead of 24 hours as it combined the time of

two separate surveys into one longer one. An April 1998 Feature Survey

was not done as available satellite imagery gave no indication of

anything of interest1 so the saved time was added to the December 1998

survey.

2.4.2 Vessel-Mounted ADCP Data .ccnmisition System

The ADCP data acquisition system was provided by LUNCON and was operated

by their technical staff during each cruise aboard the R/V PELICAN. The

system consisted of two narrowband RDI hull-mounted VN-ADCP systems (150

Hz and 600 KHz) configured so that both could operate at the same time.

DGPS navigation data were input from the vessels navigation system and

ADCP data were processed using RDI Transect software. The data

acquisition setup for each instrument is presented in Table 24-l.

10
R/V PEL!CAN PE-9820 9OW 890W 33 O°N 29°N1 2 8'N9 O°W Feature Survey I / 20-22 November 1997 89GW 11/20/1997 TO 12/11/199

88GW 87GW 86°W 88GW C1ó163 D 81 XX )y'x87 Feature Survey 2 10-11 December 1998 87GW 

8 6°W 

Figure 2.3-1. Map showing the two ADCP/XBT Feature Surveys conducted in Novemb

1997 and December 1998 with nearby mooring locations.
Table 2.4-1. Data acq-uisition setup for vessel-mounted ADCPs used

during the DeSoto Canyon Eddy Intrusion Study Feature

Surveys -

2.4.3 Vessel-Mounted .ADCP Performance and Data Return

Data were collected from both ADCPS simultaneously during the two

Feature Surveys. This amounted to a total of approximately 72 hours of

continuous operations during which the data were displayed on the

monitor and stored as 5-minute averages. Due to the greater range of

the 150 KHz ADCP, which permitted bottom track for more extended periods

during the surveys, only the 150 1KHz data were considered for later

processing.

2.5 Moored Instrument Data

2.5.1 Introduction

Moored current, temperature, conductivity and pressure measurements were

made from thirteen mooring sites in the study area along the 100, 200,

500 and 1300m isobaths. These sites were previously identified in

Figure 2.2-1. Measurements were made continuously from March 1997 to

April 1999 with six mooring deployments at approximately four-month

intervals (see Table 2.5-1) - Table 2.5-1. Approximate mooring deployment periods for the DeSoto

Canyon Eddy Intrusion Study.

The overlap in dates from the end of one deployment to the beginning

of the next is due to the actual time the last and first moorings were

actually deployed and recovered during each cruise.

Except Mooring Bl (03/27/97 - 08/22/97) and Mooring Dl (03/25/97 -

08/21/97)

The mooring configurations remained unchanged for the entirety of the

study except that a Trawl Resistant Bottom Mount (TREM) at Mooring Dl

was replaced in August 1997 with an inline mooring configuration similar

to that at the other lOOm sites. Also, at various times, the level of

the upper measurements of conductivity and temperature at the lOOm Sites

moved up or down (between 62 and 20m) depending on the availability of a

12

- iO 6ØO.

Time Between Pings (seconds) . 0.65 0.16

Depth Cell Length (meters) 8 4

Transmit Pulse Length (meters) 8 4 Blanking (meters) 4 4 Number of Depth Cells 40 40

Number of Pings Per Ensemble 10 10

- pifle epQ inba .. ... - 1 (March 1997) 03/19/97 - 07/17/97**

2 (July 1997) 07/09/97 - 11/20/97

3 (November 1997) 11/13/97 - 04/08/98

4 (April 1998) 04/02/98 - 08/12/98

5 (August 1998) 08/05/98 - 12/12/98

6 (December 1998) 12/03/98 - 04/06/99
surface marker buoy for that site. This was because the Coast Guard

required surface markers for all subsurface moorings extending to within

200 feet (Glm) of the surface. Three of the five lOOm sites (El, Cl and

El) were initially configured with surface markers and the El marker was

shifted to the Al site in August 1998. Also, in August 1998, the 20m

level on the Al and Cl moorings was shifted upward to 16m at the request

of the Review Board and program Principal Investigators (PIs).

2.5.2 Moored Xnstrumentation

The moorings consisted of a variety of current measuring instruments.

These included Aanderaa RCM-7's and 8's, General Oceanics Mk2s,

InterOcean S4s, and RD Instruments 300 kHz Workhorse and 150 kHz

Narrowband ADCP5. Additional measurements were made using Hu.grdn Seamon

Mini Temperature Recorders and SeaBird MicroCat and SeaCat Conductivity

and Temperature Recorders. The Aanderaa, General Oceanics and

InterOcean instruments were outfitted with conductivity sensors and

some, but not all, of the Aanderaa, General Oceanics and SeaBird

instruments were outfitted with pressure.

As a general rule (by the end of the field program), Aanderaa current

meters were deployed at the 200 and 300 meter levels on the 500m

moorings (A2, B2, C2, and D2) and at 500 meters on the 1300m moorings

(A3, B3, and C3); General Oceanics current meters were deployed

approximately lOm above bottom on the 500m and l300m moorings, and

InterOcean current meters were deployed approximately six meters above

bottom on the lOOm moorings (Al, El, Cl, Dl, and El) . RD Instruments

Workhorse ADCPS were deployed at 80m or 90m depth on each of the above

moorings and the 150 kHz Narrowband ADCP was deployed at 180m depth on

the 200m D9 mooring. Hugrthi Mini Temperature Recorders were deployed at

62, 150 and 250m depths on the 500m moorings and the SeaBird

Conductivity and Temperature Recorders were deployed at l6m (or 20m),

and/or 62m and/or 82m depth on the lOOm moorings. Table 2.5-2

summarizes this information for the last four-month deployment period.

2.5.3 Moored Instrument Performance and Data Return

A total of 322 instrument deployments were made over the course of the

two-year field program as approximately 54 instruments were deployed or

rotated every four months. The total good data' data return was

approximately 95.6% This return was calculated based on the maximum

number of data points expected for the various type instruments at their

respective settings. However, since an ADCP generally works or doesn't

work, the data return for these instruments reflects only that 'good

data' were obtained for at least one level, though up to 19 or 20 levels

may have provided useful data.

Initially, no data were obtained from the April-August 1998 deployment

of the Bl Mooring as it was lost in its entirety, except that the

surface marker buoy and one of the subsurface steel buoys were found

13
Table 2.5-2. Mooring instrumentation and deployment levels during the

last deployment period (Lecernber 1998 - April 1999) of

the DeSoto Canyon Eddy Instrusion Study.

adrift in the Gulf of Mexico and recovered by other parties in July and

October 1998, respectively. Surprisingly, in January 2000, some 16

months after this mooring was verified as missing, its bottom steel buoy

element and an S4 current meter were recovered off Key Largo, FL.

Approximately 2.5 months of useful data were recovered. from this

instrument. Also, in March 1999, about one month before the end of the

field program, a replacement for this same 31 Mooring was trawled up (in

14

-: -

f

- ø) 1r,- -EPTI (14} (SJ --",N; : 29°14.521'N 100 16 C/T (1719)

88°29.068'W 20 511(2 (452) 80 ADCP (214)

94 84 (08161753)

A2 2903.565'N 500 62 Mini TR (C929)

88°23.352'w 90 ADCP (197)

150 Mini TP. (C932)

200 RCM-7 (10350)

250 Mini TR (C933)

300 RCM-7 (6892)

490 S4 (08161758)

A3 28°46.051'N 1300 80 ADCP (196)

88°17.327'W 500 RCM-8 (10533)

1290 MK2 (447)

B1 29'20.565'19 100 62 RCM-7 (9950)

87°54.539'W 94 114 (07801745)

B2 29°12.720'N 500 62 Mini TR (C937)

87°52.279W 90 A]DCP (206)

150 Mini TR (c939)

200 RCM-7 (9524)

250 Mini TR (C940)

300 RCM-7 (9525)

490 51(2 (443)

B3 2904285'N 1300 80 ADCP (211)

8751.S02W 500 RCM-5/8 (7582)

1290 511(2 (450)

Cl 29°35157'N 100 16 C/T/D (0059)

872O.993W 80 ADCP (200)

94 S4 (08161755)

C2 29°22.309N 500 62 Mini TR (C946)

87°21.412W 90 ADCP (224)

150 Mini TP. (C947)

200 RCM-7 (9949)

250 Mini TR (C950)

300 RCSI-5/8 (7528)

490 552 (453)

C3 29°00327'N 1300 80 ADCP (207)

8721.260'W 8CM-4/7 (6922)

1290 141(2 (444)

ID1 30°04.128'N 100 62 C/T/D (0057)

86°50.507'w 80 ADCP (209)

94 84 (08111746)

112 29°20.144N 500 62 Mini TR (C919)

8651.149'w 90 ADCP (212)

150 Mini TF. (C959)

200 RCM-7 (10881)

250 Mini TP. (C960)

300 RCM-7 (9948)

490 511(2 (457)

119 29°42.450N 200 180 ADCP (322)

8650 .81914

El 29°42.001'N 100 20 c/T (1720)

86°19-734'W 80 ALICP (213)

94 84 (08161757)
its entirety, including the anchor) by a commercial shrimping vesel.

In this latter case, these instruments and the corresponding data were

retrieved a few months after the field program was concluded.

Approximately 22.5% of the 4.4% data loss, was due to the missing Bl

Mooring. The remaining data loss was due to various instrument

malfunctions including defective DSU5 (3), fouled or broken rotors (2),

defective tilt channels (2), stuck wings (3), leaks (1), and battery

failures (2). The 72 Hugrün temperature recorder deployments provided

100% data return as did 75 of 76 RDI ADCP deployments, the data loss

here being due to the loss of the WorkHorse ADCP at the 31 Mooring site.

The worst instrument data return (89.3%) was for the InterOcean S4

current meter where two units had battery pack failures early in their

deployments and only 2.5 months (out of four months) of useful data were

obtained from the 84 recovered in January 2000 from the missing AprilAugust 1998 deployment of the 31 Mooring. Table 2.5-3 provides a

complete summary of the data return for all of the instruments deployed

during this program, and Figure 2.5-la,b provides a time line of the

data return by instrument level on each mooring.

2.6 GeoSat Altimeter Data

Since 1993, tandem sampling of the gulf of Mexico by satellite

altimeters has allowed continuous mapping of the mesoscale circulation

in the region at unprecedented resolution. This operational capability

has been used during the DeSoto Canyon field program to monitor the Loop

Current and Loop Current intrusions into the eastern Gulf of Mexico. The

altimeter missions and the data acquired for this study are described

below.

2..6.i. TOPEX/POSEIDON

TOPEX/POSEIDON(T/P) is a joint IJ.S.-France satellite mission managed in

partnership by the National Aeronautics and Space Administration (NASA)

and the Centre National dEtudes Spatiales (CNES) . The TIP satellite was

launched aboard an Ariane 42P launch vehicle on August 10, 1992 from

Kourou, French Guiana. Two altimeters are carried aboard the satellite

but only one is active at any one time because of on-board sharing of

the altimeter antenna. The primary instrument is the dual-frequency NASA

altimeter, commonly referred to as TOPEX. TOPEX sends microwave pulses

to the ocean surface at two frequencies so that a correction can be made

to the altimeter range to account for the delay caused by free electrons

in the ionosphere. The CNES altimeter, commonly referred to as POSEIDON,

is a single freqvency, solid-state, low-power, low-mass sensor. The

antenna sharing agreement between NASA and CNES provides for the

POSEIDON altimeter to be turned on approximately 10% of the time. The

current operational procedure is to turn on POSEIDON for a complete

cycle (10 days) approximately every 10 cycles, though not exactly every

10 cycles to prevent aliasing problems.

After an initial assessment phase (Table 2.6-1), the satellite was

placed into a 10-day exact repeat orbit in late September 1992 to begin

operational sampling of the ocean and has remained in this orbit through

15
Figure 25-la. Timeline of data return by instrument level for the

indicated DeSoto Canyon Eddy Intrusion Study moorings.

Arrows indicate the approximate dates of the

hydrographic cruises.

16

Measurement Timeline

DeSoto Canyon Eddy Intrusion Study

I I I I

I I I I I I j J I I 4 Not Deployed DSC-At-0 16r CTD/CT F -I F DSC-AI-9 2ee NotlJeployed - D5C-AI-I 80'n A)CP

I 41- 1 1. 11- I -

050A1 -2 S4 I: 1

' Battery Failure Bottety Failure 1 000-A2-1 62,, TEMP F DSC-A2-2 ODe, AtICP

1-

I 05C-A2-3 150,, TE0oO' DSC-A2-4 ZOOm RCM 1. 4 Bad 1)50 1- 1 05C-A2'-S 250,, TEMP. -

050-A2-5 300'e 10CM I- I ttSC-A2---7 49Dm M1t2/54 1- Leak

1- - 1 050-63-1 at" ADCP -

I 050A3-2 500re M1I2/RCM 1- 1 050-63-3 1310,,, IOCM/1v102 Wiug Stuck

1- 0 0 No' Deployed -

I ( Lot, Not Deploye.l DSC-Bt-1 25,,, CTD 4

500-91-9 croIMl21 62m FICM 4 Not Deployed - Not Deployed

I- 1- I 050-91-2 80n, ADCP Lost Not DeplOyd -

1- DSC-8l-3 52re CTICTD F Lost Not Deployd -

I 4 9 Foaled -_ 550-91-4 I 95rn RCMJS4 Rater 4 Lost I

-I 050-62-1 62,, TEMP - 550-95-2 OOrn ADCP 1- 4 Dsc-92-3 tue,, TEMP F -1 550-92-4 200,,, RCM 1- -I 500-92-5 250', TEMP F

-I 550-62-6 30Dm RCM 1- I

050-92-7 4Mm Mk2 1- 4 050-93.-I Sore ADO'

1- I 150-63-2 50Dm MJt2/RCM 1- 11- Wl., , I OSC-93-3 1290,,, RCMIM102 - WingStuck

I I

1- I ss,,

I I I I I I i I

I

r i i i I I M AM J JASON 0 J F MA M J JASON 0 JF MA

1997 1998 1999 + +

)f.
17

Measurement Timeline

DeSoto Canyon Eddy Intrusion Study

I I I I I I I I I J I I F

I I j I I i I Not Depoyed -

5CC-Cl-C 1Cm Clii Not Deptoyed 4

0CC-Cl-I 2Cm Clii I- 0CC-Cl-C San ADCP I- DCC-C1-3 02 CT I- 1NotDeployed -

DSC-Cl-4 9Cm RCMJS4 Bod DSU - r r -I

I DCC-C2-1 0Cm TEMP F- DCC-C2-2 8Cr,, AlICE

i =

DCD-C2-3 15Cm TEtiCli F-- DSC-C2-4 200,,, RCM

Bod =

I -

DSC-C2-5 3COm TEIvIF1 F DCC-C2-5 30Cm RCM I osu I- I OCC-C2-7 490,,, tvDtZ

- I 000-03-I 0Cm ADCP I. - I -

DCC-CC-2 SOOn, Mk2JICCM - ReoordiogClmnnelI 000-03-3 129Cm RCMjEItc2

NolDeploved =

0CC-Ct-i 62,,, 10:2/COD Not DipIoyed> tr-1

000-Dl--2 COre 25C-D1-3 95rr 452/54 i

Not Deployed

1

I

0CC-Cl -4 COre ADCP I- I OCC-02t 62m rMi 1 DCCt)2-2 COrn AlICE =

000-02-3 10Dm TEMP I I- 000-02-4 20Cm RCM II tlCC-02-5 250,,, TEIoER F CCC-02-6 300,,, Mk2/RCM j Bad Channels =

OCC-02--7 48Cm Mk2 - 250-09-1 18Cm ADCP

F- NtDeployed

Nol Deployed

i =

I --

0CC-El-i 2Cm OSC-El -9 62m COlD Not Deployed 4 I- I 0CC-El -2 00,,,

- Not Dop)Oyedi =

0CC-El -3 02,,, CT F- Cktn I DCC-EI-4 RCMJS4 ! °°

I I I I _I I F I I I I I i =

I I I I I I I HAM J J A SO ND J F HAM

+ +

J J 1993 A SO 140 JF 1.1 1599 A

Figure 2.5-lb. Timeline of data return by instrument level on

indicated moorings for the DeSoto Canyon Eddy Intrusion

Study. Arrows indicate the approximate dates of the

hydrograpliic cruises.
Table 2.5-3. Moored instrument data return (by good record count)

during the reSoto Canyon Eddy Intrusion Study.

Grand TOTAL 3,554,636/3,717,857 = 95.61%

* All ADCP levels for each instrument counted as one (1) time series record.

(#) = Nuxnjjer of Instniinents

the entire mission to date. The primary science objective of the T/P

mission is to observe accurately basin-scale to global-scale circulation

variability. The operational orbit, which repeats every 10 days along

the ground track (Figure 2.6-1), does not result in sufficiently dense

spatial sampling for accurate mapping of mesoscale circulation

variability. As a result, data from TOPEX must be combined with

altimeter data from other satellites for mesoscale circulation

monitoring.

2.6.2 ERS-l&2

The first and second European Remote-Sensing (ERS) satellites, ERS-1 and

ERS-2, were developed by the European Space Agency (ESA) as a sequence

of multidisciplinary earth observing satellites devoted to remote

sensing from a polar orbit. These two satellites are nearly identical

and have identical payload instruments including satellite altimeters.

The only difference in the payloads is an ozone mapping instrument that

was added to ERS-2, which was not available on ERS-l. By flying a

sequence of identical instruments, ESA has been able to cross-calibrate

and extend a very useful time series of global remote sensing

observations. Only ERS-2 is currently operational. ERS-1 was placed in

18

Aaxi4eraa GeAe3aL Qdeanios 2

- ;- 1 223565/246355 (13) 74452/75359 (10) 64038/64038 (12)

2 209215/216395 (11) 73778/83631 (9) 72120/72120 (12)

3 256274/274213 (12) 63484/63484 (6) 80267/80267 (12)

4 213975/228239 (11) 63045/66049 (7) 72117/72117 (12)

5 209905/216469 (11) 66097/74206 (8) 68389/68389 (12)

6 214273/214273 (12) 50594/61821 (7) 67206/67206 (12)

TOTALS 1327207/1395944 (70) 391450/424550 (47) 424137/424137 (72)

Percent

Good

95.08% 92.20% 100%

Dep1o'ment

XntGrQea* i 84 - -- Sea-B].rd

NicxoCat/Seacat

RD Instrumen1s ..:'- .. 1 7977/7977 (1) 49905/49905 (6) 146571/146571 (13)

2 24532/24532 (3) 58932/60944 (5) 148537/148537 (13)

3 49692/49692 (5) 80317/80317 (6) 173190/173190 (13)

4 32817/44628 (5) 48181/72181 (6) 143576/155576 (13)

5 34515/42366 (5) 52993/52993 (5) 135942/135942 (12)

6 44458/48168 (6) 44927/44927 (4) 134780/134780 (12)

TOTALS 193991/217363 (25) 335255/361267 (32) 882596/894596 (76)

Percent

Good

89.25% 92.80% 98.66%
Figure 2.6-1. TOPEX/POSEIDON ground track over the Gulf of Mexico from

the operational 10-day exact repeat orbit.

19
a safe-hold state in May 1995 and served as a backup satellite until it

failed in January 2000.

Table 2.6-1. TOPEX/POSEIDON Mission Phases

ERS-1 was launched on July 17, 1991. Several observational periods,

called "mission phases", were flown from orbits designed to meet the

multidisciplinary objectives of the mission and optimize the scientific

usefulness of the data collected (Table 2.6-2). Only the 3-day repeat

orbits used during the commissioning and ice mission phases are

unsuitable for mesoscale mapping. Both the multidisciplinary and

geodetic phases provide altimeter data at sufficiently dense spatial

sampling to be useful for mesoscale monitoring. During the

multidisciplinary mission phases, both ERS-1 and ERS-2 sample along the

35-day repeat ground track shown in Figure 2.6-2. The 168-day repeat

orbits flown during the geodetic phases can also be exploited for

mesoscale mapping by using the 37-day near-repeat cycles within this

orbit. An example of this spatial sampling over the Gulf of Mexico is

shown in the ground track/coverage map for ERS-1 during the second cycle

of the first geodetic phase (Figure 2.6-3)

ERS-2 was launched on April 21, 1995 and immediately placed into the 35-

day repeat orbit used for the multidisciplinary phases of the ERS

missions (Table 2.6-3). The satellite has remained in this orbit

throughout the entire mission, which simplifies the processing of ERS-2

altimeter data.

2.6.3 Data Sources

TOPEX data are distributed by the Physical Oceanography Distributed

Active Archive Center (PO.DAAC) at the Jet Propulsion Laboratory. ERS

data are distributed by Centre ERS d'Archivage et de

Traitement" (CERSAT), the French Processing and Archiving Facility for

ERS-1 and ERS-2.

20

Missifl Phse flurati*n eiod ' Dosaipoj Maneuver 8/10/92 Initial phase to maneuver into

to operational orbit.

9/23/92

Assessment 8/10/92 10-day Assessment phase to evaluate the

to performance of the satellite.

12/31/92

Primary and 1/1/93 10-day Primary and extended mission

Extended

Missions

to EOM phase dedicated to collection of

an extended time series of

altimeter data from the

TOPEX/POSEIDON operational 10-

day repeat orbit.
Figure 2.6-2. ERS-1 and ERS-2 ground track over the Gulf of Mexico from

the 35-day exact repeat orbit used during

multidisciplinary mission phases. ERS-2 was in this

orbit during the entire DeSoto Canyon field program.

21
Table 2.6-2. ERS-1 Mission Phases

Table 2.6-3. ERS-2 Mission Phases

2..7 Ancillary Data

2.7.1 Introduction

A number of ancillary data sets are available from other programs

coincident in time with the DeSoto Canyon Eddy Intrusion Study. The

applicable data from these programs are identified below.

22

X.ssLón P1jse.' I Vuratiou

Reea I Desrition :j

7/31/91 Phase dedicated to observing very

Commissioning to 3-day frequently the calibration sites.

12/20/91

12/28/91 Phase dedicated to observing

Ice 1 to 3-day specific ice zones.

3/30/92

Multidisciplinary phase dedicated

4/14/92 to satisfying most of the

Multidisciplinary 1 to 35-day applications including land/ice

12/20/93 mapping with SAR and ocean

circulation mapping with

altimetry.

12/23/93 Phase dedicated to resampling ice

Ice 2 to 3-day zones after two years.

3/10/94

4/10/94 Phase dedicated to providing very

Geodetic 1 to 168-daydense spatial observations from

9/27/94 the altimeter for mapping the

geold.

Second phase of geoid mapping with

Geodetic 2 168-daythe 168-day ground track shifted

3'21/95

/ longitudinally by 8 km to doublespatial resolution.

3/21/95 Final phase to perform

Multidisciplinary 2 to 35-day multidisciplinary mission and

5/16/96 permit cross-calibration and

tandem operation with ERS-2.

Miss.ofl 1aSe ... .epeat Durato er'.od

- - Commissioning

5/1/95

to

11/1/95

35-day

Phase dedicated to commissioning

satellite.

Tandem

11/1/95

to

5/15/96

35-day Phase flown in tandem with ERS-1

for tandem SAR

Multidisciplinary

5/16/96

to

EOM

35-day

Phase dedicated to performing

multidisciplinary mission. The

satellite is expected to remain in

the 35-day repeat orbit to the end.

of mission (EUM) -
Figure 2.6-3. ERS-1 ground track showing data coverage for the first

37-day near repeat from the 168-day exact repeat orbit

used during the ERS-1 Geodetic 1 mission phase.

23
2.7.2 icil1ary Data - SCTJLP drifters

The Surface Current and Lagrangian-drift Program (SCULP) was developed

to improve the understanding of ocean circulation over continental

margins in the northern Gulf of Nexico. Drifters deployed during SCULP

are similar in design to those used during the U.S. Coastal Dynamics

Experiment (Davis, 1985) . The drifters are composed of four rectangular

vanes (50 cm wide, 90 cm tall) extending radially from a thin,

vertically oriented, tube held in place at a depth of 0.5 meters by four

small surface floats. The drifters are initially packaged in soluble

cardboard boxes with attached parachutes and dropped from aircraft.

Continuous position transmissions are through service ARGOS for a period

of 90 days. Drifter slip has previously been observed to be less than 3

cm/s (Davis, 1985) . The data were collected and processed by Carter

Ohlmann and Peter Niiler of the Scripps Institution of Oceanography.

The SCULP field program consists of three distinct segments identified

here as SCTJLP-I, -II, and -III. The SCTJLP-I drifters were deployed

primarily at 15 stations (distributed as a 3 by 5 grid) within a 125-km

square on the Louisiana-Texas shelf from October 1993 through July 1994.

The grid was reinitialized weekly for the first three months, then

biweekly for three months, and finally monthly, giving roughly one year

of data. The SCTJLP-II drifters were deployed within a -400 by 150 km

rectangle on the northwest Florida shelf to investigate cross-shore

flows. The initial SCTJLP-II deployment occurred in February 1996, and

consisted of 15 units- The grid was reinitialized every two weeks for

roughly a year. The SCTJLP-III study was specifically concerned with

eddies on the shelf-rise of the Louisiana and north Florida coasts. Four

deployments of 20 drifters seeded the edge of warm eddies identified

with remotely sensed sea-surface temperature (AVHRR) and the near realtime TIP and ERS-2 SSI-1 data. Drifters were released during April 1998,

on the Louisiana shelf, and during July 1998, on the continental margin

south of the Florida-Louisiana border.

Drifter positions, recorded at various times throughout each day, were

ordered in time and edited by eliminating displacements requiring

velocities greater than 250 cm/sec. Data points recorded within 15

minutes were averaged together to eliminate erroneous displacements

associated with observations by multiple satellites. The data were then

interpolated onto a uniform time grid and daily averaged values were

computed. This was done by calculating an analytic correlation function

for each day from the Fourier transform of a model spectrum obtained

from ten days of unequally spaced observations centered on the day of

interest (Van Meurs, 1995) . Each correlation function was then used to

produce an interpolated location time series. Interpolated values were

sub-sampled every three hours and daily averaged velocities calculated.

Tidal and inertial signals are effectively removed in the daily

averages.

24
2.7.2.1 Argos-Tracked Thifting Buoys

Thirty-two Davis-type Argos drifters were deployed in the study area

between March 1997 and December 1998 as part of an internal MMS-funded

oil spill model verification effort. The deployments were made in

conjunction with scheduled cruises of the MMS-sponsored NEGOM Chemical

Oceanography and Hydrography Study and the present DeSoto Canyon Eddy

Intrusion Study. These data are identified in Table 2.7-1 and a

composite of all of the Davis-type drifter tracks is shown in Figiire

2.7-1. Additional oil-following drifters were deployed as part of this

internal MMS effort, but these data are not considered useful to the

present study.

Table 2.7-1. MMS Davis-type Argos drifting buoys deployed during the

DeSoto Canyon Eddy Intrusion Study.

2.7.3 GulfCet II Study

Two MMS/TJSGS-sponsored cruises were conducted in the study area in MayJune and August 1997 (OREGON II Cruise 225, Leg 3 and GYRE Cruise 97G08, respectively) . These cruises were made as part of an effort to

25

Dxi.fter Xfl begin Eatö End at No, Zrt.rg

14709 03/24/97 04/23/97 30

14701 07/13/97 08/10/97 28

14702D 07/13/97 08/10/97 28

14703 07/13/97 07/18/97 28

14710 11/16/97 12/15/97 29

14711 11/16/97 12/14/97 28

17445 11/25/97 12/25/97 30

18814 11/22/97 02/19/98 89

18840 11/24/97 01/27/98 64

20505 11/24/97 03/30/98 126

20011 05/11/98 08/20/98 101

20012 05/12/98 08/19/98 99

20013 05/12/98 09/29/98 140

20014 05/16/98 08/12/98 88

20015D 05/15/98 09/23/98 131

20016 05/13/98 08/26/98 105

20017 05/13/98 09/22/98 132

20018 05/14/98 09/23/98 132

20019 05/15/98 08/11/98 88

11887 08/05/98 09/04/98 30

11888 08/03/98 09/02/98 30

11889 08/01/98 08/31/98 30

11890 08/02/98 09/01/98 30

11895 08/03/98 09/02/98 30

11896 08/05/98 09/04/98 30

11897 08/05/98 09/03/98 29

11898 08/04/98 09/03/98 30

20562 08/03/98 09/03/98 31

09700 11/21/98 12/21/98 30

09702 11/22/98 12/22/98 30

12061 11/21/98 04/01/99 131

20019D 11/23/98 01/06/99 44
Figure 2.7-1. Composite track plot of all Davis-type ARGOS drifters deployed by MM

contractors in the study area during the DeSoto Canyon Eddy Intrusion 

31°N 30 N 29 N :::i 26°N0 ::: 23°Nglow11887 11888 90°W 11889 89°W 11890 58°W 11895 87°W 1189686°W118 85°W 84°W 3/24/1997 TO 4/ 1/1999 83°W 82°W siew 80°W3D 

29 

26°2°N '23°N / I AY%i %, ç0 I I 91°W 90°W 89°W 88°W 87°W 86°W 85°W 64°W 63°W 82°W 81GW 8O°W
assess the effects of oil and gas exploration and production on the

distribution and abundance of marine mammals in the Northern Gulf of

Mexico. Table 2.7-2 summarizes this hydrographic data collection.

Table 2.7-2. MMS/USGS GulfCet II cruises.

2.7.4 Northeastern Gulf of Mexico Chemical Oceanocrraphy and

Hydroqraphy Study

As part of a separate MMS-supported study of chemical and hydropgraphic

conditions in the NEGOM being conducted by Texas A&M University, four

chemical oceanography and hydrography cruises were conducted in the

study area in November 1997 and May, July-August and November 1998.

These cruises included a sampling grid of CTD and XBT stations. Many of

the sections coincided with the same sections sampled as part of the

DeSoto Canyon Eddy Intrusion Study, except that they extended much

further up onto the shelf, but offshore only out to the 1000 meter

isobath. In addition, underway thermosalinograph and ADCP data were

collected during each cruise. Table 2.7-3 summarizes the data

collection and Figure 2.7-2 shows the hydrographic grid for the CTD

casts. XBT Stations (not shown) were generally made between most, but

not all, of the CTD stations.

Table 2.7-3. MvXS Chemical Oceanography and Hydrography cruises.

2.7.5 NEGOM Marine Ecosystem Proqrain: MS III

As part of the MANES III being conducted by Continental Shelf Associates

and Texas A&M University, four sites (1, 4, 5, and 9) along the

Mississippi/Alabama shelf break were instrumented with near bottom

current meters (4 and 16m above the bottom) from May 1997 to July 1999.

These moorings were maintained near features (pinnacles) of varying

heights rising up Out of the bottom. Table 2.7-4 lists the mooring

locations, CTD casts were also made at these four mooring sites during

each deployment/rotation cruise.

27

--

1

I

--pates

- :1l,

I 4 Cruise ID T

J

I XBTy

I 1

I ' OREGON 11-225 05/29/97 06/09/97 4 77

970-08 08/06/97 08/21/97 12 169

Cruise ID

H Dates Cs H1'

1 XBTs

I 'I

I trnezay Tlierinosalingrapi

II &

Ni 11/16/97 11/27/97 96 79 Yes

N2 05/04/98 05/15/98 99 96 Yes

N3 07/25/98 08/09/98 100 102 Yes

N4 11/12/98 11/25/98 99 114 Yes
Texas A&M CTD Stations Nov 98 9OW 89°W 88°W 870W 31°N 3 ON 2 9N 2 8N 2 7N 8 6°W 8 5OW 8 4W 83°W 

Figure 2.7-2. Map showing CTD station locations for the indicated cruise on the MM

NE Gulf of Mexico Chemical Oceanography and Hydrography Study being Texas A&M University with DeSoto moorings locations suuperimposed. 9 00W 8 9W 8 8°W 8 7W 8 6°W 8 5°W
Taizle 2.7-4. Northeastern Gulf of Mexico Coastal Marine and Ecosystem

Program; Ecosystem Monitoring Mississippi/Alabama Shelf moorings.

= Aanderaa RCM current mater

OEI Oregon Environmental Instruments current meter

2.7.6 Meteoroloqical Data

Meteorological data came from the National Data Buoy Center (NDBC)

There were two types of stations used, the shore/platform based C-MAN

stations and the moored 3-meter discus buoys.

2.7.6.1 C-MAN

Six coastal C-MAN stations were available to provide data in the NEGOM

region. They are listed in Table 2.7-5. All stations reported wind

speed and direction, air temperature and atmospheric pressure. The

sensor height above mean sea level varied from station to station.

Grand Isle, LA also reported water level data.

Table 2.7-5. C-MAN stations used during DeSoto Canyon Eddy Intrusion

Study.

29

-

, rjpe I Lati.tude 1 :-c

- : Iude pxmate Water Deth

l

tnstz1ane11

DStanc

Above

1A (A/OEI) 29°26.479'N 87°34322'W 75m (16m/4m)

lB (AA/OEI) 29°26.113'N 87°34.305'w 80m (16m/4in)

1C (AA/OEI) 29°26.243'N 87°34.79l'W 80m (16m/4m)

4 (AA/OEI) 29°l9.860'N 87°45.360'W 115m (16m/4m)

5 (AA/OEI) 29°23.516'N 87°58.660'W 80m (16m/4m)

9 (AA/OEI) 29°14.415'N 88°l9.388'w 95m (16m/4m)

Stati,on Name

-, Stat.ouXD

Sensor eights above mean sea lev'e3.

Ariemuster

- 2Lr

Temperature Pressure,

Cape San Bias, FL CSBF1 11.6 7.9 5.2

Keaton Eeach, FL KTNF1 11.8 10.9 4.8

Cedar Key, FL CDRF1 11.8 1O9 4.8

Dauphin Island, AL DPIA1 17.4 9.1 8.5

Southwest Pass, LA BLIRL1 30.5 11.9 12.5

Grand Isle, LA GDIL1 17.6 17.0 10.9
2.7.6.2 3-Meter Buoy

Four moored 3-meter discus buoy stations were used to provide offshore

meteorological and wave data in the NEGOM region (Table 2.7-6). All

stations reported wind speed and direction (5m), air temperature (5m),

sea surface temperature (-0.6m), atmospheric pressure (Om) and wave data

(height, average period, direction (only at 42007 and 42036) and

dominant period] - Wave directional data were only available for 42007

(OTP) and 42036 (W. Tampa) and both had short gaps. Wind data were

unavailable for 42007 (OTP) during Hurricane Georges in late-Septeiniber

1998. The anemometer apparently was damaged as the hurricane

approached. Station 42039 (Pensacola S.) had a two-month outage of all

data from early February to early March 1998, and its barometer data

never returned. Station 42040 (Mobile South) had a three-month outage

of its wind data from mid-November 1998 through mid-January 1999.

Table 2.7-6. Offshore meteorological buoys used during the DeSoto

Canyon Eddy Intrusion Study.

2.7.7 Water Level Data

Hourly water-level data were obtained from the National Ocean Service

(NOS) and the National Data Buoy Center (NDBC) - The stations are listed

in Table 2.7-7.

Table 2.7-7. Water-level stations used during the DeSoto Canyon Eddy

Intrusion Study.

At the start of this field program, seven water-level stations were

reporting data in the NEGOM region. By the end of the program, only

three were still active and two of those were located at Grand Isle, LA.

The NOS station at Grand Isle, LA was the only one that had a continuous

record for the study period.

30

tat XD.......... Pensacola S. 42039

W. Tampa 42036

Mobile South 42040

OTP 42007

$tatQz3Name Stat]on ID SQurceo Data

Cedar Key, FL (CE) 8727520 NOS

Panama City Beach, FL (PAl) 8729210 NOS

Panama City Beach, FL (PA2) 8735180 NOS

Dauphin Island, AL (DA) DPIA1 NDBC

South Pass, LA (SO) 8760551 NOS

Grand Isle, LA (GR1) 8761724 NOS

Grand Isle, LA (GR2) GDIL1 NDBC
2.7.8 River Runoff Data

River discharge data for eight rivers were obtained from the U.S. Army

Corps of Engineers (New Orleans District) and the U.S. Geological

Survey. These rivers are listed in Table 2.7-8. The Mississippi River

dominates the region for discharge rate.

Table 2.7-8. River discharge stations used during the DeSoto Canyon

Eddy Intrusion Study.

2.7.9 Satellite Imagery

Satellite sea surface temperature imagery was provided by the USGS via

an online collection of real time and archived images. Several images

per day in GeoTiff format were available through the following web site:

httj://coasta1.erusps.gov/east gulflhtmllsst,html - Each image had been calibrated and navigated. More information

concerning the processing procedures are available from this web site.

2.7.3.0 Iurricanes

Three hurricanes passed through the Desoto Canyon region during this

study. These were Hurricanes Danny, Earl and Georges (Table 2.7-9) -

Data on their position and intensity were obtained from Johns Hopkins

University, Applied Physics Laboratory.

Table 2.7-9. Hurricanes that passed through the study area during the

DeSoto Canyon Eddy Intrusion Study.

31

I - $tatinpmber -I$reof Daa

Ai'ea.

Mississippi (MI) Tarbert Landing USACOE 1,129,940

Atchafalaya (AT) 07381490 USGS 87,570

Bogue Chitto (BO) 02792000 USGS 1,213

Pearl (PE) 02489500 USGS 8,494

Pascagoula (PA) 02479310 USGS 8,204

Perdido (PR) 02376500 USGS 394

Apalachicola (AP) 02359230 USGS 19,500

Suwannee (STJ) 02323500 USGS 9,640

Lu na Speed 3e- Axea (m1z)

Danny 18-20 July 1997 60-80

Earl 2-3 September 1998 60-100

Georges 27-29 September 1998 105-110
III. DATA QUALITY ASSURANCE AND PROCESSING

3.]. Introduction

Data quality assurance and processing techniques are discussed in this

chapter. Where possible, detailed calibration and/or processing methods

are described. Applicable methods for ancillary data are presented from

the point in processing at which the data were obtained.

3..2 CTD Systems

3.2.1 CTD Sensor Calibration

Sea-Bird Electronics, Inc. CTD systems are designed in such a way as to

permit easy processing and reprocessing of the raw data with new

calibration coefficients. Subsequently, following a cruise, if the

sensor calibration is in question, or the calibration is rather old, the

raw data can be easily reprocessed with new calibration coefficients.

During this study, the Sea-Bird 911 Plus CTD sensors were calibrated at

the manufacturer's facilities on numerous occasions. The primary and/or

secondary temperature and conductivity sensors were calibrated in

December 1996, March 1997, February and May 1998, and again in

February/March 1999. In all cases, the conductivity and temperature

sensors were found to be within the manufacturer's specifications.

Pressure sensor calibrations were done in August 1996 and again in

January 1999. Table 3.2-1 summarizes the manufacturer's specifications.

Table 3.2-1. Manufacturer's specifications for SeaBird

Electronics,Inc., 911 Plus CTD.

As a further CTD calibration check, water samples were drawn from mixed

layers (where possible) for each cast for each cruise to determine a

separate calculated CTD salinity minus bottle salinity intercomparison.

These results (by cruise) for the 911 Plus CTD are presented in Table

3.2-2. The bottle salinities were run at Texas A&M University The

resulting data reveal that the calculated mean salinity differences for

these mixed-layer comparisons for each cruise (including the pump

plumbing plagued cruise of December 1998) ranged between +0.009 and -

0.014 psu. Subsequently, no additional processing or reprocessing of

the 911 Plus CTD data was required. Unfortunately, efforts to overcome

the December 1998 pump plumbing problem by various means of lagging and

offsetting the data were unsuccessful in eliminating the noisy salinity

signal. Subsequently, these salinity data were discarded. A Sea-Bird

SBE 19 SeaCat CTD was used, but only for a small number of casts

(Stations 12-15 and Station 41) during the last cruise (March-April

1999) when the 911 Plus CTD was unavailable due to wire termination

33

Sensor Initial cauracy (pér onth) Resà1iofl

Conductivity 0.003 mmho/cm 0002 niinho/cm 0.0004 mmho/cm

Tem.erature 0. 002C 0.0003 C 0.0002 2C

Pressure

10,000 psia

0.015% PS

(l.02N)

0.0015% FS

(0.lO2M)

0.001% FS
problems. The SeaCats specifications are presented in Table 3.2-3.

This instrument had previously been calibrated in January 1998, but was

post calibrated in May 1999. Prior to recalibration, a TS plot revealed

a significant offset in the data. Following processing and

recaljbratjon, the corrected data plot compared favorably with the TS

plot for the 911 Plus. This comparison is shown in Figure 3.2-1.

Table 3.2-2. Time (in months) of most recent calibration of 911 Plus

CTD sensors before and after each cruise during the

DeSoto Canyon Eddy Intrusion Study.

NA = NotAvailable

Table 3.2-3. Mixed layer 911 Plus CTD salinity minus Bottle salinity

comparisons by cruise during the DeSoto Canyon Eddy

Intrusion Study.

34

Date - 10, 000 ps.a unary

Temp

Pnay

Conduct

Secondaxy

Temp.

Seidar

03/18/97 -

03/28/97

7/22 3/11 3/11 0/11 0/11

07/08/97 - 11/18 7/7 7/7 4/7 4/7

07/19/97

11/11/97 - 15/14 8/NA 8/NA 8/3 8/3

11/22/97

03/31/98 - 20/9 1/NA 1/3 1/3 1/NA

04/11/98

08/04/98 - 24/5 3/6 3/7 1/6 1/7

08/15/98

12/01/98 - 28/1 7/2 7/3 5/2 5/3

12/13/98

03/30/99 - 2/NA 1/NA 1/NA 1/NA 1/NA

04/06 / 99

Cruise ± Dates ?A. (psti) St ev'. (pii

PB 9722 03/18/97 - 03/28/97 -0.014 ±0.007

PE 9803 07/08/97 07/19/97 -0.009 ±0.008

PE 9820 11/10/97 11/22/97 -0.007 ±0.008

PB 9830 03/31/98 - 04/10/98 -0.002 ±0.007

PE 9908 08/03/98 - 08/14/98 0.001 ±0.010

PE 9923 12/01/98 - 12/13/98 0.009 ±0013

PB 9932 03/29/99 - 04/06/99 -0.002 ±0.006
32.0 28.0 24 0 0Cw 20.0 .4-. 1C.. 16.0 E 12.0 0.0 4.033.0 34.0 35.0 Salinity (psu) 36.0 Salinity (psu) Figure 3.2-1. Final temperature/salinity (TIS) plots of CTD data from the Sea-Bird Sea

(left panel) and 911 Plus CTD (right panel) during the March-April 1999 

Pelican Cruise PE-9932 Sta. 12-15, 41 Pelican Cruise PE-9932 All 91
3.2.2 CTD Data Processing

Raw CTD data files were ingested into SAIC's Physical Oceanographic Data

Base Management (PODBM) system. In this database the type of station

(CTD or XBT) was identified as well as the latitude, longitude, depth of

cast and water depth for each cast. These data were then averaged to

one-meter levels and sigma-t, temperature and salinity data were plotted

versus depth for each cast to serve as an initial quality check.

