OCS Study

MMS 2001-103

Northeastern Gulf of Mexico

Inner Shelf Circulation Study

 

U.S. Department of the Interior

Minerals Management Service
 OCS Report 

 MMS 2001-103

Northeastern Gulf of Mexico

Inner Shelf Circulation Study

Authors and Co-Principal Investigators

W. Sturges

Florida State University

Pearn P. Niiler 

Scripps Institution of Oceanography 

R. H. Weisberg 

University of South Florida

Florida State University accounting number 1369-763-29

Prepared under MMS Cooperative Agreement 

Number 14-35-0001-30787

By

Department of Oceanography

Florida State University

Tallahassee, FL 32306

Published by

U.S. Department of the Interior

Minerals Management Service
Disclaimer

This report was prepared under a Cooperative Agreement between the Minerals 

Management Service and the Florida State University. 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 MMS editorial standards.

Report Availability

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

following address:

U.S. Department of the Interior

Minerals Management Service

381 Elden Street

Herndon, VA, 20170 

Citation

Suggested citation:

Sturges, W., P.P. Niiler, and R.H. Weisberg (2001). Northeastern Gulf of 

Mexico Inner Shelf Circulation Study. Final Report, MMS Cooperative 

Agreement 14-35-0001-30787. OCS Report MMS 2001 – 103, U.S. 

Minerals Management Service, Herndon, VA, 90pp.

About the Cover

The map of drifter tracks in the Gulf of Mexico shown on the cover of this report was 

prepared by Dr. Walter Johnson, the COTR for this Cooperative Agreement. All the 

drifter tracks for the 13-month long series of drifter deployments are shown. The drifters 

were deployed in depths of 50m or less along the upper west coast of Florida between 

Cedar Key and Pensacola. They spread over the full Gulf and into the open Atlantic.
Acknowledgements

This project was carried out cooperatively between MMS and Scripps 

Institute of Oceanography, The Florida State University, and The University 

of South Florida. Dr. Walter Johnson, MMS, while not officially an author 

of this report, made major contributions to this project. Dr. Alexis LugoFernandez of the New Orleans office and Dr. Ronald Lai, Washington, D.C., 

were also instrumental in major ways that enabled the success of the work, 

providing both technical and administrative support. We are deeply grateful 

for their contributions to the project.

At Scripps, P. Niiler was assisted by Dr. Carter Ohlman, who made major 

contributions to the work. The assistance of Sharon Lukas, Judy IllemanGaukel and Andy Sybrandy is acknowledged throughout the progress of the 

SCULP experiments. The drifters were the work of Technocean, Inc., of 

Cape Coral, Florida, as mentioned in the text.

At Florida State University, W. Sturges was assisted by Mr. Jianguo 

Wang, who provided significant, ongoing scientific programming assistance. 

The current meters were launched and recovered with the assistance of 

David Hunley, the Senior Engineer at the FSU Current Meter Facility, 

together with Ken Jones and Nick Wooten, NWFRWQM, as well as by Mr. 

C.J. DeHaan. We especially wish to thank Mrs. Diane Stuart for 

administrative oversight.

At the University of South Florida, R. H. Weisberg was assisted by B. 

Black, E.M. Siegel, and H. Yang. We also benefited from MMS project 14-

35-0001-30804 through Florida State University that provided support for 

our simultaneous numerical modeling program.
Table of Contents

Abstract and Executive Summary 1

I. Introduction 4

II. The Drifter Experiment 5

III. The Moored Current Meter Experiment 27

IV. Comparing Drifter Results with Current Meter Results 40

V. Practical Results: Comparing Drifter Results with 

Wind Forcing 48

VI. An annotated bibliography of published papers, 

reports, and theses from this program 61

VII. References 68

VIII. Appendix A. 

Time-series Plots of Currents and Temperature 

from Moored Instruments. 70
Executive Summary

This report presents the results of an experiment involving several 

hundred satellite-tracked surface drifters used, together with an auxiliary 

series of moored current meters and meteorological buoys, to measure the 

near-shore circulation in the coastal waters of the Northeastern Gulf of 

Mexico. The drifter experiment was performed under the direction of Dr. P. 

P. Niiler of Scripps. Each weekly set of results, determined by satellite, was 

mapped in near-real time by Dr. Walter Johnson of the Minerals 

Management Service (MMS) and then made available to interested parties 

on a web page set up for this project at http://gulf.ocean.fsu.edu. 

In support of the oil and natural gas leasing activities in the Gulf 

of Mexico, the MMS has funded a number of intensive studies of the 

circulation of the US coastal economic zone. This study, termed SCULP 

– II, obtained and analyzed data from 342 ARGOS-tracked CODE-type 

drifters off the northwest coast of Florida between February 6, 1996 and 

June 28, 1997. 

These drifters were deployed from aircraft to 26 stations on the 

continental shelf by the manufacturer, Technocean, Inc., with a 99% 

success rate. The average at-sea life of a drifter that survived the air 

deployment was 73 days. Over a 12-month period at periods of 

approximately every two weeks a new drifter was deployed by 

parachute to one of these 26 stations if no drifter was found within a 5-

km distance. In all, 25,100 one-day average displacements were 

obtained. The processed data on drifter location and velocity was 

transmitted in near real time to MMS/HQ and has been in use there in 

oil-spill modeling activities for the past four years. Abstracts of the 

scientific publications that have resulted from these data refer to 

specific dynamical inferences drawn from these data. This report 

presents a summary of the statistical analyses.

