DeSoto Canyon

Eddy Intrusion Study

Final Report

Volume 1: Executive Summary

U.S. Department of the ?nterior

rIioraIs Manement Service Gull of MBxi OS Rion

OCS SL.d

Mr1S 2000-079
DeSoto Canyon

Eddy Intrusion Study

Final Report

Volume I: Executive Summary

Authors

Thomas J. Berger

Peter Hamilton

James J. Singer

Evans Waddell

Science Applications International Corporation

James H. Churchill

Woods Hole Oceanographic Institute

Robert Ft 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 2000-079

New Orleans

November 2000
DISCLAIR

This report was prepared under contract between the Minerals Management

Service (MMS) and Science Applications International Corporation (SAIC) - This report has been technically reviewed by the MNS 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 MMS

editorial standards.

REPORT AVAILABILITY

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

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

CITAXON

Suggested Citation:

Hamilton, P., T.J. Berger, J.J. Singer, E. Waddell, J.H. Churchill, R.R.

Leben, T.N. Lee, and W. Sturges. DeSoto Canyon Eddy Intrusion

Study; Final Report Volume I: Executive Summary. OCS Study

MMS 2000-079. U.S. Dept. of the Interior, Minerals Management

Service, Gulf of Mexico OCS Region, New Orleans, LA. 37 pp.

ABOUT T 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 sguares) 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 SSH. 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
ACKNOWLEDGEMEiS

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 Harris

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

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

provided administrative 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 Mf4S Contract Officer's Technical Representative, during

all phases of this project is gratefully acknowledged.

V
TLE OF CONTENTS

VOLTi I PAGE

ACKNOWLDGEMENTS

LIST OF FIGURES ix

LIST OF TAELES xi

1.0 INTRODUCTION 1 1.1 Background 1 1.2 Project Objectives 2 2.0 MEASUREMENTS MJD OBSERVATIONS 5 2.1 Methods 5 3.0 LOOP CURRENT DYNAJ1ICS 11

3.1 Introduction 11

3.2 LC Metrics and Statistics 11

3.3 LC Energetics and Dynamics 14

3.3.1 Introduction 14

3.3.2 Ring Separations 14

3.3.3 LC Forcing by the Wind Curl 17

4.0 SLOPE CIRCULATION PATTERNS 21

4.1 Introduction 21

4.2 ypes and Sources of Eddy Influences 21

4.2.1 Loop Current Frontal Eddies 21

4.2.2 Cyclonic and Anticyclonic Eddies 29

4.2.3 Mean and Varying Circulation Patterns 29

4.2.4 Slope Dynamics and Fluxes 33

4.2.5 Deep Currents 33

5.0 ATMOSPHERIC FORCING 35

5.1 Synoptic Scale Forcing 35

5.2 Seasonal Wind Response on the Slope 35

5.3 Canyon Response to Hurricanes 36

5.4 Non-hurricane Inertial Currents 39

6.0 SUMMARY 41

vii
LIST OF FIGORES

Fiqure No. CaJ)tion Paqe

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 12-1. 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

2.1-1 Map showing standard hydrographic grid for the

DeSoto Canyon Eddy Intrusion Study with feature

survey stations for December 1998 cruise

(Stations 81-87) and all 13 current meter mooring

locations 6

2.1-2a. Timeline of data return by instrument level for

the indicated DeSoto Canyon Eddy Intrusion Study

moorings 8 2.l-2b. Timeline of data return by instrument level on

indicated moorings for the DeSoto Canyon Eddy

Intrusion Study. 9 3.2-la. Time series of Loop Current area, volume, and

circulation for 1/1/1993 through 4/30/1999. 12

3.2-lb. Time series of Loop Current northward and westward

penetration, and length for 1/1/1993 through

4/30/1999. 13

3.2-2. Percent occurrence of LC waters for the interval

from 1/1/1993 through 4/30/1999. 15

3.2-3. Mean SSH for the interval from April 1997 to

April 1998. 16

3.3-1. Periodicity of ring separations from the Loop

Current. 18

33-2. Power spectra of the north-south motion of the

Loop Current and of wind stress curl over the full

Gulf of Mexico. 19

33-3. Cross spectra between the wind curl and Loop

Current variability.. 20

ix
luST OF FIGURES (continued)

Ficrure No. Caption Paqe

4.2-1. SST image for April 12, 1998 (0247 CST) . 22

4.2-2. Conceptual model of a Loop Current frontal eddy

flow interaction with the slope in the study

region. 24

4.2-3. SST image for April 30, 1997 (1355 CST) 25

4.2-4. Conceptual model of flow interaction with the

slope in the study region from a lcfe on the

boundary of a warm LC ring. 26

4.2-5. Conceptual model of flow interaction with the

slope from a Loop Current frontal eddy close to

the study region. 27

4.2-6. Conceptualization of flow interaction with the

slope from a Loop Current frontal eddy on the

boundary of a warm LC ring located west of the

Miss. delta and blocked by the Loop Current. 28

4.2-7. Geostrophic velocities at 6m from the

hydrographic surveys.. 30

4.2-8. Geostrophic velocities on Section B for the

indicated surveys.. 31

4.2-9. Seven-DLP current vectors and isotherm depths

for the upper 300m of C2. 32

5.3-1. Comparison of inertial current magnitudes seen

near the surface at moorings C3, D2 and El

during late summer-early autumn 1998 37

5.3-2. Surface wind stress together with near-bottom

and near-surface inertial current magnitude

observed at the indicated moorings over the

study period. 38

x
LIST O TABLES

Table No. Caption Paqe

3.2-1. Statistics for Loop Current metrics

(1/1/1993 - 4/30/1999) 14

xi
1. INTRODUCTION

1.1 Backcround

The iJeSoto Canyon Eddy Intrusion Study will provide the Minerals

Management Service (MMS) with information and analysis 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 MMS. 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 climatological circulation

characterization of the area (see Figure 1.1-1 as an example) - To gather the data needed tb 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 DeSoto 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 S and others are sponsoring in the

northeastern Gulf.