Finally, a composite Temperature-Salinity (T-S) plot was produced for

each cruise.

3.3 XBT System

3..3.i. XBT Probe Calibration

The T-7 XBT probe is rated by the manufacturer as having a temperature

accuracy of ± 0.2 °C over the range -2 to +35 °C. No effort was made to

provide an independent calibration check of this accuracy, but CTD data

collected at the same location as XBT data (but not at the same time)

during the second Feature Survey in December 1998 ranged from 0.1°C to

0.7°C lower in temperature than the corresponding XBT data. These

results were derived from a comparison of temperatures at 25 and 40m

depth in a 50 meter thick mixed layer along a repeated section that was sampled twice over twelve hours.

Separately, it has been well documented (Hanawa et al. 1995) that the

manufacturer's original drop-rate equation (z 6.472 t - 0.00216 t2)

under-estimates the actual drop rate of the T-7 XBT probe, tending to

artificially lift the isotherms approximately 3 4 % from their actual

depths. The data obtained in this program were collected using this

same historical standard equation: Subsequently, in processing, a linear

correction (Z1 = 1.0336 z (from Hanawa et al. 1995)) was applied to more

accurately present the depth/temperature pairs.

3.3.2 XBT Data Processing

SAIC uses a systematic procedure for the processing of CTD and XBT

profiles. The following list suxrunarizes the processing steps for

CTD/XBT profile data respectively.

Data are logged into the SAIC Physical Oceanographic Data Base and the

cruise is assigned a unique cruise ID. Next, the data are read into

binary disk files on the local server. These files are created by

storing the data scans as ASCII characters in a sequential file using

the system file utility program. During sequential file creation,

checks are made on the input parameters (temperature, conductivity and

depth) for large spikes, data gaps and number of data scans for each

individual cast.

Vertical profiles of temperature and conductivity are plotted and

checked for spikes or obviously questionable data. The bad data scans

are removed or the entire cast is discarded depending on the number of

bad data points. T/S diagrams are created for each cast to assure

consistency with known characteristics of water masses in the Gulf.

36
From the corrected data set, both vertical and horizontal contour maps

are produced for individual sections and the entire cruise,

respectively. These maps are checked visually against the profiles as a

quality control measure. Other data products (geostrophic velocity,

integrated transports, dynamic heights) are produced as required for

further analysis.

3.4 Altimetxy

Sea surface height (SSH) analysis maps were produced from archival

altimetric measurements based on the latest versions of the TOPEX and

ERS geophysical data records (GDRs). TOPEX data were obtained from the

Physical Oceanography Listributed Active Archive Center (P0 DAC) at the

Jet Propulsion Laboratory, and ERS data from the 'Centre ERS d'Archivage

et de Traitement" (CERSAT), the French Processing and Archiving Facility

for ERS-1 and ERS-2. Both data sets are processed in as consistent a

fashion as possible to produce accurate analysis maps based on the

blended tandem altimetric observations.

3.4.1 Data rocessin

The TOPEX data were corrected using standard corrections supplied on the

JPL/P0.DAAC TOPEX GDRs, including inverted barometer, electromagnetic

bias, ionosphere and wet/dry tropospheric corrections, as recommended in

the GDR handhook (Callahan, 1993). The mean sea surface included on the

GDRs (Basic and Rapp, 1992) was subtracted from each sub-satellite data

point to apply an implicit cross-track geoid gradient correction. Ocean

tides were removed using the tidal solution derived from the Colorado

Center for Astrodynamics Research (CCAR) barotropic tide model (version

1.0) assimilating TOPEX data (Tierney et al., 1998).

The ERS-1 and ERS-2 Altimeter Ocean Products (ALTOPR) CD-ROMs were

obtained from CERSAT. The ERS altimeter data were corrected using

standard corrections supplied on the ALTOPR GDRs, including inverted

barometer, electromagnetic bias, ionosphere and wet/dry tropospheric

corrections. The data were also corrected using the CCAR Version 1.0

tide model to be consistent with the TOPEX processing.

Each cycle of corrected 10-day repeat TOPEX and 35-day repeat ERS data

was linearly interpolated to reference ground tracks based on computed

orbits for the satellites. The TOPEX reference track was based on a

ground track computed for cycle 18, with a fixed spacing of the subsatellite reference points at once per second along-track which is

approximately a 577 km spacing over the Gulf of Mexico. The ERS 35-day

reference ground track is based on 1/second along-track points computed

for cycle 6 of the ERS-1 Multidisciplinary 1 mission phase. No gridding

of the non-repeat ERS-1 data was performed. An empirical orbit error

correction was applied to the ERS along-track data, after gridding when

appropriate, to remove residual ERS orbit error. An empirical correction

of the TOPEX data was not needed; however, to consistently 'filter both

data sets the empirical correction was also applied to the TOPEX data.

This correction was based on an along-track "bess' filter, which

37
removed a running least squares fit of a tilt plus bias within a sliding

window from the along-track data. The filter window is approximately 15

degrees of latitude (200 second along-track), passing the short

wavelength mesoscale signals while filtering the longer wavelength orbit

and environmental correction errors.

This processing procedure references the along-track data to an accurate

high resolution mean sea surface based on altimeter data collected from

the TOPEX/POSEIDON, ERS-1 and GEOSAT Exact Repeat missions (Yi, 1995).

By referencing the data to an independent mean sea surface, a

climatology based on multiple altimeter mission data sets can be

developed to include past, present and future altimeter data referenced

in the same manner. This includes the historical GEOSAT data (Berger et

al.1 1996) and the TOPEX/POSEIDON and ERS-2 data available during the

DeSoto Canyon field program.

3.42 Blended Tf and ERS 1&2 Cliniato1oqr

An archive of the Gulf of Mexico SSH maps from January 1, 1993 to April

30, 1999 has been produced from the corrected sea surface height

anomalies from TOPEX/POSEIDON and ERS-l&-2. Daily analysis maps of

height anomaly relative to the mean sea surface have been created using

an obective analysis procedure (Cressman, 1959) to interpolate the

along-track data to a 1/4 degree grid over the Gulf. The method used an

iterative difference-correction scheme to update an initial guess field

and converge to a final gridded map. A multigrid procedure was used to

provide the initial guess. The complete multigrid procedure is described

in an appendix to Hendricks et al. (1996) . Five Cressman iterations were

used with radii of influences of 200, 175, 150, 125 and 100km, while

employing a 100 km spatial decorrelation length scale in the isotropic

Cressman weighting function. Data were also weighted in time using a 12-

day decorrelation time scale relative to the analysis date. A model mean

is added the sea surface height anomaly maps to estimate the total

dynamic topography. This mean was computed for the time period 1993

through 1998 from a data assimilation hindcast of the general

circulation in the Gulf. The 1993 through 1998 hindcast was made for

the MMS Deepwater Reanalysis and Synthesis Project by Lakshmi Kantha and

Jei Choi using the University of Colorado Princeton Ocean Model (CUPOM).

The residual mean in the sea surface height anomaly maps over the time

period 1993-1998 is removed before adding the model mean to produce the

synthetic height estimate. A sample map is shown in Figure 34-l.

3.5 Moored Instrumentation

3.5.1 Moored Instrumentation Calibration

Each of the moored instruments deployed in this program, with the

exception of the Aanderaa RCM current meters, was calibrated and

serviced by its manufacturer prior to initial deployment. The Aanderaa

instruments were serviced and recalibrated by Environmental Sensors,

Inc., (ESI) formerly the Canadian distributor and service provider

38
Figure 3.4-1. Sea surface height map for January 12, 1998. Sea surface

height anomalies referenced to the 1993-1998 time period

have been added to a model mean based on an assimilation

hindcast for the same time interval.

39
for Aanderaa instruments in North America. In the

Seamon Mini Temperature Recorders, the RDI 300 KHz

the Sea-Bird MicroCats, these instruments were all

the program. Periodically, over the course of the

instruments were

servicing and/or

most instruments

servicing and re

returned to the various manufactur

re-calibration. At the conclusion

were returned to the same entities

calibration. Table 3.5-1 summarizes

40

cases of the Hugrün

WorkHorse ADCPs and

newly purchased for

program, a number of

ers or ESI for

of the field effort,

for a post-program

the manufacturer I s

specifications and the following sections discuss specific calibration

issues for each type instrument.

In addition to the above, the conductivity data from the Aanderaa,

General Oceanics and InterOcean current meters, when deployed in the

upper 20Dm of the water column, tended to show some data quality

degradation over the four-month deployment period due to fouling.

Subsequently, the calculated salinity data obtained from the temperature

and conductivity data collected by these current meters were adjusted

following comparisons with nearby CTD casts taken at the beginning and

end of each mooring deployment.

3.5.1.1 Aanderaa Current Meters

All of the Aanderaa current meters were calibrated at ESI for wide range

temperature, wide range conductivity and direction as well as an

operational test of speed in February or July 1997 and again at the

conclusion of the field program in June or July 1999. Temperature

calibration stability over the study period was excellent for all but

two of the instruments. These two fell just outside the rated accuracy

(±0.05°C) with differences on the order of ±0.07°C when compared at the

high end (near 28°C) of the calibration scale. Conductivity calibration

stability was significantly worse, as only three instruments were within

the rated accuracy (±0.074 mrrtho/cm) at the conclusion of the program.

Seven sensors had drifted producing conductivity differences ranging

from ±0.078 to ±0.230 rnirtho/cm and one sensor had drifted by as much as

1.241 mrnho/cm. Two conductivity sensors had failed during the program

and were replaced with newly calibrated sensors. A compass calibration

check revealed that all of the instruments were within the stated

calibration accuracy (within ±5°) over the course of the program, and an operational test of speed (counting rotor turns) revealed that all of

the instruments were functioning properly prior to their initial

deployment and following final recovery.

3.5.1.2 General Oceanics Current Meters

The Niskin winged current meters were calibrated by the manufacturer in

January or February 1997 and again in August 1999. Three units were

removed from service due to malfunctions or a leak prior to the final

calibration check and two units still require a final servicing. To

date, all of the tested instruments were within their respective sensor specifications at the conclusion of the field program.
Table 3.5-i. Manufacturer's specifications for moored instruments used during DeSoto Can

Intrusion Study. aug C C) AcCuJ/ C cOIdUC R3ga Cz8±/ft) Conduot. 1cctrayf e$o.L*tion (flhb/cm) Speed Rq(c) Ccuracyf lW8olutIon - A.CoUrecy/ Ro2ut.ou (Pgree5) - ras

psL

Aanderae ±1 cm/s or ±5 for 5-100 cm/s RCM 7/8 ±2% of speed! ±7.5 for 2.5 to 5 030Current Meter -0.34 to ±0.05/0.1% 0-74 ±0.074/0.074 2-295 starting speed and 100 0-50

32.17 Range is 2 cm/s to 200 cm/s/0.35 

enera1 Oceanics 

981(2 Current Meter -5 to 45 ±0.25/0.016 0-75 ±2.5/0.1 0-300 ±1/1 ±2/1 0-20HugrOn Temperature Recorder -2 to 40 ±0.1/0,025 --- --- --- --- Thermistor ±2% of reading InterOcean ±0.05/0 .003 ±lcm/s/0, 2 54 Current Standard cm/s Meter -5 to 45 ±0.2/0.05 5-65 ±0.2/0.01 0-350 ±2/0.5 RDI Marrowband ±0.2% of measured ADCP -5 to 45 ±0.2/0.012 --- --- 0-1000 velocity ±5/0.005 ±0.5cm/s MDI 9iorkhorse ±0.5% of measured ADCP -5 to 45 ±0.4/0.01 --- --- 0-4000 velocity ±5/0.01 ±0.5cm/s SeaBird MicroCat ±0.002/0.0001 ±0.003/month 0-3CT Recorder 5 to 40 0100 Rem 0.0001 SeaBird SeaCat CT Recorder -5 to 35 ±0.01/0001 0-70 ±0.01/0.001 -- --- --- 0-3
3.5.1.3 Buqriin TemDerature Recorders

These instruments were calibrated by the manufacturer in January 1997

and again in July 1999. A comparison of reprocessed data from the last

deployment with both the new and old calibration coefficients revealed

that there had been no significant change/drift in calibration over the

study period. All comparisons were found to be within the instruments

rated accuracy, generally varying by no more than ± 0.025°C.

Subsequently, no adjustments were made in these data following

instrument recalibration.

3.5.1.4 InterOcean Current Meters

The 84 current meters were initially calibrated by the manufacturer

between March 1997 and March 1998. Two were recalibrated in July and

November 1998 and one of these same units (that had been recovered by a

shrimpirig vessel) was recalibrated again in June 1999 following final

recovery. Two other units were returned to I4NS after only one

deployment. All of the instruments were equipped with conductivity

sensors. 3.5.1.5 RDI .ADCPs

All of the instruments were calibrated/serviced by the manufacturer in

January or February 1997, and all but one of the recovered units were

serviced again in June or November 1999. One instrument is yet to be

serviced. The remaining eleven instruments were found to be within

specification, except that four of the WorkHorse units (197, 200, 209

and 211) were found to exceed the manufacturer suggested accuracy of

less than ±5 degrees for compass error when tested in a downward-looking

mode. However, these same compasses were within specifications when

tested in an upward-looking mode, which is how each was used in this

study. Subsequently, all of the ADCP data are considered to be within

instrument specifications, and there is no indication of any problem

with data from the unit which is still awaiting servicing at RDI.

3.5.1.6 SeaBird CT Recorders

The SeaCat and MicroCat instruments were calibrated between December

1996 and June 1997, prior to initial deployment, and again in May or

June 1999 following final recovery. Three of the SeaCat units were

borrowed from the manufacturer and only deployed once during the initial

deployment period, and a fourth borrowed unit was only deployed once

during the fifth deployment period. The conductivity cells on all of

these instruments were protected by antifouling cylinders which were

replaced after a maximum of eight months use (two deployments) . These

cylinders appear to have worked well as there was no buildup of

biological material inside the conductivity cells. In addition, the

post-program calibrations reveal that the total conductivity sensor

drift over the two-year field effort was less than 0.045 rnrnho/cm at 60

rnrrtho/cm and less than 0.020 mmho/cm at 30 inmho/cm. Temperature drift on

these same units was less than 0.003°C. Subsequently, the salinity

drift is on the order of 0.020 to 0.050 psu.

42
3.5.2 Moored Instz-uinentation Data Processinq

The following Instruments were used on the moorings: InterOceari's S-4,

Hugrun's Seamen-mini, Sea-Bird's SeaCat and NicroCat, RDI's ADCP,

Aanderaa's RCM-7&8 and General Oceanics' MK2. Each required a somewhat

different processing scheme to bring the data into SAIC's database.

Some instruments, such as the S4, have their calibrations internally

recorded. Others, like the Aanderaas, have the calibrations applied

during data processing.

The SAIC database utilizes a consistent format whereby each filename has

two data fields representing either vector or scalar data. For a

vector, the u-v components are presented. Scalar data may be either a

single parameter in each of the two fields, such as temperature, or two

different parameters, such as temperature and salinity. Times for all

data are in GMT.

Once in SAIC's database, each file for each deployment is analyzed for

proper processing, missing or erroneous data, calibration errors or

trends in the data. Missing data may be patched using similar data.

Erroneous data may be bridged by interpolation. Data, such as salinity,

may require detrending due to drift of the sensor. This is accomplished

by applying an offset to the time series by comparison to adjacent CTD

records, as required. Once the raw time series files have been cleaned

up, they are filtered using 3-HLP and 40-HLP filters. The vector data

are rotated to align the data with the local bathymetry.

All time series data for all deployments are concatenated into a single

record covering the entire two-year study period. Where data are

missing, the existing files are concatenated into as long a record as

possible. Some single deployment files remain due to data gaps.

3.6 Driftinq-Buoy Data

Drifter data from MMS included all transmissions received from each

drifter and each position of the drifter as determined by Service Argos.

Interactive procedures were then used to remove all duplicate positions,

verify the validity of each position £ ix, sort the data into a timeordered sequence and archive the data into the PODBM system. Once

loaded onto the system, a final visual check was made of the data by

plotting each individual buoy trajectory on a high-resolution map of the

study area containing detailed bathymetry and coastline. This was used

to identify spurious changes in the buoy's movement which could then be

removed -

43
XV. LOOP CURRENT ND ITS INTRUSIONS

4.1 Introduction

The LOOP Current (LC) and related processes are the primary sources of

energy and momentum for circulation features in the NE Gulf of Mexico.

With this as background a concerted effort was made to develop a more

complete description of the morphology (boundary length, enclosed area,

etc.) of the LC, the recent history of change associated with ring

shedding and some possible dynamics associated with ring assimilation

into Gulf waters. This information is basic to developing and

understanding processes affecting circulation patterns in the study

area. 4.2 Loop Current/Loop Current Intrusions from Altimetry

4.2.1 Introduction

Continuous monitoring of the Loop Current is possible from January 1993

to the present using the tandem sampling provided by microwave

altimeters aboard the T/P and ERS-l&2 satellites. Satellite altimetry is

a uniciue remote sensing technique because it is an all-weather observing

system that directly measures a dynamic variable of the ocean state, the

sea surface height (SSH) . A number of useful metrics can be derived from

altimeter data that allow quantitative and continuous monitoring of the

Loop Current intrusions into the Gulf. In this report, we describe the

techniques and altimeter-derived metrics that were developed for

monitoring the Loop Current and Loop Current intrusions.

4.2.2 Loop Current Monitoring

Altimetric monitoring of the LC is difficult because of the large

contribution of the mean circulation to the total dynamic topography in

the eastern Gulf of Mexico. Although altimeter systems measure

variations in SSH very accurately, imprecise knowledge of the marine

geoid makes accurate absolute measurements of the total sea surface

topography associated with ocean circulation impossible at this time.

Synthetic observations of the total dynamic topography can be

constructed by adding an independent estimate of the mean height to the

height deviation measured by the satellites relative to a long-term

altimetric mean.

For this study, we have selected a model mean SSH computed for the time

period 1993 through 1998 from a data assimilation hindcast of the

general circulation in the Gulf (Figure 4.2-1) - The 1993 through 1998

hindcast was made for the 4S-funded Deepwater Physical Oceanography

Reanalysis and Synthesis Program by Lakshmi Kantha and Jei Choi using

the University of Colorado-Princeton Ocean Model (CUPOM). Along-track

TOPEX and ERS-l&2 SSH anomalies are assimilated into CTJPOM on a trackby-track basis as subsurface temperature anomalies. Before adding the

mean to the gridded SSH anomaly fields, we averaged the 1993 through

1998 fields and removed any residual mean. By following this procedure,

we have referenced the anomaly fields to a mean spanning the same time

period as computed from the hindcast assimilation. The qualitative

differences in synthetic maps created using various model means and

45
20°N

92°W 90"W 88°W 86°W 84"W 82°W 80°W

Figure 4.2-1. 1993-1998 mean sea surface height from a hindcast data

assimilation experiment for the 4S Deepwater Physical

Oceanography Reanalysis and Synthesis Project by Lakshmi

Kantha and Jei Choi using the University of Colorado

Princeton Ocean Model (CUPOM) - The contour increment is

2.5 cm with positive and negative contours shown as

solid and dashed lines, respectively. The 17-cm contour,

which approximates the high velocity core of the LC, is

shown as a thick solid line.

46
climatological means are not very remarkable, however, the derived

quantitative metrics can be quite sensitive to the mean selected. For

example, the sensitivity of the analysis of eddy shedding using the

altimetric record discussed in Chapter 3 of this report has been checked

and found to have little dependence on the mean. The quantitative

metrics show much more sensitivity to the mean used to estimate a

synthetic height, however, using the data assimilation model mean and

referencing the altimeter data to a mean over the same time period

produced robust results.

Altimetric monitoring of LCrings in the western Gulf of Mexico relied

on the small contribution of the mean circulation to the total dynamic

topography (Berger et al., l996a,b). For that study, the 17-cm SSH

anomaly contour was selected to derive quantitative metrics for the LC

because the contour closely matched the location of maximum gradients in

the topography, and allowed continuous tracking of LC rings during their

translation through the western Gulf of Mexico. The 17-cm contour of

total dynamic topography also works well as a definition for the high

velocity core of the LC in the eastern Gulf of Mexico (Figure 4.2-2).

A direct comparison of the location of the 17-cm contour relative to the

sea surface temperature (SST) front can be made at the northernmost

point of the LC intrusion into the Gulf (Figure 4.2-3) . SST images

coincident with the altimeter maps were sampled along longitude

meridians within 30 and -2° degrees of the 17-cm contour. The time

series of coincident data over the time period of the DeSoto Canyon

program shows a consistent bias between the two estimates of the LC

location. In general, the agreement is good during times of strong

theLltLal contrast, however, the 17-cm contour of SSH is consistently

south of the SST front by about one degree of latitude or approximately

100 km. This is to be expected because of the difference in location

between the high velocity core of the current and the surface thermal

fronts. The variability about this bias offset is remarkably small given

the occurrence of warm water filaments and other fine scale frontal

variations on the periphery of the LC that are not well sampled by the

altimeters. No comparison can be made during much of the time period,

which highlights the difficulty of continuously monitoring the LC with

thermal imagery.

4.2.3 Loop Current Metrics and Statistics

A variety of LC metrics can be computed from the SSH maps given the

reliability of the 17-cm contour as a proxy for the location of the LC

in the eastern Gulf of Mexico. These metrics include the length, area,

volume and circulation associated with the LC within the Gulf of Mexico,

and its maximum northward and westward extent.

The procedure for computing the metrics from the SSH fields has been

automated by a MATLAB program that accesses the altimeter data archive

and objectively computes the values. Daily values for each metric are

computed as follows:

47
Figure 4.2-2. The 17-cm sea surface height contour from the altimetry

map for January 12, 1998 overlaid on the composite SST

imagery from the 2.98 days ending January 12, 1998 at

6:40 liT. SST data courtesy of the Johns Hopkins

University Applied Physics Laboratory Ocean Remote

Sensing Group.

48
SST S uiroumhng N r [ hcrnmnsI. I 7ni CL.nk ur PtI nt Actti,jl 1.JflLILk r Ni iiUernrnci.t 1 7t.rn Coui.t'ur Ikint i)i LC CaIci::pI fliry !lr t:wri fcr rtir 2 t-iot poirt r.r rrn sr-ri irf.,rr brcjIit- ,-cTII:ur c1f r rsq I hc TC 

n' rc'r t-r j.rr s I iiu r: ri tiie X- cii :ic:u:id I 

Cu1 I:I,rj.z .1V..
Load the 0.25° gridded SSH field and generate the coordinates of

the 17-cm contours within the Gulf.

Identify the LC core, which is defined as the continuous 17-cm

contour that enters the Gulf through the Yucatan Channel and exits

through the Florida Straits.

Find the maximum west longitude and north latitude coordinates to

determine the extent of westward and northward penetration of the

LC.

Compute the length of the LC by summing the distances between the

coordinates on the 17-cm Contour.

Identify all 0.25° grid cells bounded by the 17-cm contour and

compute the total Loop Current area by summing the areas of the

individual cells.

Estimate the LC volume, assuming a one and a half layer ocean and

a reduced gravity approximation, by evaluating the following area

integral over the region bounded by the 17-cm contour:

ff-hdy

g

where h is the sea surface height; g is the acceleration of

gravity; and g' the reduced gravity. (A value of 0.03 rn/s2 was

used for g.)

Estimate the LC circulation by the line integral of the

geostrophic velocity along the 17-cm contour:

3cV.d Judx+vdy

where u and v are the geostrophic velocity components and dx and

dy are the coordinate spacing in the east/west and north/south

directions respectively. The geostrophic velocity components at

the midpoint locations are found by bilinear interpolation from

the gridded geostrophic velocity components computed from the

height field. (The sign convention employed is such that the

anticyclonic vorticity associated with the LC is positive.)

The metric values have been computed for the time period from January 1,

1993 to April 30, 1999 and are shown in Figures 4.2-4 and 4.2-5. Summary

statistics for this time interval are shown in Table 4.2-1. The total

area of open water on the 0.25° grid in the Gulf of Mexico is

1,512,000 km2. During this time interval, the average area covered by

the LC was 142,000 km2 or approximately 9% of the Gulf. The average

length was 1390 kin and average circulation was 1,271,000 m2/s. The

total estimated LC volume was 2.17 x 1013 m3. This volume would take over

8 days to fill assuming an average inflow of 30 Sv, which is typical of

the transport into the Gulf through the Yucatan Channel. The average

maximum northward and westward extension of the LC intrusion was 26.1°N

and 87.7°W. The maximum and minimum northward extension into the Gulf

observed during the time interval was 28°N and. 24.l°N, respectively. The

maximum and minimum westward extension was 92°W and 85.8°W.

50
1.8

1.6

0.8

2

' 1.4

-'-U

Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

0.8

5 x 10

6 x 10

0.6

Jan93 Jul93

Area of Loop Current, Mean = 142,000 kn

Circulation Along Loop Current, Mean = 1,271,000 (m2 s-')

Figure 4.2-4. Time series of LC area, volume, and circulation for

1/1/1993 through 4/30/1999. Ring shedding events are

shown by vertical gray lines immediately following the

separation.

51

Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Volume of Loop Current, Mean = 2,170000 x 1013 (m3) 13 x 10

Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99
28

27.5

26,5

26

25.5

92

91

a 90

0

cS 89

88

87

2200

2000

1800

1600

1400

1200

1000

800

Jan93

Maximum Latitude of Loop Current Intrusion, Mean 26. 1°N

86

Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Length of Loop Current, Mean = 1,390 km

Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Figure 4.2-5. Time series of Loop Current northward and westward

penetration, and length for 1/1/1993 through 4/30/1999.

Ring shedding events are shown by vertical gray lines.

52

Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

MaximumWest Longitude of Loop Current Intrusion, Mean = 87.7°W
Table 4.2-1. Statistics for Loop Current Metrics (1/1/1993 -4/30/1999).

Similar statistics were derived from 10 years (1976-1985) of monthly

frontal analysis maps based on AVHRR thermal images (MNS 89-0068) . The

summary LC statistics from that study reported an average area of

210,000 km2 and 29.75°N and 91.25°W for the maximum northward and

westward extension. The average area derived from thermal imagery was

nearly 50% larger than the altimetric estimate reported here. This

difference is attributable to the offset between the surface thermal

fronts and the location of the core of the LC determined from altimetry.

A similar offset of 50 to 100 km was found between the surface frontal

analysis maps and subsurface temperature gradients at 125 meters

determined from coincident XBT survey transects. Assuming the mean

statistics over the two time intervals are comparable, we can estimate

the relative offset by the difference of the averages divided by the

average length of the Loop Current. This gives an estimate of

approximately 50 kin, which is comparable to the other estimates of the

relative offset. The integrated effect of this offset is to increase

estimates of the areal extent of the LC from thermal imagery relative to

the value determined from altimetry.

The percent occurrence maps for the 17-cm contour and waters associated

with the LC from altimetry are shoim in Figures 4.2-6 and 4.2-7. These

maps can be compared to the occurrence maps computed from the monthly

frontal analysis maps from AVHRR thermal imagery (Figures 4.2-6&7 from

MMS 89-0068) . The occurrence of the 17-cm contour and waters associated

with the LC from altimetry is well within the 200-meter isobath, unlike

the estimates from thermal imagery that show significant probabilities

of the front and LC waters being on the west Florida and Campeche

shelves. This again is likely a result of warm filaments or shallow

thermal features in the frontal analysis derived from the thermal

imagery being interpreted as the main front of the LC.

4.2.4 Loop Current Ring Shedding Cycle

A primary reason for computing objective metrics for the LC and its

penetration into the Gulf is to monitor the time-dependent behavior of

the LC as it sheds rings that affect currents in the DeSoto Canyon study

area. The LC ring shedding cycle has been studied extensively and an

updated list of individual shedding events and spectrum are discussed in

this report. The identified shedding events during the altimetric

monitoring time period are plotted on each of the subplots of the metric

time series. A

53

Ldqitude

North./

Lati.tide th : krea Volume C].rc1Lat1or

Mean 87.7°W 26.1°N 1390 142,000

kin2 2.l7xlQl3

m3

1,271,000

m2/sec

Std. 0.97° 0.93° 321 km 28,300 0.37x10"13 321,190

0ev. kin2 in3 m2/sec

Maximum 92.0°W 28.0°N 2016 km 194,600 2.81x1Q'13 2,016,200

km2 in3 m2/sec

Minimum 85.8°W 24.l°N 593 km 58,600

km2

0.91x10"13

m3

593,480

m2/sec
20°N

92°W 90°W 88°W 86°W 84°W 82°W 80°W

Figure 4.2-6. Percent occurence of 17-cm contour defining the high

velocity core of the LC for the interval from 1/1/1993

through 4/30/1999.

54
30°N

28°N

26°N

24°N

22°N

20°N

92°W 90°W 88°W 86°W 84°W 82°W 80°W

Figure 4.2-7. Percent occurrence of LC weters for the interval from

1/1/1993 through 4/30/1999.

55

_____________

I___________

I-_ ___________

N 4

5 -'-

I __

I

1
shedding event is associated with each of the dominant peaks in the time

series. The exact timing, however, is dependent on the criteria selected

for the definition of separation and is complicated by the ambiguity of

associating a discrete time with what is clearly a continuous process.

The method employed to identify separation tends to select the times of

minimums in the metric time series immediately after an extended

intrusion, which is a conservative estimate that is unlikely to over

count shedding events.

The matrix of correlation coefficients computed from the altimetric

estimates of LC metrics is shown in Table 4.2-2. Identifying a best

metric for monitoring the LC is difficult because of the high

correlation between the individual time series. Dynamical considerations

may favor the circulation metric, which shows the most consistent growth

throughout an intrusion event and the greatest increase just before

separation. The volume of the LC is the least correlated with the other

metrics and exhibits large variations independent of shedding events.

There has been a single estimate of the correlation coefficient between

area and northward extend of the LC reported, based on frontal analysis

derived from the thermal imagery. The value found was 0.76, which is

significantly less than the 0.955 determined from the altimetric

analysis and may be related to the difficulties associated with

identifying frontal boundaries in thermal imagery. The high correlation

found between the altimeter-derived metrics support the use of surrogate

time series from the historical record as proxies for shedding events.

For example, there is very high correlation between area, length and

circulation and the northward penetration of the LC, a metric that is

available from the historical record.

Table 4.2-2. Correlation Matrix for Loop Current Metrics

4.2.5 Geostrophic vs. SCULP Drifter Velocities

The SSH maps were used to monitor the LC, LC rings and secondary eddies

in the northeastern Gulf of Mexico during the field program. To assess

the accuracy of the altimetry, we compared geostrophic velocities

calculated from the objectively analyzed maps of SSH to the coincident

drifter estimates. The daily 0.25 resolution gridded SSH anomaly

fields, based on archival TOPEX and ERS--2 altimeter data, were added to

the hindcast model mean to give an estimate of the total SSH.

Geostrophic velocities were calculated from the height fields using

finite differences. Similar comparisons have been made of the gridded

56

Area

I Volume Clrculat].on

Mazimuin

North

Latitude

Maximum

West

Longj.tude

Length

Area 1.000 0.950 0.932 0.955 0.917 0.948

Volume 0.950 1.000 0.891 0.872 0.828 0.846

Circulation 0.932 0.891 1.000 0.931 0.908 0.948

Max. North

Lat. 0.872 0.931 1000 0.876 0.955

Max. West

Lan. 0.917 0.828 0.908 0.876 1.000 0.950

Length 0.948 0846 0.948 0.955 0.950 1.000
altimeter geostrophic velocities to drifters velocities (Ohlmann et al.,

submitted to JGR Oceans), and found that surface currents from drifting

buoys agreed well with geostrophic velocities derived from the gridded

(0.25°) SSH maps over the deep basin. In that study, however, 14-day

averages of the daily drifter velocities within a cell were used for the

comparisons. Here, we directly compared the daily averaged values.

The total number of daily average surface velocities from the combined

SCULP-I, -lit and -III data sets is 39,362, with 52% (20,517) located in

0-50m depths, 16% (6,203) in 50-lOOm depths, 22% (8,801) in 100 to 1500m

depths, and 10% (3,841) in depths greater than iSOOm. The map of the

vector correlation coefficient (Crosby et al., 1993) between the

altimeter-derived velocities and the daily-averaged surface velocities

from the drifters is shown in Figure 42-8. The two estimates are in

better agreement over the deep Gulf and along the continental slope in

water depths greater than lOOm, but not on the continental shelf.

4.2.6 Loop Current Characteristics During Field Measurements

The over 6-year long time series of LC metrics can be used to

characterize the LC behavior during the present field program: March,

1997 through April, 1999. Metric time series for the 2-year time period

are shown in Figures 4.2-9 and 4.2-10.

Only one LC metric achieved an extremum during the field program - the

circulation, which reached a maximum for the over 6-year time period in

late September 1997 during the first intrusion in the first year of the

DeSoto program. Two ring shedding events occurred during the field

program, both in the first year. This is in contrast to the second year

of field measurements during which the LC retreated to a far more

southerly position for nearly 9 months, the longest time period of

retreat observed in the altimetric record.

The mean SSH over the two-year record is shown in Figure 4.2-11. The

two-year mean differs little from the hindcast model assimilation mean

(Figure 4.2-1) added to altimeter data to produce an estimate of the

synthetic height field. The anticyclonic circulation over the DeSoto

Canyon study area was only slightly stronger than that during the 6-year

mean time period. A distinct region of cyclonic circulation, between the

study area and the mean LC, can be discerned in both means, though the

feature is displaced to the west during the field program time

intervals. Caution should be exercised when interpreting subtle

differences in these maps since most of the "signal' is dependent on the

fidelity of the model assimilation hindcast.

Significantly stronger differences were found over the study region in

the 1-year means from April 1997 through April 1998 (Figure 4.2-12) and

April 1998 through April 1999 (Figure 4.2-13). The distinct behavior of

the LC in each year of the two-year field program clearly affected the

circulation structure captured in the 1-year mean SSH fields. Weaker

anticyclonic circulation in the study area and stronger cyclonic

circulation to the south of the study area are seen in the first year

mean, associated with a stronger and slightly intruded JIC. The cyclonic

circulation, just north of the mean LC, also displaced to the east. When

57
1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

58

Figure 4.2-8. Map of the vector correlation coefficient between daily

averaged drifter velocities and coincident geostrophic

velocities computed from maps of sea surface heights.
1.8

1.6

2.6

2.4

E

2.2

1.8

1.6

2

1.2

U1

0.8

5 x 10

13 x 10

Apr97 Jul97 Oct97

Apr97 Jul97 Oct97 Jan98

Area of Loop Current, Mean 135,000 ku?

'I. Volume of Loop Current, Mean = 2.120000 x 1013 (m3)

Jan98 Apr98 Jul98

Apr98 Jul98

/

I'. I Oct98 Jan99 Apr99

Oct98 Jan99 Apr99

Figure 4.2-9. Time series of LC northward and westward penetration, and

length during the field program. Ring shedding events

are shown by vertical gray lines.

59

x 106 Circulation Along Loop Current, Mean = 1,232,000 (m2 s)

I }

Apr97 Jul97 Oct97 Jan98 Apr98 Jul98 Oct98 Jan99 Apr99
27.5

27

z

j26.5

'26

25.5

25

24.5-

Apr97 Jul97

89.5

89

°88.5

5 88

87.5pJ'

87

86.5

Apr97

2000

1800

1600

1400

1200

1000

Maximum Latitude of Loop Current Intrusion, Mean = 25.8°N

Oct97 Jan98 Apr98 Jul98

Length of Loop Current, Mean = 1,310 km

Maximum West Longitude of Loop Current Intrusion, Mean = 87,4°W

Jul97 Oct97 Jan98 Apr98 Jul98 Oct98 Jan99 Apr99

Apr97 Jul97 Oct97 Jan98 Apr98 Jul98 Oct98 Jan99 Apr99

Figure 4.2-lU. Time series of LC northward and westward penetration,

and length during the field program. Ring shedding

events are shown by vertical gray lines.

60

Oct98 Jan99 Apr99
lo _d 'jktTj

20ON

92°W 90W 88°W 86°W 84°W 82GW 80°W

Figure 42-ll. Mean SSH for interval from April l97 to April 1999.

61
2ON.'-

92°W 90°W 88°W 86'W 84°W 82°W 80°W

Figure 4.2-12. Mean SSH for the interval from April 1997 to April 1998.

62
-:' /

. \

-

- -

S

', S ,

' - 4

= : = &1L -T

20°N - ____

92GW 9OW 88°W 86°W 84GW 82GW 80°W

Figure 42-13. Mean SSH for the interval from April 1998 to April 1999.

63
the LC is retreated during the second year of the field program, the

circulation pattern in the mean field showed stronger anticyclonic

circulation over the study region and a displacement of the cyclonic

circulation south to the west.

4.3 Loop Current and LC Rings - Enerqetics and tynajnics

4.3.1. Introduction

The most energetic events in the circulation of the Gulf of Mexico are the separation of large anticyclonic rings from the LC. The discussion

below about the LC and its rings consists of two parts. The first part

is analysis, based on a much longer and improved dataset, of the

freguency of ring shedding. The second is a discussion of the forcing

of LC variability by the wind curl. We find a remarkable coherence

between the variability of the LC and that of wind curl over the whole

Gulf. The coherence is near the -95% confidence limits in several bands

at which there is power in both signals.

Building on previous work, we examined all the apparent rings since

July, 1973. This new data set includes using satellite altimetry since

1992 and provides a set of 34 known ring formations. The primary

advantage of altimetry is that the data remain available in the summer.

One finding is that the ambiguity of whether or not a ring has separated

is reduced, but not eliminated; the uncertainty with which separation

"events" can be specified remains approximately 4 weeks, even with

nearly-continuous data. The distribution of separation intervals has

peaks near 11 and 14 months; if the distribution is smoothed, as in the

usual spectral analysis, a large peak in the distribution of separation

intervals is formed near, but not exactly at, 12 months. A large

secondary peak is at 6 months, with a smaller peak at 9 months.

4.3.2 Backqround and procedures

4.3.2.1 Historical Analysis

Before the main flow from the Caribbean Sea becomes the Florida Current

and then the Gulf Stream, it first passes through the Gulf of Mexico.

When this powerful current from the Caribbean first comes into the Gulf,

the flow is directed primarily towards the north. This flow must

accomplish a 9Q0 turn to the east to pass between Cuba and the Florida

IKeys. The dynamical balance of this turn has been discussed by Pichevin

and Nof (1997) . In the course of making this turn, the current pattern

takes the shape of a portion of a large circle, or loop. Over the

course of several months, the part of the flow field in this loop

extends farther into the Gulf (see, for example, Reid, 1972) until it

becomes sufficiently unstable. At this point a large anti-cyclonic ring

gradually separates from the main flow and drifts to the west. In many

respects these rings are analogous to Gulf-Stream rings.

There are several puzzling features of these ring separation events.

They are not quick, simple events, but can be long and drawn out; the

separation process generally taking several months or more. It is not

unusual f or observers of some satellite IR images to conclude "The ring

64
has now separated, yet an image several months later may show the ring

still attached to the main flow. Clearly, we do not understand the

separation process well. Nevertheless, for any representation of Gulf

circulation (whether analytical or numerical) to be capable of modeling

the circulation competently, it should be able to handle ring separation

events correctly. It seems important to us, therefore, to determine

this fundamental time scale of ring separation events as accurately as

can be done based on the available data.

Since the l970s, much of our knowledge of ring-shedding behavior has

been based heavily on satellite infra-red (i.e. SST) data (see, for

example, Maul and Vukovich, 1993). SST data continue to be extremely

valuable, because the horizontal resolution is unsurpassed. However,

SST data suffer from a major limitation: the uniformly warm surface

temperatures in the Gulf of Mexico during summer (i.e. no thermal

gradients) allow no inferences about the flow field for 3-4 months in

the summer.

Data from satellite altimeters, however, are not subject to this

seasonal limitation. Some unresolved problems with determining a

proper geoid surface for interpreting the altimetry data remain, but

these are gradually being reduced to an uncertainty level of a few cm.

The present level of accuracy suggests that, while there may be details

of the SSH field that cannot be resolved exactly, large-scale features

such as whether a ring has separated from the LC can now be determined

unambiguously.

Satellite altimeter data are now routinely available in both near-realtime (Lillibridge et a1, 1997) and as archival data records. Leben

maintains a web page (http://www-ccar.colorado.edu/---altimetry/) on which

daily maps of the Gulf of Mexico sea surface topography have been posted

since 1996. Multi-satellite sampling by the altimeters aboard the

TOPEX/POSEIDON and ERS 1 and 2 satellites has been used to map the Gulf

of Mexico sea surface topography over the interval from April 1992 to

the present. As an example, these maps have been useful for the study

of LC rings in the western Gulf (Biggs et al., 1996).

Previously, a data set based on ring separations between 1973-1993

(Sturges, 1994) suggested that the time intervals between ring shedding

events had two modes, one at 8-9 months and the second at 13-14 months.

These data were based heavily on satellite IR data. It is possible that

some separations included on that list, particularly those iii the spring

or early summer, may not have separated. Since that analysis was

completed, three changes have taken place. First, the earlier

"community approved consensus" list has been subject to several

corrections. Second, the satellite altimetry data has made it possible

to insure that summer-time events are followed reliably. And third,

several years elapsed during which more separation events have taken

place. Consequently this new data set contains over 50% more ring

events than in the previous compilation. This new, expanded data set is

a key focus of this presentation.

65
4.3.2.2 New Data and Processinci

We have reviewed maps of SSH in the Gulf to estimate the times of ring

separation events. These maps are based on geophysical data records

(GOR) from the TOPEX/POSEIDON (T/P) and ERS 1 and 2 satellite

altimeters, processed in a manner consistent with the near real-time

monitoring described by Lillibridge et al., (1997).

TOPEX data were adjusted using standard corrections supplied on the

JPL/PO.DAC TOPEX GDRs, including inverted barometer, electromagnetic

bias, ionosphere and wet/dry tropospheric corrections, as recommended in

the GDR handbook (Callahan, 1993) . Several additional corrections not

found on the original GDRs were also applied to the TOPEX data. These

corrections included orbits based on the JGM-3 gravity model and an

empirical ocean tide model (Desai and Wahr, 1995) based on TOPEX

altimeter data computed using the JGM-3 orbits. The ERS-1 and ERS-2

Altimeter Ocean Products (ALTOPR) CD-ROMs were obtained from CERSAT. The

ERS altimeter data were corrected using standardcorrections supplied on

the ALTOPR GDR5, including inverted barometer, electromagnetic bias,

ionosphere and wet/dry tropospheric corrections. The data were also

corrected using JGM-3 orbits and the Desai and Wahr (1995) tide model to

be consistent with the TOPEX processing. After extracting the data from

the GDRs and applying corrections, the corrected SSHs from both

satellites were referenced to an accurate high resolution mean sea

surface based on altimeter data collected from the TOPEX/POSEIDON, ERS-1

and GEOSAT Exact Repeat missions (Yi, 1995).