The displacements of the drifters on the West Florida Shelf were 

variable in time and occurred principally along bottom topographic 

contours. The monthly mean currents were the strongest along the shelf 

break and west of Cape San Blas. Significant mean flows occurred on 

the shelf break to the south or southeast and near shore along the 
Florida panhandle to the west. Across-shelf currents were strongest off 

Cape San Blas and the Mississippi Sound. The largest excursion from 

the general oscillating drift pattern occurred in early October 1996 

when, during a tropical storm, 35 drifters moved with speeds in excess 

of 50 cm/sec westward from the West Florida Shelf past the Mississippi 

River delta as far as the Louisiana-Texas shelf. One unexpected 

finding was a “forbidden” zone that emerged north of the Florida Bay, 

south of Tampa Bay, suggesting that a region exists where no drifters 

ventures. 

About 47% of the velocity variance can be explained by spatialscale coherent patterns that occupy the entire horizontal extent of the 

northeast Gulf shelf. The spatial correlation scale of the alongtopography currents along isobaths is about 50 km in water depths 

between 10-30m and 350 km in water depths of 50-80m. Both local 

winds and remote forcing from the Loop Current were shown to 

produce the observed strong events in the large spatial-scale flow 

patterns. The region of the shelf between 100m and 30m depth exhibited 

no annual mean velocity, but drifters from that region were ejected to 

all four corners of the Gulf of Mexico in less than 120 days by shortlived strong events in circulation.

In addition to the surface drifters program, a program of moored 

current meters was carried out cooperatively between Florida State 

University and the University of South Florida. The mooring program was 

designed to supplement previous moored current data as well as to 

complement the drifter study. These results are described in detail in 

Section III. 

It is of considerable scientific and engineering interest to know how 

the measurements taken from the two different observation systems 

compare; this study is described in Section IV. The two different sets of 

measurements agree quite well in the expected ways and show the expected 

shear and veering within boundary layers. 

Discussed in Section V is an issue of considerable practical interest 

and a fundamental question: whether if the wind blows at a certain speed in 

a given location we can then compute the near-surface currents with any 

reasonable accuracy. The answer is "yes," and the coefficients we obtained 

are presented here. The results have been presented in near-real time on a 
web page, http://gulf.ocean.fsu.edu, as mentioned above, and it is our 

expectation that this page will be maintained for several years beyond the 

formal end of the study. The mooring program on the west Florida shelf has 

been continued under the ECOHAB study, and these results are maintained 

and made available to interested parties on the web page: 

http://ocg6.marine.usf.edu .

Over the past few decades, we have learned to compute the purely 

wind-driven currents on the continental shelves quite well. Numerical 

models are beginning to be able to include the influence of offshore eddylike features on shelf currents. One awkward technical problem still 

remains, however, because much of our knowledge of the eddies beyond the 

shelf break depends on results of satellite altimetry. We learned from the 

MMS-sponsored DeSoto Canyon study that a dominant eddy size is in the 

100-150 km range. Casual study of the ground tracks of the Topex 

(altimeter) satellite shows that eddies of this size can fit comfortably inside 

the track-line pattern in such a way that they remain essentially hidden from 

view. And since many energetic events of shelf currents seem to be driven 

by these offshore eddies, a very great deal remains to be learned about the 

shelf currents along the Florida coast.
I. Introduction

This report presents the results of an experiment involving more than 

300 satellite-tracked surface drifters used together with an auxiliary series of 

moored current meters and meteorological buoys to measure the near-shore 

circulation in the upper waters of the Northeastern Gulf of Mexico. The 

drifter experiment was conducted under the direction of Dr. P. P. Niiler of 

Scripps. Each weekly set of results, determined via ARGOS satellite, was 

mapped in near-real time by Dr. Walter Johnson, MMS, and made available 

to interested parties on a web page, http://gulf.ocean.fsu.edu, that was 

established for this project. As the project developed, both the near-surface 

data collected by the drifters and the currents measured at the fixed 

moorings were made available on that web page. It is our intention to 

maintain this web page and make the data and primary plots available for at 

least two years after the end of the project.

The primary purpose of this experiment was the measurement of nearshore surface currents at depths of less than 50 -100m. The drifters perform 

this work with great accuracy in the upper meter of the water column. This 

work and the results are described in the Section II of this report. The 

current meter moorings and some brief results are described in Section III. 

It is of great interest to know how the measurements taken from the 

two different observation systems compare; this is described in Section IV. 

An issue of considerable practical interest, and fundamental question, 

discussed in Section V, is whether if the wind blows at a certain speed in a 

given location, we can then compute the near-surface currents with any 

reasonable accuracy. The answer is "yes", and the coefficients we obtained 

are presented here.

Section IV presents an initial list of the reports and publications that 

have come from this work. In Appendix A we show a lengthy series of plots 

that describe the observations of currents at a selection of the various 

moorings. The initial copies of this report also include a compact disk that 

contains the data from the drifters, the current meter experiments, and the 

figures from the web page.
5

Section II. The Drifter Experiment

Also known as the

“SURFACE CURRENT LAGRANGIAN-DRIFT PROGRAM”

SCULP – II

II.1 Summary of the Drifter Work

 In support of the oil and natural gas leasing activities in the Gulf of 

Mexico, the Minerals Management Service (MMS) has funded a number of 

intensive studies of the circulation of the US coastal economic zone. In this 

study, termed SCULP – II, data were obtained and analyzed from 342 

ARGOS-tracked, CODE-type drifters off the northwest coast of Florida in the 

period February 6, 1996 to June 28, 1997. These drifters were deployed from 

aircraft to 26 stations on the continental shelf by the manufacturer, 

Technocean, Inc., with a 99% success rate. The average at-sea life of a drifter 

that survived the air deployment was 73 days. Over a 12-month period, at 

periods of approximately every two weeks, if no drifter remained within 5 km 

of any one of these 26 stations on the continental shelf a replacement drifter 

was deployed by parachute . In all, 25,100 one-day average displacements 

were obtained. The processed data on drifter location and velocity was 

transmitted in near-real time to MMS/HQ and has been in use there for the 

past four years in oil spill modeling activities. Abstracts of the scientific 

publications that have resulted from these data refer to specific dynamical 

inferences drawn from them. This is a summary report of the statistical 

analyses.