1.2 Project 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, and mass fields associated with the eddyslope interactions.

1
Gulf of Mexico Sea Surface Height

,pu TOPEXIES-2 sea surfacE height aromay

pus 10-year c1irnatoIogic model mean

25

U:

-25

FLt-it 1.1-1. c i.ir ea BrLc:e cit wit xurp1

lhe yp d of ecdy-1ik t.urc' tt r ur in Ut d rtheatrr: j1f of Mexiro. Th ad ar oter 3ti1f cui hvr i 'ric irt nri eddy iiLd.
DeSoto Canyon Eddy Intri..jsor Study

. C.tt ttr tlrt,;

iUA I .J-1. Uae JI the .tuy with c.or nd mgA 1

0!: the bed. :ce (wxmr co çhrri1er

by te.1i:e 41tiuctr, vrrt! ky 3cgrAph

teazes &re on t cc crc of I ot: ths

tnd the V.cr..d ,.l1 .irc }±wn th.? i.cain o curr?t. -te ortc d xctI iri ng

th tc'. :uyI diy 1t.o study.
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 (Figure 1.2-1). The general

location and configration of moorings were specified in the Scope of

Work provided by the NNS. 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 Mt4S is funding Texas A&M University to conduct

a study titled " Northeastern Gulf of Mexico Chemical Oceanography and

Hydrodgraphy Study" (Contract No. 1435-01-97-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

(CSA).

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

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 - UMiami) - 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
2.0 MEAST.TRNTS AND OBSERVArIONS

2.1 Methods

The DeSoto Canyon Eddy Intrusion Study involved acquisition and use of a

broad range of ocean and remotely sensed measurements over a two-year

field season. Measurements were begun on March 18, 1997 and completed on

approximately April 6, 1999. Field activities were conducted during

seven mooring maintenance and hydrographic cruises conducted

approximately every four months over the two-year interval.

Types of field measurements included:

In-situ obsex-v-atjons

Current velocities using normal current meters that measure

velocity and temperature at the level of the instrument (Figure

2.1-1);

Acoustic Doppler current profiler (ADCP) which use a transmitted

acoustic signal to estimate current velocities over a range of

depths above the transducer,

Stand alone thermistor that record temperature time series at

positions between other instruments;

Conductivity/temperature sensors that can with data conversion

provide time series estimates of temperature and salinity; and

Ship-based survey observations

As part of the seven ship surveys, profiles of conductivity (salinity)

and temperature were measured over a repeated grid of stations (Figure

2.1-1) . It was also during these cruises that drifters were often

released. To supplement the regular grid hydrography taken during the

seven cruises, two special studies/surveys were conducted to help

rosolve finer scale features that may occur over the slope in the study

area. Remotely sensed (satellite-based) observations

An important element in reaching the goal of this program was access to

remotely sensed sea surface temperature (SST) and sea surface height

(SSH) observations. A multi-spectral radiometer on a polar orbiting

satellite provided observations that were calibrated to estimate the

temperature of the sea surface (skin) temperature. Given a clear sky

(no clouds to intercept the thermal radiation), this provides at least

twice daily essentially instantaneous SST patterns in the eastern Gulf

of Mexico. When ocean surface temperature gradients exist (nominally

October - May) SST patterns help identify evolving surface current

patterns. The IS has a contract with the US Geological Survey (USGS) to

provide, via the Internet, navigated and calibrated images of SST in the

eastern Gulf. These images are publicly accessible at:

http://coastalerusgsqov/et qulf/html/gst.hml

S
33 O°N 29N 2899 O°W 90°W 8 9W/ 8 9DW 6 9 LI 78[I] 70 AD> LI - B2 LI D2 75 LI 074 LI C3 A 880W 88W C / 87°WLJ LIE LI 0 D9 E1 0 S LI LI LI LI 0 87GW LI 86GW 

8 6W 8 

Figure 2.1-i Map showing standard hydrographic grid for the DeSoto Canyon Eddy Intrusion Study with feature survey stations for December 1998 cru

(Stations 8l-87) and all 13 current meter mooring locations (solid circles) 

- 
SSH data are provided by at least two satellites having different

coverage patterns and schedules. The resulting altimetry database has

been integrated into an estimate of the Gulf-wide pattern of SSH. This

methodology, which has been evaluated against actual sea surface current

patterns (e.g. drifting buoy trajectories) is best left for description

in the main body of this report. Suffice it to say, the data processing

scheme produces a once a day estimate of the SSH. Using these height

estimate, the pattern and magnitude of pressure driven (geostrophic)

surface currents can be estimated. These SSH images are publicly

available from the following Internet site:

http: //www-ccar.coloradc.edu/realtime/gom/qomhtm

The processing of the altimetry as well as maintenance of this Internet

site were and are being done by the Colorado Center for Astrodynamics

Research (CCAR) at the University of Colorado, one of the participants

in this project.

Lagrangian observations

Drifters that move with the ambient surface currents and

periodically report their positions via a satellite link.

Al). the above types observations are supplemented by enviromnental

observations being made and reported routinely by NOAZ/NWS and

NOAA/1IDBC, such as coastal and at-sea wind velocity, and air and water

temperatures, hurricane trajectories and characteristics and coastal

sealevel.

As illustrated in the project time lines (Figures 2.l-2a,b), excellent

data return was maintained by the field measurement program. As a

result, a comprehensive rrtultivariate data set was available to the

program Principal Investigators to help reach the program's goal of

describing oceanographic features and processes occurring over the

continental slope in the oceanographically complex DeSoto Canyon region

of the NE Gulf of Mexico.

7
Figure 21-2a. Timeline of data return by instrument level on the

indicated DeSoto Canyon Eddy Intrusion Study moorings.