These corrections allow data from both repeat and non-repeat ground

tracks to be combined by objective analysis, so that the non-repeating

ground tracks covered during Geodetic Phase 1 and 2 of the ERS-1 mission

(April 10, 1994 to March 21 1995) can be used to augment T/P sampling

for more accurate mesoscale mapping.

The final step in the along-track processing is to apply an empirical

orbit error correction to the ERS 1 and 2 along-track data to remove

residual ERS orbit error. 2n empirical correction of the TOPEX orbits is

not needed; however, to 'filter' both data sets consistently, the

empirical correction is also applied to the TOPEX data. This correction

was based on an along-track "bess" filter, which removed a running

least squares fit of a tilt plus bias within a sliding window from the

along-track data. The filter window is approximately 15 degrees of

latitude (200 seconds along-track), passing the ocean mesoscale signals

much shorter than this, while removing the longer wavelength orbit and

environmental correction errors.

Daily maps of the Gulf of Mexico SSH anomaly relative to the mean sea

surface were created using an objective analysis procedure (Cressman,

1959). This method interpolates the along-track data to a 0.25° grid

over the Gulf. The method uses an iterative difference-correction scheme

to update an "initial guess" field and then converges to a final gridded

map. A multigrid procedure was used to provide the initial guess, as

described in the appendix to Hendricks et al. (1996) . Five Cressman

iterations are used with radii-of-influence of 200, 175, 150 , 125 and

100 km, while employing a 100 km spatial decorrelation length scale in

66
the isotropic Cressman weighting function. Data were also weighted in

time using a 12-day decorrelation time scale relative to the analysis

date.

The altimetry data are used here to determine only departures from the

mean SSH. To estimate the full SSH, or total dynamic topography,

relative to a level surface, the mean surface from a 10-year numerical

ocean model (courtesy of L. Kantha and U.K. Choi), has been added to the

height anomaly maps. To test the reliability of this surface for the

purposes of this work, we found that the timing of ring-shedding events

was insensitive to the mean used to produce the synthetic product,

though the local structure of the LC was affected by differences in the

mean fields on the order 10 cm.

Altimetric mapping provides all-weather and all-season data. While it

shows less horizontal detail than the IR data, the altimetric data used

here are less likely to miss the shedding or re-attachment of a ring.

The primary advantages of the new list of sheddings, obviously, are that

there are no data losses from clouds, or from the usual summertime

spatial uniformity of SST in the Gulf.

4.3.2.3 Methods

The method used in this analyses is similar to that employed by Sturges

(1974) . By examining the maps as a time sequence, it is clear when ring

begins to separate. However, new rings often begin the separation

process, only to reconnect with the LC as much as a month or more later.

To tell for certain that a ring will separate, it is necessary to

observe the nascent ring as it continues to form, pulling away clearly

and drifting to the west as a separate feature. One advantage of this

altimetry-based data set, never obscured by summertime problems, is that

the continued examination of the rings as they pull away allows us to be

certain that the separation process goes to completion.

All available frames of contoured data since the spring of 1992 were

evaluated. The method was simply to examine each map to determine

subjectively whether the ring appeared to be separating, and continue to

track it as the separation process continued. A surprising feature of

these results is that, even with a significantly improved data set, the

exact time of separation of a ring remains elusive. As has been

reported earlier, based on numerical modeling results (e.g., Sturges et

al., 1995) the separation process is not a single abrupt event, but a

gradual pulling away, with parts of the flow connecting the main portion

of the LC to the separating ring for a significant part of its travel

time to the west. When the time sequence of maps are examined, it is

clear that this separation process takes many weeks. It is rarely

possible, on the basis even of this new data set, to point to a

particular time in the separation sequence and say "At this point the

separation is complete."

The most striking example of this ambiguity took place during 1996. It

appeared that a ring was beginning to separate, and over the course of

nearly a year the "ring" hovered in essentially the same general

location. On the basis of many subjective viewings, we conclude that a

67
ring did separate in September 1996, but the ambiguity is bothersome.

Awareness of this phenomenon is certainly not new, as it has been

observed to take place previously in the Gulf of Mexico and for Gulf

Stream rings as well.

To review briefly the method used earlier by Sturges (1974), the

separation times shown in Table 4.3-1 were used to determine a

histogram. To make the results more nearly in keeping with the usual

analyses of time-series data, however, the inverse of the separation

interval is plotted; the resulting values were then smoothed with 5

Manning passes to give the form of a power spectrum. The basis for this

procedure was described by Silverman (1986)

Table 4.31. A compilation of ring-separation events; i.e., times when

data are available to show a ring separating reliably

from the Loop Current.

Entries through Oct. 1986 are from Vukovich (1988 a&b);

other entries prior to July 1992 are from Table 1 of

Sturges (1994) using corrections based on Berger (1993).

Data beginning 1992 are from satellite altimetry, this

work.

68

Vear

-

.. unerta.nty, weeks

1973 July

1974 April 9

1975 January 9

1975 July 6

1976 August 13

1977 March 7 1978 June 15

1979 April 10

1980 January 9

1981 March 14

1981 November 8 1982 May 6 1983 March 10

1984 February 11

1984 August 6 1985 July 11

1986 January 6 1986 October 9 1987 September 11

1988 May 8 1989 Nay -June (?) 12.5

1990 August 14.5

1991 Aug - Sept 12.5

1992 July 19 11.5 4 1993 June 22 11 4

1993 Sept 19 3 1 1994 Sept 22 12 4

1995 April 8 7 4

1995 October 18 6 4

1996 April 30 6 5

1996 September 5 4

1997 October 11 13 3

1998 March 14 5 2.5

1999 August 22 17 2
Nevertheless we emphasize that what is recorded in Table 4.3-1 is not a

time series and Figure 4.3-1 is not based on the usual spectral

calculations.

4.3.3 Discussion of Ring Shedding reguency

The complete list of ring separation times is shown in Table 4.3-1.

Other ring separation events were recorded a decade or more earlier, but

we show here only those for which we believe that successive separations

are reliably known. There are 34 separation events in the 26-year

interval July 1973 - August 1999. For the recent separations, beginning

in 1992, the apparent uncertainty interval is shown explicitly. The

separation times listed are the midpoint of that interval.

Figure 4.3-1 shows the frequency distribution of ring separations in the

form of a spectrum. The results shown provide conclusions that are

somewhat more reliable than in the previous, similar calculation. The

most noticeable region of power is near one year. The peak in the lowfrequency mode is at 11.6 ma; this specific peak does not change if

smoothing is done by only 3 Hanning passes for higher resolution. We

attach no significance to the precise value 11.6 mo, as most of the

separations are unknown to /- at least a month. The 80% confidence

limits, estimated by many iterations with a bootstrap method, extend to

1.7 times the power found in any peak. The mean value, for 33

separation intervals in 313 months, is near 9.5 months, yet there is

little power at that value. The distribution has one major mode near

annual, with half the separations at periods shorter and longer than

l0.5 months.

Figure 4.3-2 shows a slightly different version of the same calculation.

The smoothed "spectrum" from Figure 4.3-1 is shown together with the

original data of the exact times of the individual separations from

Table 4.3-1. It is obvious that the smoothing process, while desirable

for many reasons, has formed a peak where none exists in the basic data.

Thus, we should be careful to note that there is no "annual cycle" in

the LC in the way that there is such a strong annual cycle in most other

variables, such as the wind forcing, temperature, etc.

One difficulty with the data of Table 4.3-1 is that the uncertainties

are perhaps large but unknown for the values based on the older data.

Even with the satellite altimetry, the separation of September, 1996

remains elusive. But by following the images carefully, we conclude that

a ring did separate. Its motion was slow, and by the following late

April or early May the ring appears almost to have been re-absorbed by

the LC. However, a ring emerges at about 93°W when this ring

disappears, leading us to the conclusion that the ring actually

separated.

What one does, when faced with such uncertainty, of course, is to try

the calculation both ways to see if it makes any significant difference.

By computing the 'spectrum" of Figure 4.3-1 with and without the elusive

ring, one finds that the differences are negligible. A small bump at

-30 months will appear, but the effect on the rest of the spectrum is

69
Ring Separations from the Loop Current since 1973

0 0.05 0.1 0.15 0.2 0.25

Frequency (cycles/month)

Figure 4.3-1. Periodicity of ring separations from the Loop Current.

This figure is actually a histogram, plotted to resemble

the usual power spectrum; both scales are linear. All

known separations since July 1973 - Aug. 1999 are

included; see Table 4.3-1. The original histogram has

been smoothed by 5 Hanning passes. A single value at (3

mo) is omitted for clarity, and is offscale. Confidence

limits of 80%, are obtained by multiplying (or dividing)

any value by 1.7; determined by a bootstrap technique.

70

-,.. 0

-E

0

a)

1.8

t6

C') C

1.2

ci)

o1

C')

O.8

0.6

0.4

0.2

0
6

0

Ring Separations from the Loop Current since 1973

14

11 9 6

41 \JV

0 0.05 0.1 0.15 0.2 0.25

Frequency (cycles per month)

Figure 4.3-2. Periodicity of ring separations from the Loop Current;

similar to Figure 4.3-1, except that the original,

unsmoothed separation data values are shown in addition

to the smoothed curve from Figure 4.3-1

71
hard to see by eye. The peak at 11.6 months is broadened slightly, but

otherwise unmoved, and the peaks at 6 and 9 months are unaffected.

It is well known that determining the true uncertainty of geophysical

spectra is fraught with problems. We often deal more with hope than

with confidence. The primary power peak here is near one year, so a 31-

year data set might seem relatively "long" in some sense. However, a

major uncertainty is that the wind system over the North Atlantic forces

a decadal-scale variability in the flow. The "typicalT' low-frequency

variability in winds, sea level, and transport of the Gulf Stream seems

to be at periods of the order of 100-200 months. We may thus suppose

that, if the climate system has substantial variability at such long

periods, the data of Table 4.3-1 may in fact now provide only two or

three "looks" at the process, rather than 31. The error bars may thus

be larger than we think. The essential result is that the primary power

lies near a period of 12 months, but there is almost no power at exactly

12.0 months There are smaller peaks at 9 and 6 months.

When the wind forcing is averaged over many years it has a very clear

annual cycle. It is surprising, therefore, that the peak here is near

one year, but quite distinctly not precisely at 12.0 months. This

feature is explored in Section 43.4.

4.3.4 Forincr of LC Variability by the Wind. Curl

4.3.4.1 Backqround

Two aspects of the LC's behavior have long been a puzzle. First, even

though the wind forcing of the ocean has a pronounced annual cycle,

there seems to be no annual cycle to the LC nor to the shedding of

rings. Data from the early l960s, when ring shedding was first

discovered, suggested an annual cycle. However, the duration of

observations is now long enough to provide much higher resolution. At

periods just shorter and just longer than annual, there is a large

amount of power, but essentially no power at exactly a year, as shown in

the preceding section.

The second point is more subtle. As the LC intrudes from its more

southerly position, around -24°N, to a more extreme northerly position

near -28°N, its front advances several hundred kilometers and a large

amount of upper-layer water is displaced. Except for the entrance and

exit ports, the LC is totally surrounded by the Gulf of Mexico.

Therefore, the displaced volume -6 x km3. of displaced water must

somehow "go away,"yet it is not clear just where it goes. Initially the

most likely option might appear to be that this upper-layer fluid is

forced Out through the Florida Straits, or back through the Yucatan

channel, but at present we have no evidence documenting such a flow

pattern. The amount of "extra" flow that must be associated with the

intrusion phase of the LC cycle can easily be estimated to be as great

as -10 Sv, during times when the LC grows quickly over an interval of

only 2 months.

The idea we wish to put forward hinges on a new and perhaps surprising

bypothesis, which admittedly is contrary to the ideas of the way the

72
ocean responds to Ekman pumping in the large mid-ocean gyres in the open

ocean. We propose that, to accommodate the inflowing Loop Current, the

displaced upper layer waters of the Gulf may be largely balanced by

outflow of deep water in Yucatan Channel. This flow takes place on two

time scales: first, on the one-to-several month time scale of Loop

Current intrusions, the mass fluxes of order 5-10 Sv.; and second, on

the long-term-mean time scales associated with the decay of old rings

(-one per year), with a mass flux of order 2 Sv. This second value is

presumably the time mean of the fluctuations of the first. We will show

(following section) that this second value is consistent both in mean

value and in energetics with the Ekrnan pumping over the interior of the

basin.

The large mass fluxes associated with the shorter time scale intrusions

are consistent with recent numerical modeling experiements (for the

NICOM, E. Chassignet, personal communication; for the Navy Ocean Model,

W. Schmitz, personal communication; for the Princeton Ocean Model, T.

Ezer, personal communication; for the Bryan-Cox model, Y. Hsueh, Y.

Golubev, personal communication) . It is startling to see large

compensating flows of deep water to the south during the times of a Loop

Current intrusion. All the models referred to are full-ocean, windforced, and do not specify input boundary conditions at Yucatan; the

fact that such flows are consistent among the different models lends a

bit of credence to the idea we have put forward, although it surely is

not proof. This information is offered to support the plausibility of

our suggested method, but a full discussion is clearly beyond the scope

of this report.

For most of the year, the majority of the surface area of the Gulf of

Mexico is forced by convergent surface wind stresses. Because the net

Ekinan pumping at the surface is downward, there must be a net inflow in

the upper layers that is balanced by a net outflow. The sign of the

deep Sverdrup interior flow would be to the south, and this flow would

presumably leave the Gulf by flowing back through the wider, deep,

Yucatan Channel. We propose that this outflow is deep water.

In the more usual case of Sverdrup flow in the main subtropical gyres of

the open ocean, one usually assumes that the resulting southerly flow

goes down, to the south and, somehow, "away." Because this transport in

the Gulf is confined and the exit through the Florida Straits is only

-850m deep, the logical outflow port is through the 2000-m deep Yucatan

Channel. We recognize that this is a novel and perhaps controversial

hypothesis; the proof will rest initially, on two ideas that can be

demonstrated. First, the energetics make it highly plausible; second,

the remarkably good agreement between wind curl variability (or Ekman

pumping) and the LC position is quite persuasive.

One of the first thing that comes to mind, when discussing such a flow

path, is whether the water mass properties make such an idea possible.

Elliott (1982) made a careful study of the decay of anticyclonic rings,

making the argument that the salty upper-layer waters in rings could be

diluted by river water to make less salty Gulf water. If upper-layer

waters are pumped down, it is likely that over the course of decades

there will be a very great deal of mixing not only with river inflow but

73

/
also with inf lowing deep Caribbean water. We do not give the details

here, but assert that a balance of water properties can be demonstrated.

4.3.4.2 Energetis of the orcinq

We first estimate the amount of energy required to pump near-surface

flufd down to depths below the main thermocline. The suggestion put

forward here is that the upper waters are forced downward, below the

inf lowing water, or to depths of roughly 900m and deeper. There are two

related ways to estimate the work required. First, we can estimate

directly the work req-uired to force a given volume of upper-layer fluid

down through the known stratification of the Gulf. Second, we can

determine the available potential energy (APE) of a detached ring. To

within the assumptions used for an order-of-magnitude calculation, these

two methods give the same value. While it is possible to make these

calculations much more precisely, it will turn out to be adequate here

to determine merely the order-of-magnitude.

The APE in rings has been studied carefully by Reid et al. (1981), Bray

and Fofonoff (1981), and others. Reid et al. show (their Table 2) a

collection of rings for which the APE above l000m is -8 x iO' J. We

will take this value to be typical for rings here.

Next we estimate the energy available by the work done by the winds on

the surface of the Gulf. Stern (1975) shows that the rate at which

energy is supplied to the ocean is given by the product of wind stress

and the near-surface geostrophic velocity. That is, the energy rate, E,

E = v = Mean (t v) + Mean < t' v' > (4.1)

where the first term on the right is the product of the mean values, and

the second is the mean of the product of the fluctuating parts. It is

quite possible to determine the fluctuating component of the wind

stress, but we are able to determine the velocity only poorly in the

mean, and even worse for the fluctuating parts. It is our suspicion

that the second teim on the right will add to the first teLln on the

right, but the error bars are quite large. We therefore estimate only

the first term on the right.

It is well known that the central and western Gulf, away from the LC, is

dominated by an anticyclone. Averaging the presence of LC rings, we

take the central high to be of order -30 cm, or somewhat less than half

the magnitude in the center of the LC itself. Over a horizontal

distance of -500 km the slope would give a surface current of -6 cm/sec.

This is a conservative estimate, assuming continuity with the observed

flow in the western boundary discussed by Sturges (1993).

The general pattern of winds over the Gulf of Mexico is dominated by the

trade winds blowing from east to west. The winds are much stronger in

the south, with velocities decreasing steadily toward the north. There

is a rotation in the far west, so that the winds blow more from

southeast toward the northwest, but this is a secondary effect. We take

1 dyne/cm2 as a typical value over the southern half of the Gulf, where

74
the trade winds are strong. To determine the amount of work supplied by

the wind, the product t v, the instantaneous value, must be integrated

in time for about one year, the time scale of ring formation. The

result is easily shown to be 12 x 1016

The winds are lighter over the northern half of the Gulf, but still to

the west, while the geosti-ophic return flow must be of opposite sign.

So the product 't v in the northern half will be of opposite sign from

that in the south. Assuming for simplicity a mean wind Stress in the

north of about half that over the south, the net work done by the winds

over the whole Gulf is thus very roughly estimated as -6 x 1016 cj- Recall that the APE in a ring is only -8 x 1015 J.

These rough estimates suggest that the work done by the wind is not only

adequate but is ample by an order-of-mag-nitude to force down the volume

of one detached ring per year. We have tried to be conservative in the

estimate of wind forcing, but even if we are in error by a factor of 2

or 3 the argument holds. While this is certainly not conclusive proof,

but seems a necessary condition. A more convincing argument is provided

by the high coherence between the variability of wind curl and LC, as

described below.

' LC Data

The monthly position of the northernmost extension of the LC front used

in these calculations is largely from the prior studies already

identified. The earlier location data were taken from the set shown by

Sturges (1992); the more recent are from satellite altimetry results

provided by Leben (see Section 42 for addition discussion of these

recent data) - In some previous studies the variability of this position

has been used as a surrogate for the times of LC separations, although

this is only an approximate measure. A better metric for the present

purpose would be the area enclosed by the full LC, but such data are not

reliably available over a sufficient interval. The newer data allow

locations of the northernmost extensions to be determined on an

approximately weekly basis, but we have used monthly positions to be

consistent with the older data.

Wind Curl Data

The wind data we have used is from the NCEP Reanalysis Project,

available at their NOAA web page (http://wwwcpc.ncepnoaagov). We

computed the stress using a constant drag coefficient. The more

involved method of a variable drag coefficient may be useful, but we are

interested here primarily in whether the curl forcing and the LC

variability are coherent. We suspect that the presumed reduced

reliability of the LC positions in earlier years obviates the need for

more sophisticated calculations of wind curl.

To determine the curl, we computed the line integral of stress along the

outer edge of the Gulf, but are restricted by the 2.5° resolution of the

NCEP winds. We computed the curl over two regions: first, the basic

area f the Gulf, including the Bay of Campeche; second, as a trial

calculation, we added an area to the west of Cuba, north of 20°N,

75
between Cuba and S. America. The northward Ekman transport in this

region is also constrained by the coastal boundaries to be forced

downward with the rest of the Gulf, so it would seem that this pumping

should also be included. As it turns out, the details of the results

from the two approaches were changed negligibly.

4..34..3 Conparison of LC Variability and Wind Curl Forcing

Figure 4.3-3 shows the power spectra of the LC variability and wind

stress curl variations. Because of the assumed physical behavior of the

system, the signal we call "LC" here is the time derivative of the LC

front position, used to determine the -S motion of the intrusion of the

LC. The lack of power at the annual period in LC position is an odd but

clear result. To make these signals comparable, the wind curl shown in

Figure 4.3-3 has been pre-filtered to remove the mean annual cycle from

the curl. It is obvious that the two spectra are quite similar.

Figure 4.3-4 shows the cross spectra between these two. The three

panels show the cross-spectral power, the coherence, and phase. The

coherence between the wind curl over the whole Gulf plus the added

region to the west of Cuba is only slightly higher than without the

added region, and that calculation is shown here. The coherence is

above the 90% confidence limits at all freq-uencies where there appears

to be substantial amounts of power in either signal. In other words, at

periods other than near annual, the two signals are highly coherent.

The coherence of the two signals is a surprising result. One important

aspect, with regard to our basic hypothesis, is that the curl forcing

and the motion of the LC front for the frequency region where most of

the power lies (periods of 9 - 14 months) are 180° out-of-phase. That

is, when the curl forcing was most strongly negative, the LC motion had

its maximum rate of advance to the north. This result is consistent

with, but does not prove, the notion that the curl forcing (or Ekman

pumping) was responsible for the variability of LC intrusions. We

should emphasize that the high coherence shown in Figure 4.3-3 is found

with the signals determined from the curl over a basic box around the

Gulf, with and without the small region in the Bay of Campeche, and with

and without the region west of Cuba. In other words, this result seems

robust.

S The nnual Cycle

We introduced a new mechanism of exchange between the LC waters and the

Caribbean Sea via a deep outflow; we have tried to explain here the

observation that the motion of the LC front is coherent with the curl

forcing over the whole basin. It may be more difficult to explain

"something that does not happen," namely the lack of coherence at the

annual period. We offer the following argument which may be plausible,

if highly speculative.

76
Figure 4.3-3 Power spectra of the north-south motion of the Loop

Current and of wind stress curl over the full Gulf of

Mexico. The Loop Current signal is from the time

derivative of the northern-most position of the Loop

Current front, from a variety of data sources. The wind

curl is computed from the NCEP reanalysis wind data over

the Gulf of Mexico plus a small region south of Yucatan

Channel and. west of Cuba. Smoothing is by 5 Hanning

passes. The mean annual cycle of the curl was removed

from the signal before computing the spectrum. The

monthly data begin in July 1973.

77
Figure 43-4. Cross spectra between the wina curl and. Loop Current

variability (the signals of 4.3-3). The upper panel

shows cross spectral power; middle panel, coherence

squared, and lower panel phase. Smoothing is by 5

Hanning passes. The 90% confidence limits are shown.

78
The arguments in the preceding sections have been made with the implicit

understanding that the Skinan pumping forces upper-layer water down and

ultimately out via some deep exit route. In a steady circulation

pattern, this would be a complete set of ideas. But the wind forcing

over the open Atlantic gives rise to the well-known annual cycle of the

Florida Current transport. The flow in the Caribbean and Gulf is the

result of the wind forcing over the whole Atlantic; the Gulf Stream is

the return flow. As the transport into the Caribbean Sea - Gulf of

Mexico increases as a result of the annual cycle of wind forcing, more

water is forced into the Gulf as LC inflow. As the flow increases, the

initial response to this increased flow through Yucatan Channel would be

for the LC to penetrate farther into the Gulf of Mexico.

If the Ekmnan pumping over the surface of the Gulf were constant, we

speculate that the LC might indeed have an annual cycle. However, the

Ekman pumping over the Gulf also has a substantial annual cycle,

separate from the annual cycle over the open Atlantic Therefore we

suggest that the deep outflow associated with the annual cycle of Ekman

pumping perhaps balances (to a first approximation) the changes in flow

associated with the annual cycle of the Gulf Stream transport.

It is appropriate to compare the phases and the amplitudes of the annual

cycles of these two flow patterns. Figure 4.3-5 shows three versions of

the annual cycle of the wind curl: one over a basic rectangle covering

most of the Gulf (the box determined largely by the 2.5°-degree

resolution of the NCEP data (22.5° - 27.5°N, 85-97°W); the second,

including the curl forcing over the Bay of Campeche; and the third

including the forcing in an area west of Cuba, as described above. The

reason for showing all three curves is to emphasize the fact that the

phase of the forcing signal is insensitive to the details of the area

selected.

The maximum downward pumping occurs during July. Recall that the annual

cycle of the Gulf Stream transport also has it maximum in approximately

July and minimum in October, despite the fact that the wind forcing over

the Atlantic is phase shifted considerably from this timing. The annual

cycle has been very well documented, from the early work of Fuglister

(1951), through the studies of Niiler and Richardson (1973), more

recently by Sate and Rossby (1995), and others. These two seemingly

unrelated processes, the curl over the Gulf and the annual transport of

the Gulf Stream, are approximately in-phase on the average; there will

be large variations from this on a year-to-year basis.

The magnitudes of the variability of various parts of the flow can be

estimated as well. The magnitude of the annual cycle of the Gulf Stream

in the Florida Straits was estimated by Niiler and Richardson (1973) to

be a peak of -33.6 Sv in early summer and a minimum of -25.4 Sv in early

winter. Sato and Rossby also find that the fluctuations are plus or

minus -4 Sv, peak to peak; the standard deviation of the monthly means,

however, is -3 Sv, despite having a large dataset based on many years of

observations -

79
Figure 4.3-5. The mean annual cycle of wind curl over the Gulf of

Mexico from the NCEP data. The dash-dot curve labelled

"Main" is for a basic rectangle extending from 22.5°N to

27.5°N, 85°W to 97.5°W. The dashed curve labeled "Main &

SW" also includes the region over the Bay of Campeche;

the full curve labeled 'Main & SW & SE" also includes a

region to the west of Cuba that also confines the

northward-directed Ekman transport.

80
For comparison, the magnitude of the annual cycle of wind curl over

Gulf, shown in Figure 4.3-5, has a peak (negative) value of --5.5 and a

minimum of --1.4. Thus, we can estimate that the forcing changes by

plus or minus -2 over its mean value of ---3.5.

There is a simple and nearly independent way to estimate the downward

transport associated with the Ekman pumping; we estimate the magnitude

of the flow forced by winds over the Gulf in comparison with the

equivalent forcing over the North Atlantic. Most maps of open ocean

wind curl show the line of maximum curl going over Bermuda and extending

into the Gulf. We can estimate the Ekrnan pumping in the Gulf merely by

knowing the ratio of the width of the Gulf to the width of the Atlantic,

and by the relative magnitudes of the curl in the Gulf to the curl over

the open Atlantic. The analyses of the COADS winds by Mayer and

Weisberg (1993) show that the curl forcing over the Gulf in summer is

essentially the same as in the western N. Atlantic. During the winter

the curl in the Gulf is roughly half that over the Atlantic. The ratio

of the width of the Gulf to the width of the Atlantic is (in degrees of

longitude) -15°/70°, or 0.2. Therefore, we may use a ratio of 0.5 -

0.75 for the curl, and estimate the Sverdrup transport in the Gulf as

-[0.2 * (.5-.75) * 303 Sv, or -3-4.5 Sv, depending on the fraction of

the curl one chooses.

Thus, by this order of magnitude calculation, the variability in wind

curl pumping over the Gulf can easily be determined. Given a mean value

of -3.5 Sv, with a seasonal variability of 2, we estimate that the

variability of the curl-forced transport over the whole Gulf that could

be associated with an annual cycle would be on the order of ±2 - 3 Sv.

While this is not exactly the same value as the annual cycle of Gulf

Stream transport, it seems close enough to provide an interesting basis

for further study into the relationship between these important

variables.

4.3.5 Discussion and Conclusions

The arguments presented support the suggestion that the curl of the wind

stress is the major contributing factor to the irregular behavior of the

northward advance of the LC front. The issue requires further study for

a clear proof and thorough understanding. The lack of an annual cycle

in the LC, as demonstrated in Figure 4.3-2, despite the strong annual

cycle in the wind forcing remains an elusive issue.

A final point to mention is the time scale of the downward pumping. The

net vertical motion forced by the Ekman pumping is on the order of 50

rn/year. Thus, to force surface water down beneath the incoming current,

to -l000m, will take a time on the order of decades. It is worth

pointing out that after about a year, the "old ring" in the western Gulf

has been sheared apart and lost its identity. The tattered remnants of

many old rings must remain in the Gulf, being flushed out at about one

per year. There is ample time for mixing.

81
V. SLOPE CIRCULATION PATTERNS

5.1 Introduction

The prior chapters of this report begin establishing the complex nature

of the often concurrent circulation producing mechanisms as well as the

three-dimensional complexity of the resulting circulation patterns.

Chapter 5 beings to describe both the documented patterns of currents as

well as the associated dynamical features that appear to produce these

patterns -

A broad range of horizontal scales-of-motion were evident in the DeSoto

Canyon during the field measurement period. The presence of

oceanographic features producing circulation patterns in the DeSoto

Canyon have been documented for at least 35 years (Figure 5.1-1),

however, access to synoptic documentation of broad regions of the Gulf

of Mexico became possible with the availability of satellite imagery and

altimetry (Chapter 4) . These remotely sensed geophysical measurements

in combination with in-situ current/temperature time series and shipsurvey observations begin to provide a basis for a systematic

descriptions, in spite of the limitations in the remotely sensed data

that result from seasonal availability (thermal) or horizontal

resolution (altimetry)

The following material in Chapter 5 presents a reconstruction of

features present in the study area during the two-year field measurement

effort, as well as a description of current and hydrographic patterns

associated with these features. Observations were analyzed to

characterize currents and transport at a number of time scales.

Additionally, the in-Situ current and temperature data have been used to

describe slope and shelf break dynamics as well as fluxes of temperature

and momentum.

5.2 Event Description

The following is a chronological description of the more significant

events observed during the two-years of field observations. The

measurements are subdivided into eight intervals representing times when

a particular process appears to have had a controlling influence on the

observed flow field. Time series of current vectors from the upper 8 to

40 m at all sites are shown with winds at buoy 42040 located on the A

mooring transect and coastal sea level at Panama City in Figures 5.2-laid. Currents at the 300m depth from all sites are shown in Figures 52-

2a-2d.

Period 1: Mardi 21 - May 1, 1997

Satellite derived sea surface temperature (SST) and sea surface height

(SSH) fields indicate that during this interval the southwest portion of

the array was influenced by a warm intrusion from a streamer trailing a

former cyclonic Loop Current (LC) frontal eddy (LCFE) that had moved

southeast of the array around a Loop Current ring (Figure 5.2-3a) This

streamer apparently spun-up into a warm anticyclonic eddy (ACE) that

caused strong eastward flow over the upper 500m at A3 (Figure 5.2-2a).

83
c. ,fl - ---- 32 - NV -'-L::- -- ___-.1_- 3 I-. ( : / ... 'I. Jinrt rhsIn '.- _1 ; L 2 _.__;, ' -- t':----- \ ' ao ' I 

thi . 1-1 Ccj:I.or : aI1nry s a 'nitt Ir tfln.En bbJ .t.ri:c . 3.

ttAE prciec. 1ce LE aaxn nt s.cri ± s1i: lr f 

,.p t t1 AcAxi:y LL Lh kJ....L L.dk :2uUt. ct.'u * Low

t.- ,rat bsi.it c: cvx th y t: +o

i t: p:r crycr rl- i: 1i:ve irP--nc w**y 1w 1i*

h.1f irto o1r1tjv' j '1 S .f. (a -7.i -a17 M C' - 5 -5
- ts-,

g

5-_

0 £ -s--

-15 - 00 -= - 50-a

40 E 30-A 2 0 20-A

inN0

I'-N

'4

6040- 1- 20- - 0 -40 -

SO

A A A ___ VT TV

A1509 isobath

c

5'

-' 44 1)L- f402Jr

yeI1 14Mt

Croon Isoboth & 0ffshor

Iii

NDBC Buoy 42040 (R75)

Panama City

1 6m

20m

29m

1 2m

12m

1 6m

I 6m

12m

8m

Bm

Figure 5.2-la. Subtidal current vectors from depths of 8 to 40 m for

all mooring sites from March to September 1997. Frame

of reference has been rotated so that for each mooring,

vectors directed upward represent along isobath flow

with an easterly component.

85

'9 10 tO 5 30 24 19 13 3 Msrch April Moy Juno July Augunt op1onnbor 1907 DeSoto Canyon, 40-HLP Upper Currents, All Sites
aN

C,

--

-IS80 -

5-

-20-

-40 -

N

410

ci N

N

N 75-

o10 5Q25-5 - E I ci =

-50 - 75

Along inobath Croco i5QbaIIl & 011ohoro

NDBC Buoy 42040 (R75)

Panama City

1.1i1AA A £ A 4A. a & A TV TV r 'r '

iP

//(J/ ,Ii,

DeSoto Canyon, 40-HLP Upper Currents, All Sites

I 6m

. L 12m

8m

Figure 5.2-lb. Subtidal current vectors from depths of 8 to 40m for

all mooring sites from September 1997 to March 1998.

86

13 7 27 21 11 10 4 1 26 Septsmb8r Oclotier Novernb.r December Jenuory F.bruery )lorch

1908
,. 15 0 10-

125 i

100-_

75- E 50:

- 25-s

0

-25

SO

I'-

0

N

"0

N

In

0

No 5) 5)0

I'- N

100-c

50-

I

-50-

-100 -

19

SS41. làJL 8m

\Phi Bm

-.1101

16 10 5 March April May 199

Alang isobath Cross Isobath & Offshore

30 June2 July

DeSoto Canyon, 40-HLP Upper Currents, All Sites

NDBC Buoy 42040 (R75)

19 13 5 August September

Panama CIty

1 6m

20m

I 6m

29m

12m

12m

8m

Figure 52-lc. Subtidal current vectors from depths of 8 to 40m for

all mooring sites from March to September 1998.

87
0) N N

0) -(

=1

C

N

c-I

C00)

LI

I00

50 Eo C) 50) 100

.1 Alon9 noboth

3m L,. jLSeptember October

I......J.

27 2* 16 10 November December January 1969 DeSoto Canyon, 40-HLP Upper Currents, All Sites

NDBC Buoy 42040 (R75)

Panams City

I 6m

12m

3m

Sm

8m

Figure 5.2-id. Subtidal current vectors from depths of 8 to 40m for all mooring sites from September 1998 to March 1999.

88

26 February March

01016

>' 20-

C, - 10_;z

-20

125 100 - 75 - 50 : 25 - 25
89

40-

300m

*iY

f'/f 40 Arôn ibth

40 t

2- Cross Isobsth & Offshore

- _A 300m

40-

40-

2:

AII& 'j//j 300m

20---

'i / f/i

40- 40

i :i-

::'-" - ::

MARCH APRIL MAY JUNE JULY AUGUST SEPTEM8R 1997

DeSoto Canyon. 300m level

Figure 5.2-2a. Subtidal current vectors from 300m depths at mid-slope

moorings (A2, B2, C2 and D2) from March to September

1997.
60-

40-

20-

0 20-

-40-

-60-

60-

40-

20

(4 - 20--

-40 -

60 -

60-

40-

20-

-20-

-40-

-60-

60 -

40-

20

a u__z0 40-

60 13 7 SPTFM BEN 1997

AIng isobath

Lcmss sobath & OffohorN -A

____ 300m

300m

DeSoto Canyon, 300m level

300m

300m

90

Figure 5.2-2b. Subtidal Current vectors from 300m depth at mid-slope

moorings (A2, 32, C2 and D2) from September 1997 to

March 1998

I1 I,

2 27 21 16 10

OCTOBER NOVEMBER DECEMBER JANUARY

I9UR 4 FEBRUARY 26

MARCH
40-

i 20-

I - 20--

-40 -

60-

20

0

60-

60-

40

'0 --

Q 20---

-40 -

60

6040

20

0 20

40

U 20

40

60

20-

19 1'

w

MARCH 1998

H

Also9 i5Rbth LCmOSIsoboth & 0ffsho

ir -1,- /j\\

I J 16 10 5 30 24

APRIL MAY JUNE JULY DeSoto Canyon, 300m !evel

'r 300m

300m

300m

II 7 2

SEPTEMBER

Figure 5.2-2c. Subtidal current vectors from 300m depth at mid-slope moorings (A2, B2, C2 and D2) from Narch to September 1998.

91

19 13

AUGUST
Figure 5.2-2d. Subtidal current vectors from 300m depths at mid-slope moorings from Septendjer 1998 to March 1999.

92

60 300m

r -

- Along isobath 50 -

50-

40 Cross Isobath & Offshnr 20

300m

20 H 300m

0

-

I

20-

0 - -. 300m

20T 40- 1) I 13 7 2 27 21 15 10 4 I 26

2PTEMRER OCTObER NOVEMRER MRER JAHUARY FERRUARY MARCH 1998 19990

DeSoto Canyon, 300m level
i r i.- rr LJss 1.0 1A. F_'t. T 1. ., 1i t4'.iL i'n rirrrnt vrtri nc wrr ii :i lis mfl iTt t L umt t ljLct.t r.idy rh. rrE prqr.
In the SSH fields this waliLt streamer is shown as a surface ridge that

wrapped around the surface low from the LCFE (Figure 5.2-3b) - Contributing to the flow variability during this time was the

development of a 100 km scale cyclonic shelf eddy (CSE) that produced a cyclonic flow pattern from mooring lines A-D and resulted in offshore

exchange of cool shelf water at line B (Figure 5.2-3a) . Currents along

the shelf break also appear to have a significant wind influence during

this time with alongshore flow events following alongshore wind events

and in-phase with coastal sea level changes. Flows deeper than 200m were mostly westward, although variable and there was a positive offshore

temperature gradient at depth that further supports the finding of a

cold eddy offshore that was responsible for entraining the warm streamer

into the array in the upper layer.

Period 2: May 1 - September 9, 1997

Period 2 is characterized by strong eastward flow at shelf break and

mid-slope sites and westward flow in the offshore (l500m) and at depths

below 20Gm (Figures 5.2-la and -2a) - Although satellite SST images w.ere

not useful during summer SSH fields show the Loop Current merging with

the ring from Period 1 and extending northward to about 27.5N. A trough

extends north of the Loop to 28.7'N and a ridge extends north of the

trough into the moored array (Figure 5.2-4a) - The trough appears to

develop from a LCFE and causes the strong westward flow at offshore and

deeper sites. The ridge most likely comes from a warm streamer that

extends into the array and evolves into an ACE that supports the strong

eastward flow in the upper layer at the shelf break and mid-slope.

Shipboard hydrographic data of July 8-16, 1997 are consistent with this

above description. A tongue of high salinity LC water extends into the

DeSoto Canyon region from offshore. Near surface geostrophic currents

showed this as an onshore intrusion of LC water (Figure 5.2-4b). A

significant eastward jet along the salinity and temperature front at the

shelfbreak and westward flowed offshore and at 500m (Figure 5.2-4c)

Cross-shelf temperature gradients from the array and shipboard data are

mostly negative in the upper layer during this time and indicate a warm

intrusion from the south. Persistent eastward winds developed over the

period June 14 to August 3 and may also have contributed to the eastward

flow in the upper layer of the shelf break.

Period 3: November 1, 3.997 - February 4, 1998

Period 3 is associated with persistent eastward flow over most of the

array. The eastward flow was stronger and more persistent in the western

part of the array where it extended to depths of 300 to 50Gm at the

offshore stations on lines A and B. (Figure 5.2-lb and -2b) At lines C

and D the flow was more variable with strong onshore and up-canyon flows

during the last half of this period. The eastward flow continued to

follow the topography to the south in the eastern part of the array

causing southward alongshore flow at mooring E. Shipboard derived

geostrophic currents also show the eastward jet during Novemher 11-19,

1997 (Figures 52-5a and -5b) to have been stronger in the western part

of the array. The jet extended along the slope throughout the array and

followed the topography toward the south in the eastern part of the

array. The vertical extent of the jet was at least 50Dm.

94
Gulf of Mexico Sea Surface Height

TOPEXJERS-2 sea surface height anomaly

plus 1 0-year chmatological model mean

cm

30

25

20

15

10

5

0

-5

-10

-15

-20

-25

30

95

Figure 5.2-3b. SSH for April 30, 1997 derived from TOPEX/ERS-2 plus

model mean. Dark contours are for SSHs that are greater

than zero; white contours are for SSHs less than zero.
30°N

28°N

26°N

Gulf of Mexico Sea Surface Height

TOPEX/ERS-2 sea surface height anomaly

plus 10-year climatological model mean

7,

20°N

90°W 88°W 86°W 84°W 82GW 80°W

cm

30

25

20

15

10

5

0

-5

96

Figure 5.2-4a. SSH for July 1, 1997 derived from TOPEX/ERS-2 plus model

mean. Dark contours are for SSHs that are greater than

zero; white contours are for SSHs less than zero.

24°N -10

-15

-20

22°N :- :

-25

30

07/01/97
C

R/V PELICAN PE-9803

90GW 69W

31 N

90W

R/V PELICAN PE-9803

90W 89W

31N

30N

29N -

28N

90.w

/

89W

8W

68W

S- .....

-- -' - / -

I

I

88W

7/ 8 TO 7/1 6/1 997

87W 86GW 85W

31 N

6 - l000m Geostrophic Velocity

7W

87W

500 - i000m Geostrophic Velocity

B5W

20 cm/S

28N

7/ 8 TO 7/16/1997

66W 85W

31N

85W

3ON

- 29N

28N

Figure 5.2-4b,c. Geostrophic currents at b)

level of no motion at 1000

during the July 8-16, 1997

are for currents measured

indicated by dots

97

6m and c) 500m, based on a

m using CTD profiles taken

cruise. Heavier vectors

at the mooring sites
6- l000m Geostrophic Velocity 11/11 10 11/1 9/1 997 90W 8W a7w 85GW 31N I 31N

a 40 cm/s

30N -

29N

28N

R/V PELICAN PE-9820

90GW 89W

31 N b)

30N

29'N

28N

90W

90W

R/V PELICAN PE-920

I

89W

/ _I.-

)(/ /

- _----: -_' . _I

-

I-.

i - /

_Sf

88GW 87W

::- v' .._/ "

/ I ' -' I -

I -

-, I I

I I 89W 87'W

300 - 100Dm Geostrophic Velocity

I 11/11 TO 11/19/1997

85W 5.W

31 N

85W

30N

85W

2SN

30N

2N

28N

Figure 52-5a,b. Geostrophic currents f or a) Gm and b) 300xn, based on

a level of no motion at l000m using CTD profiles

taken during the November 11-19, 1997 cruise.

Heavier vectors are for currents measured at the

mooring sites indicated by dots.

98

88W 87W 85W 85GW
The SSH fields indicate that a large LC ring formed in October just

prior to this period (Figure 5.2-5c). A LCFE was present northeast of

the ring, with a filament ridge wrapping around the eddy and into the

array. By mid-December when strong east flows were widespread, this

ridge had spun-up into an ACE over the array, with a cyclonic feature

located offshore separating the ACE from the LC ring and the LC (Figure

5.2-6). These features continued while the LC extended northward into

the eastern Gulf, eventually reaching 28' with a trough at its northern

extreme that extends into the array. Apparently, the northward intrusion

of the LC and trough caused the ACE over the array to move to the west

ending the eastward flow event.