The displacements of the drifters on the West Florida Shelf were 

variable in time and occurred principally along bottom topographic contours. 

The monthly mean currents were the strongest along the shelf break and west 

of Cape San Blas. Significant annual mean flows were on the shelf break to 

the south or southeast and near shore along the Florida panhandle to the west. 

Across-shelf currents were strongest off Cape San Blas and the Mississippi 

Sound. The largest excursion from the general oscillating drift pattern 

occurred in early October, 1996 during a tropical storm when 35 drifters 

moved with speeds in excess of 50 cm/sec westward from the West Florida 

Shelf past the Mississippi River delta to the Louisiana-Texas shelf. A 

“forbidden zone where no drifter ventures" emerged north of the Florida Bay, 
6

halfway to Tampa Bay, a region to About 47% of the velocity variance can be 

explained by spatial-scale coherent patterns that occupy the entire horizontal 

extent of the northeast Gulf shelf. The spatial correlation scale of the alongtopography currents along isobaths is about 50 km in water depths between 

10-30m and 350 km in water depths of 50-80m. Both local winds and remote 

forcing from the Loop Current were shown to produce the observed strong 

events in the large spatial-scale flow patterns. The region of the shelf between 

100m and 30m depth exhibited no annual mean velocity, but drifters from that 

area were ejected to all four corners of the Gulf of Mexico in less than 120 

days by short-lived, strong events in circulation.

II.2. Background

Direct observations of the circulation on the continental shelf of the 

Gulf of Mexico have been going on since the early 1970s. Since that time, the 

MMS has sponsored several efforts to collect and synthesize these data 

(Niiler and Koblinsky, 1978) and to carry out comprehensive field studies 

where circulation data were lacking (LATEX, 1995). Of primary importance 

were data on the movement of the very-near-surface water parcels that could 

be used most directly to assess the fate of spilled oil. MMS has also sponsored 

the development of numerical models of the surface circulation on continental 

margins, and data sets were needed to assess their validity and usefulness in 

modeling particle dispersal in the Gulf of Mexico (Herring et al., 2001). To 

help accomplish these objectives, a field program entitled “Surface Current 

and Lagrangian-Drift Program (SCULP)” commenced in June 1993 to deploy 

and analyze data from arrays of ARGOS fixed-surface drifters in the US 

economic zone of the Gulf to observe the patterns of near-surface Lagrangian 

displacement and circulation. This is a report on the overview of the data 

synthesis of SCULP II that was carried out on the west Florida and Alabama 

continental shelves from February 1996 to June 1997. 

The data in this study were obtained with air-deployed ARGOStracked drifters similar to CODE drifter developed by Davis (1985). The 

objective of SCULP-II was to observe the monthly to seasonal time-scale 

evolution of the circulation on the entire northeast Gulf of Mexico shelf. 

Drifters released in this area were transported in several surprising ways to 

other distant parts of the Gulf, resulting in a more comprehensive description 

than originally planned, especially of the appearance and mechanisms of the 

forcing of strong currents along the shelf break. 
7

The experimental plan was to fly over a 26-station pattern on the shelf, 

between 50m depth and the coast twice a month and parachute a drifter on to 

any station where none was found within a 5-km distance. In this way, an 

array of drifters was maintained over the shelf for a 12-month period. Each 

month, the processed data was forwarded to MMS/HQ so that the cooperating 

scientists at MMS could use these in near-real time for analyses relative to 

potential oil spills. (The complete processed file has also been released by 

MMS/HQ for public use.) Section 2 describes the drifters and the data 

processing. Section 3 describes the monthly mean drifter displacements, 

velocity and the velocity variances. In Section 4 are derived several 

characteristics of coherent patterns of flow. The abstracts of the published 

papers that deal with several dynamical causes for the observed flow patterns 

are presented in a later chapter of this report.

II.3. The methods and data processing

i) The drifters: Technocean, Inc. of Cape Coral, Florida, built the drifters used 

in this study (Figure 1). These were packed in a folded configuration in 

soluble cardboard "coffins" and made air-deployable by the use of degradable 

cotton parachutes. The tubular PVC body of the drifter, from which the selfdeploying arms emanate after the coffin disintegrates in seawater, contains the 

ARGOS transmitter, controller and batteries. The overall dimension of the 

square, cross-shaped drogues was 92 cm high and 100 wide. The controller 

within the drifter shut off the power after 120 days of transmission. The 

design and testing of these devices was done in SCULP I (Niiler et al., 1997). 

Their half-life at sea was 73 days. The slip of these drifters as a function of 

wind speed or sea-state is not known quantitatively, but is estimated by Davis 

(1984) to be less than 3 cm/sec in the coastal upwelling winds off northern 

California.