Arrows indicate the approximate dates of the

hydrographic cruises -

8

Measurement Timeline

IJeSoto Canyon Eddy Intrusion Study

I I I I I I I

I I I I I I I NotDeplcyed 1

I I --

1

-

-

--

-

-

-

-

-

-

-

-

DSC-A1-D lS,n 0/05

Not De.lo ed DSC-Al-9 20,,, Mk2 550-Al -1 80 ADCP 4- ISOC-Al -2 9Dm S4 4

I Battery Failure Battcry Failure DSC-A2-1 52m TEMP. F I DSC-A2-2 SUm ADCP DSC-A2--3 15Dm I : DSC-A2-4 20Dm RCM 4 Bad DSU I DSC-A2-5 25Dm TEMP F , SOC-AZ-S 30Dm RcM I 050-A2-7 490nm Mk2/54 - r 1 Icck I- - I DSC-A3-t 80,,, ADCP -1

050-A3-2 50Cm Mit2/RCM Wing 5(0011

I' D50-A33 131Cm RCM/Mk2 j Not Dcpteyed Lost Not Deployed DSC-81-t 20,,, CTD F 4 NotDeploycd

b 4 5 4 . NotDeployed9 : DSC-1-9 62m crDn4ja/ Lssl Not Deployd DSC-BI-2 8Cm 'oci' I: Furled Lost Not Deployd

550-81-3 82,,, Cl/CrD - 1 DSC-Bt-4 95,', RCMJS4 I Otetor I 4 Lost I DSC-B2-1 52m I1DMB I F 05C-SZ-2 9Cm ADCP -! 550-82-3 150,,, TEMP F 950-02-4 200,,, RCM I

-I 050-82-5 250,,, TEMP. F DSC-82-6 30Dm SCM -i SOC-SO-? 450nm M0t2 DSC-B3-t 8Cm ADCP I 550-83-2 50Dm Mk2/RCM II. I- Wa., Wmg Stuck

I I I DSC-93-3 129Dm RCMJMD2 I

I

I I

I I I I I I I I I I I I I I I I I HAM J JASON D J F HAM .J JAB 0 ND JIMA

1997 1990 1999 + -
9

Measurement Timeline

DeSoto Canyon Eddy Intrusion Study

I I I I I

I I I j I I I I I I I I I

5 -

--

055-Ct-fl 1001 CFD Not Deployed 4

Not Depoyed

1 4 ltSC-Cl-1 20m CUD I- DOS-Cl-S 8Dm ADCP Not IDe <

055-51-3 ployed) -

005-51-4 95°' RCMJS4 Bad DSU - r SOC-CS-i 52m TEM] DSC-C2-2 DOn' ADCP

I 955-52-3 15Dm TEM_R F DSC-C2-4 20Dm RCM

Bad

i =

-

005-52-5 25Dm TEMP F DSC-C2-fl 30Cm 0CM DSU '- 1 DSC-'C2-7 49Dm Mk2

I 005-CS-i 811" aD(p '-. -

05C-C3-2 SOOn, M1c2/RCM

DSC-C3-3 I 29Dm RCMIMk2 Rneordiog Channel I i Not Deployed

i =

fl,,, O.VCID

Not Deployed ( > I:-1

SOC-SI -2 DOn, AIDCP

F

I- 1155-01-3 5n Mk2/S4 Not Dcployed - II DSC-DI-4 99mn ADCP I- I 95C-D2-1 62,, TPJtDJi I- 055-02-2 9On ADCP =

I DSC-D2-3 15Dm TEMP I- 005-02-4 20Dm RCM

-

DSC-D0-5 250,,, TEMP I- 055-02-6 30Dm Mk2/RCM Bad Channels i

I tISC-02-7 490m MkZ

- i =

ItSC-99-1 180,, ADCP Deloed> F

Not Deployed

1 ==

055-El-i 2Dm CTD/CT N,t 055-91-9 G2mmm CFD Not Deployed 4 SF

055-El -2 BOmm, ADCP

: Not IDepoyod

I

I DOS-El -3 82m CT F DSC-El-4 95°' RCM/54 Booken

I:-.__.I _otor I

I I r i i j i =

i I I I I I I M AM ii A SO Hill F MA Mi

+

199 JASON IDJ F MA

1999

Figure 2.l-2b. Timeline of data return by instrument level on the

indicated moorings for the DeSoto Canyon Eddy Intrusion

Study. Arrows indicate the approximate dates of the

hydrographic cruises -
3.0 LOOP CURRENT DYMICS

3..1 Introduction

The Loop Current (LC) and related processes are key source 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 comprehensive

description of the morphology (boundary length, enclosed area, etc.) of

the LC, the recent history of change associated with ring shedding and

some of the dynamics associated with ring assimilation into Gulf waters.

3.2 LC Metrics and Statistics.

A variety of LC metrics and statistics were computed using SSH

information. To facilitate joint use of data sets a comparison was made

of the LC frontal boundary location as estimated by SST imagery and the

location of the 17 cm SSH contour. These two methods of estimating LC

position were generally in good agreement, however, the 17 cm height

contour was consistently south of the SST front by about 10 of latitude

or approximately 100 km. This is consistent with other definitions of

the LC and LC ring boundaries as estimated by subsurface temperature

structure. Often the location of the primary current field associated

with the LC and LC rings is defined as where the 15°C isotherm drops

below 200m depth. This location will also consistently be south

(inside) of the location of the surface thermal front. Note that the 17

cm height contour usually enters the Gulf through the Yucatan Channel,

follows the LC boundary and exits the Gulf through the Straits of

Florida.