Satellite derived SST patterns in early December clearly show the

presence of a warm eastward jet over the array, as well as the LC ring

southwest of the Mississippi River delta and the immerging new LC south

of the array (Figure 5.2-7). Also shown is a long streamer of slope

water that was entrained offshore along the east side of the ring south

of the delta. In addition, the SST patterns indicate the presence of 50

km scale cold, cyclonic eddies that developed on the temperature front

on the north side of the warm jet. These features can enlarge up to 100

km over time and appear to have been active in shelf/slope exchange

judging from their association with offshore streamers or squirts of

cold shelf waters. Near the end of the period, SST images clearly show

the LC extended near 28N with a LCFE at the northern extreme and just

south of the instrument array (Figure 5.2-8) . Also visible in the image

is the cold shelf eddy that had enlarged to about 100 km and caused a

cyclonic circulation pattern between mooring lines B to D. The ring had

moved further to the west increasing the separation from the LC. A

strong LCFE was present southwest of the Tortugas, producing a narrowing

of the LC.

Winter wind forcing patterns apparently contributed to the increased

flow variability at the shelf break although many times overshadowed by

the eddy-induced flow changes. A particularly strong westward wind event

occurred near the end of the period from February 2-4 that caused

westward flow through the array coupled to a coastal sea level rise

(Figure 5.2-ib)

Period 4: February 4 - Jtily 7, 1998

During Period 4, flow was generally westward throughout the array,

except in the upper 30 to 40m along the shelf break where the flow was

eastward, although with more variability. SST was especially clear up

until early May and shows strong influence of frontal eddies and

filaments that extended into the array from a recently separated ring,

resulting in the westward flow at the offshore sites (Figure 5.2-9)

Considerable offshore exchange is indicated during this time as shown by

the presence of elongated offshore squirts of cold shelf water in the

vicinity of the array. The young LC showed strong interaction with the

southwest Florida shelf near the Tortugas and a Tortugas cyclonic eddy

was well-developed south of the western Florida Keys. By mid-April the

frontal eddy south of the array was well-developed and a warm streamer

wrapped around the eastern side of the eddy and into the array (Figure

52-lOa) - The streamer appears to have caused the westward flow to split

99
30°N

28N

26N

24°N

2TN

10/01/97

20N

90GW 88GW 86°W 84GW 82GW 80°W

Gulf of Mexico Sea Surface Height

TOPEX/ERS-2 sea surface height anomaly

plus 10-year chmatological model mean

I

100

Figure 5.2-Sc. SSH for October 1, 1997 derived from TOPEX/ERS-2 plus

model mean. Dark contours are for SSIs that are greater

than zero; white contours are for SSHs less than zero.

cm

30

25

20

15

10

5

0

-5 -10

-15

-20

-25

-30
26°N

24N

22N

Gulf of Mexico Sea Surface Height

TOPEX!ERS-2 ea surface height anomaly

plus 10-year climatological model mean

çc

I,

e

100

-

- Ii;-t.-

12/15/97

20°N

90GW 88GW 86GW 84°W 82GW 80°W

30

5

-15

101

Figure 5.2-6. SSH for December 15, 1997 derived from TOPEX/ERS-2 plus

model mean. Dark contours are for SSHs that are greater

than zero; white contours are for SSHs less than zero.

s. cm

30N

28°N

25

20

15

10

0

-5

-10

-20

-25

-30
2 ,ZJ3 5'3L' 081)ec17. U23CST SST (C11u J4J '-. - + $ .- __L Piuu- 5.2-7. T b . I97 iiur f'i br VA 'ar. UM iI 

Le_ iav4 1 frr rThr&ty Th gbtwa1d CC'd 

nvirrd b L itvt1y 1rrnr -i.ur L -cL w
OeMP OTh)CSi' ICo1MiuRj T 'c:r for Pi r;, ire ci1 the i ..ri.t11:i rh h.ivr irrr*
=3CS USGS. I =: .. t:jE. 4' - A ., : t .3 1 Ijrnrt rj T i c: an rr r''r' ., 1 r r.r m porz The :.3p u.-i-t :C. cta= &.zy LL!C b1e.
1. T :waq hpr11 :i 'U4 s:r:s Mr::i:rwI irrri,r r. th. ime i :h' t.'- TAfltifY. :h+ J:i cLc ,r
within the array with up-canyon and eastward flow on C, D and E lines

and westward and offshore flow on A and B lines. Similar flow properties

were observed for geostrophic currents derived from shipboard data

(Figures 5.2-lOb and -lOc), where westward flow extended to at least

500m in the western part of the array and up-canyon flow was estimated

for the eastern part. The young LC was located near 25'N west of the

Tortugas. A band of cold shelf water appears to have been moving

southward all along the west Florida shelf and into the inner shelf. The

SSH and SST fields for mid-April show similar patterns with a. large

cyclonic eddy on the northeast side of a newly formed ring and a ridge

(streamer) that wrapped around the eddy and into the array (Figure 5.2-

11) - This pattern continued throughout Period 4.

Period 5: Jifly 7 - August 29, 1998

Period 5 is characterized by eastward currents in the upper layer at

most of the mooring sites, except for Cl, which had westward flow

(Figure 5.2-ic) . At 300m the flow was also eastward until late July when

it shifted to be mostly westward (Figure 5.2-2c) . SST images are not

available for this period, however, the SSH fields indicate that a LCFE

was located offshore of the array on the boundary of the previously

separated ring. A ridge wrapped around the east side of the LCFE and

appears to spin-up into a ACE close to the array (Figure 5.2-l2a)

Eastward flow that is expected to develop on the north side of the ACE

would be consistent with the observed eastward flow pattern in the upper

layer, where as at 300m the flow was more influenced by the offshore

cyclonic eddy. Shipboard measurements during August 4-12, 1998 further

support this interpretation. Geostrophic currents near the surface show

a strong eastward jet along the shelfbreak and mid-slope, with a small

cyclonic eddy in the nearshore part of the canyon (Figure 5.2-12b). The

eastward flow decreased below 300m and was replaced by westward flow

over the deeper region of the slope (Figure 5.2-12c) - The clockwise

circulation within the ACE was also shown by near surface drifter

patterns from two drifters launched in early August east of the

Mississippi delta and later entrained in the ACE, making several

clockwise circuits from August 10-30 (Figure 5.2-12d) - This interval is

also noted for a time of prolonged eastward winds. However, winds speeds

were not strong and did not appear to have a significant influence on

observed currents.

Period 6: August 29 - October 29, 1998

Period 6 is noted for its strong wind forcing resulting from two

hurricanes that passed over the array. Hurricane Earl moved from west to

east through the outer part of the array on September 2 and 3. Hurricane

Georges moved from south to north directly over line A on September 27-

28. A strong current response was observed at inertial frequencies for

the deeper sites and is discussed elsewhere in this report. The

subinertial currents were generally westward over most of the array

during this period (Figure 5.2-id and -2d). The two hurricanes are

indicated by strong westward flow events during their passage over the

array. Surprisingly, little eastward flow occurred. SSH fields indicate

that there was a weak ridge over the array with a stronger offshore

trough that may have had a controlling influence on the persistent

westward flow. Although the winds were also westward, except during the

106
R/V PELICAN PE-9830

90GW 9W

31 N

C

30N

29N

28N

R/v PELICAN PE-9B30

90W 89W

31 N

30N

29N

28N

90W

88GW

6 - 1000 m. Geostrophlc Veloctty

3/31 TO 4/ 8/1998

87W 85W

31 N

1 //1 /'\-'-'---

89W 87W

300 - 1000 m. Geostrophic Velocity

sew

40 cm/s

>

85W

28N

3/31 TO 4/ 8/1998

86W 5'W

31 N

ç 30N

- 29N

28N

Figure 5.210b,c. Shipboard geostrophic currents for b) 6m depth and

c) 300m depth, based on a level of no motion at

l000m for March 31 April 8, 1998. Heavier vectors

represent currents measured at the mooring sites

indicated by dots.

107

0w 89W 87W 86W

88W 87W
30N

28N

26'N

24N

22N

Gulf of Mexico Sea Surface Height

TOPEXTERS-2 ea surface height anomaly

plus 10-year chmatological model mean

I

SS

20N

90°W 88°W 86GW

04/12/98

84°W 82°W 80°W

cm

30

25

20

15

10

5

0

-5

-10

-15

-20

-25

-30

108

Figure 5.2-11. SSH for April 12, 1998 derived from TOPEX/ERS-2 plus

model mean. Dark contours are for SSIIs that are greater

than zero; white contours are for SSHs less than zero.
30°N

28°N

24N

22N

Gulf of Mexico Sea Surface Height

TOPEX/ERS-2 sea surface height anomaly C

plus 10-year climatological model mean

k)

08/01/98

20N

90°W 88°W 86GW 84°W 82GW 80W

cm

30

-15

-20

-25

-30

Figure 5.2-12a. SSH for August 1, 1998 derived from TOPEX/ERS-2 plus

model mean. Dark contours are for SSHs that are

greater than zero; white contours are for SSHs less

than zero.

109

26 N I'

25

20

15

10

5

0

-5

-10
R/V PELICAN PE-9908

90W 89W

31N

30N

29N

25N

90W 89W

R/V PELICAN PE-9908

0W 89W

31 N

30°N

29N -

28N

90W

I

8gw

88W

88W 87W

6 - 1000 m. Geostrophic Velocity

68W 87W

8/ 4 TO 8/12/1998

87W 86W 85W

88W 87W

500 - 1000 m. Geostrophic Velocity

S6.W

31 N

40 cr0/s

>

20 cm/s

>

- 29N

85W

8/ 4 TO 8/12/1 998

86W 85W

31 N

85W

30N

28N

30°N

- 29N

25N

Figure S.2-42b,c. Shipboard geostrophic currents for b) 6m depth and c)

500m depth, based on a level of no motion at l000m

for August 4-12, 1998. Heavier vectors represent

currents measured at the mooring sites indicated by

dots.

110
DRIFTERS 11887 11896 11897 8/5 TO 9/4/1998

90'W 89'W 85'W 87'W 86°W 85'W 84W 83°W 82W

31N I 31'N

Figure 5.2-12d. Trajectories of satellite tracked near surface drifters

11887, 11896 and 11897 from August 5 - September 4,

1998. Arrow heads are placed every ten days of the

plotted trajectories.

111

goow 89.W 880W 87W 8 6'W 85'W 84°W 83'W 82W
second half of Hurricane Georges, which could also have contributed to

the westward flow at the shelf break.

Period 7: October 29, 1998 - January 15 1999

During Period 7, flow was generally eastward over the upper 3COm for

most of the array (Figures 5.2-id and -2d). The shelf break stations

displayed more variability than offshore sites. This pattern may have

been partly due to local wind forcing. Satellite SST imagery shows a

warm 200 km scale eddy or remnant ring to the south of the array during

this time that appears to have been influential in driving the eastward

flow (Figure 5.2-13) . The SSH and SST fields generally show similar ACE

and LC patterns (Figure 5.2-14a). Shipboard derived geostrophic currents

between December 1-9, 1998 show the strong eastward currents over the

upper 500m of the slope (Figure 5.2-14b and -14c) . A satellite tracked

near surface drifter, launched near the head of the Desoto Canyon on

November 21, was entrained by this warm eddy in mid-December of f the

middle of the west Florida shelf (Figure 5.2-14d) . The drifter was

carried around the eddy in a clockwise direction over the next several

weeks and then moved back to the north into the eastern region of the

array. This northward intrusion into the array appears to have been

assisted with a warm streamer that extended into the array from a

frontal eddy moving around the remnant ring. In March, the drifter moved

back onshore close to the region where it was originally launched. A

strong negative cross-shelf temperature gradient occured over the array

with considerable small-scale structure indicating water mass

interaction and cross-shelf exchange. The LC had begun to reform and

extended northward to about 26.3°N just south of the warm eddy.

Period 8: January 15 - March 15, 1999

Period 8 was a somewhat confusing interval with variable flows over the

array. At mooring A3, currents were eastward over the upper 50Cm for the

whole period, whereas at the shelf break and mid-slope, the flow was

more often toward the west although with occasional eastward flow

(Figures 5.2-ld and -2d) . Fortunately, there were many days with clear

SST images during the period that helps explain the observed

variability. Apparently, the warm eddy, mentioned above, continued to

influence currents in the array and caused the persistent flow at A3

(Figure 5.2-15) . A series of small (50-60 km) cyclonic eddies developed

on the shoreward thermal front of the warm intrusion causing flow

variability within the array. The LC extended north to about 26.7N in

late January, with a large frontal eddy centered near 26°N on its

eastern border, trailing a filament into the region of the warm eddy.

This pattern continued for most of February. By the end of February and

early March the warm eddy moved to the southwest away from the array and

caused strong offshore flow to the southeast at A3 (Figure 5.2-16a)

Shipboard estimates of geostrophic currents for March 28 - April 5, 1999

also show generally offshore flow near the surface in the western half

of the array with a cyclonic shelf eddy in the DeSoto Canyon that caused

up-canyon flow (Figure 5.2-l6b). Ceostrophic currents at 300 and 500m

depths were weaker and toward the west (Figure 5.2-16c). The LC extended

to about 27°N at this time and apparently forced the

112
- LIIj tJJL iu it I4IrI1 rT:1r if.rt 

vrLic -hi tnigc. NcLc U LC u 

c
30N

22°N

Gulf of Mexico Sea Surface Height

TOPEX/ERS-2 ea surface height anomaly

plus 10-year chmatollogical model mean

12/30/98

20°N

90°W 88GW 86GW 84°W 82°W 80°W

cm

30

25

-25

-30

114

Figure 5-2-14a. SSH for December 30, 1998 derived from TOPEX/ERS-2 plus

model mean. Dark Contours are for SSHs that are

greater than zero; white contours are for SSHs less

than zero.

20

15

10

5

0

-5

-10

-15

-20
6 - I000m Geostrophic Velocity

8W 87W

Figure 5.2-14b,c. Shipboard geostrophic currents for b) Em depth and

c) 500m depth, based on a level of no motion at

l000m for December 1-9, 1998. Heavier vectors

represent currents measured at the mooring sites

indicated by dots.

115
DRIFTER 12061

9OW 89°W 3N

3ON

2 6N

29N

28°N

27°N

/ a

11/21/1998 TO 4/ 1/1999

88W 87W 86W 85W 84°W 83GW 82W

I 31N

90°W 89GW 88W 87°W 86°W 85GW 84W 83°W 82GW

3ON

29N

28N

27N

26 N

Figure 5.2-14d. Trajectories of satellite tracked near surface drifter

12061 from November 21, 1998 - April 1, 1999. Arrow

heads are plotted at trajectory locations occuring

every ten days.

116
25Jan99 OJOCST ZUSGS I I 4,.Q * * 4 i5.ib I tUL 8IT rvc f*-r 'r-1 1 u-.- ti -c,i-::u: WL 1JpCr ILLVC1 (L..LL LL. th.t! ll1: ray t tie thre .t te
a 1I t 5. USGS ---Y t' + .--+... .p I) -. :. \ j- r j- I.JL 1-'J IJ.9 i?I 11r:u cii'rrnf wpi .1 nn & 

1:4prg to g4at.:icl ocu ccit1r r Iticfl ro.1y trig
C

R/v PELICAN PE9932

90W 89W

31N

R/V PELICAN PE9932

90W 89W

3

30N

29N -

28N

90W

/

89W

,1

3/28 TO 4/ 5/1 999

88W 87W 8W 8W

6 - 1000 m. Geostrophk Velocify

c) \\1/ I, - - -____..,_-s. - / \ - I't" / ///-'y - X'

88W 87W

500 - 1000 m. Geostrophic Velocity

86'W

3/28 TO 4/ 5/1999

8&W 85W3N

/

30N

L

- 29N

85W

28N

Figure 5.2-16b,c. Shipboard geostrophic currents for b) 6m depth and

c) 500m depth, based on a level of no motion at

l000m for March 28 - April 6, 1999. Heavier vectors

represent currents measured at the mooring sites

indicated by dots.

119

88W 87W
warm eddy near the array to move westward to allow for further growth.

Frontal eddies and streamers are obvious around the perimeter of the LC.

A large frontal eddy was southwest of the Tortugas, appears to have had

a role in the narrowing of the LC neck, which can be a forerunner of

ring separation. Similar patterns of eddy structures and LC extension

are shown by the SSH fields (Figures 52-17 arid -18)

5.3 Loop Current Frontal Eddies

5.3.1 BackcTround

Flow in the eastern Gulf of Mexico and southern Straits of Florida (5SF)

is constrained by the configuration of the two broad shallow shelves,

the Campeche Bank and the west Florida shelf (Figure 5.3-1). The

Campeche Bank extends approximately 280 km north of the Yucatan

Peninsula, bounding the flow entering the Gulf of Mexico through the

Yucatan Channel. Likewise, the west Florida shelf steers the flow along

the west coast of Florida. Both shelves are clearly marked by steep

escarpments on their seaward edge, plunging to depths beyond 3000m in

the central Gulf of Mexico. Sill depths in the Yucatan Channel, the

channel connecting the Caribbean with the Gulf of Mexico, reach

approximately 2000 m. The 5SF is bounded on the north by the Florida

Keys and on the south by the coast of Cuba. The width of the 5SF

decreases from 150 km at the western entrance to approximately 85 km

near Cay Sal Bank, while water depths decrease from 2000 m at the

western entrance to 800 m in the east.

Circulation in the eastern Gulf of Mexico is dominated by the LC, a

portion of the Gulf Stream system that closes the subtropical gyre in

the north Atlantic. Warm water flows from the Caribbean through the

Yucatan Channel, where it is called the Yucatan Current, "loops"

anticyclonically through the northeastern Gulf of Mexico, and exits

through the SSF, where it becomes the Florida Current (FC) . The LC

penetrates into the northeastern Gulf of Mexico to varying degrees

throughout the year, and occasionally an anticyclonic ring will separate

from the LC [Leipper, 1970; Maul, 1977; Muller-Karger et al., 1991;

Sturges, 1992; Sturges and Leben, 2000] - Typically, ring separation

causes an abrupt southward retreat of the northern LC boundary and,

consequently, the establishment of more direct flow between the Yucatan

Channel and the SSF [Leipper, 1970; Ichiye et aL,l973; Hurlburt and

Thompson,1980; Muller-Karger et al., 1991, see also Chapter 4 of this

report] . After a ring separates, the LC may take several months to

reestablish its anticyclonic circulation in the northeastern Gulf of

Mexico. However, occasionally, anticyclonic rings with diameters less

than 250 km separate from the LC and do not produce a significant change

in the northern position of the LC boundary [Vukovich, 1995]. In this

scenario the current still penetrates into the northeastern Gulf of

Mexico, even after the ring has separated. The northward penetration of

the LC has significant variability with a mean period around 12 months

[Sturges and Leben, 20001

Early observational programs in the Gulf of Mexico were primarily

focused on quantifying the cycle of northward penetration and

anticyclonic ring shedding by the LC. However, observations revealed a

120
24°N

22N

20°N

90°W

Gulf of Mexico Sea Surface Height

TOPEX/ERS-2 §ea surface height anomaly g

plus 10-year climatological model mean n

S S

I

oQ

01/25/99

cm

30

25

20

15

10

5

0

-5

-10

-15

-20

-25

-30

121

Figure 5.2-17. SSH for January 25, 1999 derived from TOPEX!ERS-2 plus

model mean. Dark Contours are for SSHs that are greater

than zero; white Contours are for SSHs less than zero.

88 °W

11

86W 84GW 82GW 80°W
30N

26N

24N

22°N

Gulf of Mexico Sea Surface Height

TOPEXIERS-2 sea surface height anomaly

plus 10-year climatological model mean

cm

30

/ 25

03/02/99

20N

90°W 88°W 86°W 84°W 82°W 80°W

20

15

10

5

0

-5

-10

-15

-20

-25

-30

Figure 5.2-18. SSH for March 2, 1999 derived from TOPEX/ERS-2 plus model mean. Dark contours are for SSHs that are greater

than zero; white Contours are for SSHs less than zero.

122
30°N

28°N

26°N

24N

22°N

Eastern Gulf of Mexico

90°W 88GW 86°W 84GW 82°W 80°W

20°N

90°W 88°W 86W 84°W 82°W 80°W

30°N

8°N

6°N

4°N

Figi.ixe 5.3-1. Bathymetry of the eastern Gulf of Mexico and southern Straits of Florida.

123

,, ;r

2

55' S.

(c

of Mexico

5'

I 'I

5, Sw

-

-

Campeche

Yucatan

Pen.

5; 555

!

'S -StrJts

-:-Z-,'

Caribbean

of

Flonda

_______5____

C LI

_ISea

ba

2

2

2

I1OOrr --5

''5,

Bank

'/,_:

,sc" ," 5,
wide range of smaller-scale features that were related to the largescale processes. For instance, Leipper [1970] investigated current

patterns in the northeastern Gulf of Mexico and found meanders and

eddies located along the northern and eastern edges of the LC. This led

to the first evidence that anticyclonic ring shedding may be preceded by

the westward propagation of cyclonic eddies across the Yucatan Channel

[Cochrane, 1972] - Several investigators have presented hydrographic data

showing a closed cyclonic feature near the Dry Tortugas, which

occasionally appears during ring separation of a LC ring [Nowlin and

McLellan, 1967; Molinari, 1977] - The dynamic height maps of Ichiye et

al. [1973] in the northeastern Gulf show evidence of cyclonic eddies

embedded in the edge of the LC just north of the Campeche Bank. No

mention of the eddies is made by the authors, but they appear to have

diameters of approximately 50-100 km.

Curiously, although the Gulf of Mexico and 5SF are geographically and

dynamically connected by the LC, previous studies concerning the

mesoscale variability associated with each have remained independent.

The most comprehensive studies of LC frontal eddies (LCFE's) and

perturbations took place in the late 1970s to early l980s with the

studies of Maul [1977], Vukovich et al. [1979], Paluszkiewicz et al.

[1983], and \Tukovich and Maul [1985]. Vukovich and Maul [1985],

combining more than 10 years of satellite infrared data and several

years of coincident hydrographic measurements, identified large cold

perturbations on the northern extreme of the LC that moved southward

along the LC boundary, adjacent to the west Florida shelf. These

meanders eventually grew into stationary meanders with closed cycloriic

streamlines and cold cores near the Dry Tortugas. tipon reaching the SSF,

they were observed in one of two possible configurations: A large

southwestward oriented protrusion of cold water near the Dry Tortugas

[e.g., Maul et al., 1984] or a cold tongue, bounded to the west by the

southward flowing LC and to the east by a ridge of warm water extending

over the west Florida shelf to as far north as 26°N [of. Vukovich and

Maul, 1985, Figure 2a]. The cold tongue and the cold meander events

observed by Vukovich et al. [1979], Maul et al. [1984], and Vukovich

and Maul [1985] always displayed the signatures of closed cyclonic

circulation. Geostrophic estimates of velocities within the LCFE

exceeded 100 cm/s on the LC side of the features where horizontal

density gradients were maximized, while velocities reached 20 cm/s over

the west Florida shelf. Diameters varied from 80 to 120 km. and

subsurface signatures extended to l000m [Vukovich and Maul, 19851

Surprisingly, in 10 years of observations, the features were never

observed moving into the SSF. Once the perturbations reached the Dry

Tortugas, they either dissipated or grew westward across the width of

the LC. Vukovich and Maul [1985] postulated that dissipation of LCFE may

involve kinetic energy transfer to the mean flow, similar to the

behavior of spin-off eddies described by Lee [1975] in the northern

Straits of Florida. The lack of prior observations of such propagations

may be an artifact due to aliasing of an imperfect sea surface

temperature time series derived from early satellite images.

In a subsequent study, Vukovich [l988a] used a wave-staff technique on 5

years of infrared satellite data to quantify the boundary variations

associated with the LCFE. Perturbations along the northward flowing limb

124
of the LC were 20-30 km near 25N and grew to 90 km near 27°N,

suggesting that perturbations generated at or entering through the

Yucatan Channel grow as they propagate toward the northern section of

the LC. The eddies observed by Maul et al. [19741 and Maul [1977],

proposed to be generated by shear instabilities, and by Cochrane [1972],

proposed to be a result of topographic vortex generation, were all of

the order of 10-20 km. Two-layer model experiments generated cyclonic

eddies downstream of the Yucatan Channel, east of the Cairipeche Bank

[Huriburt, 19863. The model eddies were generated by baroclinic

instabilities in the vicinity of the steep topography of Campeche Bank

[Huriburt, 1986].

To summarize, cyclonic eddies are common along the Outer LC boundary,

and observations have suggested that they may be important in the LC

anticyclonic ring-shedding process. Model studies and observations

suggest that these LCFE's form along the northward flowing branch of the

LC and grow in an unstable manner as they propagate downstream along the

frontal boundary. Recent studies (Fratantoni et al., 1998) have shown

that these eddies can continue around the Loop Current and enter the 5SF

where they have been termed Tortugas eddies (Lee et al., 1995).

Fratantoni et al. (1998) used AVHRR imagery to show that Tortugas eddies

are LCFE's that become nearly stationary off the Dry Tortugas.

Fratantoni also identified two modes of interaction of LCFE's with the

Tortugas eddies: mode-1, a southward advancing LCFE forces the Tortugas

eddy downstream into the Straits of Florida; and mode-2 the LCFE takes

part in a ring separation without interaction with the Tortugas eddy

which results in stationary Tortugas eddies for periods up to 140 days.

5.3.2 Influence of Loop Current Frontal Eddies on the Study Area

Due to their large spatial scales and rapid evolution LCFE are best

observed with synoptic, satellite derived SST images. A good example is

shown in Figure 5.2-8 for February 5, 1998. At that time there were

three well-developed LCFE's visible in the image: one at the northern

extreme of the Loop Current just south of the array; another at the

northwest corner of the Loop; and a third over the outer part of the

Campeche Bank. These eddies are identified by the warm streamers of LC

water that wrapped cyclonically around and into their cold cores. There

was also a fourth LCFE on the eastern side of the LC, somewhat hidden by

the clouds, and a fifth LCFE southwest of the Tortugas that is

participating in the narrowing of the LC neck that can be a forerunner

of ring separation. As mentioned in the previous section, SSI-I fields are

also useful in identifying LCFE and their filament structures.

Typically, a LCFE will appear as a closed low near the boundary of the

LC high. Comparison of LCFE signatures in same day SST and SSH fields

show generally good agreement in location and size of features (Figures

5.2-6, -7, -10 and. -11 are good examples) . The warm filaments are

generally shown as ridges wrapping around the closed lows.

Investigation of the evolution of LCFE from a succession of SST images,

SSH fields and movies of the same, shows that these features travel

around the LC at speeds of 2 to 24 km/d. It generally takes several

months for a LCFE to travel completely around the LC from the Yucatan

Channel to the southern Straits of Florida. However, the features can

125
shear apart before they accomplish the complete circuit around the LC.

LCFE's are commonly observed to form along the outer edge of the

Campeche Bank as the Yucatan Current enters the eastern Gulf of Mexico

and perturbations of the flow are no longer constrained by topography.

It is also possible for LCFE to form anywhere along the LC frontal zone.

After formation they go through a rapid growth stage along the western

and northern frontal boundaries of the LC reaching dimensions of 100 to

300 kin in about one month. The decay stage of LCFE is indicated by the

elongation of the warm streamers that trail in their wake. This often

occurs along the eastern side of the LC where the LCFE's merge onto the

steep escarpment of the west Florida shelf. Interestingly, the warm

filaments trailing the LCFE's as they move off to the south are often

observed to extend into the DeSoto Canyon moored array area.

Available SST and SSH imagery for the study area indicate that there

were at least 12 LCFE's in the vicinity of the moored array during the

study period, 11 of which either directly or indirectly influenced the

observed flow and temperature variability. Table 5.3-1 summarizes the

relevant .information pertaining to each eddy as estimated from the

evolution of SST and SSH fields. During the warm summer months (June -

Septernber) only SSH data were used due to the absence of surface thermal

gradients. A more detailed description of these LCFE is provided in

Appendix A.

Table 5.3-1. Characteristics of Loop Current frontal eddies (LCFE) that

influence the study area as determined from satellite derived SST and

SSH fields.

5.4 Characteristic Slope Circulation Patterns

5.4..l Geostrophic Current Patterns

Data from hydrographic surveys during this project provide a basis for

computing several high resolution snapshots of the upper layer

circulation in the study area. Patterns observed during these cruises are similar to mean circulations obtained from the moored

126

Peziod,, QIserre :..($):i

Eddr

Duration

Pr3.od of

Ix1flence ox

.:I:/tLid .rsa

Estimated

Max. Size

E-W x N-S

(]un)

Esta.anatd translation

Spied

(1n/t)

1 1/1-8/15/97 3.8 3/21-7/14/97 300 x 200 5 2 7/1-10/15/97 1.8 7/14-9/9/97 270 x 140 4

3 8/10/97-1/15/98 5 9/9-12/15/97 110 x 75 12

4 12/6/97-2/10/98 2 12/19/97-2/1/98 140 x 60 17

5 1/11-2/25/98 1.5 2/5-2/18198 130 x 75 17

6 1/16-3/25/98 2.3 3/1-3124/98 280 x 140 20

7 2/5-3/14/98 1.3

No Influence

Detected 110 x 55 24

8 3/4-6/1/98 2.9 3/16-5/1/98 300 x 245 2.4

9 3/23-6/15/98 2.8 5/1-6/24/98 110 x 110 10-16

10 7/1/98-2/1/99 8 7/7-9/1/98 200 x 2d0 Stationary

11 1/7-3/2/99 18 1/6-2/8/99 150 x 110 10

12 2/15-4/30/99 2.5 2/20-3/10/99 110 x 110 9
instrumentation during a number of different periods (1 to 3 months

long) of sustained flows as discussed in Section 5.4.4. Thus, the

geostrophic flows, derived from the CTD surveys, reveal some of the

details of the eddy field over the Northeastern Gulf of Mexico slope.

Figure 5.4-1 and 5.4-2 show the near surface and 500m level geostrophic

flows (relative to 1000 in) for each of the seven surveys. These

geostrophic velocities compare well with the 5-day averaged, 40-HLP

currents from the moorings, centered on the cruise periods, that are

overlaid on the maps. The near-surface velocity maps show complex eddy

circulations with diameters of order the width of the slope (50 to 100

kin) . Smaller scale features may have been present on the upper slope and

at the head of the canyon.

Five of the seven maps (Figures 5.4-la,b,c,e & t) show predominantly

eastward flows along the upper slope, south of Mobile Bay, apparently

driven by the circulation features located offshore of the area. These

eastward jet-like flows had differing configurations depending on the

sense of rotation of circulation patterns further south. In March 1997

(Figure 5.4-la), the eastward flow, south of Mobile, diverged from the

shelf break to bypass the head of Desoto Canyon. A weak cycloneanticyclone pair of small eddies occupied the canyon. A SST image for

March 24, 1997 shows a streamer of cold shelf water that moved southeast

along the northern edge of the jet as it turned away from the shelf

break. The image shows two moderately-sized warm eddies centered at

28°N, 88.7°W (LC ring D - for "Deviant") and 28°I\T, 87°W, respectively.

The survey documented the northern part of this latter circulation. The

northern front of the LC was south of 26°N at that time. Similar

circulations occurred during the November 1997 and August 1998 surveys

(Figure 5.4-ic & e) . In the former, the eastward flow turned sharply to

the south at approximately 87°W. In the latter, the flow was driven by

a vigorous anticyclone to the south, and the canyon circulation was

cyclonic. According to the map of SSJ-I, an elongated warm anomaly formed

parallel to the west Florida escarpment in the wake of the detachment of

ring F ("Fourchon" -400 km diameter) from the LC in April 1998. This

anomaly remained for most of the year and was probably a direct

contributor to the strong anticyclonic flows observed in the August

survey (see Sections 4.2.2 and 5.2).

The remaining two surveys that show eastward flows (Figures 5.4-lb & f)

are similar in that the jet-like flow roughly followed the shelf break

around to the eastern side of the canyon and, cycionic flows were

present over the deep water south of Mobile Bay. In July 1997, the

cyclone and anticyclone on the west Florida slope appears to have

generated strong flows toward the shelf break that enhanced flow around

the head of the canyon. In December, however, the interaction of the

deep water cyclone and a weaker west Florida slope anticyclone (Figure

5.4-if) did not generate onshore flow that could transport warm salty

water to the outer shelf.

127
5/V FEUCAN PE-9722 9091 5l'W 311 )0011 2911 3311 - / I--- \ ( - ,frV2' f 2 \ \ ? ' / / .1 J \\ 5/V PEUCAlI PE-9820 100 69W 31'S 3/19 TO 3/27/1997 9/V PEUCAPO PE-11503 !rw 51W 55'l11 115'W 1O'W erw "t17' 1- LI\\\ j \. "\. 9.W 97'W 319 40 cn/. 85W40 -N 25 9 il/li TO 11/19/1997 R/V P(11CA9 PE-9830 86'W 5-I'W 10W !I'W 31'H 30'*l 25 1 95'W 10'91 SOW / 08'S 7/8 50 7/15

07'W 86W 

S6.w40 Crn/, 3/32 TO 4/ 5

56W 

Figure 5.4-1. Geostrophic velocities at 6m depth from the hydrographic surveys. 5-day average velocity vectors (heavy arrows) from the Bin depth of the ADCP's 

shown at stations marked with a dot. 6 1000 m. Geostrophic Veiccily 6 - 1000 m. Geootrophic Velocity
8/ 4 TO 8/12/1998 R/i PEUCA1I PE9923 I W 908 800 319 800 lOW 870 lOW 6 - 1COC m. Geostrophic Ve'ocity Figure 5.4-1(contd) Geostrophic velocities at 6m from the hydrographic surveys. 5

average velocity vectors (heavy arrows) from the 8m depth of ADCP's are shown at stations marked with a dot.
arw - 000 m. C.oslrophrc Vilocily 3/f9 to 3/27/17 R/V PCUCAH PE-9803 6'W 5W QW 9W 31t1 Il/Il 10 1/0/1997 I/VPEUCAN PE-30 0W rw Figure 5.4-2 Geostrophic velocities at 500m from the hycirographic surveys. 5-day average velocity vectors (heavy arrows)from the 500m level current met

are shown at stations marked with a dot.
R/l FEUCAN FE 1)00 318 ) - 2}) I / ,-- '? / - 1)/V FEUCAN FE1)1)32 g) 90W 9W 318 K 8/4108/10/11)98 0/V PELICAN FE9923 17'W 00W !OW 1)00 I 31N 21) -- - 8lW 20.91 f (_;. '.-...... _'___\ I -.......1-;1.- / .... '..-. . \ -- \ 

,-:-.---- N 3/2810 4/ 5/1991) 07.99 0.9921.91 20 m/. 29.91 .._. 2.91 \. '... I I I N., 209 ,., 289 8OW arw 85.9) 500 1000 rn Goosirophlc V010011y 12/1 1012/0/

Ill) 00.90 I) I 

21) crn/. .9

Figure 5.4-2(cont'd) Geostrophic velocities at 500m from the hydrographic surveys. (See previous figure for complete caption.) J ....-'/ //..C_ \ \ '','.---- -. / .-!'-"r--l/\ fl- I1' \ "\.
The map from March 1998 (Figure 5.4-id) shows flow to the west that is

driven by a compact and vigorous slope cyclone in the center of the

study area. There was another region of cyclonic flow to the west that

appears in the altimetry as a cold frontal eddy on the northern

periphery of ring F. Ring F was in the process of detaching from the

LC. In the SST image for April 4 (16h CST) the two cyclones can be

identified clearly. There was also an anticyclone over the west Florida

slope that interacted with the cyclone to the west to promote onshore

flow toward mooring Cl. The March 1999 survey, however, shows a

Situation with weak, confused flows consisting of small scale cyclones

and anticyclones along the upper slope on both sides of the canyon

(Figure 5.4-ig).

The SOOm geostrophic velocity maps (Figure 5.4-2) do not always have the

currents flowing in the same direction as in the upper layer. A

westward current in opposition to surface eastward flows was observed

close to the Alabama slope (Figures 5.4-2a,c & e) - In deeper water, the

500m level flows were often in the same direction as the surface flows.

Examples are the westward cyclonic flows in March and July 1997, and

March 1999 (Figures 5.4-2a, b & g) . In other cases, the circulation at

depth was similar to the near-surface flows across the slope. The two

vigorous cyclones were observed at both levels in March 1998 (Figures

54-ld and 5.4-2d). The opposite case is found in December 1998

(Figures 5.4-lf and 5.4-2f) where slope flow to the east occurred at

both levels. The 500m flows, however, turned south along the west

Florida escarpment at a more westerly position than the near-surface

jet. An important feature of all these 500m depth velocity maps is the

presence of eddy motions with smaller length scales than at the surface.

It is also noteworthy that the smaller scale eddies that were present at

both depths, such as the canyon anticyclones in Figures 5.4-la & b and

54-2a & b, had their centers displaced towards deeper water at the

lower level. This was presumably the influence of the slope topography

on the eddy circulations. The tilting of the vertical axis of

peripheral eddies has been observed on the LATEX slope and attributed to

the interaction of the smaller eddy with the larger (Berger et al,

1996; Hamilton et al., 2000).

Perhaps the clearest picture of the generation of a westward slope

countercurrent at depth are shown in maps for August 1998 (Figures 5.4-

le and 5.4-2e) - The anticyclonic flow over the canyon was observed at

both depths. At the surface, the flow turned south. At depth, the flow

was blocked by the eastern side of the canyon and turned north and

joined a cyclonic circulation which then fed a countercurrent along the

slope. The westward flowing countercurrent seems to have become

entrained into the anticyclone south of the Mississippi Delta that was

observed in the southwest corner of the study area. A similar

circulation pattern was also observed in March 1997 (Figures 5.4-la and

5. 4-2a)

The depth dependence of the slope flow patterns is illustrated in Figure

5.4-3 where the geostrophic velocity component parallel to the isobaths

is contoured for section B. This section, due south of Mobile Bay,

often crossed the jet-like flows, discussed above, and extended into

offshore waters deeper than 2000m. Observations from March 1997 and

132
a) 0 0 10 20 30 40 50 60 70 Disc no. (1cm) R/V PELICAN PE-9B20 11/14/1997 t 11/14/1197 9 0.olrlphIc V.IonftV (Onc/.) ln.I 0000 46 kIln 10.64 Mn 27.94 0 100 200 300 400 500 600 700 800 900 10000 100 200 300 400 500 600 700 800 900 1000 7 72 11 2 7 2 7 0; 0; 00 -- 2/V PELICAN PE-9803 7/Il/loll. 7/11/1997 Tr,....1 9 0.1mph!. V.l.lty (cm!.) R.inr..c. L.v.I 1000 db 141. 77.14 11.. 21,94 R/V/ PELICAN PE-9830 4/ 2/1999 I. 4/1/1999 1rcn...t GnoclmcpOlc Y.lccIIy (crr./.) 0.1. r.rm. 1.1.1 1000 lb Olin = 29.64 II.. 4.94 5m !. ,1 5 I 

Figure 5.4-3 Geostrophic velocities on Section 3 for the indicated surveys. Positive vel

eastward, normal to the section. Higher westward (negative) velocities are line on the left of each section indicates the depth of the local bottom. R/V PELiCAN PE-972Z ( 3/21/1997 1 3/21/1997 Tn..cI 0 0.1rlph11 0.IlclIy (lrn/.) R.1.r.m. L.v.1 1000 lb kIln 24.12 kIo = 35.03 E-caQ 100 200 300 400 500 600 700 800 900 1000 0 10 20 30 40 c) 0 80 90 100 110 120 E-ca 100 200 300 400 500 600 700 800 900 1000 100 110 120 c,_.-i--1--- 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 100 110 

0 10 20 30 40 50 60 70 80 90 100 110 

Dj9Ianco. (1cm)
e) 0 100 200 300 400 - C 500 a 600 700 800 900 1000 0 R/V FEUCAN PE-g908 5/ 7/1155 lo 5/ 7/lIss Trans.oI B 0.o.Irophio V.Inolty (nm/n) R.t.m.,mn. L.v.l 1000db 91mm 2542 }lnn 56.41 20 $1 0 100 200 300 400 500 600 700 800 900 1000 110 120 0 R/V PELICAN PE9923 2/ 3/1555 In 12/ 4/1195 Tmoninet B G.onlrnphlc V.Inclly (cm/n) B.1.r.nc. mcd 1000 45 Sin 01.05 Onc 32.00 C R/V PEUCAN pEgg3z 4/ 1/1915 Ic 4/ 2/1599 Irnanunt U Onc.Ircphlc V. molly (nm-mm/n) Rnf.r.mmn. Lnv.I 1000db Sin 11,19990 10.07 60 70 80 90 100 110 

Figure 5.43-(contd) Geostrophic velocities on Section B for the indicated sur

Positive velocities are eastward, normal to the section. Higher westward velocities are shaded. The bold line on left of each section indicates the depth of the local bot

30 40 50 60 70 80 90 100 CCa g) 0 1 00 200 300 400 500 600 700 800 900 1000 a 10 20 30 40 50 60 70 80 90 100 110 120 Drntor. (Am)
August 1998 (Figures 5.4-3a & e) most clearly show the surface jet and

the slope counter current below 200 to 300m. The core of the westward

flow was at a depth of about 300 and 400in and had a vertical extent of

500 to 600m for March 1997 and Aug-ust 1998, respectively. Further

offshore the observed eastward flow had a subsurface maximum at a depth

similar to the maximum westward flow, that was part of the deeper

cyclonic flow discussed above. The last survey in April 1999 (Figure

5.4-3g) documented a weak two-layer structure with the westward current

maximum at the shallow depth of 200m. Similarly, a two-layer structure

is suggested for November 1997 by the mean, westward current velocity

vectors at 500m in Figure 5.4--2c that occurred close to the slope but

were missed by the geostrophic calculations (Figure 5.4-3c) -

In contrast, the purely cyclonic and anticyclonic cases (Figures 5.4-3d

& f, respectively) showed no reversals with depth. The velocity

maximum, over the upper slope, was near-surface and eastward for the

anticyclone, and subsurface, at about lOOm, and westward for the cyclone

as expected for these type of eddies. The July 1997 section (Figure

5.4-3b) seems similar, showing eastward and westward flows over the

slope. However, the eastward upper slope jet was separate from the

westward flow offshore. The latter resulted from the northern part of

an offshore cyclonic eddy as Figure 5.4-lb makes clear.

The description of the geostrophic flows indicates that an eastward

near-surface, jet-like flow often occurred along the upper slope

offshore of Mississippi and Alabama. Further east on the slope this

flow took various paths, including following the canyon rim, depending

on the configuration of the eddies. This jet-like flow was sometimes

accompanied by a westward countercurrent at depth. The depth of the

countercurrent seems to have varied with different circulation patterns.

However, it generally coincided with the occurrence of a small-scale

cyclonic circulation at depth over the west Florida escarpment. The

upper slope jet was often opposed by westward flows over deep water that

were caused by cyclonic eddies related to the LC or LC ring fronts (see

Section 5.2>. In other situations, the circulation was dominated by one

or more cyclones or anticyclones that occupied the slope. In these

cases the direction of flow at the shelf break was controlled by these

eddies.