A 26-station deployment pattern was laid out over the northeast Florida 

shelf (Figure 2). The deployments were carried out by a crew supervised by 

Technocean, Inc in a twin-engine Piper-Navajo aircraft operated by Cherry 

Air of Lafayette, LA on a schedule specified several days before by the PI's at 

the Scripps Institution of Oceanography (SIO). Cherry Air made 23 flights 

commencing on 5 February 1996. It began the first deployment with 15 

drifters at the easternmost stations, to which the 11 westernmost stations were 

added by 2 April 1996 after it became apparent that drifters moved rapidly 

westward to the Mississippi Sound, the objective being to obtain a complete 

data set during the time that these currents would 

90 ̊W 85 ̊W

25 ̊N30 ̊N

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Pensacola 

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Figure 2. Number of daily averaged surface current observations from Lagrangian driftex 0.25 degree bins. Drifter deployment locations are indicated with circles. Bathymetry care shown at 50 and 200 m. 
10

reverse. Fully loaded, the plane could carry 20 drifters at a time, but usually 

only 10-18 drifters were required to re-initialize the array every two weeks. 

In the 12-month period, the plane deployed 344 drifters, of which 342 (99%) 

produced data. 

ii) Data Processing: Once a month, Service Argos delivered to Technocean, 

Inc. a data tape that contained the times of ARGOS receptions and derived 

locations of the drifters and estimated to have a minimum uncertainty of 

300m. Technocean produced processed data from the raw data tapes. This raw 

location data was put into correct time sequence and de-spiked with an 

algorithm, which eliminated displacements that yielded velocities in excess of 

250 cm/sec. Observations with less than 15 min. time separation from 

different satellites were averaged together. Next, an analytic correlation 

function was computed every day from the Fourier transform of a model 

spectrum based on 10 days of unequally spaced data centered on the day of 

observation (VanMeurs, 1996). This correlation function was used to produce 

an interpolated location time series, sub-sampled every three hours. Velocity 

components were computed from center time differences. Daily average 

locations and velocities are used in this presentation.

The following data sets were derived at SIO and linearly interpolated 

on to the drifters' daily average locations: (a) the 0.08 degree resolution 

TOPO .08 bottom topography, (b) bottom topography gradient vector, 

computed from 0.25 latitude x 0.25 longitude sub-sampled bottom depth data, 

(c) long-topography and cross-topography velocity vectors.

The data density map on a 0.25 degree resolution (Figure 2) shows that 

the largest number of daily average observations come from a region 

shallower than 50 m. This was no surprise, as the deployment stations were 

shallower than 50m. Drifters cross over to the shelf break with ease as the 

region between 50m and 200m depths appears contain about 50% of the data 

even though they were not deployed there. The statistics of flow will be 

developed for all 0.25 x 0.25-degree regions if there were more than 5 oneday average observations in them.

II.4. Circulation patterns

The ensemble of drifter tracks (Figure 3) shows that while the 

predominant motion is along the shelf, drifters cross isobaths and become 
95 ̊W 90 ̊W 85 ̊W 

25 ̊N30 ̊N

Start LocationLast Location Campeche Bank

Figure 3. Tracks for each of the 342 drifters deployed during the SCULP-II experiment. Starting (endshown with circles (diamonds). Bathymetry contours are shown at 200 and 2000 m. 
12

entrained in the circulation patterns in the deep Gulf (Ohlmann et al., 2001) or 

cross the outflow of Mississippi River to the Louisiana-Texas (LATEX) shelf 

(Ohlmann and Niiler, 2001). Prominent in this display is a "forbidden zone" 

on the west Florida in the coastal waters (shallower than 10m) and a large 

area of the shelf directly north of the Florida Keys, nominally the Florida Bay 

(Yang et al., 1999). On their passage southward to join the Florida Current 

some are entrapped into cyclonic eddies emanating from the westernmost tip 

of the Florida Keys and are transported by these eddies to the Campeche 

Banks. Several of these drifters are also entrained into the shoreward side of 

the Florida Current or a jet along the shelf escarpment (Hetland et al., 1999). 

These latter, together with the drifters that move southward at the 50m isobath 

in the fall and winter seasons, travel to the North Atlantic or become 

entrained into the spin-off eddies on the shelf off eastern Florida or the 

Florida Keys. Thus it is seen that particles from the deployment region 

traverse the entire Gulf of Mexico and the US economic zone of bounding 

Florida on a time scale of less than 120 days.

A variety of shorter-period circulation phenomena are displayed in 

monthly tracks (Figure 4). Note that drifters on large segments of the shelf 

appear to move together, with reversals on time scales shorter than a month. 

The large displacements often occur during ten-day and even smaller periods. 

The largest coherent excursions occur in October '96 and November '96, when 

drifters move rapidly westward across the mouth of the Mississippi. Of the 

342 drifters deployed, 59 travel westward across 90W to the LATEX shelf. 

These motions reflect wind-forced motions that have synoptic meteorological 

time scales and have been reported from analyses of current meter data (e.g., 

Niiler, 1975). During the months of November through March, drifters moved 

southward on the west-Florida shelf in a well defined "stream" just shoreward 

of the 50m depth contour, the explanation for which has not emerged from 

dynamical considerations. 

In July 1996 a strong jet to the south was centered on the shelf-break at 

150m depth. This feature could have been related to the encroachment of the 

Loop Current on the Tortugas shelf (Hetland et al., 1999). In the period of 

April-October, there was a general westward movement of drifters south of 

the Florida Panhandle and west of Cape San Blas. This coastal flow to the 

west in water depth less than 20 m was most well organized in June 1996 

when the coastal winds in this area are strongly from the east. 
25 ̊N

30 ̊N

March 96 

Start LocationEnd Location 

April 96

Start LocationEnd Location

M

Start LocationEnd Location

90 ̊W 85 ̊W

25 ̊N30 ̊N

June 96 

Start LocationEnd Location

Figure 4. SCULP-II drifter tracks shown by month for a complete year (from March 96 through February locations on the first (last) day of each month are in denoted with circles (diamonds). Bathymetry contour200 and 2000 m. 