Shown in Figure 3.2-la,b are time series of LC area, LC volume, maximum

northward LC intrusion, maximum westward LC intrusion and. length of the

LC. The occurrence of ring shedding events has been indicated so this

influence can readily be identified. Over the six years available for

examination with these techniques, the length of the LC varied between

slightly less than 800 km and approximately 2300 km - a change of almost

300% with a time mean length of 1390 km. Ring shedding events caused

length reductions of between 600 km and 1100 km. Using the 17 cm height

contour, the maximum northern and western excursions were 27.5°N and

90°W respectively. The most southern and eastern maximum excursions

were approximately 24°N and 86°W. These latter "lesser" maximums

immediately followed a ring separation. LC area varied between 0.8 -

1.9 x l0 km2 with a mean area of l4 x 1O km2 The range of area would

have been less and the mean area larger but for shedding of two large

rings. Some of these statistics are summarized in Table 3.2-1 (4.2-1).

An examination of the time series shown in the Figure 3.2-i panels,

suggests the strong correlation between these variables. Computations

of correlations showed a minimum coefficient of 0.82 between any of

these descriptive variables.

To help provide an understanding of when and where LC water may be

found, the percent occurrence has been mapped and contoured (Figure 3.2-

2) . As expected, LC water occurs at more northerly positions less often

than positions to the south. This drawing also provides an impression

for the general shapes that the LC assumes as it grows from the most

11
1.6

0.8

2

L8

"t. 1.6

0

0.8

0.6

1.8

5 x 10

0.6

Jan93 Jul93 Jan94 Jul94

13 Volume of Loop Current, Mean = 2.170000 x 1013 (m3)

1 Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

106 Circulation Along Loop Current, Mean = 1,271,000 (m2 s1)

Area of Loop Current, Mean = 142,000 kni?

Figure 3.2-la. Time series of Loop Current area, volume, and circulation

for 1/1/1993 through 4/30/1999. Ring shedding events are

shown by vertical gray lines.

12

Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99 Jan95 Jul95

Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99
28

27.5

Z27

26.5

26

255

25

24.5

92

91

90

589 0

88

0

87

2200

2000

1800

'- 1600

1400

1200

1000

800

Jan93 Jul93 Jan94

Maximum Latitude of Loop Current Intrusion, Mean 26. 1N

Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Maximum West Longitude of Loop Cuirent Intrusion, Mean = 87.7°W

86

Jan93 Jul93 Jan94 Jul94 Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Jul94

Length of Loop Current, Mean 1,390 km

Jan95 Jul95 Jan96 Jul96 Jan97 Jul97 Jan98 Jul98 Jan99

Figuro 3.2-lb. 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.

13
southerly position following ring shedding to a more northerly and

westerly position expected prior to shedding. A contour map of the mean

SSH fields during this project are shown in Figure 3.2-3.

3.3 LC Enerqetjcs and Dynamjcs

3.3.1 Introduction

Because LC ring shedding is one of the most important and dynamic events

in the Gulf of Mexico, an effort was made as part of this program to

understand the dynamic magnitude of the process. It should be noted

that access to remotely sensed SSH observations were an important

contributor to this line of investigation.

Table 3.2-1. Statistics for Loop Current Metrics (1/1/1993 -4/30/1999).

3.3.2 Ring Separations

To understand the ring shedding process, an historical perspective is

essential. Over the years a number of investigators have looked at the

ring shedding cycle (interval between shedding events). With

availability of SSH information, in particular for the summer months

when SST imagery provides minimal information, the prior estimates of

ring shedding can be expanded confidently. Figure 3.3-1 (4.3-1) shows a

smoothed freQuency plot - actually a manipulated relative frequency plot

- showing separations since 1973.

In a geophysical context where ring shedding may be linked to the larger

basin-scale patterns having decadal time scales, ring shedding cycles of

approximately one year as measured over 25 years may be misleading. The

proper time scale for evaluation of confidence may be such that only two

or three cycles of basin-scale patterns may have occurred over the 25

years of ring shedding information. Regardless, the data in Figure

3.3-1 shows that the most noticeable region of power is near one year -

11.6 months - but specifically not at one year. The mean value, for 33

separation intervals in 313 months is near 9.5 months, although there is

little power at this period. The absence of power at the mean is

neither surprising nor unusual for events having a range of values such

as these separation intervals.

14

Wst.

Longtude Lat.tude Length Area Volume C1.rculat].on

Mean 87.7°W 26.1°N 1390 142,000 217xj0l3

m3

1,271,000

m2/sec

Std.

ev. 0.97° 0.93° 321 28,300 0.37xl0A13

m3

321,190

m2/sec

Maximum 92.0°W 28.0°N 2016 ] 194,600 2.81xlO"l3 2,016,200

m2/sec

inimum 85.8°W 24.l°N 593 58,600 0.9lxlO"13 593,480

j k2 m3 m2/sec
2ON.

92°W 90OW 88W 86°W 84°W 82°W 8OW

Figure 32-2. Percent occurrence of LC waters for the interval from

1/1/1993 through 4/30/1999.

15
3ON

28°N

26°N

24°N

22°N

20°N

92°W 90°W 88W 86°W 84GW 82°W 80°W

Figure 3.2-3. Mean SS-I for the interval from April 1997 to April 1998.

16

-. .. - - -' - - - -

'p I ' -I

c

-' - I

I t

I

'I

I

--

I

I

\\' c I 1,:; I)

,.

U
3.3.3 LC Forcing by the Wind curl

The premise of this section is that most of the Gulf of Mexico is forced

downward by Ekman pumping and that this fluid volume is compensated by

the upper-layer fluid displaced by the advancing LC front. It is

suggested that the fluid that is thus pumped downward, on a time scale

of decades eventually leaves beneath the incoming LC at depths below the

main thermocline.

An order-of-magnitude analysis of the Ekman pumping by the wind curl

shows that the work reqjuired to force a given volume of upper-layer

fluid down through the known stratification of the Gulf is comparable to

the available energy from work done by winds on the surface of the Gulf.

This quantity agrees with the available potential energy of a LC ring.