Consequences of these circulation patterns included the transport of low

salinity water from the shelf to the slope, and the transport of warm,

salty offshore water toward the shelf break. Figure 5.4-4a shows the

near-surface salinity for July 1997. The eastward shelf break flow

(Figure 5.4-ib), assisted by eastward flow on the shelf that was driven

by the prevailing westerly winds, distributed brackish water from the

Mississippi delta along the canyon rim. This could be an important

mechanism by which nutrient-laden, river-derived water was input to the

shelf on the east side of the canyon. There are no major river

discharges into the shelf on the east side of the DeSoto Canyon. The

northward flow between the two counter rotating eddies (Figure 5.4-lb)

caused a warm salty intrusion to approach the shelf break.

135
a

R/V PELICAN PE-9803 7/ 8 TO 7/17/1997

90W 89'W 7W 86W 85W

31W

I 31W

30N

29'N

28 N R/V PELICAN PE-9722

90W 89W

31 N

30N

29N

28N

90.W 89W

Surface Salinity (psu)

3/18 TO 3/28/1997

8W 56W 5W

31N

88W

100 Meter Temperature (Deg C)

30N

85W

28N

30N

- 29N

28N

Figure 5.4-4 (a) surface salinity from July 1997 (PE-9803), and (b)

temperature at lOOm from March 1997 (Cruise PE-9722) -

136

90W 89W 88W 87W 8W 85W

18 19 20 21°C

b)
The eastward flow that prevailed along the shelf break in the western

portion the study area, and the occurrence of small scale anticyclones

around the head of the canyon, caused isotherms to be upwelled over the

upper slope. Figure 5.4-4b shows contours of the temperature at lOOm

for the March 1997 survey. Figure 5.4-la gives the surface velocity

vectors. Cooler water was present on the northern edge of the eastward

current that, in this case, bypassed the canyon head. However, the

small anticyclone in the canyon also raised the isotherms to produce

water colder than 18°C.

5.4.2 Lonq Period Events

Snapshots of the slope circulation raise questions on the persistence of

various flow patterns and their frequency of occurrence. This section

examines the moored data in order to derive some characteristic time

scales and select appropriate periods for the calculation of useful mean

flow patterns. The emphasis is on flows that have some degree of

persistence over periods of a few months with higher frequency motions

being discussed in later sections.

The middle slope (500m) moorings were the most heavily instrumented and

allow both the temperature and velocity structure to be documented for

the upper 300rn of the water column. Figure 5.4-5 shows the 7-day low

pass (DLP) temperature and velocity profiles at C2 over the entire two

years of the study. The records from the 500m moorings at C2 are

similar to the data from the other three 500m moorings. During the

study, three periods appear to have had sustained flows to the east or

west, or both at different depths. The isotherms at different depths

had similar displacements indicating that the whole of the upper water

column warmed and cooled in response to the advection of the barotropic

flows. A small amount of seasonal waLlning and cooling was evident at

the 62m level. It is noted that the first year of the study (March 1987

- March 1998) was cooler than the second year when the mean depth of the

15°C isotherm was about 50m deeper. The difference in the flows is that

eastward, upper layer currents predominated in the first year whereas

more sustained episodes of westward flow occurred in the second year.

Generally, eastward and westward flow events raised and lowered the 15°C

isotherm, respectively. Presumably, the cyclonic flow episodes were

effective in advecting warmer water from the deep eastern basin to the

northeastern slope.

As shown in Figure 5.4-5 the first period of sustained flows occurred in

the summer of 1997 (S97 in Figure 5.4-5) with eastward flow above 200m

and weaker westward currents at depth. The July 1997 survey occurred in

the middle of this period (Figures 5.4-lb and 5.4-2b) and documented

westward flow further offshore resulting from the northern edge of a

cyclone. The means and standard deviations (plotted as ellipses) are

given for this period in Figure 5.4-6. The l5m and 70m levels show

eastward flow following the trend ot the upper slope isobaths with

westward flow further offshore. The mean currents at A3, B3 and C3

indicate that some of this westward flow was being entrained into the

upper slope jet. At depths of 200, 300 and SOOm the flow was westward

with the 300m level giving the maximum mean currents of -10 cm/s. The

500m mean flows (Figure 5.4-6c) should be compared with Figures 5.4-2a,

137
i\ -

7-DLP Temperatures at Mooring C2

19 10 30 19 7 27 16 4 26 15 4 23 12 1 20 II 30

N A U J J A S 0 N S J F U A H 1 J A S 0 N 0 J F U A 1997 1995 1999

7-Day Low Pass Currents at Mooring C2

Figure 5.4-5 Seven-DLP current vectors and isotherm depths for the

upper 30Dm of 02. Vertically upward vectors are directed

along isobath with an easterly component.

138
5/10 TO 8/17/1997

90W 89W 8W U7W 86W

31W I

I I 31W

28N

9OW 89W 8.W 87W

5/10 TO 8/17/1997

86W a5W

31 N

90W 39W

Summer 1997 40I-ILP Statisfics cii 70 m

86W 85W

28N

139

Figure 54-6a,b. Mean currents and standard deviation ellipses for

summer 1997 (5/10 to 8/16/97) for: (a) iSm, (b) 70m

(c) 200m (solid.) and 300m (dashed), and (d) 500m

(solid) and 1290m (dashed) depths.

OW 89W 88W 87W 55W

Summer 1997 40HLP Sfatiflcs cii 15 m
9ow

90.w

89W

5/10 10 8/17/1997

87'W 86W 85W

31 N

Summerl997 40HLP Statistics at 200 (Solid) and

8BW

Summer 1997 40HLP Statistics of 500m (Solid) and 1290m (Dashed)

300m (Dashed)

5/10 TO 8/1 7/1 997

BW 85W

3 N

85W

2N

Figure 5.4-6c,d. Means and standard deviation ellipses at indicated

depth.

140

90.W 89W

9OW 89W 8W 87W
b & e. Though in general the means were comparable to the standard

deviations it is noteworthy that the cross-slope variances were a

substantial fraction of the along-isobath variances, particularly

offshore arid in the canyon. The fluctuation kinetic energy, which is

proportional to the area of the ellipses, showed no marked variation

across the array at any given depth level. However, there was a strong

decrease with increasing depth. This representative mean flow pattern

had an upper slope eastward jet opposed by reverse flows below 200m and

westward flow over the lower slope caused by a cyclone. It occurred at

other times in the array such as December 1997 to January 1998, and July

1998 (Figure 5.4-5) and was related to the occurrence of LCFE being

present in the region to the south between 28°N and 29°N (see Section

5.2).

Between February 1 and April 30, 1998, an interval denoted by C iii

Figure 5.4-5, flow was westward at all depths with speeds at 300m

equaling or slightly exceeding those at the surface. The early April

1998 survey showed a vigorous cyclone over the western part of the

canyon (Figure 5.4-id) . Velocities, and also temperatures,. had

energetic short time-scale (-10 to 20 days) fluctuations that rarely

reversed the direction of the prevailing westward currents (Figure 5.4-

5) . Similar periods with all flows at C2 toward the west included

August and September 1998 and February and March 1999. The short period

fluctuations in the latter interval were not as energetic as during

interval C.

The mean and standard deviation ellipses, for the standard depth levels,

are given in Figure 5.4-7. Mean flows at 16m and 40m below the surface

were toward the west with a magnitude of approximately about 10 cm/s

over the Alabama slope and an anticyclonic circulation over the head of

the canyon. The smallest westward velocities were at the shelf break.

The standard deviation ellipses, in the upper lOOm, show that the

kinetic energy increased from the shelf break toward deeper water. The

energy levels were also greater in the western part of the array than

for the summer 1997 interval (compare Figures 5.4-6a and 5.4-7a) . In

the canyon, upper layer mean flows were northward. At the 16m level,

flow turned east, with some returning to the offshore at El. At 70m,

the flow turned westward towards Pensacola. Thus, cyclonic flows iii

deep water can generated onshore velocities along the axis of the canyon

(transect D) . At 200m and 300m, the westward means exceeded 25 cm/s in

the center of the slope (82) and approached 10 cm/s at 500m depths.

This period is characterized by a rapid succession of cyclonic frontal

eddies propagating around a well extended LC or ring F which at this

time was in the process of forming and detaching from the LC (see

Section 5.2). This explains the predominance of cyclonic flow and the

presence of short period fluctuations during the February to April 1998

interval. The intrusion of LCFE Onto the slope during this period

caused the steady deepening of the 15°C isotherm at mid-slope locations

(Figure 5.4-5)

141
31 N

30N

29N

28N

31 N

30N

2N

28 N

2/ 1 TO 4/30/1998

90W 8W 88W 87W 86W 85GW

9OW

9UW 89W

2/ 1 TO 4/30/1998

88W 87W

3VN

Figure 5.4-7a,b. Mean currents and standard deviation ellipses for the

cyclonic flow period (2/1 to 4/30/98) at (a) 16 m, (b)

70m,(c) 200m(soljd) and 300m(dashedj,and (d)

500m(solLid) and 1290m(dashed) depths.

142

90W 88W 87W 85W

Cycloriic Flow 40HLP Statistics at 70m

89W 88GW 87W 86W 85W

Cycloriic Flow 40HLP Statistics at 1 6m
C

31N

30N

29N

g0w

90W

88W

2/ 1 TO 4/30/1998

87'W 86W 85W

I............ 28N .. - - . 28N

90W 89W 88W 87W 86W 85W

Cyclonic Flow 40HLP Scflstics at 200 (Solid) and 30Gm (Dashed)

2/ 1 TO 4/30/1998

90W 89W 88W 7W 85W

31 N

31N

10 cm/s

Cyclonic Flow 40HLP Statistics at 500 (Solid) and 129Gm (Dashed)

30N

29N

85W

28 N

Figure 5.4-7c,d. Mean currents and standard deviation ellipses for the

indicated depths.

143
-The last interval, (interval A in Fig-ure 5.4-5), was also characterized

by along slope flows toward the east. The difference from the S97

period is that flows decreased little with depth and no undercurrent was

present. Observations from the December 1998 survey (Figures 5.4-if and

5.4-2f) show an anticyclonic circulation extending vertically through

the water column over the central part of the canyon. The mean

velocities and standard deviation ellipses for this period are plotted

in Figure 5.4-8. The mean velocities in the upper lOOm were eastward

with northward inflow occurring on the western edge. The main current

bypassed the head of the canyon but a subsidiary loop maintained the

anticyclonic flow along the rim.

A similar situation is illustrated by the geostrophic currents in Figure

5.4-lf. At the 7Gm level, there is a suggestion that the canyon

subsidiary loop may have become a cyclonic recirculation eddy. This

again induces northward currents at D9 and Dl. Unlike the cyclonic flow

case, the sources for these onshore flows is the Alabama slope rather

than deep eastern basin water. Eddy kinetic energy was again relatively

uniform across the array, but the energy levels were less than for the

S97 interval. At 200m and 30Gm depths, the means were eastward but, at

B2, they exceeded the means in the upper lOOm. At 50Gm (Figure 5.4-Gd),

however, a narrow region of westward flow existed next to the slope.

The eastward mean flow over the 120Gm isobath appears to have been

blocked by the west Florida slope and was partly returned to the west

via a cyclonic circulation in the canyon. Thus, the circulation was

similar to, but less energetic than that occurring during the S97

interval, except that there was no cyclonic circulation offshore and the

undercurrent was restricted to a narrow region close to the bottom at

the 50Gm isobath. The anticyclonic circulation implies uplifted

isotherms on the outer edges of the eddy. The water at 20Gm rapidly

cooled during this event (Figure 5.4-5). This particular anticyclone

was not directly associated with the LC. SSH and SST images show the LC

mostly south of 25°N at this time. Examination of the time series at

C2 (Figure 5.4-5) shows a few similar periods in the record but they

were of short duration. Similar events include the latter halves of May

and Aug-ust 1997.

Analysis of these three mean circulation patterns show that the eddy

circulations derived from the geostrophic currents may have been

persistent for periods on the order of months. The large-scale

features, such as the upper slope iet and its deep counter current,

occurred in more than one survey. They have been shown to be general

features of the northeast Gulf slope circulation. Forcing for these

flows seems to depend on the presence of eddies over the lower slope.

These deep water eddies may sometimes be related to LCFE, however, even

when the LC was not present in the northeast Gulf, similar eddies were

still present. For example, in the last nine months of the field

observations, the LC front did not extend north of 26°N and Ring F was

west of the delta. However, cyclonic and anticyclonic flow regimes were

still observed over the northeast Gulf slope. These eddies may

sometimes be still related to peripheral circulations of a major LC ring

even after it has moved out of the eastern basin.

144
90W

90W

89W

89W

11/ 1/1998 TO 1/16/1999

88W 87W 86W 85W

31 N

88W

AnfiCyclonic Flow 40HLP Statistics at 70m

86W

AntiCyclonic Flow 40HLF Statistics at 1 6m

11/ 1/1998 TO 1/16/1999

85W

28N

Figure 5.4-Sa,b. Mean currents and standard deviation ellipses for the

anticyclonic flow poriod (11/1/98 to 1/16/99) for (a)

16m, (b) 70w, (c) 200m (solid) and 300m (dashed) and

(d) 50Cm (solid) and 129Cm (dashed) depths.

145

90.w 89W 88W 87W 88W 85W
31 N

30N

29N

28N

90W 89W

11/ 1/1998 To 1/16/1999

88W 87W 85W

31 N

10 cm/s

28N

87W 5W

at 200 (Solid) and 300m (Dashed)

11/ 1/1 998 TO 1/16/1999

7W 86W e5W

31 N

6W

30N

- 29N

28N

90W 89W 87W

AntCyclonic Flow 40HLP Statistics at 500 (Solid) and 129Gm (Dashed)

85W

25N

146

Figure 5.4-8c,d. Mean currents and standard deviation ellipses for the

anticyclonic flow period (11/1/98 to 1/16/99) at

indicated depths.

90.w 89W 88W

AnfiCyclonic Flow 40HLP Statistics

90W 89GW
5.4.3 Two-Year Statistic Properties

After the analysis of means and standard deviations for specific kinds

of flow episodes, it is useful to present statistics for the complete

two-year study period. Figure 5.4-9 gives the means and standard

deviations for the currents at various depths. The standard deviations

greatly exceeded the means at all depth. Unlike the episode statistics,

however, the two-year records contained both eastward and westward flow

events. The means were quite robust, and subdividing the record into

two separate 12-month periods produced very similar statistics. Upper

layer means were eastward and tended to follow the isobaths at the

surface. At 70m, the eastward flow was a little weaker and had an

indication of a residual cyclonic circulation over the head of the

canyon. There was a similar splitting of upper and lower layers at Dl

into eastward means that occurred in Figure 5.4-8 during the

anticyclonic flow period. Mean flows at 70m along transect D were

directed northward across isobath. Therefore, it seems that both

cyclonic and anticyclonic episodes contribute to the upper layer,

northward mean velocities along transect D in the canyon. Mean flows at

D9 had a up-slope component above about 120m. At 200m and below, the

flow was all to the west with slightly larger magnitudes at the 500m

isobath than at the l300m isobath. Therefore, the Counter current at

depth was a distinct feature of the mean slope circulation.

The anticyclonic and cyclonic nature of the upper and lower layer flows,

respectively, are also reflected in the two-year mean temperature fields

at 90m and 500m (Figure 5.4-10) . The 90m temperatures had a strong

cross-slope gradient with the warmest water on the offshore side of the

Alabama slope. The water at the head of the canyon was more than 2°C

colder. Thus, persistent anticyclonic flows caused upwelling at the

head of the canyon to a greater degree than elsewhere along the shelf

break. At 500m, the mean, cross-slope temperature gradient, though

small, had the opposite sign consistent with the westward flows found in

the lower layer. The mean anticyclonic, upper layer circulation over

the slope was consistent with the SSH data. A two-year mean SSH anomaly

for the study period is given in Figure 5.4-11, A positive anomaly was

found over the slope and separated from the mean LC by a negative

anomaly. The latter resulted from the frequent passage of DC frontal

eddies (see Section 5.2). These cyclones were an important forcing to,

as well as a direct influence on, the slope circulation.

5.4.4 Long Period Variability

Spectra were calculated for the two-year long velocity records.

Examples of the kinetic energy spectra are given for the 40m level in

the upper water column (Figure 5.4-12a, b & C), and 200m and deeper

(Figure 5.4-l2c, d & e) . The spectra are in variance preserving form

such that equal areas under the curve have equal contributions to the

total kinetic energy. Attention is restricted to periodicities longer

than 15 days (0.065 cpd) . Higher frequency motions are discussed in the

context of seasonal variability in Section 6 of this report.

147
Velocity Statistics at I 5m

90W 89GW

31N

30N -

29N

28N

30N

29N

Velocity Statistics at 70m

90W 89W

31N

28N

90W

crw 89W

4/ 1/1997 TO 4/ 1/i9

SSW 87W

31 N

88W

8W

87W

7W

86W

86W

10 cm/s

10 cm/s

- 2Y'N

85W

30N

28N

4/ 1/1 997 TO 4/ 1/1999

87W 86W

31N

ç 30N

L

29N

28N

5.W

Figure 5.4-9a,b. Mean current vectors and standard deviation ellipses

for the two-year study period at the indicated

depths.

148
90W 89W

3

90W 89W

31 N

30N

29N

2 N

Velocity Statistics

Velocity Statistics

4/ 1/1997 TO 3/31/1999

87W 86W 85W

88W 87W

4/ 1/1 997 TO 3/31/1999

85W

31 N

10 cm/s -

28N

30N

- 29N

28N

5W

Figure 54-9c,d. Mean current vectors and. standard deviation ellipses

for the two-year study period at the indicated

depths -

149

90W 89W 87W 86W 85W

200 m(SoIid) & 300m (Dashed)

90W 89W 87W 8W

500m (Solid) & 1290m (Dashed)
a)

31N

30W

29N

28N

gow

40HLP Mean Temperature at 500m

40HLP Mean Temperature at 90m

4/ 1/1 997 TO 3/31/1 999

88W 7'W 85W

31 N

Figure 5.4-10 Contours of mean temperatures for the two-year study

period (4/2/97 to 3/31/99) at (a) 9Dm and (b) 50Dm.

150

4/ 1/1997 TO 4/ 1/1999

9o.w 8&W 87W 86W 85W

9OW 89W 87W 8BW 85W

gow 89W 87W 8.W 85W
Gulf of Mexico Sea Surface Height

2-year mean TOPEX/{RS-2 sea surface hightanorna]y

plus 10-year chniatolog]ca] model mean

cm

30

25

20

Li

LO

S

0

-5

-LO

- L5

-20

-25

-30

Figure 5.4-11 [To-year mean TOPEX/ERS-2 SSH anomaly added to a 10-year

climatological LC derived from numerical model studies.

151
200 10 a)150 10050 10-2 CYCLES/DAY Li ci z> b) c) 10050 10-2 CYCLES/DAY Li 0z> 150 100 0 0 0 1 O 2/ II III III IIII IIII II 32,. 1.I IAjA1I II 

1111

Figure 5.4-12. Kinetic energy spectra in variance preserving form for two-year velocity 

(in panels a, b and c), Cd) 200m and 300m, (e) 490rn, and (f) 50 Urn. The 

the frequency bands used for the ECF analysis are shown by heavy dashed 

KINETIC ENERGY SPECTRA KINETIC ENERGY SPECTRA KINETIC EN

CYC

I OSCA2-3R 70 40 44. SDcIO I 500-01-29 45 45 4.4. SOLID I DSCD1ZR 75 40 14. 

2 050-02-2575 3 000ASIS 70 40 44. DASHED 2 350-02-29 75 40 4.1. DASHED 2 DSCD0-1R125 27 14. 

40 44. SOlID 3 500-03-10175 40 4.4. SOLID 3 550-02-20145 40 4.4. 

4 DSCB0-15 75 40 H, DASHED DATE 1997/ 4/ 20000 RIME SERIES LEIEDT4.4 724 DAYS 4 0SCE1-2R170 40 H. DATE SF05005 OF FSEEDOI1 1997/ 4/ 2,0000 TIME SERIES LE1AOTI4 724 DAYS DEGREES OF F9EECOI4 22 ANDWIDT44 0.01519337 COD DATE 1997/4/ 20000 T

22 5,0509415TH 0.01011337 SF0 SEGREES OF FREEDOM 22 R
300 Figure 5.4-12. Kinetic energy spectra in variance preserving form for two-year velocity re

(in panels a, b and c), (d) 200m and 300rti, (e) 490rn, and (f) 500m. The bo

the frequency bands used for the EOF analysis are shown by heavy dashed li

I 131 I I 41 2.JPII 11 I F I 2d) 11h11u1 I /. I 

21 

1111111 

d) KINETIC ENERGY SPECTRA e) KINETIC ENERGY SPECTRA f) KINETIC ENER

10-2 10-0 1 

CYCLES/SAY CYCLES/DAY CYCLE

I DSC-#2-69 70 300 H. SOUD I SSE-B2-7R 75 490 4. SOLID 1 050-03-ZR 70 500 01. SCU2 DSC-C2-4# 75 200 9. 04.51-lED 2 DSC-C2-7R 75 490 0. DASHED 2 050-03-211175 500 1. DASF3 2SC-112-6R 75 300 9, SCUD 3 DSC-D2--7R945 490 10. SOLID DATE 1997/4/20000 lINE 4 DSC-D2-41145 200 H. DASHED 001 1997/4/ 2.0000 lIME SERIES LENGTH 724. DAYS DEGREES OF FREEDOM 22 11AI4

DATE 1997/4/ 20000 TIME SERIES LENGTH 724. DAYS 50000(5 OF FREEDOM 22 11AIODWIOTH 0.01519337 CR0 DEGREES OF FREEDOM 22 HANDWIDOH 0.01519337 CR0 I 25 100 tnJ 75 0) za: > 50250 20 Ui Uza: > 10 4030 Ui 0z 20 a: > 100
These spectra have prominent peaks that do not always appear at the same

frequencies. This seems to be caused by movement of eddies on the

slope. Moorings near the edge and center of eddy circulations have

different spectra with the former having more energy at higher - frequencies. Even so, three frequency bands were chosen so that they

spanned the major spectral peaks for the majority of the records. These

are 0-0.02 (724-50 day period), 0.02-0.043 (50-24 days), 0.43-0.65 (24-

15 days) cpd. The boundaries of these bands are marked on Figure 54-

12.

In the lowest band (0-0.022 cpdj, energetic motions were found between

-70 and 100 days at the mid-and offshore moorings at and above 300m.

The shelf-break and deep slope (500m) velocities had less energy in this

band. The B2 mooring had the most energetic fluctuations and energy

decreased in both along slope directions. The canyon region had less

energy than over the Alabama slope. These motions, with periods longer

than 45 days, corresponded to some of the major events discussed in the

previous sections. The two higher frequency bands had prominent peaks

at 200m and below. At the upper levels, motions at 25 to 35 days and 16.

to 20 days were not as prevalent and organized as at depth (compare

Figure 5.4-12a & b with 5.4-12 d & e)

EOF analysis was performed for the three, equal-width frequency bands

using all available velocity records (both U and V-components) that

spanned the two-years of measurements. Three levels (16, 40 and 68m)

were used from the 300 kEz ADCPs (all moorings except D9), and four

levels (37, 72, 98 and l68m) from the 150 kHz ADCP on D9. Analysis

using all the levels of individual ADCP's has shown a high degree of

coherence between levels. The various levels at C2 are seen to have

been visually coherent in Figure 5.4-5. This analysis showed that the

two higher frequency bands had very similar patterns and therefore they

were combined in order to increase the significance of the modes. The

EOF analysis presented here is for the two frequency bands 0-0.02 and

0.02-0.065 cpd, respectively. The elgenvector components were multiplied

by the square root of the eigenvalue to give amplitudes in velocity

units (cm/s) . The significant modes and their eigenvalues are summarized in Table 5.4-1. Error estimates for the eigenvalues were

determined from the number of degrees-of-freedom in each band and the

requirement that the eigenvalues not overlap, and thus, be considered

distinct (North et al. 1982) . Under this strict criteria modes 2 and

higher, and modes 3 and higher are not significant for the lower and

higher frequency bands, respectively.

For 50- to 15-day period (0.02-0.065 cpd) motions, more than half the

variance was not accounted for by the two significant modes. This

implies that a majority of the energy was contained in small-scale

motions that were not coherent across the array. At longer periods, the

first mode dominated and accounted for slightly more than half the

observed variance. The structure of the modes was examined by

decomposing the amplitude and phases of the U and V components at each

position into vector hodographs. Essentially, these EOF hodographs use

the same formulation as tidal current ellipses (Foreman 1979) where the

phase is relative to the major axis. The ellipses represent coherent

154
current fluctuations at the central period of the band about the twoyear mean flow.

Table 5.4-1 Frequency domain EOF analysis of tWO-year velocity records.

Figure 5.4-13 shows the EOF ellipses at selected upper and lower layer

depths for mode 1 of the lower frequency band. The fluctuations did not

have a great deal of variability with depth. For this first mode, the

fluctuations followed the trend of the isobaths and were most energetic

along the Alabama slope. These fluctuations had little or no influence

on the canyon and shelf break moorings Cl, Ll, D9 and El and thus

represent large scale, low-frequency (-400-day period) eastward and

westward flows along the slope. Phase changes from east to west were

small but show some interesting features. Along the 500m isobath, the

east side led the west at both the 40m and deeper depths, but for the

l300m moorings the west had a small lead or was nearly in-phase with the

east. This implies offshore and westward propagation of phase which in

turn suggests propagating topographic waves (Eogg 1981). Unlike

topographic Rossby waves, these propagating topographic wave motions

have the largest amplitudes near the surface.

A similar EOF analysis using the same 0-0.02 cpd frequency band was

performed for all the temperature records at 90m. The mode structure

was similar to that given in Table 5.4-1 with the first mode accounting

for 63.6% of the total normalized variance. The amplitude and phase of

mode 1 of the 90m temperatures are given in Figure 5.4-14. The

amplitudes of the fluctuations were quite uniform with the maxima on the

western rim of the canyon and the offshore side of the Alabama slope.

Again, the phase changes indicate offshore and westward propagation of

the coherent signals. A least squares fit of wave numbers to the array

of phases obtained from the velocity and temperature modes (Hamilton

1984) are given in Table 5.4-2. Estimates of wavelengths range from 250

to 600 km with the wave vector directed approximately due south. This

155

enter Feqiency 0 003$ aiidwadti = 0 O3.94 p6

No.

Eigenvalue

(cm2/s2)

Percent of Total

Variance

Cumulative

Percent Error

1 487.2 57.7 57.7 130.2

2 128.1 15.2 72.9 34.2

3 80.3 9.5 82.4 21.5

4 57.6 6.8 89.2 15.4

Center reuency 0 04282 Bandwidth = O.O453

No. Eigenvalue

Percent of Total

Variance

Cumulative

Percent Error

1 41.32 27.6 27.6 7.19

2 23.24 15.5 43.1 4.05

3 17.31 11.6 54.6 3.01

4 14.22 9.5 64.1 2.48

S 10.06 6.7 70.8 1.75
31 N

30N

29N

28N

31 N

90W

9OW

9OW

99'W

59W

88W

724-50 Day Periods Mode 1 at 40 m 57.7 Variance

4/ 2/1997 TO 3/27/1999

88W 87GW 86W 85W

31 N

87W

4/ 2/1997 TO 3/27/1999

85W

31N

724-50 Day Periods Mode 1 at 200-500 m 57.7 Variance

Figure 5.4-13 EOF ellipses for mode 1 of the 724 to 50-day period band.

(a) is the subset for 40m depth, and (b) for 200m (C2 and

D2), 300m (A2 and B2) and 500m (A3 and C3) depths. The

relative phase of the motions is given by the arrow

heads -

156

30N

29N

28N

ow 89°W 88W 87W 86W 85W
28N

9OW 89W 88GW 87W

gow

90W

724-50 Days Temperature Amplitude at 90m Mode 1 63.6% N.Var.

4/ 2/1 997 TO 3/27/1999

89GW 88W 87W 86GW 85GW

31 N

90W

89.w 88W

4/ 2/1997 TO 3/27/1999

87W 86GW 85GW

31 N

86GW

- I I 89W 88W 57GW 8&W

724-50 Days Temperature Phase at 90m Mode 1 63.6% N.Var.

85W

85W

3ON

2'N

Figure 5.4-14 EOF Mode 1 (a) amplitude (°C), and (b) phase (degrees)

of the 90m temperatures for the 724-50 day period band.

Positive phase angles lead.

157
is consistent with the energy of planetary waves propagating westward

along the slope. This could be an indication that the slope acted as a topographic waveguide for long period (-400 day) disturbances. The time

and length scales are appropriate to LC or LC ring induced motions.

Table 5.4-2 Wave nurribers from EOF Mode 1 phases for 724 to 50-day

period fluctuations

The 50- to 15-day period band was much less energetic than the low

frequency fluctuations discussed above. Mode 1 ellipses for the upper

and lower layers are given in Figure 5.4-15. For this mode, the

velocity fluctuations were essentially in-phase, both horizontally and

with depth. Therefore, there is little evidence of propagating signals.

At the start of the cycle (e.g. --0° phase angle) the flow consisted of

two similar-sized (-400 km diameter) anticyclones, one over the Alabama

slope with a center over or seaward of the 2000m isobath, and the other

weaker circulation over the head of the canyon with a center between D9

and D2 (Figure 5.4-iSa). At -180° phase angle the situation was reversed

with two cyclones present. At depth, only the slope anticyclonic/

cyclonic circulation was present (Figure 5.4-lsb). Mode 2 (Figure 5.4-

16) reversed the sign of the slope circulation such that there was

cycloriic flow when there was anticyclonic flow over the head of the

canyon and vice-versa. The flows were also more perpendicular to the

isobaths on the lower slope (1300m moorings) than for the first mode.

Because the second mode accounted for less of the total variance, the

occurrence of counter rotating eddies was less prevalent than eddies of

the same sign. The flow patterns for both modes indicate that the two

eddies were interacting through flows along the upper Alabama slope.

The implication of this analysis is that slope and canyon eddy

circulations were relatively short lived (-10 to 25 days) and of

considerable complexity. This may account for the shifting position of

the peaks in this 0.02-0.065 frequency band. The relatively low

percentage of the total variance accounted for by the significant modes

also implies that smaller scale eddies were poorly resolved by the

array. This is in contrast with the more organized, energetic, largescale wave-like mOtions diagnosed in the 0-0.02 cpd frequency range.

158

I

i Eas' Wave

Number,

I

' I I

I North Wave

Number, .1

'H'

'

I I Wavelength

, it()

Di.rection

Prpaatn

1rue)

U, V-Cmpts

at 40 m

0.000046 -0.002597 385 179

U, V-Cmpts

at 200 to

500 m

0.000305 -0.004348 229 176

Temperature

at 90 m

0.000014 -0.001637 611 180
31 N

30N

29N

30N

2N

2eN

90W

90W

4/ 2/1997 TO 3727/1999

87W 86GW 85W

31 N

B8W

87'W 86W

50-15 Day Periods Mode 1 at 200-500 m 27.6% Variance

85W

30N

Figure 54-15 EOF ellipses for mode 1 of the 50 to 15-day period band.

(a) is the subset for 40m depth, and (b) for 200m (C2 and

L2), 300m (A2 and B2) and 500m (A3 and C3) depths. The

relative phase of the motions is given by the arrow

heads.

159

28N -. 28N

9OW 89W 87W 86W 85W

50-15 Day Periods Mode 1 at 40 m 27.6% Varionce

4/ 2/1997 TO 3/27/1999

9OW 89W 88W 87W 85W

31N 31N
a

28 N

90°W

o.w

90W

89.W

89W

4/ 2/1997 TO 3/27/1999

88W 87W 86GW 85W

31 N

88W 86'W

50-15 Day Periods Mode 2 at 200-500 m 15.5% Variance

85W

2BN

50-15 Day Periods Mode 2 of 40 m 15.57W Variance

4/ 2/1997 TO 3/27/1 999

89W 88GW 87W 8W 85W

31 N

Figure 5.4-16 EOF ellipses for Mode 2 of the 50 to 15-day period band.

(a) is the subset for 40m depth, and (b) for 200m (C2

and D2), 300m (A2 and B2) and 500m (A3 and C3) depths.

The relative phase of the motions is given by the arrow

heads.

160

9OW 9W 88W 87W 85W
5.5 Slope Dynamics and Fluxes

5.51 Divergence and Relative Vorticity

The arrangement of the moored array allowed resolution of both northsouth and east-west spatial gradients of velocity. The divergence (D)

and relative vorticity () are given by:

D au/ax + av,iay, and aviax -

respectively, where (u,v) and (x,y) refer to the geographic coordinate

system. Direct calculation of these quantities is expected to be noisy.

Therefore, following Chereskin et al. (2000), least square planes were

fitted to arrays of 4 to 6 velocity components at a given depth level.

Thus,

u(x,y,t) u0 + x au/ax + y auiay + HOT, etc.,

where (x,y) were measured from the center position of the array. In the

least square fit, the velocity components were weighted by their

standard deviations. The resulting standard deviations of the gradient

terms in the plane model were of order 0.05 to 0.09f, where f is the

Coriolis parameter (07 l0 s-') . Standard least square methods

(Chapter 15, Press et al., 1992), rather than the matrix inversion

method described by Chereskin et aL (2000) were used for these

calculations. The array positions used in the calculation of D and

for various depth levels at B3, C2 and D9 are shown in Figure 5.5-1.

The time series of divergence and relative vorticity at 40, 70 and 500m

depths are given in Figures 5.5-2 and 5.5-3, respectively. Substantial

fluctuations of D and occurred at both the 40 and 70m level for B3 and

C2. The signals were coherent with depth except at D9 where both D and

fluctuations were small. Vorticity was also coherent between the

upper levels of B3 and C2. The divergence at the upper levels of B3 and

C2 were weakly positively correlated (R = -0.25 and 0.35, respectively)

with temperature records at the same positions. The 80m temperature

record at B3 was, however, negatively correlated (R -0.48) with the

vorticity. The same relation does not hold at C2. The implication is

that only a small part of the temperature signal could be attributed to

local vertical velocities. At B3, there was a stronger relationship to

whether a cold or warm eddy (positive or negative vorticity,

respectively) was present on the lower slope The same relation did not

hold for C2 because it was often situated in the outer parts of

different eddy flows.

It is clear from Figure 5.5-3 that the vorticity fluctuations at B3 were

closely related to the along-slope currents. Thus, eastward flow

corresponded to negative vorticity and vice-versa. The large negative

anomaly in July and August 1998 corresponded to the intense anticyclonic

circulation observed over the slope (see Figure 54-le) . Even though

the vorticity at C2 was similar to that at B3, it was much less closely

related to the C2 currents. Again, this most likely occurred because of

the position of C2 on the edges of the major deep-water eddy

circulations (Figure 5.4-1). The small magnitude

161
Figure 55-1 Positions used for divergence and vorticity calculated at

the circled points.

162
I-00

°

0 -

I

'I20 -

3=

5

-- - IA*AAI I.. - D9

A.

W 1'

- - M. LA ...

80,..

'r''''w- .'

'''''

- L ....

- L7m

D9 =

-

-

.

72ni

C2

:1

r

- -,.

0 01 h. th.i. A..0

- r

A

- fl

gg

40m

C2 VT,. r'6 r!p'I

. .r G8rfl

26 -

24-

22 1 . vw

- VT

8, Cr3

3 1! '"

0.. j. .o A1..

V''

. . 83 Fl !'' 68,,,

Divergence (Scaled by f) & Temperature

L'

163 /

Figure 5.5-2. Divergence (normalized by t) and 40-HLP temperature time

series for the indicated positions.
Aloog isobath

LCross Isobath & Offshore

AIon8 issbativ LCmos losbath & Offshom

I F ...L.,I F F, I ,l I 5 13 30 15 7 27 IS 4 a. 1! 4 23 12 1 20 11 3! Mor Apr Mny Jan JIy Aug Sep Out Noy Des J55 Fob 51cr Apr May Jun JIy Aug S.F 01st Nov D.c Jon Feb Hur Ap

1997 1998 1999 Relative Vorticity (Scaled by f)

83

Figure 55-3. Relative vorticity (normalized by f) and 40-HLP current

vectors for the indicated moorings and measurement

depths. Relative vorticity is such that a positive value

indicates a tendency for counterclockwise rotation and a

negative value for clockwise rotation.

164
of the vorticity fluctuations at D9 is somewhat surprising given that

flows following the rim of the canyon should have considerable

curvature. However, major flow events often bipassed the head of the

canyon, and the eddy scales were often smaller than the separation

between the moorings used for these calculations (Figure 5.5-1)

The spectra of D and show that most of the energy was at periods

longer than 50 days (Figi.ire 5.5-4a). The peak at l00 days is similar

to those found in the velocity spectra (Fig-ure 5.4-12) and suggests that

the major vorticity fluctuations were associated with the long period

propagating slope waves that are probably related to LC and LC ring

effects. Figure 5.5-4 also shows the coherence squared and phase

differences of the most coherent records with D and at B3 and C2 (40m

depth) - At long periods, the vorticity was coherent and 1800 out-ofphase with the along-slope current at B3. This confirms the visual

correlations of the time series in Figure 5 .5-3. However, D and B3 were

coherent and in-phase with the along-slope current at C2 at both long

periods and periods of order 10 days. D and were not significantly

correlated at B3 and because at B3 and C2 were highly correlated

(Figure 55-4c), at C2 was only weakly correlated with the along-slope

currents at C2. A dynamical explanation of why the divergence of the

eddy circulations over the lower slope were closely related to the

current over the middle slope is not apparent at the present time. The

other interesting relation to come from the coherence analysis is that

the divergence at C2 was most strongly related to the across-slope

current at C2 (Figure 5.5-4c) . This seems to indicate that part of the

up- and downwelling that occurs along the edge of eddies on the lower

slope was induced by cross-slope exchange.

5.5.2 Across Margin Transport of Momentum at the Canyon Edge

Cross-margin flows at the canyon edge can also result in exchange of momentum

between the canyon and the adjacent shelf. Using the velocity data from

canyon edge moorings, the total cross-margin fluxes of along-isobath velocity,

and its eddy contribution were computed according to:

FM uv, and FM UV

where FM and FMM are the total and eddy fluxes, respectively; u is alongisobath velocity; and u' and v' are fluctuations about the means of u

and v. Hereafter, FM and FMe are referred to as momentum fluxes

(although, technically, they are related to actual momentum fluxes

through sea water density) - The velocity rich data set from the canyon

edge moorings allowed for the computations of momentum fluxes throughout

most of the water column. These data further enabled us to compute

"momentum transport" at each of the moorings by integrating the momentum

fluxes over depth, ie.

TM = JFMdz, and TM - JFMdZ

where TM and TN, are referred to as the total and eddy cross-margin

momentum transports, respectively. The ADCP data from each of the shelfedge moorings were used to compute these TM and TM, time series. These

data spanned a depth range of roughly l2-72m.

165
.004 .003 Li U< .002 > .001 .000 10 1O 10 CYCLES/DAY Ua- 0 95 A 1' ..-- -4 1JI CYCLES/DAY 1 900-R3-1 .40 IV. SCUD VELOCITY TIVEROENCE DSC-B3-1 40 .1. RELATIVE VOI4TICITY 2 I]SC-R3-1 40 IV. DASHED RELATIVE VORTICITY DOC-93-IFT 75 40 4.1. V COMPONENT TATE 957/ 4/ 20000 TIVVE SERIES LENGTH 724 DAYS DSC-93-1 45 IV. VELOCITY DIVEI1GENC ECOREES CF FREEDOM 22 RANEWIOTH 0.01511337 OrE 2 000-02-211 75 49 IV. V COMPONENT 110 RATE 1997/4/ 2.0000 TIME SERIES LENGTH 724 OATS DEGREES OF FREEDOM 22 NAHOWIDTH 0.01519337 CPO Li (1) a- CYCLE

DOC-.53--I 40 1.4. DSC-C2-2 40 4, 050-02-2 40 H, 

2 000-02-OR 75 40 H, 

DAlE 1997/ 4/ 2.0000 TIME S

DECREES OF FFEEECII 22 RAHDW

Figure 5.5-4 (a) Spectra of the divergence and relative vorticity at E3. Coherence squa

differences between the indicated records at (b) B3 and (c) C2. Li 2 Al A iIiI!__ 80 I1IAIJU ________________________ ''-f 

a) SPECTRA b)
Integration of TM over time gives an estimate of the quantity of

momentum transferred across the canyon edge per unit along-canyon

distance. When considering the direction of this momentum transport it

is necessary to take into account the direction of the along-isobath

flow, u. The coordinate system used in these calculations has the

positive u direction set such that deeper water is to its right(i.e., to

the northeast at moorings Al, El and Cl; and to the southeast at mooring

El). With this convention, the positive v direction is always onshelt.

In this rotated coordinate system, positive momentum fluxes and

transports represent the onsheli flow of positive along-isobath momentum

(positive U) or the of fshelf flow of negative along-isobath momentum.

Consider, for example, momentum fluxes occurring at mooring Cl when the

flow at the mooring is northeastward (positive U) - Positive momentum

fluxes represent an onsheif transfer of the momentum of this flow.

However, if the flow at the mooring is to the southwest (negative U),

then a positive momentum flux represents an off shelf transfer of

momentum from this flow (because, in this case, Iuv] is positive for an

offshelf velocity)

IJnlike the cross-margin fluxes of temperature and salinity discussed

later in Section 5.5.3, the cross-margin momentum fluxes and transports

computed from the canyon edge mooring data were dominated by the eddy

contributions. For all computed momentum transport time series, the

ratio of TMe to TM variance was 0.82-0.95.

Time series of vertically averaged along-isobath current and TM,

computed from the canyon edge ADCP velocity profiles (Figure 5.5-5)

reveal a number of events of strong along-isobath flow coupled with

large cross-margin momentum transports. At all canyon-edge moorings,

the largest vertically averaged along-margin flows occurred during

Hurricane Georges (late September 1998) . These flows were in the

negative u direction (having an alongshore westerly component) at all

canyon edge moorings, indicating a hurricane-induced flow moving

counterclockwise along the canyon rim. As with the cross-margin

salinity and temperature fluxes discussed in Section 5.5.3, the computed

canyon edge momentum transports differed greatly from mooring to mooring

during the hurricane.