90 ̊W 85 ̊W

July 96 

Start LocationEnd Location

90 ̊W 

Aug

Start LocationEnd Location
25 ̊N

30 ̊N

September 96 

Start LocationEnd Location 

October 96

Start LocationEnd Location

Nove

Start LocationEnd Location

90 ̊W 85 ̊W

25 ̊N30 ̊N

December 96 

Start LocationEnd Location

Figure 4 continued. 

90 ̊W 85 ̊W

January 97 

Start LocationEnd Location

90 ̊W 

Febr

Start LocationEnd Location
15

The sequence of events that compose the October 1996 movement of 

35 drifters from the shelf south of the Panhandle to the LATEX shelf was 

clearly related to forcing by the tropical storm "Josephine" (Figure 5). 

Between October 3 and October 10, the entire west Florida shelf from Tampa 

Bay to the Mississippi Sound is accelerated to the west during the strong 

tropical storm (Ohlmann and Niiler, 2001). Similar movements occur in the 

period of November. 15-30 when 12 drifters make a rapid excursion to the 

west. These rapid displacements, with daily movements in excess of 100 

km/day across the mouth of the Mississippi River, had not been known before 

these drifter data were assembled. 

In summary, the drifter displacements appeared to be characterized by 

6-12 day vacillation along the shelf isobaths. In the winter months a persistent 

stream toward the south, just east of the 50m isobath. Occasional strong 

events occur during which the waters across the entire width of the shelf 

participate in the motion, either forced locally by storms or by Loop Current 

interactions with the shelf. The dispersions of drifters over large distances 

were caused primarily by these events.

II-4. Velocity statistics

The monthly mean velocity vectors (Figure 6) show that the strongest 

currents were found in water depths greater than 200 m. On the shelf, several 

spatially-coherent patterns appear in all months except April, May and 

September. In these months the mean velocity field was weak and contained 

many small spatial-scale features. In the months of August, October, 

November and January, large velocities in excess on 40 cm/sec, were 

observed to the west past the mouth of the Mississippi River. In April and 

May, monthly mean velocities in excess of 20 cm/sec were observed directly 

west of Cape San Blas. Because of the predominance of strong events with 

time scales shorter than a month, a smooth seasonal cycle cannot be defined 

well from viewing these data directly, but such a cycle may have existed on 

the northeast Gulf Shelf in 1996-97, as seen in the analysis below.

The annual mean velocity map, made from the entire record length of 

data (Figure 7), shows the strong currents that were caused by the short lived 

events discussed above. From this picture it is apparent that drifters cross the 

shelf break at the head of the DeSoto Canyon along the westernmost margin 
25 ̊N

30 ̊N

Sept 29 - Oct 2, 1996 

start locationend location 

Oct 3 - Oct 6, 1996 

start locationend location 

95 ̊W 90 ̊W 85 ̊W

25 ̊N30 ̊N

Oct 7 - Oct 10, 1996 

start locationend location 

Figure 5. SCULP-II drifter tracks recorded from a) 29 September 1996 through 2 October 1996, b) 3 OctOctober 1996, c) 7 October through 10 October 1996, and d) 11 October through 14 October 1996. Drifter continental shelf (water depths < 200 m) are illustrated in black. Tracks for regions offshore of the shelf-b(> 200 m) are gray. Starting (ending) drifter positions are denoted by circles (diamonds). Bathymetry contare shown at 200 and 2000 m.

95 ̊W 90 ̊W 

Oct 11 - Oct 14, 199

start locationend location 
25 ̊N

30 ̊N

50 cm/s

March 96 

50 cm/s

April 96 

M

90 ̊W 85 ̊W

25 ̊N30 ̊N

50 cm/s

June 96 

Figure 6. Monthly mean surface current vectors computed from SCULP-II drifter data. Mean velocities a0.25 x 0.25 degree bins over a complete year (March 96 through February 97). Velocity vectors over the c(water depths < 200 m) are illustrated in black. Vectors for regions offshore of the shelf-break (> 200 m) acontours are shown at 200 and 2000 m.

90 ̊W 85 ̊W

50 cm/s

July 96 

90 ̊W 

Aug
25 ̊N

30 ̊N

50 cm/s

September 96 

50 cm/s

October 96 

Nove

90 ̊W 85 ̊W

25 ̊N30 ̊N

50 cm/s

December 96 

Figure 6 continued. 

90 ̊W 85 ̊W

50 cm/s

January 97 

90 ̊W 

Febr
90 ̊W 85 ̊W

25 ̊N

30 ̊N

50 cm/s

Figure 7. Mean surface current vectors from the SCULP-II drifter data. Mean values are 

calculated for 0.25 x 0.25 degree bins with 5 or more observations. Vectors inshore 

(offshore) of the shelf-break (200 m) are shown in black (gray). Bathymetry contours

are shown at 50, 200, and 2000 m. 
20

of the Mississippi Sound and directly west of the Dry Tortugas. The jet just 

shoreward of the shelf break was not due to the local presence of the Loop 

Current along the shelf, but is a shelf circulation feature caused by the 

interaction of the Loop Current with the shelf near the Tortugas in June 1996 

and July 1996 On the central shelf, where the deployment strategy obtained a 

uniformly sampled data set, no resulting significant mean currents were 

displayed.. When Figure 7 is viewed together with Figure 2 it is apparent that 

the strong mean circulation features here were not sampled as well as on the 

northeast shelf, and drifters entered these features only when they were 

carried away from the deployment area. These mean current features could 

well be a result of the biased sampling that resulted from the fact that drifters 

were not deployed on a regular basis in the regions where they occurred. It is 

quite remarkable, however, that the mid-shelf region of no mean flow was 

able to eject drifters to the four corners of the Gulf via Lagrangian transport 

processes of short-lived, strong events on a time scale less than 120 days. 