Consequently, on an annual basis the work done by the wind is sufficient

to force down the volume of one ring per year.

An examination of the LC position (north-south) and the wind stress curl

is shown in Figure 3.3-2. The coherence and phase of the two signals is

shown in Figure 3.3-3. The strong coherence and appropriate phase

relations provide additional support for the possible role of Ekinan

pumping and LC dynamics.

The various calculations and comparisons suggest a possible relationship

between LC ring shedding and the curl of the wind stress over the Gulf

of Mexico. This relationship is not 'proven," rather the patterns and

magnitudes of dynamic variables are such that they suggest such a

relationship may exist. Further study is needed to evaluate and

possibly show this linkage.

17
1.8

1.6

1.4

1.2

1 0.8

0.6

0.4

0.2

0

0

Ring Separations from the Loop Current since 1973

0.05 0.1 0.15 0.2 0.25

Frequency (cycles/month)

Figure 3.3-1. 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. The original histogram has been smoothed by 5

Hanning passes. A single value at (3 mo) is omitted for

clarity, and is of f scale. Confidence limits of 80%, are

obtained by multiplying (or dividing) any value by 1.7;

determined by a bootstrap technique.

18
Figure 3.3-2. 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.

19
Figure 3.3-3. Cross spectra between the wind curl and Loop Current

variability. 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.

20
4.0 SLOPE CXRCUL.TION PATTERNS

4.1 Introduction

A detailed examination and description is provided of the vertical and

horizontal patterns of currents over the slope, from the shelf break

into the deeper water of the mid- and lower slope. Patterns are

complex, having time and space scales covering a broad range. As

developed, it is clear that a eddies, both anticyclonic and cyclonic,

have a substantial influence on observed current patterns.

4.2 Types and Sources of Eddy Inf].uences

4.2.1 Loop Current Frontal Eddies

A variety of differing types of eddy features were seen to affect the

circulation patterns documented by the in-situ observations, the shipbased surveys and the satellite remotely sensed observations. Of

particular relevance are LC frontal eddies (LCFE) that form on the

boundary of the LC and on the boundary of LC rings. A LCFE is an

essentially cyclonic feature that forms along a boundary. They have

describe and documented extensively along the Gulf Stream and to a

lesser extent along the LC but along the west Florida escarpment. The

LCFE affecting the northeastern Gulf can be much larger than those along

the west Florida shelf/slope due to the lack of a proximate constraining

boundary. An example of a LCFE is shown in Figure 4.2-1. The

cyclonic/counterclockwise flow is indicated by an arrow. As is often

the case, a trailing filament of warmer water extends Out from the LC or

ring in advance of the cold core cyclonic eddy. These features

move/migrate along the frontal boundary and evolve as they move. The

filaments can extend further. At times, the filament can become almost

detached from the frontal boundary and end. up a relatively thinner lens

of warmer water ballooning out from the frontal boundary. The warm

filament/streamer can extend the influence of the LC frontal boundary

well away from the actual position of the main LC boundary.

The cause of the frontal boundary features is a topic of open

discussion. On the Gulf Stream warm/western wall, the filament is

associated with the crest of a migrating wave motion and the cyclone

associated with the trough of the wavelike oscillation. Such

associations are not so readily resolved along the LC, however, the

similarity between the features would suggest similar causes that remain

to be well defined. The presence of LCFEs along separated LC rings

points to the role of stability of the frontal boundary as a

contributing factor in their formation and evolution.

As might be expected, the potential direct influence of LCFE5 within the

present study area is greater when the LC is well extended to the north.

In contrast, the influence of frontal eddies on rings are associated

with the local presence of a ring, prior to having migrated westward out

of the eastern Gulf. The specific influence of the various component

elements of a J.JCFE (cold pool, filament, etc) depend on factors such as

the location relative to instrument and bottom topography, the distance

21
L2-L. ?.iri1 1 i:;. r1r-iurr 1rrcj1t v

n Lb tLiiIy P:I.urci rni

t text have bn Ut1 L.t
from the study area to the frontal boundary, the presence or absence of

other dynamic features in the region

Clearly, it is not possible to develop a one or two scenarios of the

influence of ICFE on conditions in the study area. An effort has been

made to conceptualize the influence of LCFEs by developing a series of

"modes" that describe the patterns identified during this study.

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 (northward extended) LC. This process is

shown schematically in Figure 42-2. LCFE's can have horizontal

scales of several hundred kilometers 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 southeast causing 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. A number of LCFE's

produced this type of current and temperature response during

their passage south of the array. An example in SST imagery are

given in Figure 4.2-3. Intrusions of these warm streamers can

sometimes result in strong up-canyon flows in the DeSoto Canyon.

Offshore flows can result from the convergence of the trailing

wake of a LCFE with an approaching ring or LC crest. 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 4.2-4).

Mode 2c. Mode 2c occurs when a LCFE is located close to the array

and is directly interacting with the slope (Figure 4.2-5) . 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.

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 4.2-6).

The LCFE produces westward flow in the deeper waters below 200m

23
22 °N

I

20°N

90°W

Yucatan

Pen.

Yucatar-: Inn!

880W 86°W 84°W 82°W

CUBA

220 N

20°N

80°W

Figure 4.2-2 Conceptual model of a Loop Current frontal eddy flow

interaction with the slope in the study region.

24

90°W 88°W 86°W 84°W 82°W 800W

30°N

280N

44

4.

.- -

$

*

Cycjonic

circulation

pattern below

2OOm

4. .

.1

,.

.1J

30°N

280N

26 °N 26°N

Surface

anticycloriic

flow

e + 4.