Large along-isobath currents and cross-margin momentum transports were

also observed during the winter of 1997-1998. As noted in Section 6.4,

this was a period when intense storms passed over the canyon and when a

vigorous eddy was seen at the western canyon edge. The vertically

averaged velocities and cross-margin momentum transports from this

period (Figure 5.5-6) reveal a response to wind forcing which varied

from storm to storm; and, for each storm, from mooring to mooring. The

late December 1997 storm generated positive along-isobath flows along

the canyon rim. Computed cross-margin momentum transports associated

with these flows were relatively large and directed onsheif at moorings

Al and El, but were negligible at mooring Cl. By contrast, the storm of

early February 1998 generated strong negative along-isobath flows along

the canyon rim. Computed cross-margin momentum transports during this

storm were unusually large only at mooring Cl. These very large

momentum transports at Cl would have carried the storm-generated

momentum onsheif. Relatively large cross-margin momentum transports

167
E

0

100

-100

100

Ec 0

-100

100

E00

c'JCl,

c)

E

-100

10

-10

10

C'JC!) c,, 0

E

-10

10

C"

Cl)

C!) 0 E

-M. i I . Ia V ii

a ai

Vertically Averaged Alongshelf Velocity

Crossshelf Momentum Flux

-) , -r4

1. -10 A MJ J A SO ND J FMA MJ J A SON D J F MA

1997 1998 1999

Figure 55-5. The top three panels show vertically averaged alongisobath velocity at the indicated moorings. The lower

three panels show estimated cross-isobath momentum

transport at the indicated moorings.

168
('U

(0 COE

Wind Stress at 42040

Cross-isobath Momentum Transport

-10 L 15 20 25 30 4 9 14 19 24 29 3 8 13 18 23 28

Dec. 1997 January 1998 February

Figure 5.5-6. For the winter of 1997-1998, computed surface wind

stress at buoy 42040 (top panel), vertically-averaged

along-isobath velocity (middle panel) and cross-isobath

momentum transport (bottom panel)

169

10

AA A4A .A.

100

50

0

-50

-100

Vertically Averaged Along-isobath Velocity

- mooring Al - mooring Cl

- - mooring El

I-'
were also observed during mid-January 1998 when the winds were mild and

an eddy was observed at the canyon edge. These large momentum

transports were confined to moorings Al and Bi, which is consistent with

satellite imagery which show the eddy in the area of these mooring

during mid-January (see Figure 6.4-14,15). The computed transports at

Al changed sign (from onsheif to of fshelf) on January 18, presumably due

translation of the eddy by the mooring.

To assess the possible influence of the computed momentum transport

"pulses" on the canyon-edge velocity field, consider an idealized

situation in which the onshore momentum flux is confined to an outer

shelf band of width, W, and mean depth, D. In the absence of friction,

the average velocity in this band will be changed by:

t2 Au = JTNdt/(DW)

ti

Integrating over the successive onshelf and off shelf momentum transport

pulses seen at Al during mid-January 1998 (associated with the eddy

encounter with the canyon edge) and setting D and W to 60m and 20 km.

respectively, gives a Av of 20 cm/s. Using the same D and W values and

integrating over the onsheif momentum pulse seen at mooring Cl during

the early February 1998 storm, gives a Au of 50 cm/s. Clearly, results

of these over-idealized calculations are not robust in a quantitative

sense. Nevertheless, they provide evidence that cross-margin momentum

fluxes during stoLlns and times of eddy contact with the canyon edge may

significantly influence flows over the shelf near the canyon edge.

To estimate the overall extent to which wind-forcing contributed to the

cross-margin momentum fluxes, we computed a wind-driven component of the

fluxes using the method outlined in Section 5.5. The relationship of

the estimated wind-driven fluxes at various depths was investigated

through cross-spectral analysis. For a particular mooring, crossspectra were computed between the shallowest and all other flux time

series. Coherence and phase relative to the shallowest flux series was

determined according to standard formulae (Bendat and Piersol, 1971; p

32) and by frequency band averaging over the entire cross-spectra. As

demonstrated here by the results for mooring Cl (Figure 5.5-7), the

estimated wind-driven fluxes accounted for roughly one half of the total

flux variance. The wind-driven fluxes were highly coherent over the

water column, with coherence always exceeding the 99% confidence

interval. Their relative phases suggest a two-layer structure to the

wind-driven momentum flux, with near-bottom fluxes nearly 180° out-ofphase with fluxes above (Figure 5.5-7) - The zone separating the layers

was relatively narrow and deep, extending over the 55-60 m depth range

at mooring Cl.

Because the cross-margin momentum fluxes were determined from both along

and across-isobath velocity components, an obvious concern is their

sensitivity to the choice of along-isobath orientation. To test this,

we computed across-margin fluxes and transports at each canyon

170
30 40 50 60 50 0 50 100 150

% variance related to wind phase relative to 16 m flux

Figure 5.5-7. Characteristics of the estimated wind-driven crossmargin fluxes of momentum at mooring Cl. The left panel

shows the ratio of the estimated wind-driven to total

flux variance. Shown in the right panel is the relative

phase of the wind-driven flux as a function of depth.

171

10 10

I U

20 . 20 U

. .

. U

30 30

. . . .

-- 40 . 40 . 0

j) 0 . .

U

50 50

. U

U U

60 U 60 U U U

U . 70 70
edge mooring using along-isobath orientations differing by ± 5°of the

orientation chosen for the calculations which produced the results

discussed above (these orientations are given in Table 6.4-2) - Although

the details of the momentum fluxes and transports changed with varying

orientation, all of the features discussed above were seen at all

orientations tested.

5.5.3 Across Margin Heat and Salt Fluxes at the Canyon Edge

Cross-margin flows at the canyon edge can effect an exchange of warm,

saline within the canyon with the fresher and cooler coastal water

onshore. Using temperature, salinity and velocity time series from the

canyon edge moorings, cross-margin fluxes of temperature and salinity

were computed using:

F1=vT, F=vS

where T, S and v represent temperature, salinity and the across-isobath

component of velocity. F and F can be converted to heat and salt

fluxes by multiplying by Seawater heat capacity and density,

respectively. The portion of the property fluxes produced by random,

"eddy" motions, was estimated in the typical fashion as the product of

property and velocity fluctuations about their means. For example, eddy

fluxes of temperature were calculated as:

FTe = v'T', with v' = v - <v>, and T' = T -

Where <> denote time averaging. With this convention, total mean fluxes

could be divided into eddy and advecti-ve components, i.e.

<v><T> <v'T'>

Computation of temperature and salinity fluxes at the canyon edge was

limited by the paucity of temperature and salinity data from the canyon

edge moorings. These allowed for determination of coincident

temperature and salinity fluxes at only 11 points over the canyon-edge

(Tables 5.5-1 and 5.5-2), concentrated near the bottom and near the

surface. In addition, times over which the fluxes could be calculated

were limited by sensor failure, and varied greatly from point to point.

At all locations, eddy fluxes made a relatively small contribution to

the total cross-margin flux of temperature and salinity (e.g. Figure

5.5-8) keeping in mind that both salinity and temperature are always

positive and always greater than some minimum value, thus the total

fluxes may be large due to the contribution of temperature and salinity

above the local minimum values. Certainly the sign of this total flux

is the sign of the cross isobath velocity. For salinity fluxes,

variance of the eddy flux was 0.1-2% of the total flux variance. Eddy

fluxes of temperature made a somewhat larger, 4-14%, contribution to the

total variance of temperature flux.

Of particular note is the strong linear relationship between temperature

and salinity fluxes, demonstrated here by the plot of all salinity

fluxes against all temperature fluxes at the canyon edge moorings

(Figure 5.5-9). Slope of the least-squares linear regression line

172
00

600

400

200

-200

-400

Temperature Flux - Mooring Cl at 16 m

I

-600

A M J J A S 0 N D J F

1997 1996

- total flux

- eddy flux

M A

Figure 5.5-8. Time series of cross-isobath temperature flux at 16 m

depth at mooring Cl. Total flux and eddy flux are shown

by the thin and bold lines,respectively.

173
1500

1000

500

E

o

-500

><

-1000 9-

-1500

(I)

-2000

-2500

-3000

Figure 5.5-9. Plot of cross-margin salinity flux against cross-margin

temperature flux. Fluxes were computed using data from

canyon edge moorings (Al, El, Cl, Dl and El). The line

was fit to the points by least-squares.

174

I I I -2000 -1500 -1000 -500 0 500 1000

Temperature flux (°C cm/s)
relating these fluxes was 1.49 psu[C with a 95% confidence interval of

only 0.004 psu/'C. In view of the much lower costs of acqviring

temperature vs salinity measurements, this result may be of value in

designing future studies in the area. Water mass fluxes may be

adeguately deteiliLined from temperature and velocity measurements alone.

When judged by their statistical errors, the computed mean fluxes of

temperature and salinity across the canyon edge were significant at

moorings Cl, Dl and El, and not at Al and Bl (Tables 5.5-1 and 5.5-2).

The mean fluxes at Cl, Dl and El were dominated by their advective

components. These means suggest that, in the long term, the water mass exchange at the canyon edge may be something of a two-layer phenomenon,

with onsheif flux near the surface and offshore flux near the bottom.

Unfortunately, the limitations in available mooring data does not

support an evaluation of how fluxes in the center of the canyon edge

water column may conform to, or clash with, this view.

Table 5.5-l. Estimated cross-margin temperature fluxes at DeSoto Canyon

edge moorings (at 100 m isobath) . Positive fluxes are

onshelf.

08/18/1998 -

12/17/1998 -

SensØ*.

flepth

(rn)

04/01/1997-

11/25/1997 -

11/25/1997 -

04/01/1997 -

08/27/1997 -

04/12/1998 -

11/23/1997 -

07/20/1997 -

F1uxe.s;

C:

08/17/1998 -

03/27/1997 -

08/14/1998 -

03/31/1997 -

Standard Errors were computed as Std. Err. SD! df)112,

where SD is the standard deviation of the fluxes about

the estimated mean and df are the number of independent

flux measurements from which the mean was calculated. df

were approximated as T/Te, where T is the length of the

flux series and T is the correlation time scale of the

series. Based on autocorrelation functions of the flux

series T was set to 60 hrs. 90 % confidence limits of

the means are roughly twice the standard errors (See

Bendat and Piersol, 1971; p 115).

An issue of importance is the extent to which canyon edge fluxes were the product of wind-induced flow or the result of other types of

175

Mean,

01/28/1998 1.1 49.6 -1.33 2 .73

04/01/1999 -28.2 20.9 -1.97 1.2

08/18/1997 -26.9 18-9 -4.66 1.98

04/04/1998 16.5 15.5 0.79 0.81

04/04/1998 1.0 9.7 1.11 0.76

07/01/1997 -0.8 12.6 0.92 0.57

04/01/1999 -51.2 25.9 -6.79 3.62

08/08/1998 -16.5 11.2 1.16 1.39

12/05/1998 40.0 5.1 -0.82 0.58

03/30/1999 -66.1 8.1 -0.09 7.38

11/14/1997 -85.7 19.1 -3.58 1.78

03/31/1999 -49.7 15-3 -5.73 1.93

04/03/1998 8.1 16.5 -1.15 1.02

08/07/1998 5.4 5-2 -0.39 0.42

Al 16

Al 16

Bi 20

Bl 62

Bi 82

82

ci 16

Cl 20

Cl 82

Dl 62

El 20

El 20

El 62

El 82

-Std. std
motions, such as those generated by LC eddies. To address this issue, a

standard statistical technique was employed to extract that portion of a

salinity or temperature flux series that was statistically related to

the surface wind stress. The procedure was carried out in two steps.

In the first, the relation between flux and wind stress was quantified

by computing a spectral transfer function with east and north wind

stress components as inputs and cross-margin flux as the output (see

Bendat and Piersol, 1971; pp 151-153) . In the second step, this

function was combined with the wind stress components to produce the

estimated wind-driven component of the flux series.

In Figure 5.5-10, supports a comparison of the total and estimated winddriven component of cross-margin temperature flux at moorings Cl, Dl and

El. Also shown are the ratios of the wind-driven flux variance to the

total flux variance, a rough estimate of the proportion of flux

generated by wind motions. These ratios suggest that wind-forced flows

may have accounted for a significantly higher proportion of the crossmargin temperature flux near the bottom than at the surface. This

disparity is best evidenced by the mooring Cl flux variance ratios. The

admittedly sparse time series suggest a seasonality in the importance of

wind-induced fluxes. The greatest mismatch between total and windforced temperature fluxes tended to occur in the spring and summer. Of

note are the large temperature fluxes seen at the 20m level of mooring

El during May-July 1998, which coincided with negligible estimates of

wind-driven temperature flux. At all moorings, the largest cross-margin

temperature fluxes occurred during Hurricanes Earl and Georges (in

September 1998) - However, the character of the fluxes during these

storms was not the same at all moorings. At mooring Cl, they were

offshore near the surface and onshore near the bottom, while the reverse

was observed at moorings Dl and El.

Table 5.5-2. Estimated cross-margin salinity fluxes at DeSoto

Canyon edge moorings (at 100 m isobath) . Positive

fluxes are onshelf.

* See Table 5.5-1 for an explanation of the standard errors.

176

oiing

Sensor

Popth

(M) Date

Total Flue

p31.1 cm/s

EddF1iié'

psu cm/s

sean Err.* tean Err..*

Al 16 08/18/1998 - 01/28/1998 1.2 64.3 -1.97 1.09

Al 16 12/17/1998 - 04/01/1999 -44.0 34.3 -0.39 0.51

. :. ............ .............................................. /. ::. - .....1-:- ..: ... .:...:... .:.: ...... 51 20 04/01/1997- 08/18/1997 -33.7 28.0 -0.98 0.45

81 62 11/25/1997 - 04/04/1998 27.9 27.2 -0.04 0.16

81 82 11/25/1997 - 04/04/1998 -0.2 19.1 0.01 0.12

81 82 04/01/1997 - 07/01/1997 -3.2 23.2 -0.03 0.03

. ,'. :.-. . . ........... :/::.-:...: '... ci 16 08/17/1998 - 04/01/1999 -66.9 35.0 -0.07 0.68

ci 20 03/27/1997 - 08/08/1998 -276 17.0 -0.55 0.27

ci 82 08/27/1997 -12/05/1998 785 9.8 -0.07 0.07

81 62 08/14/1998 - 03/30/1999 -114 8 13 9 0 02 0 10

El 20 03/31/1997-11/14/1997 -- -119.3 27.2 -107 0.43

El 20 04/12/1998 - 03/31/1999 -68.6 23.1 -0.24 0.25

El 62 11/23/1997 - 04/03/1998 17.1 31.1 0.40 0.19

El 82 07/20/1997 - 08/07/1998 10.8 10.3 -0.21 0.09
20

-1000

0

-2000

1000

0

- 400

E 200

o 0

-200

-400

o -600

1000

0

-1000

500

0

Surface Wind Stress eastward

- northward

I I

I I

I I I I Cl 82 m

I-i

500 I I I I I

I I I

I I

I I I A MJ J A SON D J F MA MJ J A SON D J F MA

1997 1998 1999

Figure 5.5-10. Time series of cross-margin temperature flux at the

indicated moorings and depths together the with surface

wind Stress components (top panel) Total fluxes are

shown by the blue lines. The red lines are the portion

of the total flux coherent with the surface wind

stress. The number above each time series is the ratio

of the wind-driven flux variance to the total flux

variance.

177

Temperature Fluxes

2000
5.6 Deep Currents

Previous sections have described the circulation in the upper 500m of

the water column. Over the slope, the circulation was dominated by

eddies and an eastward flowing current with reversed flows at depths

below about 200m. A major source of energy for these flows appears to

have been eddy circulations, including LC/LCFE, in the deep eastern

basin of the Gulf. The LC and LC rings penetrate to depths of 800 to

l000m. Below these depths, lower-layer currents have been obser-ved to

have the characteristics of topographic Rossby waves (TRW) (Hamilton

1990) with periods ranging from about 10 to 100 days. TRW motions are

columnar, slightly bottom intensified, have wavelengths of order 50 to

250 kin and propagate with group velocities of order 10 to 20 km/day.

The majority of studies of TRW's have been in the Middle Atlantic Bight

(e.g. Thompson 1977; Hogg 1981) where there has been some success in

relating deep TRW motions to Gulf Stream meanders (Pickart 1995). The

source of TRW motions in the Gulf is presumed to be the LC though the

generation mechanisms have not been established. Muxnerical model

studies, on the other hand, have predicted deep eddy circulations

associated with the detachment and westward propagation of LC rings.

These lower water column eddies have similar propagation speeds to their

overlying LC anticyclone (Welsh and Inoue 2000) - The existence of these

eddies has not yet been established from observations. If the source of

TRW's is primarily in deep water, then wave motions propagating towards

the slope will tend to be refracted by the increasing bottom slope back

into deep water (Shaw and Csanady 1988) - Limited observations of nearbottom currents in depths of about 100Cm on the northern slope have

shown very small velocities (<10 cm/s) compared to current magnitudes of

order 30 cm/s under the LC (Hamilton 1990) - The present study made velocity and temperature measurements at 1Cm

above the bottom at three positions (A3, B3 and C3) on the 1300 m

isobath along the northeast Gulf slope. The 40-HLP time series for the

two-year study period are shown in Figure 5.6-1. These velocity records

were not significantly correlated with the 500m level currents on the

same moorings and, therefore, they have been analyzed separately. There

were differences between the first six months (April to September 1997)

of the A3 velocities and the remainder of the record. In the first six

months there was a westward mean flow of 2.1 ± 0.5 cm/s which was

similar to the mean at SOOm. The fluctuations were not significantly

correlated between the two levels of A3. The significant westward mean

could be attributed to the cyclonic circulation that was present on the

lower slope for most of this period (see Figure 5.4-6 for the summer

1997 mean currents and Figure 5.4-2a & b for the 500m level geostrophic

flows in March and July 1997) - After October 1997, flows at all three

positions had more variability in direction and became more energetic.

The eastern most station (C3) and the western most (A3) had the highest

and least fluctuation energy, respectively. In examining these figures,

there is also visual coherence of some of the events in both the

currents and temperature fluctuations.

The kinetic energy and temperature Spectra are given in Figure 5.6-2a

and b. For the analyzed interval, there was a peak in the spectra for

C3 at about 20 days Hamilton (1990) showed that the 20- to 30-day

178
N 20-

20

Along isobOth

'I, A L Gross Isobath & Offshore N C3

10 ___________________________________________ '5 ' 1290m '5

C-, 33

1 290m

A3

1310m

I liI_,,, II

19 16 10 30 24 19 13 7 2 March April May June July Auguat September

1997

40HIP Bottom Currents and Temperatures

Figure 5.6-la 40-HLP velocity vectors and temperatures for the bottom

current meters on the 1300m moorings. Plotted in four,

six-month segments (a-d) - Vectors directed vertically

upward are along isobath with an easterly component.

179
4.0

A !WWA '! Yrr

A3

131 Om

Along isoboth

, Cross Isobath & Offshore C3

1310m

Figure 5.6-lb. 40-FILP velocity vectors and temperatures for the

bottom current meters on the 1300m moorings. Plotted

in four, six-month segments (a-d) -

180

13 7 2 27 21 16 10 4 1 26

September October November December Jonuory Fobruery kto rob 1958 40I-h.P Bottom Curronto and Temperatures

4.6

4.5

) 4.4

1

4.2

4.1

B3 Om
4.6-

4.5

n 6

4-3

Ui 0

19 16 10 5 30 24 19 13 7 Murch April My June July Au9ust S.pemb.r 199

'Iong isubath

LCmsslssbalh &

40HLP BQftom Currents and Temperatures

B3 Om

A3

1310m

Figure 56-lc 40-HLP velocity vectors and temperatures for the bottom

current meters on the 1300m moorings. Plotted in four,

six-month segments (a-d) -

181
50

4.2

I-. 4.1 4.0

' " '! !W1V

10-

20: .I

13 7

5apternbar

Juuan Day 225 Ia B/13/ih98

2October

iJong isobath

LCrass Isobath & Offshore

10

January

1 B99 40HIP Bottom Currents and Temperatures

27 21 Navemb.r is

Dmamber 4Fobruery

1 26 March

C3

1290m

A3

131Cm

Figure 5.6-id 40-HLP velocity vectors and temperatures for the bottom

current meters on the 1300m moorings. Plotted in four,

six-month segments (a-d).

182

03

4.6 . Cm

4.5

p.44 A3 4.3 1 ol Cm

C3

Cm
a) 3020100 b) .006 .002 .000 180 UIll <0a- -180 Figure 5.6-2 For bottom instruitients on 1300m moorings: (a) Kinetic energy spectra (b) te

spectra. In panel (C), coherence squared and phase differences are shown along isobath (V) components at 500m and l300m for the three offshore moorin

32 r- 11.. Pt4 I 

10-0 101 CYCLES/DAY 10-a CYCLES/DAY 10-2 I 0

CYCLE 

I 052-43-3670 1310 U. 2 002-63-36 75 290 U. DOUG 005600 1 DSC-A3-3 ISIS U. 2 DSC-63-3 1290 U. SDUD TEMPERATUR( DAShED TEMPERATURE 02-05-26 70 500 U. 52-05-36 70 1310 U. 

3 002-23-36373 1290 U. 0010 1997/11/21,0000 SOliD lIME SERIES 106010 300 llOYD 3 252-23-3 1200 U. DATE 1007/11/21.0222 SOUD TGIIP060TUOE TIME 2ERIES LENOTH 300 DAYS 02-63-26 75 500 U. 52-63-36 75 1290 U, 

DEGREES OF EREEIIOM 22 RAN DWIOIH 0.23666667 GPO DEGREES OF FROEIIOM 22 RAI900IOTH 0.03666667 CPII SC-C3-26175 500 U. SC-23-SRT7S 29GM. 

DO E 1997/I1/21,0000 TIME GE 6(03 OF IOEE0D}A 22 RAND

KINETIC ENERGY SPECTRA S P E CTRA
period TRW's dominate the spectra in water depths greater than 2000m,

particularly in the central and western Gulf. On the northeast Gulf

slope, the energ-y at these periods decreased rapidly towards the west

indicating that the source could have been further to the south along

the west Florida escarpment and thus may have been related to LC frontal

eddies on the east side of the LC. At higher frequencies the spectra

had a number of small peaks. There was little energy associated with

these motions though there were intervals in the records when short

period fluctuations (amplitudes -5 cm/s) dominated at one or two of the

moorings. An example is the May-June 1998 period at A3 and B3 (Figure

5.6-ic) . The phase and coherence between the 500 and l300in alongisobath (V) components (Figure 5.6-2c) confiLins that the lower slope

bottom currents were largely decoupled from flows in the upper layers.

An EOF analysis for the 10- to 50-day fluctuations (0.02 to 0.1 cpd) was

performed for the bottom velocity and temperature records at the three

moorings. A common interval of 300 days beginning on November 21, 1997

was used for the analysis. This avoids the period at the end of

September 1998 where relatively large velocities of -20-25 cm/s (Figure

5.6-id) caused by Hurricane Georges were observed (see Section 6-3).

The spectra were noiivalized by the variance of the frequency band so

that U, V and T had equal weight in the EOF decomposition. The results

are given in Figure 5.6-3 where the top panel shows mode 1 velocity

ellipses and the lower panel the normalized amplitude and phase of

temperature (T), and the cross (U) and along (V) isobath velocity

components. Only the first mode, which accounts for 48.8% of the

normalized variance in the frequency band, was significant The

velocity ellipses show the rapid decay of kinetic energy from east to

west with the signal at A3 lagging that of C3. The major axes also

rotated clockwise with respect to the local isobath direction. In the

east, at C3, the major axis of the ellipse was at a small angle to the

north-south trend of the west Florida escarpment isobaths. In the west,

at A3, the fluctuations were almost perpendicular to the slope. If

these motions were TRW's, then this must have been caused by changing

bottom slopes from steep to less steep so that the wave frequency moved

closer to the cut-off frequency (Rhines 1970) above which TRW motions

were not supported.

The phase of T and V show a linear increase with distance along the

isobath (Figure 5.6-3b) where positive phase differences lead. Thus,

westward propagation is clearly indicated with an average phase

difference between C3 and A3 of 58°. This results in an along-isobath

wavelength of 620 kin. The across-isobath wave number is the larger

component for short-wavelength TRW s. Therefore, the true wavelength

will have been less than 620 km. These westward propagating wave-like

motions appear similar to TRWs found in deeper water. There is no

information on the variability with depth in the bottom few hundred

meters of the water column, and therefore, it is not know-n if there was

any bottom trapping of these fluctuations that is characteristic of

TRW'S. It is however noted that the EOF analyses of the upper-layer

currents at these frequencies showed no indication of any westward

propagating signals. In this upper layer, propagating wave-like

fluctuations were restricted to periods longer than 50 days. This

184
31N

90W

.00

89GW 88GW 87W

50-10 Day Periods Mode 1 48.8% N.Var. at 1300 m

11/21/1 997 TO 9/17/1998

56W 85GW

31 N

1300 m Currents and Temperatures

Frequency Domain Analysis 1997/11/21 0 300 Days

Amplitude Phase Differences

.25 .50 .75 1.00 -200 0

0

50

200

a

100

3

150

Center Frequency .0583 Bandwidth .0800 cpd 48 DOF

Mode No 1 Percentage of Total Normalized Variance 46.8

200

Figure 5.6-3 50 to 10.-day frequency domain EOF analysis. The upper

panel (a) shows mode 1 velocity e11ipss. The lower

panels (b) show normalized amplitude and phase versus

distance along the isobath for the bottom instruments on

the 1300m moorings.

185

a

3ON

29N

28N

90W 89W 88GW 87W 8SW 85°W

b)

A3

03

C3
separation by frequency and characteristics of upper and lower layer

waves indicates that they had different sources and generation

mechanisms. These are unknown at the present time.

186
VI. SLOPE RESPONSE TO ATMOSPHERIC FORCING

6.1 Introduction

A detailed evaluation of the in-situ and survey data revealed that

atmospheric processes at the ocean surface appeared to have influenced

to varying degrees the circulation and transport patterns in the study

area. As evidenced from the presentation in Chapter 5, eddy-like

features having several origins were present and affecting the observed

currents. The actions of multiple forcings by these eddy motions as well as atmospheric forcing tends to complicate isolation of particular

process-response models. When the meteorological forcing is large as it

was when Hurricanes Earl and Georges traversed the study area, linkages

between forcing and response can be clearer, although understanding the

movement throughout the water column to this forcing remains a

substantial undertaking. Strong storm systems are not uncommon in the

northeastern Gulf. Figure 6.1-1 shows the number of storms with

tropical status or greater per decade that moved through two subregions

(Florida Panhandle and Big Bend) of the NE Gulf over a 110-year

interval. Most of the storms contributing to this total could have had

an effect on circulation patterns in the study area.

Chapter 6 examines several time scales as well as the spatial extent of

an oceanographic response to atmospheric forcing. With the increased

distance to the shoreline and the increased water depth, the relative

importance of wind stress to circulation over the slope is often assumed

to be lessened. Results from the present study will indicate that wind

stress often had a significant influence seaward of the shelf, Le. over

the slope.

6.2 Slope Response to Synoptic Wind Forcinq

The typical response of shelf currents to synoptic scale wind forcing is

an energetic barotropic response of alongshelf currents to local

alongshelf winds that are coherent over the shelf domain. This response

is known as the arrested topographic wave (Csanady, 1978) and generally

dominates the low-freqvency current variability at most mid- and inner

shelf locations (Beardsley and Butman, 1974 for the Middle Atlantic

Bight, NAB; Mitchum and Sturges, 1982 for the west Florida shelf; and

Lee et al., 1989 for the South Atlantic Bight, SAB) . The response is

different from a propagating continental shelf wave in that the forcing

is local and the response is phase-locked to the forcing. The resulting

amplitude of alongshore currents are uniform in the vertical and over the domain of the forcing, which is generally larger than the shelf

scale for synoptic forcing events. The phase response of alongshore

currents is also uniform over the water column and along isobaths, but

increases with water depth. lJpwelling (downwelling) alongshore winds

will accelerate offshore (onshore) flows in the upper Ekman layer that

are near in-phase with the wind forcing. Due to the coastal constraint

these cross-shelf flows will cause a onshore (offshore) "adjustment

drift" below the Ekman layer that in turn drives a uniform alongshore

flow in the same adjusted geostrophically to the alongshore flows

resulting in a set-down (set-up) of coastal sea level from the upwelling

(downwelling) wind-driven alongshore currents. This is referred to as

187
28°N Occurrence of Tropical Storms* 90°W 88°W 86°W 840W I V 260N90°W 042B01 A 42039Q *Tropical Storm has wind speed? 18 mIs I OPIA \caParke 4\ -. D2 E1 CSBF 42O4O.6I_'., AlW -;-, 141. 13 - 12 (012 7- .,_ 6-4-- ........ . a....L,...4q.. (I) -.......................- 3 ------------------- . I. - .'>- F- F --- t- - --I - 7800 1910 1920 9839 9940 1950 8960 1972 1990 1990 Moo1na o ND8OBUoy o 041 NSl.,n 84°WDiamond = Paniarodle Square Big Bend Circle = To1aF 82°W 80°W 82°W Figure 6.1-1. Map which partitions the storms occurring in each moorings and present key storms represents storms NE Gulf of Mexico and indicates the number of 

partition per decade. Also shown are DeSoto meteorological measurement sites. The number 

that reached tropical storm status or stronger

I 30°N 88°W 86°W
the Ekman frictional equilibrium response" (Beardsley and Butman, 1974;

Scott and Csanady, 1976) - Typically, the response is limited to the

shelf and pressure fluctuations at the shelfbreak are assumed zero. However, Lee et al. (1989) observed this response at the shelf break and

over the slope in the SAB with strongest response in the lower layer.

They attributed the spread of the response over the slope to the weaker

bottom gradient in the SAB slope, which resulted in the cross-shelf

scale of the slope being equivalent to that of the shelf. The arrested

topographic wave response is often difficult to observe at shelfbreak or slope locations due to the strong influences from boundary currents and

their eddies. Certainly, the SAB is no exception and the influence of

eddies there is notorious and well observed. However, the first-order

influences of boundary currents and their eddies tend to be baroclinic

so that the barotropic response to alongshore wind forcing becomes

relatively stronger closer to the bottom.

In the DeSoto Canyon study area, the shelfbreak and slope region are

subject to strong influences from the Loop Current, rings, cyclonic

frontal eddies, anticyclonic eddies (ACE) evolving from warm streamers

and smaller scale cyclonic shelf eddies that can develop on intruding

warm fronts. This mixture of eddy processes produces strong current and

temperature variability over the slope and can account for considerable

cross-slope exchange. The time scale of variability from forcing by the

LC and mesoscale eddies (LC frontal eddies (LCFE) and ACES) tends to be

longer than the synoptic wind event band of 5 to 15 days. Thus, current

and temperature variability within the synoptic band are more likely due

to a combination of wind forcing and submnesoscale shelf eddies that are

observed to develop on thermal fronts in the array. Also one would

expect the wind response to be stronger and more observable at the

sheltbreak sites compared to mid-slope or offshore moorings where the

influence of mesoscale eddies is proportionally greater.

The shelf region, shoreward of the present mooring array, is generally

oriented in an east-west direction, but is closed at its western end by

the Mississippi River delta and curves toward the southeast at its

eastern end, although still open. The shelfbreak tends to be oriented in

a northeast to southwest direction from moorings A to D and then curves

to the southeast past site E. The slope tends to follow the orientation

of the shelfbreak, although the slope is narrower in the western region

of the array at sites A-C and widens out as the shelf curves to the

southeast near D and E. Due to the general east-west orientation of the

shelf one would expect the strongest response to occur for east-west

(alongshore) wind events and that this response would be strongest

toward the center of the shelf where the topography is more uniform,

i.e near sites B, C and D. The most observable wind response should be

found at site D, which is furthest from the influence of mesoscale

eddies and at the narrowest section of the shelf, where alongshelf flows

could be amplified. The weakest response should occur at sites A and E,

near the western boundary of the shelf and southerly alignment,

respectively.

Visual inspection of the upper level current vectors from all

measurement Sites together with the winds from offshore buoy 42040 near

mooring line A and coastal sea level time series at Panama City, FL,

189
generally support the above expectations (Figures 6.2-la-id) - There are

at times significant correlations between alongshore wind, current and

sea level events in agreement with the arrested topographic wave

response. This response appears to be better defined at the shelfbreak,

particularly at sites D and C and less so at shelibreak (A and E) and

offshore sites. The response is stronger during winter and spring

seasons when wind forcing is greater due to the passage of strong cold

fronts and winter storms. The winter of 1998 is a good example, wherein

a storm in early February caused strong westward alongshore wind forcing

and westward currents at most of the mooring sites together with a sharp

rise in coastal sea level.

Seasonal wind forcing showing an interesting pattern in which winds

shift from their strong oscillatory nature in winter and spring to a

more persistent eastward direction in summer and southwestward direction

in fall. The shelfbreak currents also tend to show these seasonal

shifts, especially at Site Dl. The barotropic nature of the alongshore

current response at the shelfbreak and the high correlation to wind

forcing and coastal sea level is clearly displayed in the first year of

current data from mooring Dl (Figure 6.2-ia and -ib). Alongshore

currents are coherent over the water column and tend to lag alongshore

winds on the order of one day. Coastal sea level is near in-phase with

the alongshore currents as expected from local wind forcing. Also, a

clear shift to persistent eastward flow occured during the summer

consistent with the eastward winds.

The vertically averaged current vectors from each shelf break site

plotted together with buoy winds near line A and Panama City coastal sea

level are shown in Figures 6.2-2a to 2d. There are numerous examples of

coherent, uniform current events that occured at all shelfbreak sites

following strong alongshore wind events, as well as the seasonal shifts

reported earlier.

5.2.1 Time Domain EOF's (TEOF)

Spectral analyses generally show significant energy peaks in the 5-15

day period band for subtidal current records from shelfbreak locations

that were strongest during winter and in the lower part of the water

column (example Figure 6.2-3a) - Similar energy peaks were also observed

in the 5-15 day period band for wind and coastal sea level records. In

comparison, energy spectra for stations further offshore typically show

the strongest current variability to be associated with motions at

periods longer than 20 days (Figure 6.2-3b). Time domain OF analysis

was used to extract the coherent part of the total ensemble of wind

stress from all wind stations and also for the ensemble of currents from

all sbelfbreak moorings. The EUF's were computed separately for the

alongshelf and cross-shelf wind Stress components and current components

and also for each summer and winter season.

During all seasons the mode EOF of the alongshelf wind stress

components were nearly equal in amplitude and similar in phase at all

stations and accounted for 70 to 80% of the total variance. The same can

be said for the amplitude and phase of the let mode EOF of cross-shelf

wind stress from all stations, which also explained about 70 to 80% of

190
15-

15-

60 -

50 -

40-5

-? 3o r 10 -

10 -

6040

20 20

40

60 1997

A

I;

DeSoto Canyon, Mooring Dl

Along isoboth

LCss I5nbnth &

NDBC Buoy 42040

Panama City

Figure 6.2-la. Subtidal current vectors between 8 to 95m at mooring

site Dl during March to September 1997 together with

subtidal winds from buoy 42040, located near the A

mooring line, and coastal sea level at Panama City1 FL.

See notations for directions of velocity vectors

relative to isobaths and geographic frame-of-reference.

191
15 -

10-

5-

-

-

6040-

200-

-20

£0

50-

-40-

75 -

50-2

0 25-

50-0

75

r4

DeSoto Canyon, Mooring Dl

NDBO Buoy 42040

Panama City

Along rcobath Cross Isubath & Offshora8m

I

BOrn

95m

Figure 6.2-lb. Subtidal current vectors between 8 and 95m depth at

mooring site Dl from September 1997 to March 1998

together with subtidal winds from buoy 42040, located

near mooring line A, and coastal sea level at Panama

City, FL. See notations for directions of velocity

vectors relative to isobaths and geographic frame-ofreference -

192

13 7 Sop 2Outobor 27 21 Novamb.r 0 acorn hi16 IS

January 1908 4

F.bruory March 26
15-r

Q 10-

. 5-;

5-;

15--

60 -

50 -

40- -? oa i 20

io-

-10 -

I

AIung isobath

LCssIsobath &

NDBC Buoy 42040

Panama City

Figure 6.2-2a. Vertically averaged, rotated, subtidal current vectors

from all shelf break moorings together with subtidal

winds from buoy 42040 located near mooring line A, and

coastal sea level at Panama City, FL. for the interval

from March to September 1997.

193

19 16 10 5 30 24 19 13 7 2

hlcrub April Moy Jun. Juiy Snp!.rnb.r

1997

DeSoto Canyon, Vertical Average at Shelf Break Moorings
15 r

5N. 10=

5-0

=

-"... Os

- > -5-0

!

20 . o-

-20-

-40

C

N. cc

vS

Aloyb sobeth Cross sobath & Offshore

NDBC Buoy 42040

Panama City

I

Figure 6.2-2b. Vertically averaged, rotated, subtidal current vectors

from all shelf break moorings together with subtidal

winds from buoy 42040, located near mooring line A, and

coastal sea level at Panama City, FL. for the interval

from September 1997 to March 1998.

194

U, N

ccr 75 - C 50-0

C 25-0 N

.

Dl

A*S!!a -1 ki 1_, cc04 "-255

Lu -50-0

-.,.--- -

,tsvwLu

75 - Ii ,I, I. 1..

7 2 27 21 16 10 4 1 26 S.pkrcb.r October November December Januory February March 1997 1998

DeSoto Canyon, Vertical Average at Shelf Break Moorings
20-

10 10

20

lao-

_4o -

- 20-

0-

-20

N

U, N

Cr c-i U, Cr LI U) N

Cr

i;i 60

40-

20-

NE 0 40-

-60

19

Panama City

NDBC Buoy 42040

Along isob.th

LCbs isobath & Al

h4srsh 1998

DeSoto Canyon, Vertical Average at Shelf Break Moorings

Figure 6.2-2c. Vertically averaged, rotated, subtidal current vectors

from all helfbreak moorings together with subtidal

winds from buoy 42040 located near mooring line A, and

coastal sea level at Panama City, FL. tor the interval

from March to Septenther 1998.

195

19 13 7 2 AuUst s.pi.ncb.r 16 10 5 30 24 April Uny Jun. July
20-

!. '-

N

U, N

100-!

100-

rc 50 N ., 0 -50 - 100

Crus subath & Olfuhurn

DeSoto Canyon, Vertical Average at Shelf Break Moorings

NDBC Buoy 42040

Panama City

/

Al

Bl

25

Figure 6.2-2d. Vertically averaged, rotated, subtidal current vectors

from all shelf break moorings together with subtidal

winds from buoy 42040, located near mooring line A, and

coastal sealevel at Panama City, FL. for the interval

from September 1998 to March 1999.

196

-25 - AIng abaUi

13 7 S.pf.,nb.r 2 27 21 15 10 4 Ontubor Navurnb.r D.c.rnb.r January F.bru,,y March 1998 1999
Seasonal Cl Near Bottom Velocity, Salinity and Temperature Spectra KINETIC ENERGY SPECTRA 101 CYCLES/DAY I DSC-C1-.2R 45 66 H. SOUD 1997105/15 2 I]SC-C1-2R 45 66 H. DASHEE 997/11(15 3 ESC-C1-2R 43 66 H. SOLID 1998/04/IS 4 650-01-2545 66 H. DASHED 1998/12101 1tME SERIES LENGTH 115 DAYS DEDREES OF FREEDOM 22 HANDW!D1H 0.09523610 CPD SPECTRA 1 T' CYCLES/DAY

I 050-01-3 62 H. SOliD SAUNITY 1997/05/15 2 DSC-C1-3 62 H. DASHED 5AUNITY 1997111115 3 DSC-Cl-3 62 H. SOLID SAL1NIIY 1998104/15 TIME SERIES LENGTH 116 DAYS DEGREES OH FNEEEDM 23 RANDWDTH 0.09523690 CPD

3

0 SPECT

Figure 6.2-3a. Kinetic energy spectra at Cl for different depths and seasons during the tw

/ 2/ 41 It

F .4 I'.

2

I j\

I

2 'N I \_ / 'Nj 150 1 00

0z> 50

0 .04 .03 LI 0

z

< .02

> .01 .00

2 LI 0z

> CYCLES/

1 550-Cl-S 62 NI. DOUG 

2 050-01-3 92 14. DASHED S 050-01-3 62 H. SOUD 4 DSC-C1-2 65 14. GASHED 

TINSE SER

DEGREES OF FREEDOM 22 BANDWIDT
tIi;Tp JZC pu sidp UO9S uiinp Bq 

q-9 axn6 11UG 3I9UTj 3 JO S5O0 50cI006J. SI 

Q!M0506 

635 0511 

160-00-050 5 5 0651 00H0Y11 

5L1-00-000 I 60l 51 onos 

150-00-000 0 II 0651 OOHSYEI 

6 0 60-00-000 5 0111 OflOS 

L- 5003354 .10 0336035 51 5101190605 Sd) 606050600515531 531060 31916. 5150 65 515-03-000 P rI 000 OOH050 56/51/5666 51165-03-350 11 000 SillS so/p /5661 51155-03-300 0 II 000 O3HSVS S /01/1861 51160-03-000 66 006 SnOS 01/s /1661 AVG/s]10A3 0 0 105336410 0336030 SI 510150555 043 506560600HISH31 S31530 35511 SAPS 56 01161-03-005 5 59 00 53190011 51/01/5661 51161-03-355 0 59 09 Sf05 50/I' /5666 01111-03-355 5 59 09 OOH550 o /01/1666 I 01151-03-355 506 01100 51/c /0666 Ava/SJ1DAD 00 001 00 1 

is / 

/ 

' I -F \ \ /t SI L III ,F! l j II II I III liii II1 I \ p VI33dS AOèI]N3 3ILN1N OLLJN]N V13dS ADd3N 

èiJN3 3LL3NJN
the total variance for all seasons, except for a 54% variance in the

summer 1997.

For alongshelf currents at the shelf break, the 1" mode EOF explained

about 60% of the total variance in winter and 45% in summer seasons. This mode was generally barotropic at all stations. Currents at stations

Bl, Cl and Dl were generally more highly correlated with the mode than

at stations Al and El, with stations Cl and Dl usually showing the

higher correlations and amplitudes. During summer seasons, a 2' mode EOF

for the alongshore currents accounted for 20 to 30% of the variance. The

mode EOF of the cross-shelf currents was also barotropic and

explained about 30% of the total variability in winter and 24 to 46% in

summer seasons. The larger amplitudes and correlations with this mode

occurred at stations A and E. The 2 mode EOF generally accounted for

about 20% of the total variability in cross-shelf currents, but with

much smaller amplitudes than mode 1 and weaker correlation between

sites. Including currents from mid-slope stations for depths less than

lOOm together with the shelfbreak currents generally reduced the percent

of total variance explained by the lt mode to 30 to 50% for the

different seasons for either current component and spread more

variability into 2' and 3 modes.

As an example of the TEOF analysis we show time series of the TEOF modes

of cross-shelf and alongshelf wind and current components from the

shelfbreak for the period of significant wind forcing, winter 1998

(Figure 6.2-4a and -4b). Also shown is the Panama City coastal sea

level. The l' mode alongshore currents have much greater amplitude than

the 2' or 3 modes and also greater than any of the cross-shelf current

modes; whereas, the amplitudes of the l' mode alongshelf and cross-shelf

winds are about equal. The l' mode alongshelf current EOF is visually

well correlated with the 1st mode alongshelf wind stress and with the

coastal Sea level.