Thus the value of a limited number of time series of currents taken on this 

shelf at various fixed locations for deriving conclusions on the fate of oil must 

be questioned severely.

Though it will be obvious to the reader, perhaps we should emphasize 

this last point. A mean signal emerges in this data set, but in many of the 

regions there are only a very few observations. One should be very reluctant, 

therefore, to interpret any mean value based on fewer than perhaps 100-150 

samples as being well observed. Furthermore, it is essential to be aware that 

the mean from even the best-sampled regions are a mean for this time period 

only. A long-term mean (assuming that one exists that is different from zero) 

would have to be established from data taken in many different years. We are 

beginning to assemble enough data – both of wind and of currents – to 

estimate how long this might require, but at this writing is remains an open 

question.

The variance ellipses (Figure 8) demonstrate that the variance along the 

bottom contours was larger than across bottom contours. The major axes of 

the variance ellipses, however, show a consistent counterclockwise rotation 

relative to the bottom contours. This could well have been due to the Ekman 

currents driven by winds that crossed isobaths. The smallest variances were 

found in the shallow water between Cape San Blas, Cedar Key and Tampa 

Bay. Variance in 50 m depths west of Cape San Blas was as vigorous as that 

on the shelf break. The topography of De Soto Canyon is only weakly felt in 

the relative different rotations of the variance ellipses on either side of the 
90 ̊W 85 ̊W

25 ̊N

30 ̊N

500 cm2 sec-2

Figure 8. Velocity variance ellipses computed from SCULP-II drifter data. Ellipses are 

calculated from daily averaged velocity values for 0.25 x 0.25 degree bins with 5 or more

observations. Bathymetry contours are shown at 50, 200, and 2000 m. 
22

Canyon wall. Drifters move across these walls without much regard to the 

severe changes of topography that occur below. 

The vector EOFs of velocity computed from the shelf less than 100m 

depth show that a pattern that was unidirectional and encompasses the entire 

shelf (Figure 9) accounts for 47% of the variance (Figure 10). The four principal 

EOFs account for a total of 75% of the variance and account for 100 km spatial 

scales of flow along topography. The time series of the amplitude of the 

principal mode has energy on time scales of 5 days and larger. It also reflects the 

strong wind events that caused the rapid movement of drifters toward the 

Mississippi Sound in October and November of 1996. 

The second principal EOF mode appears to have a shift in August from a 

predominantly positive value to a negative value and is perhaps the best representation of the "seasonal cycle" of the circulation. This pattern shows 

convergence toward Cape San Blas from both sides of the Cape with a strong 

current to the northwest on its western side.

The residual of the energy, 25% of the initial energy, after the currents 

composed of the four principal modes were subtracted, still shows a preponderance of motion along topographic contours (Figure 11). This represents spatial 

scale motions with coherence scales less than 50 km, or two grid points of the 

averaging scale used. Thus there were significant energetic motions, such as the 

coastal current west of Cape San Blas, that were not coherent with the rest of the 

motions on the shelf on a 100 km spatial scale and were not well represented in 

the principal modes. There were motions that were not well represented by the 

decomposition of the principal components. The assumption of separability into 

functions of space and time in constructing EOFs may not be valid here.

To characterize the space and time scales of motion, the auto-correlation 

function of the along-topographic current was computed as a function of time

lag and spatial lag along topographic contours (Figure 12). Significant 

differences occur in the transition from 10-30 m depth region to 100-150 m 

depth region. The time scales of coherence increase from 10 days to 15 days and 

then decrease to 7 days. The space scales increase from 25 km to 300 km and 

then decrease to 100 km. In summary, the currents in shallow water persist for 

10 days, but have short space scales along topographic contours. On the midshelf, the time scale is several weeks and the space scales encompass the entire 

study area. On the shelf break the time scales are much reduced, as are the space 

scales, perhaps due to the small scale eddies of the shelf-break region. There is a 

tendency of the signals to propagate to the west along the mid-shelf, much more 

slowly than continental shelf waves would in this region. 
30 ̊N

50 cm/s

mode 1 

mode 2 

90 ̊W 85 ̊W

30 ̊N

50 cm/s

mode 3 

Figure 9. Spatial patterns of the first four empirical orthogonal eigenfunctions. EOFs are calculated witvelocity data on a 0.25 x 0.25 degree grid. Modes 1 through 4 explain 47.2, 12.3, 8.5, and 6.2% of the variThe 200 m isobath is shown. 

90 ̊W 85 ̊W

mode 4 
-0.2

-0.1

0.0

0.1

0.2

-0.2

-0.1

0.0

0.1

0.2

Feb Apr Jun Aug Oct 

1996

Dec Feb 

1997

Mode 4

6.22% of variance

Figure 10. Temporal amplitudes corresponding to the first 4 spatial EOF modes shown in Figure 9. 