S

4

S

24°N

ch0"' 0 S S

--

- Straits

of

FIoid

240N
FIQ..Lt T iuir rDr pri1 0. ..997 flI!b cr :4 iu 

,*L141i iru Jii.' iimjc n 'h tucii A L:Iri t.r- ii iri 

.0 LLU ..4iJ. u rh Lo *t pro,r.
28°N

26°N

24°N

22°N

20°N

90°W 88°W 86°W 84°W 82°W 80°W

30°N 30°N

;

.s-. - ./ Surface Anticyclone .-": .

*1'

Figure 42-4. Conceptual model of flow interaction with the slope in

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

LC ring.

26

'--:'.

'2000m LC Ring

wI Frontat

Eddies

UyCIonICTiOWatWater

- depths 2OO m

2

:--

IT5" \

, S; "'S

.- 5'

2

I

2

5' q

Warm Filament

%' I

. 5" T _1

Sfra!tS

CUBA

I

90°W 88°W 86°W 84°W 82°W 80°W

8°N

6°N

4°N

2°N

O°N
30°N

28°N

26°N

24°N

22° N

90°W 88°W 86°W 84°W 82°W 80°W

20°

90°W

- Warm

Filament

88°W 86°W

Cyclonic circulaltion at

water depths 2OQm

84°W 82°W 80°W

30°N

8°N

6°N

D°N

Figure 42-5. Conceptual model of flow interaction with the slope

from a Loop Current Frontal Eddy close to the study

region -

27

Oçç/ 55c 0

I2

2

_20001fl- - 1

S/

-

.,i Si Westward surface \ currents associated with

flow in warm 'filament

of Iorida-'

CUBA

Yucatan ',-: /

Pen.

!neI S'- - - - 2

2
28°N

26 °N

24°N

22°N

20°N

goow 88°W

*

' -e

SOOm'

86W

Aflticyclonic currents

fl upper lOOm

CycTonic currents

at water depths

2OOm

84GW 82°W

28°N

26 °N

24N

22°N

20°N

80°W

28

Figure 42-6. 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.

90°W 88°W 86°W 84°W 82°W 80°W

30°N

4. 30°N
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 the study region.

4.2.2 Cyclonic and Anticyclonic Eddies

Remote sensing helped identify eddy like features that do not have a

direct link to the LC or a LC ring. This is not to say that these

features did not evolve as features in some way due to the LC as a

source of energy and momentum; however, at the time they are identified

this possible linkage is not readily apparent. Both cyclonic

(counterclockwise) and anticyclonic (clockwise) rotating features were

documented during this study and an effort was made to resolve their

nature and influence. The horizontal scales of these features with both

senses or rotation are comparable. Due to limitations in horizontal

scale resolution with altimetry, the lower eddy diameter is smaller than

can be well resolved by available altimetry. The upper length scale is

on the order of 100 - 200 km.

These features evolve with time and migrate. The specific evolutionary

course as well as the path and rate of movement are often dependent on

the location and types of other features which surround the specific

feature. Thus, there is not a common pattern for these features, rather

the specific influence is described on a case-by-case basis. As

determined from ship-survey data, currents in the upper layers and midto lower layers (500-l000m geostropic velocity) can be in the opposite

along shore directions. (Figure 4.2-7) . Examples with no reversals with

depth are shown in Figures 4.2-8.

4.2.3 Mean and Vaxyinq Circulation Patterns

Long-term (two year) mean current patterns were quite robust.

Subdividing the records into two separate 12-month intervals produced

results similar to the complete two-year record. In all cases the

standard deviations were substantially larger than the means which

points to the inherent importance of the variable flow patterns

occurring in the study area. Near surface mean flows generally had an

eastward component while below 70mg westward means occurred. Even such

long-term averaging maintained a distinct counter current at depth.

Superimposed on the means are long term but variable current and

temperature patterns. Throughout the study, several intervals with

durations of two months or greater of relatively sustained or persistent

conditions (events) were identified (Figure 4.2-9) . Averages and

variances for each of these events were computed to characterize

conditions during each event. The events represent a "mixed" event with

cyclonic and anticyclonic conditions (S97), a cyclonic event (C) and an

anticyclonic event (A). Very low frequency variability may indicate the

presence of propagating topographic wave motions. This analysis also

indicated to variance that was unexplained by low frequency EOF

analyses, which pointed toward higher frequency, shorter scale motions

as a possible source for a substantial component of the observed

variability.

29
8/ 410 8/l2/199 8/V PEL(CAII PEO23 86W 85W SOW SOW 6 - 1030 m. Geostrophic Velocity Figure 4.2-7. Geostrophic velocities at Gm from the hydrographic surveys. 5-day average velocity vectors (heavy arrows) from the Sm depth of the ADCP

are shown at stations marked with a dot.
R/V PELICAN PE-9722 5/21/1997 In 3/21/1997 Trnn,.nO B G.celrnpftle V.IocIIy (crn/. FE.I.r.nne L.e.I 1000 db kiln 04.13 Mon = 35.03 0 10 20 30 40 R/V PELICAN P-982O 11/14/1997 to 19/14/1997 Tron,.of B 0.oelr9pftic Voloolly (nm/c) R.I.r.no, U,.! 9000 dl, Urn = 10.64 Moo 27.90 0 ( 50 60 70 80 90 100 110 120 0 10 20 30 40 Dicturoce (cm) Stotin, 7; P/V PELICAN F°E-9803 7/i /9997 to 7/11/1997 Tr,n,,nt B G.o.hophlc V.10,117 (am/c) R.!.r.r,c. Lead 1000 db kiln 27.10 kIn, 21.90 l 50 60 70 Diotunc (km) P/V/ PELICAN PE-9830 4/ 2/1999 In 4/4/1199 Tronnal B Seoctrophic V.lool17 (nm/c) R.I.r.no. Len.! 100044 kiln 29.6 lion 4.94 1 if i ii\I II 2 2:, 5 I I Figure 4.2-8. Geostrophic velocities on Section B for the indicated surveys. Positive velocities are eastward, norirtal to the section. Highe

westward (negative) velocities are shaded. 0 10 20 30 40 50 60 70 80 90 100 110 1