Spectra of the l' mode alongshelf current has a well-defined energy peak

centered at 10 days, whereas, energy peaks of the l' mode wind stress

EQF's are spread over a period band of 8-10 days (Figure 6.2-5) . The 1

mode alongshelf current EOF is highly coherent with the l' mode

alongshelf wind stress over a wide period band with a phase lag that

increases with frequency, suggesting a near constant time lag on the

order of one day (Figure 6.2-6) - Even greater coherence occurred

between the l' and 2' mode cross-shelf wind Stress EOF's with the 15t

mode alongshelf current. This may be explained by the curving nature of

the shelfbreak study area where cross-shelf winds (toward l55'T) would

have a significant alongshelf component over the eastern portion of the

shelfbreak that could add to the alongshelf current modal variability. A

similar response of alongshelt current modes to alongshelt and crossshelf wind Stress modes was observed when mid-slope currents in the

upper lOOm were included in the TEOF analysis indicating that the

current response to synoptic wind forcing extends over the slope.

6.2.2 Frequency Eomain EOF's (FEOF)

Energy spectra of shelfbreak currents and wind stress records and their

mode TEOF's consistently show well-defined spectral peaks centered

199
755.o- 2.5- ) .0. -7.5 - 60- 60- 40- 20 -20- -40- 00 L,___ . - - A. VVT y - V V W L. . yV' ! 26 6 D.csrnbr 16 26 5 Jariury 1998 15 25 Shelf Break EOFs, U-componentaridTauX(t) 14 24 6 MQrvh Figure 6.2-4a. Cross-shelf components of TEOF modes of shelf break currents and wi

stress and Panama City, FL coastal sea level for winter 1998.
C0 60- 40- 20 20- -40-- 200- - A_ r'1v __A L_ a- A&A ALA A V -w w-. y -t - A L A A A A kAk 'r wyyv!y 400 - 200- -400 26 6 16 December 25 4 14 February Shelf Break EOFs V-component and TauY (Ty) 24 6 March 16 Figure 6.2-4b. Alongshelf components of TEOF modes of shelf break currents and 

stress and Panama City, FL coastal sea level for winter 1998. 26 5 15 Jan uc r-y 1998
6000 20000 23 1 09 MODE 1 100 N SOUD V COA4PONENT 2 SI MODE 2 100 N DASHED V COMPONENT 3 SR AIODE 3 100 N SOUl) V COMPONENT DATE 1997/10/ 50000 TIME DENIES LENGTH 19 DAYS DEGREES CE FREEDOM 19 BANDWIDTH O,O9010909 CPD 2.0 1 .5.5.0 10-1 CYCLES/DAY 2.0 1 .5.5.0 I WS MODE 1 TAIlS SCUD U COMPONENT 1 WS MODE 1 TAUT SOUD 

2 WO MODE 2 TAME DASHED U COMPONENT 2 *5 MODE 2 TAUY DASHEDATE 1117/12/ 5,0000 TIME SERIES LENGTH 99 SAYS DATE 1197/12/ 5.0000 I1ME DDEGREES OF FREEDOM 11 BANDWIDTH 0,09090909 CpD DEOI1EES OF FREEDOM lB BANDW

Figure 6.2-5. Spectra of TEOF modes of shelf break alongaheif currents and and 'y. wind 

components for winter 1998. 1/r 2 

a) SPECTRA b) c) SPECTRA SPEC

102 101 CYCLES/DAY I O_2 1 -1 CYCLES
180 uJlIT<0 -180MS hOOT I TAUT SRI4OIDEI 90014 WO MORE 1 TAUT 2 SEMOEE2 lOOM WO MODE 1 TAUT SNMORE3 lOOM DATE 1997/12/ 5.0020 DEGREES OF FREEDOM 18CYCLES/DAYV COMPONENT V CO1.{PON(NT V CO]APOI4ENT V COMPONENT V COMPONENT V COMPONENT TIME SERIES LENOTIE 99 DAYS RANDWIDTH 009090909 CPDVSS TED TRS 1 80 Li (IT. 5 I-180MS MODE 1 TAUS 1 SRMODE1 10099 MS MODE I TAGS SBMODE2 10014 WS MODE I TAGS SNMODE3 10014 DATE 1997/12/ 5.0000 GEDREES OF FREEDOM iNCYCLES/DAYU COMPONENT V COMTONENT U COMPONENT V COMPONENT U COMPONENT V COMPONENT TINE DENIED LENGTH 19 DAYS RANDWIOTH 0.09090909 CPDAND YRS VRS 0 180 -180WD lOOSE 2 TAGS SNMODE( lOOM MS MODE 2 TAGS 2 SRMOOE2 lOOM WS MODE 2 CASE SNIIODE3 10044 DATE 1997/42/ 20000 DEGREES OF FREEDOM 183/ ,f 2 jF i CYCLES

TINE DBANDW

23/1 Ii Ii __ I 10_a II ;_l 2 

3 _-..-

ia-S 10_

Figure 6.2-6. Coherence and phase spectra of TEOF modes of a) alongshelf wind stress and. 

mode 1 cross-shelf wind stress and alongshelf currents; and c) mode 2 cross

stress and. alongshelf currents for winter 1998. a) b) c)
within a 5-15 day period band. Frequency domain EOF (FEOF) analysis was conducted on the combined along isobath (v) and cross-isobath (u)

current component time series within this band. The analysis was done

separately for each mooring over the upper lOOm for each of the four

seasons of measurements.

Results of this analysis are summarized as ellipses in Figures 6.2-7a to

-7d for the let mode FEOF's at selected depths on the shelibreak moorings

and for each season. Mode 1 has a barotropic Structure at all

shelfbreak sites with near constant phase with depth and small decrease

in amplitude near the bottom. The orientations of the ellipses are

aligned with the shelfbreak isobaths and indicates that largest

amplitude occurs with the alongshelf component of the mode. The mode

explains a large percentage of the total variance at each site. The

percentage of the total variance explained is greater in winter than

summer and is maximized at sites Cl and Dl, where approximately 90% of

the variance was explained during winter seasons and 60 to 80% was

explained during summer. At station Al and El, the lt mode FEOF

accounted for about 60 - 70% of the total variance during winter and 50

- 70% in summer. The percentage of total variance explained generally

decreased with offshore distance, but still there was 50 - 70% of the

variance explained by the let mode from all sites for the upper lOOm.

6.2.3 DiScussion

The above analyses indicate that shelfbreak currents and upper level

slope currents had a significant response to coherent synotpic-scale

wind events similar to that observed on other wide, shallow continental

shelves, the so called 'Ekman frictional equilibrium response" to local

alongshelf wind forcing (Beardsley and utman, 1974; Scott and Csanady,

1976) Alongshelf winds that were coherent over the shelf domain,

generated cross-shelf Ekman transports in the surface layer with

opposite return flows beneath which accelerated barotropic alongshore

currents that were balanced by cross-shelf pressure gradients from

cross-shelf sea level slopes. The geostrophic adjustment time needed for

sea level to balance alongshore flow is t = L/(gh)112 (Winant, 1980). For

the DeSoto Canyon shelf region with shelf width (L) of 100 km and mean

shelf depth (h) of 40m, the geostrophic adjustment time is less than 2

hours, which is in good agreement with the observations. The time

necessary for the alongshelf currents to reach equilibrium with the

alongshelf wind stress is the frictional adjustment time t H/r, where

"H" is the water depth and "r" is the bottom resistance coefficient. At

similar outer shelf locations, "r" has been found to be approximately

0.1 (cm/s) (Lee et al., 1989). Using this value for r at the shelfbreak

(lOOm) gives a frictional adjustment time of about 28 hours. This

adjustment time is in reasonable agreement with the time lag of about 33

hours estimated from the phase lag of the 1st mode TEOF of shelfbreak

alongshelf currents to alongshelf wind stress at the 10-day energetic

period during winter 1998 (Figure 6.2-5)

This type of barotropic response to wind forcing is often observed to be

trapped over the shelf on wide, shallow shelves with steep bottom slopes

seaward of the shelf break like the Middle Atlantic Bight and South

Australian shelf (Clark and Brink, 1985) - Csanady (1978) found that the

204
Frequency Domain EOF Analysis of Velocity Profiles for DeSoto ADCP Currents

Frequency Band 0.065 - 0.210 cpd

Mode I

Al 12m

Al 32.rn

Al 68.rn

18-5 DAYS

-ID 5 10 /

Al 95.nr Al 95rn

Summer 1997

% of Normalized

Variance = 62.2

Al 12rn Al 12.m

Al 32rn

Al 68.rn

Winter 1998 Summer 1998

% of Normalized % of Normalized

Variance = 66.4 Variance = 69.3

Al 12.rn

Al 32.nr Al 32rn

Al 68.rn Al 68.m

Al 95.m

Winter 1999

% of Normalized

Variance = 62.5

Figure 6.2-7a. Results of FEOF analyis for mode 1 at mooring Al for

summer and winter seasons presented as ellipses for

selected depths. The length of the major/minor axes

represent the amplitudes of the mode for the v and u

current components. The arrow gives the relative phase

of the mode.

205
Frequency Domain EOF Analysis of Velocity Profiles for DeSoto ADCP Currents

Frequency Band 0.065 - 0.210 cpd

Mode I

-15 5

Cl 12n

Cl 36.n

18-5 DAYS

5 10

Cl 12.m

Cl 36m

7

Cl 12.m

Cl 36n

Cl 68.m

/

Figure 6.2-7b. Results of FEOF analyis for mode 1 at mooring Cl for

summer and winter seasons presented as ellipses for

selected depths. The length of the major/minor axes

represent the amplitudes of the mode for the v/u

current components. The arrow give the relative phase

of the mode.

206

Cl 95.m Cl 95.m Cl 95.m

Summer 1997 Winter 1998 Summer 1998 Winter 1999

% of Normalized % of Normalized % of Normalized % of Normalized

Variance 56.7 Variance 89.9 Variance = 75.2 Variance = 84.3
Frequency Domain EOF Analysis of Velocity Profiles for De Soto ADCP Currents

Frequency Band 0.065 - 0.210 cpd

Mode I

-10 5

10-5 DAYS

Summer 1997

% of Normalized

Variance 76.4

5 10

Dl 95.m Dl 95.

Winter 1998

% of Normalized

Variance = 91.9

Dl 95.n, Dl '5.m

Summer 1998

% of Normalized

Variance = 79.2

Winter 1999

°h of Normalized

Variance = 88.3

Figure 6.2-7c. Results of FEOF analyis for mode 1 at mooring Dl for

summer and winter seasons presented as ellipses for

selected depths. The length of the major/minor axes

represent the amplitudes of the mode for the v/u

current components. The arrow give the relative phase

of the mode.

207

Dl l2.m Dl 12.m Dl 12s Dl l2.m

Dl 32n Dl 32.m Dl 32.s Dl 32.r

Dl 64m Dl 64., Dl 64., Dl 64.n
Frequency Domain EOF Analysis of Velocity Profiles for DeSoto ADCP Currents

Frequency Band 0.065 - 0.210 cpd

Mode I

Figure 6.2-7d. Results of FEOF analyis for mode 1 at mooring El for

summer and winter seasons presented as ellipses for

selected depths. The length of the major/minor axes

represent the amplitudes of the mode for the v/u

current components. The arrow give the relative phase

of the mode.

208

El 95.m El 9_m El 95.m Summer 1997 Winter 1998 Summer 1998 Winter 1999

% of Normalized % of Normalized % of Normalized % of Normalized

Variance = 65.3 Variance 71.6 Variance = 50.1 Variance = 71.5

El 12.m El 12.,, El El l2.n

El 32., El 32,, El 32.m

El 64 El 64., El 64.,,
trapping width (L,,) associated with the arrested topographic wave (the

barotropic response) could be estimated from L= (2rL/f5)"2, where L is

the alongshore length scale of the wind forcing and S is the average

cross-shelf bottom slope. For the shelf in the DeSoto Canyon study area

I = 800 km and S = l0, which gives'a trapping width for the barotropic

response on the order of 170 km. This trapping width is almost twice the

shelf width and may explain why barotropic currents at the slielfbreak

and even over the slope were coherent with the alongshore wind stress

and coastal sea level variations. A similar finding of sig-nificant

barotropic response to alongshelf wind stress was reported for the

shelfbreak region of the South Atlantic Bight (Lee et al., 1989).

However, in the SAB the Gulf Stream influence occurs within the same period band as the wind forcing, making it difficult to separate wind

and Gulf Stream forcing. In the DeSoto Canyon shelf area the offshore

Loop Current and eddy forcing generally occurred at periods greater than

20 days, whereas, the synoptic wind forcing band is 5 to 15 days. In the

DeSoto Canyon study area, the primary forcing mechanism in the 5 - 15-

day period band appears to have been synoptic wind events, which makes

the current response at the outer shelf and slope relatively more

observable.

6.3 Seasonal Wind Response on the Slope

Seasonal EOF analyses were performed for four 99-day segments of the

two-year study period. Two summer (1997 and 1998) and two winter

periods (1998 and 1999) were used. They were chosen to avoid the short,

rather ill-defined, spring and fall transitions in the wind regimes as

well as hurricanes Earl and Georges, and to capture major differences in

the current velocity characteristics. As has already been noted, 5 - 15

day period fluctuations were much more prominent in the winter than the

summer periods. There were also differences in the offshore deep-water

circulation between seasons. For example, LC was well extended into the

eastern Gulf from the beginning of the study to the end of summer 1998.

In fall 1998, the LC retreated south of 25°N and remained there until

the end of the study in April 1999.

The time and frequency domain EOF analyses performed for each mooring

separately, using the individual velocity records from the upper lOOm,

showed a strong wind-forced response over the slope in the two winter

periods (see Section 6.2) . Similar analysis for the two summer periods

showed no significant coherence between the along- and cross-slope wind

stress modes and the principal velocity modes for the shelf-break and

upper-slope moorings.

The present section examines the current patterns for the summer and

winter seasons to see if there were significant differences between weak

and strong wind-forcing, respectively. Because the previous analyses

have shown that the current fluctuations were primarily barotropic in

the upper lOOm, the time domain EUF analysis uses the 0 to lOOm depth

mean U- and V-components of the velocity records for all 13 moorings.

The eigenvectors (multiplied by the square root of the eigenvalue to

give units of clri/s) are represented by vectors in a similar manner to

Merrifield and Winant (1989). The time variation of each spatial

209
pattern is both positive and negative. Therefore, the mode amplitude vectors are reversed for about half of the total period.

The first two modes for the two summer periods are given in Figure 6.3- 1. Though they differ in details, all these modes show fluctuations parallel to the isobaths over the Alabama slope. The major differences occurred in the east, over the head of the canyon, where suggestions of eddy-like circulations generate flows in opposition to the general trends of the slope currents. These patterns are similar to the 50 to

15-day period frequency domain EOF modes, derived from the two-year long velocity data, that are discussed in Section 5.4 (see Fig-ures 5.4-15a and 16a) . The two modes for summer 1998 have similar patterns to the modes for summer 1997, if the signs of the vectors are reversed in the latter. Mode 1. for summer 1998 accounts for 52.3% of the total variance with mode 2 being barely significant. Both modes are significant for

summer 1997. This suggests that these type of patterns were typical, to

some extent, but the degree to which they dominated at different seasons seems to depend upon the characteristics of the offshore eddy

circulations. Recall that cyclonic and anticyclonic eddies dominated in

the 1997 and 1998 summer periods, respectively. In both periods, the mean flow was eastward along the slope (see Figure 5.4-6 for summer

1997) . Thus, eastward and westward fluctuations reinforced and reduced, respectively, the prevailing eastward flows. The first mode for summer

1998 accounts for the greatest fraction of the total variance (52.3%)

for any of the seasons. When the fluctuations were eastward over the

Alabama slope, a cyclonic flow was generated encompassing Cl, C2, D2, D9 and Dl. The strong northward flow through the canyon (D2 and D9) bifurcated producing strong onshore flow on the eastern canyon rim

around El. When the flow is reversed, the canyon circulation would be clockwise and the slope flow to the west. This flow pattern could be

responsible for large on-off shelf exchanges on the eastern canyon rim.

Mode 1 for the two winters (Figure 6.3-2) shows that the fluctuations generally followed the trend of the isobaths along the slope and Onto the west Florida slope. Unlike the summer periods, the canyon head was not separated from this flow pattern. Mode 2 on the other hand, has more complicated structures with opposing fluctuations and across isobath flows in the canyon. Mode 1 for winter 1998 accounts for 42.8%

of the total variance and is clearly much more energetic than mode 1 for winter 1999. Winter 1999 had the highest total variance of any of the

four seasons that were analyzed. The highest amplitudes range from 15

to 20 cm/s along the 500m isobath. The time series of normalized mode

amplitudes for the four seasons (Figure 6.3-3) also show strong

differences between summer and winter. The mode 1 amplitudes for the

summer show relatively slow changes with time that are more consistent with slowly changing eddy-driven circulations. Mode 2 is similar with a little more variability at shorter periods. In winter, the slow

underlying changes were still present but they were accompanied by more energetic fluctuations with short periods of order three to ten days. The short period fluctuations also had some degree of visual coherence between modes 1 and 2 for each winter period. This would indicate that they had similar forcing and the long period variability was the primary

reason that the modes were not correlated over the 99-day record. The analysis of the winter wind-forced response, discussed in Section 6.2,

210
219 - 11.31 97'W / ('_-1_r, I 5/IS 10 8/17/1997 8611 95'W 9011 3111 N 329N 8911 8IW 9711 9911 6511 90W S8m,.1- 1997 0-100r6 9.01 CrerI Mo4. I 33.42 or. .1/22 TO 7/30/1998 89'W 8rw 89W 65W 3011 31 N 'I 07W r p..' . I " c \\ \ \. .3..N N 

8814 9 8714 8814 0So,,ro., 1997 0-100r# 9.on Clr,.5I. Mo4. 2 19.40 9cr. . --. 29.3 J J 2911 / 3.., .. ç ' I N' 28.3 I . 2991 90W 8911 8914 0711 86W 8511 9911 0914 8814 8711 8614 Summ.r 1999 0-9O0r .l.c Cwr.rltl Ho4e I 52.30 Ver-. SIlmm.r 1999 0l00rr M.cr, C rT,99! Mcd. 2 12,00 Vci. 9/10 10 8/17/199

9611 

4/22 90 7/30/199

6611 0

19 r./. 2

Figure 6.3-1. Time domain EOF eigenvector components represented as vectors multiplied by the standard deviation of the modes. Analysis is for the 40-HLP, 0 to lOOm depth mean currents. Summer 1997 mod

1 (a) and 2 (b), and summer 1998 modes 1 (c) and 2 )d) 6911 98W .rw '19, N 'N N. 2
95W 3V94 OW.O.W 31W 219.O.W .9' ... uw 87W 86W WIot.r 1099 0loOm Moon Corr.ntn 1lo,1. 426% V,.. 12/16 TO 3/27/1999 99W 91W lOW lOW 90Wf 31W 12/ 5 10 3/14/1906 lOW nrw 8GW 85W lOW 31W 89W 81W 67W 66W WInI., 1999 2lOOn, l.o Csrr.nl, Mod. 1 296% V,.. 2814 65W 90W N-...,\\ 2991 0 9.'. 2114 15W 1014 or. '- 

I j ) \ \\ 6 .1 \. 'N. N. 'ii 'i) I .:------._ 

nOw lOW 1714 

W1.+.. 1999 OISOm M.on C,r.ntn Mod. 2 155 Vor. 12/510 3/14/199

17W lOW 

12/19 TO 3/27/139

87W 8514 9

fl. 9914 9914 8714 8614 Wl.I., 1999 0lOOm 14... Con..t. dod. 2 14.9% Vnr. 

Figure 6.3-2. Time domain EOF eigenvector components represented as vectors multiplied by the standard deviation of the modes. Analysis is for the 40-HLP, 0 to lOOm depth mean currents. winter 1998 mode

1 (a) and 2 (b) , and winter 1999 modes 1 (c) and 2 )d) IYW 91W 95 on,/ 
32-

1-E

0i

5(

I' I' ,_ 5' " At l I' I %I\

tS/%I - _II

(

Normalized Seasonal Mode Amplitudes

ft '-'i i\ j_ I

/ 'I -

I ,/ t,'551,j ' Si , ' w99 S S

IS_. ,

I

\_._,___ M2 5. / II \,_f II, 5) LA W99

I,'-'-.

/ I

__/'\ 'I S

Days

W98

Ml

S98 -.'-S-.--\-.\ 55_/5 M2

S98

Ml

"5--- M2 'I

Figure 6.3-3. Time series of the normalized amplitudes of modes 1

(solid) and 2 (dashed) for the two summer and two Winter

seasons.

213

3-

0 20 40 60 80 100
found high correlations with the currents at individual shelf-break and

500m isobath moorings. It will be demonstrated that the coherent windforced response was applicable to the shorter period motions of both

modes in the Winter seasons and thus, wind-driven currents were

important across the slope in the upper layer.

EOFTS were calculated for the east and north components of the windstress considered together. The eigenvectors for mode 1 in both winters

account for more than 60% of the variance of the four shelf and slope

wind stations used in the analysis (Figure 6.3-4) - In both seasons, the

fluctuations were directed across the Alabama slope and approximately

parallel to the west Florida slope. The differences are that the

amplitudes were slightly larger in winter 1998 than in winter 1999, and

the vectors were rotated a little more to the east-west direction and so had a larger component parallel to the Alabama slope. This may explain

why there are larger eigenvector amplitudes parallel to the Alabama

slope in winter 1998 than in winter 1999 (Figure 6.3-2). The spectra,

coherence squared and phase differences for the wind stress mode 1 and

the 0-lOOm mean current modes are given in Figure 6.3-5. The wind

stress spectra have energy from -3 to 12 and -3 to 7-day periods for the

winters of 1998 and 1999, respectively. The winter of 1998 was also the

more energetic, particularly at longer periods. This is reflected in

the mode 1 spectra for the winter 1998 currents which has peaks around

10 and 3 days which are not observed in the other modes (Figure 6.3-5b).

The coherences between wind stress and current modes were high where the

wind stress modes have energy. The phase differences show that the wind

stress led by greater than 90° which implies a lag of the currents of

about 1 to 2 days. This is probably a reflection of the -50Dm average

water depth of the moored array, and the topography of the canyon with

respect to the direction of the wind stress fluctuations. The lag could

also be interpreted as the result of an arrested topographic wave

generated by the along-slope wind stress over the west Florida side of

the canyon. The propagation of this response is to the west where the

along-slope wind stress fluctuations were small.

6.4 Canyon Response to Storms

6.4.1 Overview

Time series of surface wind stress computed from wind speeds measured at

NOAA buoys 42040 and 42039 (Figure 6.4-1) reveal that, by far, the

strongest wind stresses influencing DeSoto Canyon during this study were

generated by Hurricane Earl, which passed the mooring array in early

September, and Hurricane Georges, which traversed the array at the end

of September. Another Earl and Georges superlative is revealed by the

results of canyon edge flux calculations presented in Section 5.5.3.

The largest cross-margin fluxes of temperature and. salinity were

observed during the hurricanes. In the present section, details of the

current and temperature response to the Hurricanes Earl and Georges are

examined, concentrating on the response at the canyon edge.

Snapshots of the surface winds measured at the NOAA buoys and the nearsurface water velocities measured by the moored array (Figures 6.4-2,3)

214
b)

30N = - - 30N

29N I ' .... ... 29N

2BN 28N

90GW 89W 88W 87W 86W 85W 84W

WinIer 1998 Windstress Mode 1 61.7% Variance

12/18/1998 TO 3/27/1999

90GW 89W 88W 87W 86W 85W 84W

31N

I I 31N

30N

29N

90W 89W B8W 87W

0

0.5 dyns/cm2

28N

90W 89W 88W 87W BS.W Winter 1999 Windstres Macfe 1 64.9% Variance

12/ 5/1997 TO 3/14/1998

86W 85W 84W

31N

85W

- 3ON

- 29N

84W

26 N

Figure 6.3-4. Time domain EOF eigenvector components represented as

vectors multiplied by the standard deviation of the

modes. Analysis is for the 40-HLP wind stress time

series at the indicated stations. (a) winter 1998, and

(b) winter 1999.

215
c)

180

0

I 0_2 DATE

1 WINDSTRESS MODE I SOLID 1997/12/05

2 WINDSTRESD MODE I DASHED 1992/12/10

TIME SEPIE5 LENOTH 99 DAYS DEGREES OF FREEDOM ID BANDWIDTH 009090909 CPD

CYCLES/DAY

1 _1 CYCLES/DAY

WII4DSTRES5 MODE 1 MODE 1 (LI,V) 0-looM

WINOSTREOS MODE 1 2 MODE 2 (U,V) 0lOOM IRS DASHED

DATE 1997/12/ 50009 TIME SERIES LENGTH 00 DAIS

DEGREES OF FREEDOM 19 MANDWIDTH 0.09090909 CEO

IRS SOLID

1500

0) i000 U

C

500

180 _________________________________ 180

C, U) .0

180

1O_2

WINOSTRESS MODE 1

1 MORE 1 (DV) 0IOou WINDSTRESS MODE I 2 MODE 2 (DV) 0lOOM

SATE 1999/12/15.0000

DEGREES OF FREEDOM 15

10'

CYCLES/DAY

DATE

I MODE I (8.9) 0lOOM SOUO 1997/12/05 O MODE 2 (U,V) 0lOON DASHED 3 MODE I (LIV) 0-1009 5OUD

4 MODE 2 (LIV) 0looM DASHED 1999/12/18

TIME SERIES LENOTH 99 DAYS DEGREES OF FREEDOM 19 NNILDWIDTI4 0.09090909 CPD

CYCLES/DAY

VHS S0UD

VRS DASHED

TIME SERIES LENGTI4 99 DAYS

BANDWIDTH 0-09090909 CPD

d)

Figure 63-5. Variance preserving spectra for EOF (a) mode 1 wind

stress, and (b) 0-lOOm depth mean current modes 1 and 2

for the winters of 1998 and 1999. Coherence squared and

phase differences between the wind stress and current

modes for the winters of (C) 1998 and (d) 1999.

216

1I1

I II)

3

2

4

I t.LI.1_ IIIrI

IT'

-10_I

a) SPECTRA SPECTRA b)
U) U) U) (I) May Jul Sep Nov Jan 1997 1998 F )AA1LM Mar May Jul Sep Nov Jan Ma

1999 Figure 6.4-1. Estimated total surface wind stress at NOAA buoys 42040 and 42039 during the DeSoto Canyon study. _. N 25 I I F F I I I EU) 20 Buoy 42039 ,_ 25 NE0 20 C) >. 15 U) (I) U) (I) -D 5-- Buoy 42040I I I ni1 C) a May Jul Sep Nov Jan Mar May Jul Sep Nov Jan Ma

1997 1998 1999
Figure 6.4-2. Wind velocities and near-surface currents (at 30 and 70m depth) meas

during the passage of Hurricane Earl. The broader line going from S

shows the approximate path of the hurricane center.
Figure 6.4-3.. Same as Figure 6.4-2 except showing wind velocities and near- surface currents measured during the passage of Hurricane Georg

as shown by the broader line going from SSE to NNW.
reveal that both storms had a spatially complex wind field over the

canyon and that this generated a complex response in the near-surface

flow field. Hurricane Earl approached the mooring array from the

southwest. Over a period of approximately one day, the center of

Hurricane Earl moved northeastward past the southern end of the array

and made landfall over the Florida Panhandle. As the storm passed the

array, its winds rotated cyclonically about the canyon The spatial

scale of wind variation was particularly short as the storm approached

the Florida coast on September 3. At 00:00 on September 3, a northward

wind was observed at the eastern end of the array (at buoy 42039) and a

southward wind was observed at the western end of the array (at buoy

42040; see Figure 6.4-2) . In spite of this short-scale variation in the

wind field, near-surface currents over the upper canyon and canyon rim

were fairly uniform in direction, tending toward the east. However,

significant variations in the canyon currents were observed as the storm

made landfall, with very large onshelt velocities appearing at the

eastern canyon edge.

Hurricane Georges approached the array from the south. Its center moved

northward along the western end of the array before making landfall near

the Mississippi-Alabama border (Figure 6.4-3) . As the storm's center

approached the array, a well-organized, cyclonic wind field was seen

over the canyon. However, as was observed during Hurricane Earl, winds

over the canyon rapidly rotated and exhibited significant small-scale

variations as Georges approached land. The near-surface currents driven

by Georges over the canyon edge differed significantly from the stormdriven currents over the interior canyon. Currents over the deeper

canyon rotated clockwise, at roughly the local inertial period, as the

storm passed. By contrast, direction of flow at the canyon edge was relatively steady during the storm's passage. This flow appeared to

move cyclonically along the canyon rim, following the early hurricane

wind pattern.

6.4.2 The Canyon Edge Response

To illustrate the effect of the hurricanes at the canyon edge, Figures

64-4,5,6 show near-surface and near-bottom temperatures and velocities

measured at the canyon rim moorings from late August through early

October 1998. The plots of velocities have been rotated into along- and

across-jsobath components (see Table 6.4-1 for rotation angles) . The

coordinate system used has the positive along-isobath velocity directed

such that deeper is to its right (i.e., to the northeast at moorings Al,

Bi and Cl; and to the southeast at mooring El), and has the positive

across-isobath velocity directed onshore.

Both storms generated a strong along-isobath flow near the surface.

This moved from east to west along the canyon rim (Figure 6.4-4),

opposite to the direction of the long-term mean flow. During Earl, the

strength of this flow varied considerably as a function of position

along the canyon edge. It was strongest at the canyon apex mooring, Dl,

where it exceeded 130 cm/s at 16m depth. The canyon edge flows were

somewhat stronger during Georges than during Earl. Of particular

significance were the strong near-bottom velocities seen at the canyon

edge during Georges. Speeds in excess of 60 cm/s were measured at Sm

220
E

C.)

C-)

E

C.,

20

c\j

E

-100

-200

100

E0

-100-

-200

100

-100

Wind Stress at Buoy 42040

- eastward

northward Earl Gorges

Aldng Isobath Velocities at Canyor Edge

-, wp.!rr!..

AM .. 4 &. %-I,! - El

-200

20

August 1998

1Gm 95 m

30 9 19

September

29 9

October

Figure 64-4. Time series of wind stress (top panel), and near-bottom

and near-surface along-isobath current velocities (lower

panels) measured at the indicated canyon-edge moorings

during the passage of Hurricanes Earl and Georges.

221

100

I

0
20

C\JE

Eo0

-50

-100

100

50

05

-100

100

50

Eo0

-50

-100

- eastward

- northward Earl Georges

r

_LaI1lL.1 p*, LALk £.kidá4 !

4&j

IFp.I , Dl

-100 -

20

August 1998

1Gm

- 95 m

- 16m - 95 m

Across sobath Velocities at Canyoti Edge

Wind Stress at Buoy 42040

19 29

October 9

Figure 6.4-5. Same as Figure 6.4-4 except showing wind stress and

across-isobath velocities measured during the passage of

Hurricanes Earl and Georges. Positive velocities are

directed onshelf.

222

30 9

September

- 1Gm Cl - 95 m
C

-o

20

30 O 25 0 20

30

O 25 0 20

30 - o 25-

0 20 -

Wind Stress at Buoy 42040

- eastward

- northward

Earl Georges

anyon Edge Temperatures

El 20 m

- 95 m

Figure 6.4-6. Same as Figure 6.4-4 except showing wind, stress and.

temperatures measured during the passage of Hurricanes

Earl and Qeorges.

223

20 30 9

August 1998 September

19 29 9

October
above bottom at moorings Cl, Dl and El as the storm approached the

array. It is likely that these strong near-bottom currents may have

mobilized a quantity of canyon edge sediments.

Strong and spatially varying across-isobath flows were seen at the

canyon edge during both staLms (Figure 6.4-5) . Mooring Cl experienced

the strongest cross-isobath currents, in excess of 60 cm/s during both

storms. With very few exceptions, near-surface and near-bottom crossisobath velocities measured at the canyon edge opposed each other during

the hurricanes, evidence of a mode-i barocliriic response to the storms.

Table 6.4-1. Net onshore and offshore volume transport per unit

alongshelf distance measured at canyon edge moorings

during early February 1998 storm and at mooring Al during

the presumed eddy passage of the mooring.

Even

February Storm

Lns1...

Orient

Ccw oi

East

Canyon Edge Eddy

Mrg.

Al

B1

Cl

Dl

El

Al

Dates:I

19$

Trans. Prans.

6 2, (xl 0).

Although no appreciable changes in canyon edge temperatures were

observed during Earl, precipitous increases in near-bottom temperature

were seen at the canyon edge moorings as Georges approached the array

(Figure 6.4-6) . These were not likely the result of local vertical

mixing with warmer water from above because they were not matched by a

coincident decline in near-surface temperatures at the canyon edge. They

were probably the result of advection of warmer water from the shelf.

All of the temperature increases coincided with of f shelf near-bottom

velocities. In addition, the large fluctuations seen in near-bottom

temperature at moorings Al and El during and after Georges were roughly

9OThut-of-phase with cross-isobath velocities at the same level. This

would be expected if the temperature fluctuations resulted from

advection of a cross-shelf temperature gradient.

Details of the canyon edge velocity response to the two hurricanes can

be seen in contour plots of velocity as a function of time and depth

(Figures 6.4-7,8). During Earl, the highest velocities at the canyon

edge tended to be concentrated in a near-surface jet. At moorings Cl

and El this jet was particularly shallow, extending to about 30 m depth

(Figvre 6.4-7). By contrast, during Georges high velocities extended

throughout the water column at all canyon edge moorings. A remarkable

mid-depth maximum of along-isobath velocity was observed at mooring Cl

during Georges (Figure 6.4-8). This suggests that the canyon edge

224

20 Feb. 2-6 0.9 1.5

15 Feb. 2-6 1.2 0.9

45 Feb. 2-6 1.9 1.2

15 Feb. 2-6 2.7 0.1

-60 Feb. 2-6 0.8 2.3

20 Jan. 10-16 2.6 2.8
September 1998 Mooring Cl during Hurricane Earl - Alongshelf Velocity Onshelf Velocity çLjJ40 -20 I 20 20 30 - 40 - E 50-i 60- 0) 0 70- ,' 8090 Figure 6.4-7. Contours of along-isobath (positive to the northeast) and across-isobath (positive onshelf) velocity recorded at moori

Cl during the passage of Hurricane Earl. September 1998 2 3 4 5
Mooring Cl during Hurricane Georges - Alongshelf Vebcity 26 September 1998 26 September1998 Onsheif Velocity

2 Figure 6.4-8. Same as Figure 6.4-7 except showing contours of velocity 

mooring Cl during the passage of Hurricane Georges. 27 26 29 30 2
currents were not simply the product of vertical momentum transfer from

local wind forcing at the surface.

No consistent trend was seen in cross-isobath velocities measured at the

moorings during either hurricane. In fact, similarities in the

hurricane-driven cross-isobath currents measured at the canyon edge

moorings were hard to discern. During Georges1 the cross-isobath flow

measured at Cl was characterized by a strong and shallow onsheif current

near the surface and a weaker onsheli current of greater vertical extent

near the bottom (Figure 6.4-8) . By contrast, near-bottom and nearsurface cross-isobath currents measured at Al changed sign during the

passage of Georges (Figure 6.4-5) and were strongest in the 40-70m depth

range. Still, different patterns were seen in the cross-isobath

currents measured at the other canyon edge moorings during Georges.

6..4..3 Cross-inarqin Voltune Transports at the Canyon Edqe

An issue of importance to this study is the extent to which coastal and

deep Gulf water masses were transported across the canyon rim as a

result of the hurricanes. Unfortunately, the details of heat and salt

transport at the canyon edge cannot be studied with the moored array

data, due to the sparseness of temperature and salinity sensors in the

array. However, the array data are well suited for the calculation of

cross-margin volume flux at the canyon edge.

Using the velocity data from the canyon edge moorings, we computed the

volumes of water transported on and off the shelf past each mooring

during the passages of Hurricanes Earl and Georges. The volume of water

transported onshelf (per unit of along-shelf distance) was computed by:

o

J 1u0dtdz

-D t1

Where u0 denotes the cross-isobath velocities measured at the mooring

that were directed onshelf, D is water depth, and t1 and t2 bracket the

time over which the hurricane affected currents at the mooring. The

integration was carried out over the portion of the water column and

times over which onsheif velocities were observed. A similar

expression was used to compute offshelf volume transport. These

integrations were carried out using the velocity data from moorings Cl,

Dl and El which extended over the depth range of 12-96m (no ADCP data

were acquired at mooring Bi during the hurricanes, and no near-bottom

velocities were measured at mooring Al during the storms) . It was

assumed that the cross-isobath velocity in the surface and bottom layers

lacking velocity measurements was uniform and equal to the velocity

measured directly below (for the surface layer) or above (for the bottom

layer) . The along-isobath orientation at each mooring, on which the

results of these calculations were sensitive, was chosen based upon the

principal axes of the mooring's low-passed filtered velocity data

These orientations have an uncertainty of at least 5. We found that

changing the along-isobath orientation by 5° altered the computed

transport by 10-35%.

227
The results (Table 6.4-2) indicate that Hurricane Georges generated

significantly greater cross-margin volume transports than Hurricane Earl

(by a factor of 2-3) . These results also provide convincing evidence

that hurricane driven onshore and offshore transports were not balanced

at all locations on the canyon edge. A greater than 3:1 ratio of

onshore to offshore transport was indicated at moorings Dl and El during

Earl and at mooring Dl during Georges. This is a much larger imbalance

than can be accounted for by the uncertainty in along-isobath

orientation. These net onshelf transports, likely balanced by net

of f shelf transport elsewhere along the canyon rim, may have been the

result of convergences and/or divergences in the flow along the canyon

edge. This type of flow pattern may be expected in a region such as the

canyon where the angle between wind direction and along-isobath

orientation changes over relatively short distances.

Table 6.4-2. Net onshore and offshore volume transport per unit

alongshelf distance measured at canyon edge moorings

during Hurricanes Earl and George. -

6.4.4 Tle Inertial Wake

Previous investigators have found that storms passing over the Gulf of

Mexico can leave vigorous inertial motions in their wake. Clear

evidence of this was presented by Brooks (1983) who examined current

meter and thermistor string records during and after Hurricane Allen's

passage over the western Gulf of Mexico in 1981. Brooks found that the

storm generated inertial currents with roughly 50 cm/s maximum

amplitude, and that these decayed with an approximate time scale of 5

days. That analysis showed that the storm-driven inertial energy

propagated downward through the thermocline at a rate which roughly

accounted for the decay of the near-inertial currents in the surface

mixed layer. A longer-term examination of inertial currents over the

Texas-Louisiana shelf and upper slope was conducted by Cheri et a)..

(1996) . They found that pulses of near-inertial currents over the shelf

tended to follow a sudden change in surface wind stress. Their analysis

indicated that the vertical structure of near-inertial currents over the

shelf typically conformed with that of a first baroclinic mode, with

close to a 180° phase difference between the near-surface and nearbottom currents. The amplitudes of near-inertial currents observed by

228

Storm. -- Mooring A1ñshe:Lf:..

Orientataon

Cow of east

Sept

1998

Transport

(xlO) m

Transport

L(K1O)

Earl Cl 45 2-4 1.3 1.0

Dl 15 2-4 1.5 0.5

El -60 2-4 1.4 0.4

Georges Cl 45 26.5- 2.0 2.7

30

Dl 15 26.5- 2.6 0.5

30

El -60 26.5- 1.4 2.0

30
Chen et al. (1996) were maximal near the shelf-edge, and decayed

gradually toward the coast but rapidly offshore.

To examine the inertial current signal in the velocity records from the

DeSoto Canyon array, we employed a method developed by Perkins (1971)

Termed "complex demodulation", this method produces a time series of

complex coefficients, DL), according to:

t+T

D(t) = l/(2T)J [c(t) - <c>]etdt,

where c = u + iv

and <c> 1/(2T)J c(t)dt,

t-T

In the above, c is a complex representation of the velocity series; £ is

the local inertial frequency; and T is the length of the demodulation

window. In essence, DUis the Fourier transform of c over the period

-T<t<+T and at the frequency f. The magnitude and argument of D(t) thus

approximate the amplitude and phase of the inertial velocity signal at

time t.

The inertial current magnitude seen during and following the passages of

Hurricanes Earl and George varied considerably from mooring to mooring

(Figures 6.4-9,10) . Hurricane Earl generated weak inertial currents

along the canyon edge. As the hurricane passed the array, slight

increases in inertial current magnitude were seen at moorings Cl and Dl,

while no appreciable changes in inertial current magnitude were observed

at moorings El and Al (Figure 6.4-9). By contrast, Hurricane Earl

generated vigorous inertial currents within the interior of the canyon.

These appeared to become stronger with increasing water depth. Along

each mooring line their strength was greatest at the deepest mooring

(near the l300m isobath) . The strongest inertial currents attributed to

the storm were seen near the surface at mooring C3. Their magnitude

peaked near 50 cm/s.. roughly equivalent to the maximum strength of the

inertial currents observed by Brooks (1983) in the wake of Hurricane

Allen.

After the passage of Hurricane Earl, lengthy pulses of strong inertial

currents appeared near the surface at moorings D2 and El. When viewed

as a function of time, the magnitude of these pulses resembled the

magnitude of the inertial current pulse seen at mooring C3, except

shifted forward in time (Figure 6.4-11). The time shift, relative to

the C3 magnitude, was 1.2 days for the D2 pulse and 2.9 days for the

pulse at El. This suggests that the inertial current pulses seen at

moorings D2 and El were not locally generated, but had propagated to the

moorings from the western canyon. Based on the relative times of the

pulse's first appearance at moorings C3, D2 and El, we have determined

pulse may have propagated with a group velocity of roughly 30 km/day to

the ENE.

229
Figure 6.4.9. Surface wind stress (top panel) together with nearbottom and near-surface inertial current magnitude

observed at the indicated canyon edge moorings during

late summer-early autumn 1998.

230

c'JE

-

'I,

20

0

40

20-

0 40

20-

040

20-

0

40

20

Wind Stress at Buoy 42040

I -

I I Earl Georges - -

- eastward

northward u,

I I 1

I

Inertial Current Magnitude

Al

I I I

.

I - 16m

Clu:I 4*

I L

16m

___________ DlI I

'WVt#1i. .\*'" A £4 - 16m

El

I I

1h11A

I - 16m

August 1998

29

September October
A2

E0

A3

B2

Wind Stress at Buoy 42040

Inertial Current Magnitude

0

16m

- 490 m

16m

- 1310m

- 16m

- 490 m

Figure 6.4-10. Same as Figure 6.4-9 except showing near-bottom and nearsurface inertial current magnitude at interior canyon

moorings. Note the strong and persistant near surface

inertial currents during and following passage of Earl

at Mooring D2.