-0.2

-0.1

0.0

0.1

0.2

-0.2

-0.1

0.0

0.1

0.2

Mode 3

8.48% of variance

-0.2

-0.1

0.0

0.1

0.2

-0.2

-0.1

0.0

0.1

0.2

Mode 2

12.33% of variance

-0.2

-0.1

0.0

0.1

0.2

-0.2

-0.1

0.0

0.1

0.2

Mode 1

47.18% of variance
90 ̊W 85 ̊W

30 ̊N

500 cm2 sec

Figure 11. Variance ellipses computed for differences between daily average surface velocifrom drifter data on a 0.25 x 0.25 degree grid, and velocities computed from the first four moof the EOF analysis. The 200 m isobath is shown.

27

III. The Moored Current Meter Experiment

The primary focus of the NEGOM program was the surface drifter 

experiment to observe currents very near the sea surface. An auxiliary 

experiment involving moored current meters, as described here, was included 

for several reasons. First, there is the basic question: Do the measurements 

from drifters tell us the same things as measurements from moorings? 

Second, as is well known, the moorings provide continuous observations at a 

point, while drifters provide sequential observations at many points. The 

Eulerian and Lagrangian frameworks provide complementary observations, 

but they are qualitatively different, and by having both kinds of observations 

we hoped to be able to map the velocity fields with better accuracy and 

resolution. Third, there is a great deal of existing data from moored 

instruments. Having at least a minimal set of moored observations here to 

augment the drifters provides a means of insuring continuity between the new 

work and historical archived results.

Figure III-1 shows the location of a variety of moored instruments. 

W36 and W39 were meteorological buoys, provided by the NDBC. M56 and 

M30 were multi-instrument moorings installed by Chevron, Inc. at the head of 

DeSoto Canyon prior to the beginning of the drifter experiment. M30 was set 

using MMS equipment, so the data are publicly available. The data from 

M56, however, were still proprietary at this writing.

Moorings 517 – 521 were simple moorings with a single near-bottom 

S4 current meter set 4 m above the bottom. These moorings suffered from 

frequent encounters with fishing trawlers. As a result, moorings designated 

Kt (K-tower), Gu (Gulf) and Pa (Panama) were set with an upward-looking 

acoustic Doppler current meter (ADCP) on the bottom, in trawl-proof 

housings. These were set by the Current Meter Facility at Florida State 

University.

The mooring designated simply as “W,” on the 50-m contour, was set 

by an instrument group headed by Weisberg at the University of South 

Florida.

Figure III-2 shows a set of time lines of the mooring emplacements. 

Figure III-3 shows the design of Weisberg’s mooring, in which the adcp was 

placed at the sea surface.

An extensive set of plots of current measurements is given in Appendix 

A. Further analyses and comparisons are given in the next two sections of 

this report. A few brief results are given here as an example of the nature of 

the results.
28
29
30
31

One of the fundamental findings of wind-driven coastal currents is that 

they have a characteristic oscillatory nature, as described in the section on 

Drifter Results. Figure III-4 shows a set of variance ellipses for results at the 

near-bottom currents. The arrows show the means for the duration of the 

mooring settings, and the ellipses show the variance in the “wind-driven 

band” of periods longer than 30 hours (to suppress tides and inertial motions) 

out to periods of 20 days. Figure III-5 shows equivalent results as measured 

in the middle of the water column by the (bottom-mounted) adcp instruments. 

It is clear that the wind-band variance is very large in comparison with the 

weak mean signals. Figure III-6 shows the large variance in the tidal-inertial 

band – also as measured in the middle of the water column.

To illustrate the nature of the large variability of the flow in 

comparison with the mean, Figure III-7 shows a plot from the Chevron 

moorings south of Pensacola. With approximately 30 months of data, we 

might expect that a mean flow either to the west or the east might emerge. 

The “x” values are the monthly means; the dashed lines show the standard 

deviation of the monthly means. The monthly values range from –7 to 19 

cm/sec, and the over-all mean is 1.5 cm/sec. The individual monthly mean 

values are essentially independent (as the autocorrelation goes to zero in 

about 30 days) so the standard error of the mean is less (by the square root of 

30) than the standard deviation shown by the dashed lines. It thus appears 

that although the mean flow is ~ 1.5 cm/sec (positive, to the east), and 

significantly different from zero by 5 standard errors, the flow in any given 

month is likely to be significantly different from the long-term mean.

The importance of this result bears further explanation. During a given 

wind event, the wind-induced flow can be computed fairly well (as shown in 

section V). It is a “meaningful question” to inquire about the flow from a 

given wind event. However, it is a much less meaningful question to inquire 

about the “mean flow” in general. No clear seasonal signal emerges from the 

results shown in Figure III-7. And while the slightly positive mean flow (to 

the east) seems to be significantly above the noise, we would be reluctant to 

assume as certain that a repeated set of observations would give a similar 

mean unless we could be assured that the wind forcing would be similar. We 

note that the mean flows from the drifter experiment (Fig 7, Section I) are 

also not significantly different from zero.
32
33
34

 
35
36

III.1 Winter versus Summer

Figures III-8 through III-10 show a set of comparisons between 

observations made in the winter versus those in the summer. Each panel 

shows two months of data from the winter and then data from two months in 

the summer. These results, while typical, are from the ADCP offshore of 

Panama City. The figures are for results at 2m (in the uppermost bin from the 

ADCP), from near the middle of the water column, and from a bin 

approximately 3 m above the bottom. We see the characteristic result of 

strong currents in the winter, weaker currents in the summer. The frontal 

systems come through a periods of 3-10 days, making the flow oscillate from 

one direction to the other. The prominent directions on the stick diagrams can 

be taken to be “alongshore.” The velocity observations are made throughout 

the water column, but the temperature (bottom panel) is measured within the 

ADCP housing, at the bottom, and thus are the same in all three figures.
37
38
39
40

IV. Comparing Drifter Results and Current-Meter Results.

If we wish to understand the environmental impact of a contaminant at 

the sea surface, it is clear that we need to know and understand the velocity in 

the uppermost layers of the ocean. Historically, however, we have had 

velocity data almost exclusively from moored current meters. Therefore it 

seems important to examine whether these two types of data “see the same 

thing.”