80 90 100 110 12

50 60 70 80 90 100 110 120 0 10 20 30 40 0 100 200 300 400 E 500 a- 600 700 800 900 10000 100 200 300 400 E 500 a 600 700 800 900 1000 0 100 200 300 400 500 600 700 800 900 10000 100 200 300 400 500 600 700 800 900 1000
150

200

250

300

60

40

20

0

-20

-40

60

50

62

90

19 30. 7 16 26

7-DLP Temperatures at Mooring C2

A S 15'J

!IH

1130

1998 1999

7-Day Low Pass Currents at Mooring C2

30

C2 6m

C2

Om

c2

8m

C2

Om

C2

Om

32

Figure 4.2-9 Seven-day low pass current vectors and isotherm depths

for the upper 300 m of C2. Vectors pointing vertically

upward are directed along isobath with an easterly

component
4.2.4 Slope Dynamics and Fluxes

Vorticity over the slope appears to be in part dependent on the presence

of eddy-like features, their location and sense of rotation. Because of

the differing sense of rotation of cyclonic and anticyclonic eddies, it

might be expected that the sign of the vorticity at the various slope

locations was dependent on the above factors. It was not possible to

develop a complete explanation that related vorticity to all specific

velocity records. Some insights remain to be developed.

Momentum flux over the shelf break was governed by the influence of

slope eddies and storm events. Eddies generally transferred momentum

onshore onto the shelf from the slope and by this method slope processes

affected the circulation patterns on the shelf. Although the rate of

transfer was not as great as that occurring in substantial storms, the

transfer due to eddies could persist for longer periods of time.

Hurricanes created significant spikes in the momentum flux across the

shelf break.

Heat and salt flux calculations were limited by placement of appropriate

sensors. However, from the available data, it appears that eddies

played a relatively minor role in the total flux of heat (0.1-2%) and

salt (4-14%). The available data did indicate a strong correlation of

salinity and temperature which was sufficient for temperature to be a

surrogate for salinity. Conseguently, water mass flux calculations may

be computed from temperature and velocity alone.

Results of computations indicated that at some stations, mean fluxes

across the shelf break were significant, while at others they were not.

These results suggest that, over the long term, the water mass exchange

may be something of a two-layer phenomenon with onshelf flux near the

surface and offshore flux near the bottom. Results also suggested that

wind-forced flows may have accounted for a significantly greater

proportion of the cross-margin temperature flux near the bottom than at

the surface. At all moorings, the largest cross-margin heat/temperature

fluxes occurred during Hurricanes Earl and Georges.

4.2.5 Deep Currents

Previously, circulation in the upper 500m has been discussed. Over the

mid- to upper slope, circulation was dominated by eddies and an eastward

flowing current with reversed flows at depths below about 200m. The

primary source of energy for these flows is the LC and LC rings/eddies

which extend down to 800-l000m. It is about this depth that a relative

minimum in kinetic energy occurs. Below this, motion does not correlate

well, i.e. are not directly coupled with those higher in the water

column. Deeper currents may be related to wave motions that could be

related in some way to the LC and the separation and migration of LC

rings. Observations did not provide information that was sufficient to

explain the apparent wavelike motions measured by the deepest current

meters on the 130Dm moorings.

33
5.0 ATMOSPHERIC FORCING

5.1 Synoptic Scale Forcinq

Shelfbreak and upper level slope currents had a significant response to

coherent synoptic-scale wind events. Alongshelf winds that were

coherent over the shelf domain generated cross-shelf transports in the

surface layer with opposite return flows beneath. The geostropic

adjustment time for sea level to balance alongshore flows is

approximately two hours. Time for the along shelf currents to reach

equilibrium with the along shelf wind stress was about 28 hours. The

separation of the periodicity of eddy forcing (greater than 20 days) and

synoptic wind forcing (5 to 15 days in Winter) facilitates to some

degree an examination of these two mechanism.

5.2 Seasonal Wind Response on the Slope

Because the transition seasons of spring and fall are short and. often

not well defined in the Gulf, two winter and two summer seasons were

partitioned and used to examine seasonal scale forcing of slope

circulations. The winter seasons typically have the more vigorous

forcing due to wind stress in the 5-15 day period band of synoptic scale

events -

In this portion of the northern Gulf, these winter atmospheric events

are low pressure systems and fronts that move over and through the

northeastern Gulf of Mexico. The interval between events can vary

within a winter season with the shortest interval often occurring in the

heart of the season. Results indicate that there was a strong slope

current response to winds in the two summer seasons. During the two

summer seasons, there was no significant coherence between the alongand cross slope wind stress modes and the principal velocity modes for

the shelf break and upper slope moorings.

It is difficult to partition eddy influences from those forced by wind

stress. The relative influence of wind forcing in summer was dependent

on the relative strength of eddy-driven motions. During both summers,

eddy influences were strong and the mean flow was toward the east.

Fluctuations in along isobath currents enhanced or opposed this mean

motion. Some of the variability due to atmospheric processes was

difficult to distinguish from eddy effects in the same frequency band.

In the two winters during which current measurements were made,

variability of the primary EOF modes generally followed the trend of the

isobaths. As during the summers, slower underlying variability occurred

due to eddies, however, more energetic fluctuations having shorter

periods (on the order of three to ten days) were associated with

atmospheric forcing. The visual correlation of this higher frequency

variability suggests that the longer period fluctuations associated with

eddies caused a reduction in the correlation between modes. During

winter, at those frequencies where wind stress EOF modes have energy,

the coherence between wind stress and current modes was high. The phase

differences show that the wind stress modes led the currents modes by 1

to 2 days.