231

50 ro 16m

490 m

E

I.-) 0 ;ilIIk

50-

C.) 50-

E0

- 50

20 30 9 19 29

August 1998 September October
10

20

30

August 1998 September

Currents

October

Figure 6.4-11. Comparison of inertial current magnitudes seen near the

surface at moorings C3, D2 and El during late summerearly fall 1998

232

30

Wind Stress at Buoy 42040

L eastward

- northward

c'J

20

E 10 Earl Georges

Cl)

a) 0 >. D
Hurricane Georges appeared to have generated strong inertial currents

all along the canyon rim. Significant increases in inertial current

magnitudes were observed at all canyon edge moorings during the

hurricane's passage (Figure 6.4-9) . However, the strongest inertial

currents were again seen at the deepest moorings (Figure 6.4-10)

Subsequent increases in near-bottom inertial current were also observed

at the deepest moorings, suggesting that the hurricane-generated

inertial motions propagated vertically to the deep canyon floor.

Strong near-bottom inertial currents were also seen at the canyon edge

moorings following the passage of Georges. However, the relative phase

of these currents was complicated and did not always conform with a

mode-i baroclinic structure (Figure 6.4-12) -

To summarize, Hurricane Earl generated relatively weak inertial currents

over the canyon edge but vigorous inertial Currents over the deep

canyon. The data suggest that the inertial motions generated over the

deep canyon by Hurricane Earl may have propagated towards the ENE to the

Florida shelf. Hurricane Georges generated vigorous inertial motions

over the entire canyon, but these were again strongest over the deep

canyon. The vertical phase of the inertial motions seen at the canyon

edge during both storms were complicated and often bore little

resemblance to a mode-1 baroclinic structure. These observations are in

marked contrast to those of Chen et al. (1996), who found that stormgenerated inertial motions over the Texas-Louisiana shelf have a mode-1

structure and decay rapidly in magnitude going off the shelf. The

difference from our observations may be storm related or could reflect

general differences in the character of inertial motions in the two

study regions. Such differences might be expected given the drastic

contrast in the bathymetric setting of the study regions.

To compare the strength of inertial currents generated by Hurricanes

Earl and Georges to inertial currents generated by other storms, or by

different mechanisms such as frontal instabilities, Figure 6.4-13 shows

time series of near-bottom and near-surface inertial Current magnitude

at selected moorings for the duration of this measurement program. At

some locations, inertial currents generated by Earl and Georges were the

most vigorous of the study period (e.g., near the surface at mooring

C3) . At other locations, inertial currents due to Earl and Georges were

matched or bested in strength by inertial currents due to other events,

such as storms during February 1998. It is also seen that the decay of

inertial energy going onshore to the canyon edge, seen during Earl and

George, was typical of most inertial current events. Careful inspection

of the wind stress and inertial current magnitude time series reveals

that pulses of strong inertial motions were often associated with

episodes of strong winds. However, there were times that strong

inertial currents appeared when winds were weak (e.g., during May 1997

and April-May 1998) . Likely causes of inertial Currents during these

times include eddy shedding from the LC and eddy.-eddy interaction over

the canyon.

233
10203040 E 506O a 7080901020304050 Q. 60 Ii) 708090 Hurricane Georges - Mooring El - Inertial Current Magnitude 26 27 September 1998 28 29 30 1 2 October 3 4 5678 Figure 6.4-12. Contours of the magnitude and phase of inertial currents observed at moorin

and after the passage of Hurricane Georges. Phase is relative to the curre

shallowest current meter and is contoured only where the magnitude exceeds 5 

26 27 28 29 30 1 2 3 4 5 6 7 8 September 1998 October Inertial Current Phase
Figure 6.4-13. Surface wind stress together with near-bottom and nearsurface inertial current magnitude observed at the

indicated moorings over the study period. Because of

gaps in the wind records, wind stresses over April

1997-November 1998 were computed from buoy 42040 wind

velocities and subsequent wind stresses were computed

using wind velocities from buoy 42039.

235

c'3ç

20-

50

E

50

E

0

50

50

E

0

50

E0I

U,

Wind Stress

I I I I I I I I

I I I I I_I

ikLJ(di.[I1Lk i jg iLL_IJ.LiJ.Lj.. ILL LI. LAIJt[a.

Inertial Current Magnitude

I I I

IOl ClI I I I I I I I I I I I I i I I

.

I .. 16m 95m

I

I C2I

I I I I I I - L4 A ,, .

I 16m 490m

I I I C3

I I I I I I 1 I I

1,

I

I I 16m - 1290m

. Dl

I I I I I I I I I

i I I I I 16m 95m

I I I

I32I I I I I I

I I I I I I I

L Lk A)

I I I 16m 490m

-

I

I I El

I I I

I I I I I I I I I I I ____I

. 16m 95m

AM

1997

J JASON D J FM AM J JASON

1998

D J FM A

1999
6.4.5 Storms vs. Canyon edge eddies during t1e winter of 1997-1998

When considering eddy vs. storm effects on the canyon rim, the winter of

1997-1998 is of particular interest. The strongest stojuls of the study

period, not designated as hurricanes., passed over DeSoto Canyon during

this time (Figure 6.4-1). The most intense of these storms occurred in

early February 1998, with the second most intense storm traversing the

canyon in late December 1997. Although severely limited by frequent

cloud cover, satellite SST imagery of the period clearly showed a LC

eddy extending northward into DeSoto Canyon and smaller eddies and/or

frontal instabilities at the canyon rim (Figures 6.4-14,15) . For

example the STT image of January 16, 1998 shows an eddy (or frontal

instability) at the western canyon rim in the vicinity of moorings Al

and l (Figure 6.4-15).

The effect of the storms and eddies on the canyon edge can be seen in

near-surface and near-bottom velocities from the canyon edge moorings

(Figures 6.4-16,17) . The strongest winds of the late December 1997

storm were directed to the ESE. These appeared to have generated an

eastward tending near-surface flow at the canyon edge (Figure 6.4-16)

However, the effect of the storm on this current was not clearly defined

by the moored array data, as they showed the strongest northeastward

velocities occurring at moorings Al and Bi shortly after the storm

abated. The early February storm generated a relatively strong current

that moved cyclonically along the canyon rim. Akin to Georges.. this

storm produced strong near-bottom currents at the canyon edge. As the

storm passed the array, current speeds in excess of 30 cm/s were

measured at 5 m above bottom at all canyon edge moorings. At mooring

Cl, near-bottom current speeds exceeded 45 cm/s for a full day during

the storm's passage. Unlike Georges, a two-layer cross-margin exchange

was not clearly evident over most of the canyon edge during the early

February storm. It was only at mooring C2 that strong and opposing nearsurface and near-bottom cross-isobath flows were observed at the canyon

edge during this storm (Figure 6.4-17)

The canyon edge mooring data showed strong cross-isobath velocities

during mid-January when winds were relatively weak and when SST imagery

revealed eddies and/or frontal instabilities at the canyon edge (Figures

6.4l4,17) . The strongest cross-isobath currents of this period were

seen at moorings Al and Bl where the SST imagery showed clearest

evidence of eddy and/or frontal instability activity (Figures 6.4-

14,15) - A particularly strong and persistent flow, not related to wind

forcing, was observed at mooring Al during 14-20 January (Figure 6.4-

18) . This had subsurface maximum in its along-isobath component of

roughly 60 cm/s within the 20-40 m depth range. It's cross-isobath

component reached nearly 40 cm/s in magnitude and underwent an abrupt

change in sign (direction) on 18 January. This is a velocity signal

that may be expected due to the passage of an eddy or frontal meander.

To compare eddy and storm driven cross-margin volume transports, we used

the method outlined in Section 6.4 to estimate these transports during

the early February 1998 storm and during the presumed mid-January canyon

236
.: . :', s1:g cT) it-L rft rurrcnt v.1s:Ltis s:.triLd 1inc zi.icu c1rfy ih. fgir.. in r.r.
SST iCe1skij (ornpote 16pn9$ IOCST I6Jaa9 1449 CST ?1r. .j4 j, ITSfl7 t:S1' izti Ld S.ULIJL u1ai'1 Gil thi i:cJP. -- 1 3dc Lo h.1p c.re treii'- i: -h -i -
Figure 6.4-16.

of 1997-1998.

measured at the canyon edge moorings during the winter

Near-bottom and near-surface, along-isobath velocities

239

-100 100

0

-

-100 100

- 0

10:

100

-100

Along Isobath Velocities at Canyon Edge

. Al

I I 1Gm

..1A

I

. A* EB

- VY

I

'. - 95m

16m

I L

It&.41&IA

I wA -

I

Iq - 95m

16m

k I --.-.-. . - _- '

I

- -'1 9Gm

1Gm

100

- 0

I

I

-100

I

I I 95m

25 4 14

December 1997 January 1998

24 3

February
E00

CO0

EO

U

E0

U

-100

100

-100

100

-100

100

-100

100

EUC-)

--4- L

-

- ------ . . - .. r-' - eastward

northward

B I 16m 95m

Dl

16m 95 m

-100

25 4

December 1997 January 1998

Wind Stress at Buoy 42040

14 24 3

February

Figure 64-17. Same as Figure 6.4-16 except showing near-bottom and

near-surface across-isobath velocities.

240

Across Isobath Velocities at Canyon Edge

100
2030 40 -50 - 60 ci) 7020304050 - 60 (1) 708090 11,0 901 ',, Ji i, I I 12 14 16 January 1998 F 12 January 1998, 4 16 LI I 0 A!ongsheif Velocity ': I. I, C ' I I ' r,. I , I 26 28 30 1 3 February Onshelf Velocity I Iii 0(N 18 20 22 24 26 28 3D 1 3 5 7 

February 0FJ 

II I II II L I 1111 ii' III 1I ,,,. Figure 6.4-18, Contours of along-isobath and across-isobath velocity measured 

mooring Al during January and February 1998. 18 20 22 24 5 7 
rim eddy event at mooring Al. Volume transports observed during the

storm were of a magnitude similar to the transports observed during

Hurricane Earl (Tables 6.4-1 and 6.4-2). Similar to the transport

observed during Earl and Georges, onshelf and offshelf transports of the

early February storm were approximately balanced at some moorings, and

were grossly mismatched at others. The onshelf and offshelf transports

observed at Al during the presumed eddy passage were roughly equivalent

and similar in magnitude, to transport observed during Hurricane

Georges. A conclusion is that the canyon edge volume exchange generated

by an eddy or frontal instability can be comparable to that generated by

a hurricane.

6.5 Inertial Currents on the Slope

The velocity records often show large amplitude fluctuations with

periods near 1 day. A typical example from the upper layer for the

summer of 1997 is given in Figure 6.5-1. The across (U) and along (V)

isobath currents have similar magnitudes and the velocity vector rotates

clockwise (V leads U by _900). Because of their intermittent nature and

variability with depth, these oscillations are attributed to wind-forced

inertial-internal waves. This section describes the characteristics of

these motions and their seasonal variability. The exceptional inertial

currents that occurred in the wakes of Hurricanes Earl and Georges in

September 1998 are discussed in Section 6.4. The inertial period (2/f)

at 29°N is 24.75 hours (0.97 cpd) which is in the center of the diurnal

tide band. However, barotropic diurnal tidal currents over the slope

have magnitudes < 1 cm/s. Internal tides may be present but should be

phase-locked to the surface tide. The variability of the daily

oscillations in Figure 6.5-1 does not show the regularity that would be

expected from internal tide motions. The spectra also do not show

distinct peaks at the K1 or P1 frequencies that would be expected for

narrow band motions such as diurnal tidal currents. Therefore, the

majority of the daily oscillations are considered to be caused by

inertial-internal waves even though diurnal tidal motions are difficult

to separate out from the records.

The time series of the velocity components have several interesting

features (Figure 6.5-1) - The strongest oscillations were at C3, the

mooring furthest offshore and in the deepest water (1300m). Here, the

inertial currents were present to some degree for most of this two-month

period despite weak winds. There is little apparent correlation of the

magnitude of the offshore oscillations with wind events. However, some

of the abrupt shifts in wind direction (e.g. August 1) seem to trigger

bursts of inertial oscillations. Eddies could generate inertial

currents by interactions with other flows, and flows with large

horizontal shears that generate negative, relative vorticity anomalies

can trap inertial oscillations by causing a local decrease in the

effective planetary vorticity (f) (Mooers 1975; Kunze 1985) . Thus, the

eastward flowing upper-slope jet that is frequently found in the study

region (see Section 5.2) could trap northward propagating inertial waves

on its southern side (Kunze 1985) . The summer 1997 clockwise rotary

spectra (Gonella 1971) for two depth levels on transect C are given in

Figure 6.5-2. The peaks around 1 cpd. are broad, reflecting the

242
1,5-

LU

LCL 1.0=

cic = ., .5-

6: 1.0-

-1.5 -

Wind Stress

NDBC Buoy 42040 ,//4. ..\\\l "== j//.

'u//p' (f/(\74flfL

U (shaded) & V (solid) Velocity Components

r L -. .LáàL4.. :1 - - /. i .yr'1c'1 ( v'yriy 1Y C il

T "

.iiL

'- r i

à\L,

D9

1 33m CI

16m

Cl

40m

CI

68m

Cs

1 6m Cs

58m

Figure 6.5-1. 3-HLP time series of the cross-isobath (U) component

(dashed and shaded) and along-isobath (V) component

(solid line) at selected depths on the indicated

moorings during a two-month period in summer 1997. The

top panel shows the 40-HLP wind stress vectors from buoy

42040. Current components are oriented so positive V

has an easterly component and positive U is of fshsore.

243

19 29 August 1997

8 18 28 7 Septembflr 17

40

LUNa

LUNC

LU N

QN

(0

LU

C, LUI'- C, fl N

T 60

C,,

in
-o

a-0

0

10

Spectra

Figure 6.5-2. Clockwise (anticyclonic) rotary spectra for two depths on

moorings Cl (dashed) and C3 (solid) for summer 1997. The

frequency axis is linear to emphasize the inertial peak

at approximately 1 cpd (see arrows)

244

+

--

I A

-

-

-1

\

I1v

t

/

I / ii'

/

'\ I I tJ\

I1 J\\/

f

=

I I I I I I

1 DSC-C3-1 16 U. SOLID - COMPONENT

2 DSC-C1 -2 16 M. DASHED - COMPONENT

3 DSC-C3-1 68 U. SOLID - COMPONENT

4 DSC-C1-2 68 M. DASHED - COMPONENT

.0 .5 1.0 1.5 2.0

Cycles/day

DATE 1997/ 5/10,0000

DEGREES OF FREEDOM 22

TIME SERIES LENGTH 99 DAYS

BANDWIDTH 0.11111111 CPD
intermittency of the inertial oscillations with the offshore being more

energetic than the shelf break- The shelf break spectra maxima also

occur at slightly higher frequencies than at the offshore mooring

because of its more northerly position and thus larger value of f.

In contrast to the summer, the winter period had much less energetic

inertial motions at the shelf break than the summer despite the more

frequent passage of storms. The upper-layer subtidal currents during

the winters of 1998 and 1999 were highly correlated with the local wind

stress (see Section 6.2.3) . Figure 6.5-3 shows the velocity components

(TJ,V) for Cl, D9 and C3 for a two-month period in the winter of 1999.

Inertial oscillations were almost none existent at the three levels of

Cl, and intermittently present with small amplitudes at D9 and C3. The

main reason for this is reduced stratification in the upper lOOm,

particularly at the shelf break. Inertial currents are not supported on

the shelf if the water column is homogeneous (Kundu 1984). Figure 6.5-4

shows the inertial current amplitudes obtained by complex demodulation

(see Section 6.4.4) at 3Orn depth at Cl and C3 for the two-year study

period. The C3 record was almost always more energetic than Cl and the

amplitudes have less seasonal variation. The exceptional peaks at the

beginning and end of September 1998 were caused by Hurricanes Earl and

Georges. The stratification at Cl is represented by the 16 and 8Orn

temperature records. The temperature difference became very small at

the beginning of January 1999 and there was correspondingly little

inertial activity at Cl despite the rapid fluctuations of wind speed

(Figure 6.5-4). In the previous winter, the vertical temperature

differences were not quite so small, even though there were more high

wind events. Therefore, the inertial amplitudes at Cl were not as well

damped.

n EOF analysis was performed using the clockwise rotary spectra

calculated for the summer 1997 interval (Figure 6.5-2) . Five depths

were used from the 13 ADCP's (16, 32, 44, 56 and 68m, except for D9 for

which 29, 63, 89, 107 and l59m were used). The frequency band analyzed

was 0.8 to 1.2 cpd. The first mode accounts for 32.5% of the total

variance in the spectra and represents the large scale coherent inertial

signal over the upper lOOm of the study area. Phases and amplitudes

consistently increase and decrease, respectively, with depth. The

upward propagation of phase (positive phase angles lead) is consistent

with surface generation and downward propagation of energy. Table 6.5-1

shows the mode 1 phase differences between the lowest and highest levels

at each station. The least square estimated vertical wave lengths range

from about 90 to 190m, and increase with the depth of water. The shelf

break moorings show approximately 180° top-to-bottom phase differences

as expected because of the shelf continuity constraint (Kundu 1984).

The horizontal distribution of amplitudes and phases for mode 1 are

shown in Figure 6.5-5 for the depth of 30m. The other depths have

similar patterns. The highest and lowest amplitudes are found on the

offshore edge of the array and the eastern side of the canyon rim (Dl

and El), respectively. The phase distribution shows offshore

propagation from the shelf break except around 83 which seems to be a

focal point for propagating inertial waves. The trapping of energy in

this area is also suggested by the high amplitudes found there.

245
60

40

20

0 20

40

60

31

Jmnuary 1999

U (shaded) & V (solid) Velocity Components

4..&

Cl

16m

Cl

40m

Cl

68m

03

16m

03

40m

03

68m

Figure 6.5-3. 3--HLP time series of cross-isobath (U) component (dashed

and shaded) and along-isobath (V) component (solid line)

at selected depths on the indicated moorings during a

two-month period in winter 1999. The top panel shows the

40-HLP wind stress vectors from buoy 42039.

246

1.5 Wind Stress NDBC Buoy 42039

1.0 -

.5 -

,.

.5-s , if

1.0 -

1.5

10 20 30 9 19 February
20- -D - ci) 15- ci) - U) - 030- ci) 25- 20- 15- 10 40- 040- 30- 'A ''L, 20m ckj \\ ./ 8Om--- Inertial Amplitudes Mooring Cl Mooring Cl 32mBURLI Mooring C3 32m 11 30 MA Figure 6.5-4. Bottom two panels show the inertial amplitude at the 32-rn depth of the Cl an

The third panel shows the 40-HLP temperature records from the 20-rn (dashed) a

(solid) depths at Cl, and the fourth panel shows the 40-HLP wind speed at the 

meteorological station. 19 H 19 J A 10 30 A H J 1997
Clockwise Inertial Amplitudes at 30m Mode I - 32.5% Variance

90W 89W 88W 87W 8BW

31N

a)

85W

I 31N

5/10 TO 8/17/1997

31N

5/10 TO 8/17/1997

Clockwise Inertial Phases at 30m Mode 1 - 32.5% Variance

Figure 65-5. Mode 1 EOF amplitudes in cm/s (a) and phases in degrees

(b) calculated from the inertial band of upper-layer

clockwise spectra for the summer of 1997. Sub-sets for

the 30-rn depth level are contoured.

248

28N

90W 89W 8W 87W 8W 5W
The mean flows for the upper-layer in the summer of 1997 are given in

Figure 5.4-6. An instantaneous geostrophic current map for early July

is given in Figure 5.4-lb. They indicate that the offshore region is

between the northern edge of a cyclone and the eastward flowing upperslope jet. This should generate a region of negative relative vorticity

that may be trapping the offshore propagating inertial energy (Kunze

1986) . Thus, eddy and jet flows may in some circumstances enhance

inertial oscillations over the slope over those at the shelf break.

Figure 6.5-4 seems to indicate that such offshore enhancement is the

rule. An estimate of the horizontal wavelength from a least square fit

of the 30 and 65m level phases gives 580 ±100 km directed along

l55°±20°(T) . Since the horizontal group velocity is proportional to the

wave number, the large estimated horizontal wavelength implies the

energy was primarily propagating in the vertical direction.

This preliminary investigation into non-hurricane inertial oscillations

has shown that they dominated the high-frequency spectrum and were

present in offshore waters throughout the year. Their spatial structure

was complex because of the characteristic intermittency and the velocity

and vorticity structure of the upper-layer flows on the slope. This is

in contrast to the Texas-Louisiana slope where Chen et al. (1996) found

that the inertial energy decayed rapidly seaward of the shelf-break.

Table 6.5-i EOF Node 1 inertial oscillation analysis vertical wavelength

estimates from Node 1 phase differences

249

Water Depth (rn)

::.BOtt0mQP. Lëv1:

::::.phs Differences., .Welength (rn)

Al 100 190 105

A2 500 140 131

A3 1300 132 148

Bl 100 174 110

B2 500 149 127

1300 122 144

Cl 100 124 146

C2 500 123 159

C3 1300 121 157

Dl 100 230 86

D9 200 251 192

D2 500 153 120

El 100 172 115

All - - 153
VII. CONCEPTUAL MOX)ELS OF CIRCULATION PATTERNS

7.1 Introduction

The material presented in this chapter is a first effort to develop an

integration of some of the observations made and used in this project.

It is the beginnings of a conceptual model that will describe many of

the various circulation patterns in an idealized/conceptualized model.

The study region has many complex oceanographic processes of varying

time and spatial scales; this is but a first attempt at a descriptive

characterization.

7.2 Modes of LC and LC Rinqs

The discussion in Chapter 5 of Loop Current Frontal Eddies (LCFE), in

conjunction with Table 5.3-1 that summarizes LCFE physical

characteristics, reveals certain consistent and repeatable patterns in

the evolution of observable features in the SST and SSH fields, and the

responses in the flow and temperature measurements of the moored array

that suggest distinct modes of interaction. The physical processes

controlling these modes of interaction will be discussed with the aid of

conceptual models.

Mode 1. irect Interaction from LC or LC Ring

Mode la. Mode la (Figure 7.2-la) represents direct interaction of the

northern edge of either the LC when it is near its maximum penetration

into the eastern Gulf, or, a warm ring that is directly to the south of

the array. In either case, the strong eastward flow near the northern

boundary of the LC/ring will cause strong eastward flow and negative

offshore temperature gradients in the array from near surface to at

least 500m. A schematic model of this process is shown in Figure 7.2-la.

A good example of the current response to a warm intrusion of this type

occurred for most of Period 7 (See Chapter 5) from early November 1998

to early January 1999 as shown by the strong, persistent eastward

alongslope flow at all sites throughout the period (Figures 5.2-lb and -

2d).

Mode lb. Mode lb is a form of remote ring interaction with the steep

slope. SST imagery together with moored current vectors indicate that

when a remnant warm LC ring interacts with the slope west of the

Mississippi River delta, a warm eastward jet can develop over the slope

and extend into the array region. A schematic representation of this

process is shown in Figure 7.2-lb. The impingement onto the slope of

strong eastward flow within the ring will set-up a negative alongslope

pressure gradient that would be trapped over the slope by the steep

topography and thus drive a warm eastward jet along the slope. A clear

example of this mode of interaction occurred during November 1997

through January 1998 (Figure 5.2-6) and resulted in strong eastward flow

over the upper 500m that continued for about 3 months at most sites in

the array (Figures 5.2-lb and --2b)

251
28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W 82°W 80°W

20°N

90°W

--

88°W 86°W

Surface jet

84°W 82°W 80°W

8°N

6°N

DN

Figure 7.2-la. Conceptual model of direct LC or LC ring flow

interaction with the slope in the study region The

black solid line defines the boundary of the LC and a

LC ring. The dashed line within the LC ring

illustrates the pattern of currents below 200 in in the

water column. The solid arrow through the mooring

array illustrates the direction of flow at water depths

of less than 100 m in the array.

252

_,- - - --- ._# .

'----

S.,..4, I II l JItI_.I II L VVQkl

- depths 20Om

C)'

So

ace

\ /

r

2

Surface boundary

of LC ring

r -

CUBA

12

Yucatan

Pen.

ucatan'

hnTiè1

,- _: 2

2i

30°N 30°N
28°N

265N

24°N

22°N

90°W 88°W 86°W 84°W 82°W 8OW

20°N

90°W

_tct\_

r00' ,SS'

88°W

Eastward directed

surface jet with small

scale boundary eddies as

.2 seen in SST imagery

°Q00'i,

Current pettern at

water depths S

2OOm

86°W 84°W 825W

28°N

26°N

24°N

22°N

20°N

80°W

Figure 7.2-lb. Conceptual model of interaction of a LC ring located

south and west of the Miss. Delta with the continental

slope. Interaction creates eastward directed surface

jet with small-scale eddies on either side.

253

30°N . 30°N

-5; 4

a U, I
Mode 2. LC frontal Eddy Interactions

Mode 2a. One of the more common events causing strong flow and

temperature variability within the moored array comes from the

interaction of LCFE'S with the slope. Mode 2a represents the interaction

that occurs from LCFE's moving around the northern boundary of a mature

LC. This process is shown schematically in Figure 7.2-2a. LCFE's can

have horizontal scales of several hundred km's and extend to depths of

600 to 800m. The cyclonic circulation in these features entrains warm LC

water around their cold cores in the upper lOOm. As the eddies move to

the east around the LC and south of the array, they leave a long

trailing warm streamer of upper level LC water that extends into the

moored array. Isobaths curvature in the array shifts from a east-west

orientation in the western part to more SE-NW alignment in the eastern

part of the array. Thus, the northwest extension of a warm streamer will

cross isobaths into shallower water. As a result the warm upper layer

will shrink and vorticity constraints will cause an anticyclonic

rotation. The Streamer continues to pump warm water into the array

region as the LCFE moves to the southeastcausing the eventual spin-up of

a warm anticyclonic eddy over the array. This produces an eastward flow

with a negative cross-slope temperature gradient in the upper lOOm of

the array. Below that depth, the cyclonic circulation in the LCFE causes

strong, persistent westward flow. LCFE's 1, 2, 4, 5, 11 and 12 (Table

5.3-i) all produced this type of current and temperature response during

their passage south of the array. Examples in SST imagery are given in

Figures 52-3a, -10, -15, and -16. Intrusions of these warm streamers

can sometimes result in strong up-canyon flows in the DeSoto Canyon

(Figure 5.2-lOa) . Offshore flows can result from the convergence of the

trailing wake of a LCFE with an approaching ring or LC crest (Fig-ure

5.2-l0a) . Both processes can result in significant cross-slope exchange.

Mode 2b. Mode 2b is essentially the same as Mode 2a except the LCFE is

located on the boundary of a LC ring south of the array instead of the

LC itself (Figure 7.2-2b). LCFE 9 appears to have this mode of

interaction.

Mode 2c. Mode 2c occurs when a LCFE is located close to the array and

is directly interacting with the slope (Figure 7.2-2c) . The warm

streamer is more parallel to the slope bathymetry and is advected to the

west around the cold core of the eddy. As a result the flow is westward

throughout the array both above and below lOOm. LCFE's 3 and 8 were of

this type (Figure 5.2-9)

Mode 2d. Mode 2d represents the type of flow interaction that can occur

from a LCFE on the eastern boundary of a warm ring, where its southward

movement is blocked by a growing LC (Figure 7.2-2d). The LCFE produces

westward flow in the deeper waters below 200m and eastward flow results

in the upper lOOm from the anticyclonic eddy that spins-up over the

array from the warm streamer. Blockage of the movement of the LCFE can

result in a near stationary eddy and prolonged influence on the flows in

254
30°N

28°N

26°N

24°N

22°N

I

90°W

20°N

90°W

.

88°W

71

Surface

anticyclonic

flow

Yucatan

Pen.

860W

Cyclonic

circulation - pattern below

2OOm

C,

84°W

S

82°W

traifs

of FIofd'

CUBA

Yucatañ: :inn1

88°W 860W 84°W 82°W

80°W

30°N

28°N

260 N

24°N

22°N

20°N

80°W

Figure 7.2-2a. Conceptual model of a LCFE flow interaction with the

slope in the study region

255
28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W 82°W 80°W

4.

20°N

90°W 88°W 86°W 84°W 82°W 80°W

8°N

6°N

4°N

Figure 7.2-2b. Conceptual model of flow interaction with the slope in

the study region from a LOFE on the boundary of a warm

LC ring.

256

_,j4..

.:k4

__S_, - -, ,eth2i

'2000m LC Ring

wI Frontal

Eddies

-

, ,)

":.

'y,

A 1

bill \

-

\Arm Filament

2

I

ca4, 2

CUBA

!4----.

- Yucatan

Pen.

-- catan;-

j-#1ê1'

----: 'LEE-= c-,

2

30°N 30°N

\ Surface Anticyclone

a,'
3ON

28°N

26°N

24°N

22°N

90°W 88°W 86°W 84°W 82°W 80°W

20°N

900w

Warm

FIament 30°N

8°N

6°N

4°N

2°N

O°N

80°W

Figure 7.2-2c Conceptual model of flow interaction with the slope

from a LCFE close to the study region.

257

t4 - -"I

"S

2

2

- -

'---- /

'_5_ Iorida-"

2

-rI CUBA

"-"--:-Y

t

2

Yucatan

Pen.

2

ft

Cyclonic circulaltion at ., water depths 2OOm

88°W 86°W 84°W 82°W
in upper lOOm

$ Cyclonic currents

atwater depths

2OOm

Figure 7.2--2d. Conceptualization of flow interaction with the slope

from a LCFE on the boundary of a warm LC ring located

south of the Mississippi Delta and blocking the LC.

258
the study region. LCFE's 3 and 10 had this mode of interaction (Figures

5.2-7, -9 and -10)

7.3 Reqional Scale Patterns

The previous sections have discussed how LCFE's affect circulation on

the lower slope, and contribute to the generation of anticyclones. The

presence of eddies on the lower slope generates circulation patterns on

the upper slope that can persist for periods of order weeks to months.

The most characteristic flow pattern is a two-layer jet with eastward

flow at the surface and a return flow at depth. The transition between

the upper and lower flows varies with the offshore forcing but is

typically between 200 and 300m. The upper-layer jet along the Alabama

slope can take various paths through the DeSoto Canyon, including

bypassing the head of the Canyon or at the Florida slope. In the former

case, the head of the canyon is often occupied by one or more eddies

that can be cyclonic or anticyclonic. These jet-like flows are

efficient at transporting Mississippi River water discharged near the

delta out over the slope, and on occasion, injecting it onto the west

Florida shelf.

The westward counter flow at depth is more complex and may have several

causes including being an entrainment flow into a deep offshore cyclonic

eddy, and the blockage of offshore eastward or northward flots by the

west Florida escarpment that seems to turn these flows cyclonically into

a westward current along the slope. This type of cyclonic turning at

depth is often accompanied by a surface-layer cyclonic eddy in the head

of the canyon. A sketch of this flow pattern is given in Figure 7.3-1.

If a cyclonic frontal eddy associated with an extended LC or a recently

detached LC ring moves onto the slope then the predominant eastward

flows at the surface are replaced by westward flows of the cyclone.

Superimposed on these relatively steady flow patterns are fluctuating

eddy-like currents that can alternately reinforce and oppose these

circulations. The energetic parts of the spectrum can vary with

position and season and the relatively poor coherence between adjacent

moorings indicates that a good portion of these type motions are

transitory and not well resolved even with the close spacing of the

array. At long periods of -100 days, there is evidence of the coherent

propagating signal. This may indicate that the slope is acting as a

topographic wave-guide allowing large-scale long-period disturbances,

that may be initiated in the northeast Gulf, to propagate onto the

Western Gulf slope.

Because the IC eddy shedding cycle is variable and does not have an

annual component, the eddy field over the slope does not have a specific

seasonal component. However, there is a difference in the circulation

characteristics between summer and winter caused by wind-forcing. In

winter (-'Lecember through March), there are frequent synoptic weather

systems passing over the northeast Gulf. These systems produce

energetic along-isobath, upper-layer flows that follow the canyon rim

259
i 

ETtLt: jin : ;c :q )jtr :ipus .; .1v 'flqr4r
with periodicities from -P3 to 15 days. These flows are largely absent

in the summer. The wind component that is most effective in generating

this slope response is directed parallel to the west Florida slope. The

maximum current fluctuations are found over the Alabama slope with a lag

of order one-day. The response is qualitatively that of an arrested

topographic wave with that maximum response downstream (in a continental

shelf-wave sense) of the maximum along isobath wind-stress. This is

attributed to the relatively narrow shelf such that the cross-shelf

length scale for continental shelf-waves extends over the slope. This

is in contrast to the wide Texas-Louisiana shelf and slope where shelfedge currents have little correlation with winter-storm winds (Nowlin et

a1 1998)

Finally, the high frequency currents are dominated by inertial

oscillations, with periods of -1 day, that are present in deep water

throughout the year. At tho shelf break, inertial oscillations are

present in the summer but not in the winter because of the lack of

stratification in the latter. There is some evidence that inertialinternal wave energy is being trapped by the eddy-jet current systems of

the slope. Hurricanes passing over the slope in summer produce a strong

inertial response, which can persist for many days.

261
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269
APPENDIX

A

271
APPENDIX A.

I.CFE 1: When LCFE 1 was first observed in the SSH field of January 1,

1997 it was centered at about 26.8°N and 88.8°W and was already welldeveloped on the north side of a recently separated LC ring. During

January and February the southward movement of this LCFE around the LC

ring was blocked as the LC ring re-merged with the LC. Following this,

the LC grew to the north and forced the LCFE to the northeast and

closer to the moored array. LCFE 1 first began to influence the flow and

temperature variability in the moored array in mid-March when a warm

streamer was observed to wrap around the east side of the LCFE and

extend into the array, causing strong eastward flow at A3 during March

and April (Figure 5.2-3a). At this time the northern edge of the LC was

at about 26.5°N or about 280 km south of the array. A smaller scale (100

km) cyclonic eddy was observed to develop on the warm front of the

streamer causing cross-shelf exchange and recirculation within the

array. During May and June the warm streamer appears to spin-up into a

ACE over the array and LCFE 1 appears to have remained south of the

array as estimated from the SSH fields. This causes strong eastward flow

in the upper lOOm at shelf break and mid-slope stations, and westward

flow below 200m and at the 1500m stations. From mid-June to mid-August

1997, LCFE 1 translated around the eastern side of the LC at an

increased forward speed of about 5 km/day. Its influence on observed

currents appears to diminish in mid-July just as LCFE 2 approached the

array. LCFE 1 was no longer observable after mid-August.

LCFE 2: LCFE 2 spun up just south of the array in the trough left by the

departure of LCFE 1 around July 1, 1997. This eddy rapidly increased in

size and continued the westward flow at the offshore sites and below

200m. LCFE 2 influenced the study area from mid-July to mid-September.

Eddy 2 appeared to divide in mid-August with part moving away from the

array to the southeast, travelling along the eastern side of the LC,

which had an extreme tilt to the northwest at the time. The portion of

the eddy that broke away was no longer identifiable in SSH by midOctober.

LCFE 3: LCFE 3 consist of the portion of LCFE 2 that remained near the

array when it separated and moved southeast in mid-August 1997. LCFE 3

was located near the array in early to mid-September and caused westward

flow at most locations. In mid-October this eddy started moving offshore

along the eastern side of a warm ring that had separated previously from

the LC and was located close to the shelf break southwest of the delta.

Interaction of the warm ring with the steep topography appears to force

a warm eastward jet that extends into the array causing strong eastward

flow to depths of 500m. However, ring separation was not completed,

which blocked the southward movement of the eddy causing it to remain

near 27°N till mid-December at which time it began moving to the

southeast around the LC at a speed of about 12 km/day (Figures 5.2-6 and

-7), while LCFE 4 was approaching the LC from the southwest. A warm

streamer wrapped around LCFE 3 and into the vicinity of the moored

array. This streamer appears to have spun-up into a ACE over the array

causing strong eastward flow to continue for most of Period 3. After

mid-January this eddy was no longer identifiable in SSH fields.

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LCFE 4: LCFE 4 was first observed on the west side of the LC in early

December 1997 (Figure 5.2-7) and moved into the region south of the

moored array in early January 1998. The eddy continued to move around

the LC at speeds reaching 17 km/d and was midway down the LC off the

west Florida shelf by early February 1998. The warm streamer from this

LCFE continued to pump warm water into the upper layer of the study area

arid helped to maintain the eastward jet in the array. It is believed

that the warm ring south of the delta would have produced eastward

currents over the slope west of the array that may also have contributed

to the warm eastward jet that occurred within the array.

LCFZ 5: LCFE 5 was first observed on the west side of the LC on January

11, 1998. It moved around the LC reaching the crest just to the south of

the array on February 5 (Figure 5.2-8) and was midway down the eastern

side off the west Florida shelf by February 25. The rapid passage of

this event with a translation speed of about 17 km/d caused increased

variability of currents in the mooring array with transient westward

flow at depths greater than 200rn from about February 5 to 18, 1998.

LCE 5: LCFE 6 was first observed in SST data on January 16, 1998. It

moved up the north side of the LC and was near the array on February 25.

The eddy influenced the study region from March 1 - 24. The LCFE

enlarged to about 280 km in the east-west direction and caused

significant westward flow in the mooring array as the warm filament was

carried around the north side of the eddy and into the array (Figure

5.2-9) - The cyclonic circulation of the eddy also appears to cause

significant offshore transport of cooler slope water into the eddy core.

The eddy moved rapidly around the east side of the LC and appears to

have participated in separation of a large new ring toward the end of

March 1998.

LCPE 7: SST imagery indicates that LCFE 7 was formed on the outer part

of the Campeche Bank on February 5, 1998. It moved northward along the

westward boundary of the LC reaching maximum size of 110 x 55 km on

February 12 while still well south of the mooring array. On February 25

it had reached the northwest corner of the LC and was extending a long

streamer into a remnant warm ring to the southwest. At a speed of about

24 km/d the eddy appears to catch up with and shear apart in the trough

of LCFE 6, without impacting the currents in the moored array.

LCFE 8: LCFE 8 was first visible in SST images at the northwest corner

of the LC on March 4, 1998. By March 20 the eddy was south of the array

and had increased in size to 244 x 240 km (Figure 5.2-9) . Westward flow

on the north side of the eddy was causing strong westward flow within

the moored array to depths of 500m. At this time the LC had separated

into a large warm ring and the eddy was moving slowly to the east along

the northern boundary of this ring at about 2.4 km!d. In early April,

the southward movement of the eddy around the ring was blocked by the

new growth of the young LC, Also a streamer extending around the eddy at

this time appeared to have two smaller scale (100 km) cyclonic eddies

form near the western tail and contributed to cross-slope exchange. On

April 12, a large streamer wrapped around the east and north side of the

eddy and into the array, where strong westward flow from the eddy

continued (Figure 5.2-lOa) . By April 25, the eddy was on the eastern

273
side of the ring and had enlarged to 300 x 245 km. The warm streamer

intruding into the array was causing flow splitting, with onshore flow

on the line B, westward flow at line A and northeast, up-canyon flow

along the lines C and D. In early May, the eddy appears to participate

in further separation of the ring from the emerging LC and was no longer

clearly identifiable in the SST fields. However, SS}{ data generally

supports the same type of feature identification and indicates that the

eddy was absorbed in the growing trough of ring separation in early

June. The SSH fields also show the general pattern of the IJCFE and

streamer influencing westward flow in the array throughout March and

most of April 1998. The cyclonic circulation within the eddy appears to

have had a strong effect on movement of slope water into the eddy core,

causing enhanced cross-slope exchange (Figure 5.2-ba) - LCFE 9: LCFE 9 was first observed in SST imagery March 23, 1998 on the

southwestern extreme of the recently separated LC ring. It moved rapidly

to the north along the ring western boundary at a speed of about 16

km/d. It reached the northwest area of the ring on April 12 and had

dimensions of approximately 110 x 110 lan (Figure 5.2-bOa and -11) - Its

movement slowed to about 10 km/d as it traveled around the northern part

of the ring and was south of the moored array in early May with a ridge

wrapped around its eastern side and into the array. The ridge appears to

have spun-up into a ACE that seems to have caused the eastward flow at

the shelf break stations and westward flow offshore and at depths deeper

than 200m. The combined influence of the newly formed ring and the young

LC appears to have formed a ridge tilting toward the northwest. The SSH

fields indicate that a series of LCFE's merged along the eastern side of

the ridge during June 1998 and are difficult to identify separately in

the imagery.

LCFE 10: SSH fields indicate that during July 1998 a cyclone formed

south of the array on the northern side of the ring that separated in

March. A ridge wrapped around the eastern side of the low and into the

array (Figure 5.2-12a). The ridge intensified into an ACE near the array

which caused eastward flow in the array that continued for most of

Period 5. Westward flow developed below 300m for part of that period,

presumably due to the cyclonic circulation in the offshore eddy. The

cyclone remained more or less stationary until early February 1999.

Apparently the interaction of the ring with the young LC caused a ridge

that prevented movement of the cyclone around the ring.

LCFE 11: LCFE 11 was observed in SET fields on January 7, 1999 south of

the array at about 26.5°N. A warm filament extended from the eddy to the

region of the array and appears to have developed into an warm ACE. SST

imagery of January 25 shows the eddy on the east side of the LC and a

large streamer extending into the array (Figure 5.2-15) - Apparently,

only the northern edge of the ACE reached the offshore Sites and caused

eastward flow. Flow at the mid-slope and shelfbreak sites did not appear

to be affected by this ACE. By February 8 the influence of the ACE on

the study region was no longer apparent. Translation speed around the LC

was approximately 10 kin/d. On March 2 the eddy was located near 25°M and

participated in narrowing of the LC neck, an indication of potential

ring separation.

274
LCFE 12: LCFE 12 was first observed in SST fields of February 5, 1999

southwest of the array on the northwest curvature of the LC. SSH data

show the eddy to be well-developed south of the array in mid-February

This eddy moved eastward around the north side of the LC at a speed of

about 9 krri/d. A ridge developed around the east side of the eddy and

extended into the array area. The SST data shows this ridge to consist

of a warm filament that contributed to the ACE in the array area,

producing eastward flow at the offshore mooring sites from February 20

to March 10 (Figure 5.2-18). The ACE moved southwest out of the array

area in mid-to late March as the eddy moved down the eastern side of the

LC, which was growing northward at the time. The eddy became difficult

to detect in SSH data after April 1999.

275
The Department of the Interior Mission

As the Nations principal conservation agency, the Department of the Interior has responsibility for

most of our nationally owned public lands and natural resources. This includes fostering 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 territories

under U.S. administration.

The Minerals Management Service Mission

As a bureau of the Department of the Interior, the Minerals Management Services (MMS) primary

responsibilities are to manage the mineral resources located on the Nations Outer Continental Shelf

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute

those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound

exploration and production of our Nations offshore natural gas, oil and other mineral resources. The

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and

accurate collection and disbursement of revenue from mineral leasing and production due to Indian

tribes and allottees, States and the U.S. Treasury.

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for

all Americans by lending MMS assistance and expertise to economic development and environmental

protection.