35
5.3 Canyon Response to Hurricanes

During these field measurements, two hurricanes (Earl and Georges) moved

over portions of the in-situ instrument arrays. This provided an

opportunity to examine the oceanographic response of the DeSoto Canyon

to at least these two exposures to extreme atmospheric forcing. Because

the storm intensity varied, as did the extent of the associated wind

field and the path, the oceanographic response differed to some degree

between the two storms.

Both hurricanes generated strong along isobath flow near the surface

with currents being somewhat stronger during Georges. Strong and

variable cross isobath flows were seen at the canyon edge during both

storms. During Earl, highest velocities at the canyon edge were in a

near surface jet. During Georges, high velocities extended throughout

the water column with a mid-depth maximum of along isobath velocity in

the vicinity of mooring Cl.

An examination of volume flux during these storms indicates that Georges

caused significantly (several times) greater cross-isobath transport at

the shelf break than did Earl. The excess onshore transport over the

passage of the storm points to greater offshore transport at a location

not documented with measurements.

Several prior studies have documented the inertial current response to

strong storms passing over the slope. The inertial response of interest

results from the relatively quick (relative to oceanographic adjustment)

impulse being provided by the combined influence of a migrating wind

stress field on the surface and the migrating oceanographic pressure

dome under the hurricanes. After the input of momentum at the surface,

the resulting motion is controlled by the Coriolis force that causes

circular like motion of the water particles with a periodicity governed

by the magnitude of the Coriolis force, which is dependent on the

latitude. In this study area at approximately 29°N the inertial current

period is 24.75 hours or 0.97 cycles per day. This motion propagates

downward in the water column, so the phase arid magnitude of currents at

the surface can differ from those occurring at various depths.

The inertial current magnitudes following passage of these two

hurricanes varied considerable between mooring/locations. The strongest

inertial currents associated with Earl occurred near the surface at

mooring C3 in l300m of water (Figure 5.3-1). For both storms,

relatively weak inertial currents were seen at the shelf break; stronger

currents occurred in the interior of the DeSoto Canyon. There is some

evidence that the inertial current pulse may have propagated

horizontally, arriving at locations further to the east at later times.

In contrast and as shown in Figure 5.3-1, inertial current from

Hurricane Georges did not display similar eastward phase propagation.

The relative strength of the inertial response to hurricanes can be seen

in Figure 5.3-2 that shows the amplitudes of inertial currents over the

duration of the measurements at several locations and depths.

36
>

-o

10

E0

20

30

Currents

60

50

40

30

20

Wind Stress at Buoy 42040

'I

-t I F

- C3 - D2

- -. El

III,, P III - t_ SI

19 29

October

Figure 53-l. Comparison of inertial current magnitudes seen near the

surface at moorings C3, D2 and El during late summerearly autumn 1998

37

30

c.'j 20

E 10

U)j)

0

10-

0

20 30 9

August 1998 September

-I- - eastward

- northward

Earl Georges
Figure 5.3-2. 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 th wind records, wind stresses over April

1997-November 1998 was computed from buoy 42040 wind

velocities and subseqvent wind stresses were computed

using wind velocities from buoy 42039.

38

ci)

.

-

Cl) E

-

-

E

o

0

c'.130

20-

50-

0

50

50

o

0

50

Wind Stress

I I I I I I I I I

tLg&.j JAkUjikL.a.L.

I I I I I I

Inertial Current Magnitude

I P I I I I I I I ci

iiL

I I I J 18m 95m

I I I I I I C2

LL iJ A i4 I

1 1 I 16m 490m

I I I I I I I I I I I I C3

L.i 11

I I I I - 1290m

- - 1-j' J

I I I I I I I I I I I Dl

AA1AA PhA&.kII.Lk _LI4 LMJAjpth.Lh.

I I I 16m 95m

I I I I I

I I I I I I P2I I I I I 16m

- 490 m

,

I I I I I I I I I I I El

L JhA AIjAJj

I

L

I I I 16m 95m

AM J JASON D J FM AM J JASON

1997 1998

tJ J FM A

1999
5.4 on-hurrieane Inertial Currents

As indicated above, the local inertial period is within the diurnal

tidal spectral peak. To separate inertial motions from internal tidal

currents relies on the regularity of the tidal signal and the lack of

corresponding regularity of inertial currents. In addition, the

internal tides should be phased locked over the vertical column, where

as inertial currents will not have that vertical phase relationship.

The strongest inertial oscillations were at C3, the mooring furthest

offshore. During the summer of 1997, inertial currents were present in

spite of the weak wind speeds. Some initiation of bursts of inertial

currents seem to have been associated with abrupt wind shifts. In

contrast, during that winter when wind speeds were greater, inertial

currents were weaker. The lack of vertical stratification at the

shallower water sites can reduce the vigor of the inertial response.

The argument is made that the vorticity associated with eddies and jets

over the slope can trap offshore propagating inertial currents, and thus

- enhance the inertial currents at the deep water sites as compared to

those at the shelf break.

These measurements show that non-hurricane inertial currents dominated

the high frequency current fluctuations and were present in the 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 the inertial energy decayed rapidly

seaward of the shelf break.

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6.0 STJARY

The DeSoto Canyon Eddy Intrusion Study established a comprehensive set

of observations to evaluate and describe the oceanographic conditions

within the study area. Each of the PIs effectively utilized

multivariate data sets to isolate and describe often complex current and

flow patterns. The availability of remotely sensed data, in particular

SSH information, as well as ship-based surveys provided essential

spatial and temporal extensions to the arrays of in-situ instruments

that provided a nearly complete set of observations

41
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural resources. This includes fostering 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 Service's (MMS) primary

responsibilities are to manage the mineral resources located on the Nation's 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 environmentallysound

exploration and production of our Nation's offshore natural gas, oil and other mineral resOurces. The

MMS Royalty Management Program meets its responsibilities by ensuringthe 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 emphasison working to enhance the quality of life for all Americans by lending MMS assistance and expertise to economic developmentand environmental protection.