We realize of course that these two different measures of currents can 

not possibly give identical results. Moorings down in the water column are 

much less able to measure the effects of the upper Ekman layer. Moreover, in 

the NEGOM experiment, our measurements for most of the duration of the 

drifter experiment were only 4 m above the bottom and therefore in the 

bottom boundary layer. The near-bottom currents will be strongly 

constrained by topography – much more so than the upper layer flow 

measured by the drifters. 

For this reason we have made the following comparisons. The two 

moorings 520 and 521 (as seen in Figure III-1) are the ones with the 

smoothest local bottom topography and therefore the most likely to show 

good comparisons. We first determine a “longshore” direction from maps of 

local bathymetry, and orient the current meter results so as to be aligned in a 

longshore coordinate system. We then rotate the coordinate system of the 

drifter velocities (at the same location) to determine the angle of maximum 

correlation between the two. Figure IV-1 shows how the correlation changes 

as a function of this angle (here measured counter-clockwise, relative to East). 

The purpose of showing this (rather bland) result is to emphasize that picking 

exactly the right angle is not crucial; 10° or so in either direction is barely 

noticeable on this plot. 

Mooring 521 is in a region of relatively straight isobaths at a depth of 

approximately 26m. The S4 current meter here is 4 m above the bottom and 

is therefore likely to be in the bottom boundary layer.

Figure IV-2 shows the primary result, a scatter plot with a correlation 

coefficient of 0.66. Using a linear least squares fit such that

 Speed at the current meter = k * speed of the drifter + offset (IV-1)

we find that the coefficient here between drifter and near-bottom current is 

0.31 (dimensionless), with an offset of –1.1 cm/sec. In other words, if the 

drifter, at the surface, shows a velocity of 10 cm/sec, we would expect, on the 
41
42
43

average, a velocity of only 3 cm/sec at the near-bottom current meter. The 

speeds here (for Figure IV-2) are daily fits for the drifters, as described in 

Section II, and “daily” filtered to suppress tides at the current meters. The 

dashed lines about the least-squares fit show an estimate of the uncertainty of 

the slope (k) as determined by the Bootstrap method.

Figure IV-3 gives a comparison of the longshore speeds as seen by the 

current meter with the actual observation times when drifters were close to 

the mooring. We see that the drifter observations, as seen by the current 

meters, do a very good sampling job for the times when they are present.

Figures IV-4 and IV-5 show equivalent results for mooring 520, which 

was inshore of 521. The observed speeds in shallower water are somewhat 

greater. Table IV-I summarizes these results. The offset speeds of (-) 1-2 

cm/sec suggest either that when the wind stops the flow at the current meters 

does not stop as quickly as the drifters do, or that there is a small mean flow 

even in the absence of wind. These observations, however, are quite limited, 

and with an over-all scatter of ~20 cm/sec the mean is of questionable value, 

although the linear coefficient between the two variables is significant.

_____________________________________________________________

Table IV-I. Relationship between daily mean longshore speeds at moored 

current meters (4 m above bottom) and speeds measured by the drifters.

Depth Heading for max R R k veering angle

 Bottom Drifter

Mooring 520 17m 305° 340° .63 .53 35°

Mooring 521 27m 305° 330° .66 .31 25°

Several features emerge from these comparisons. In shallow water 

(520) the offset angle appears slightly greater, although these calculations are 

done with only a resolution of 10°. This can be the effect of the higher speeds 

at 520, giving rise to greater veering in the bottom boundary layer. The 

coefficient (k) is much greater at 520. This suggests that higher speeds in the 

upper Ekman layer are influencing the lower layers to a greater extent at 520 
44
45
46
47

than at 521. The nice result, however, is that – despite an expected amount of 

scatter in the observations, the two kinds of measurements do tend to “see the 

same thing” rather well in the longshore direction.

Similar calculations were made for the cross-shore component of 

velocity, which were not reliably coherent. The correlation coefficients were 

–0.02 (at 520) and –0.16 (at 521). Part of this lack of correlation is that the 

upper veering to longshore winds is to the right of the wind at the top, and to 

the left at the bottom. When the winds are not blowing strongly, there is little 

or no veering to the right at the surface. 

It will be shown in the next section that the cross-shelf velocity 

component, as determined by the drifters, actually is found to be coherent 

with the wind forcing. However, the angle of the wind must be adjusted 

differently for the cross-shelf component than for the alongshore component.

We also should point out one detail of these calculations. In the results 

shown here (Table IV-I) we fixed the angle of orientation of the current meter 

results and rotated the drifter velocity components to find the best agreement. 

There is a reason for this, of course, because the current meters, near the 

bottom, are constrained by bottom topography. It could be argued, however, 

that this choice is somewhat arbitrary, and a better fit might possibly be 

obtained by choosing the “longshore” orientation for the drifters first, then 

rotating the current meter data to find an angle of orientation for maximum 

correlation. We did this. The results of this second calculation, however, 

differ from Table IV-I in only the second or third decimal place, with random 

signs, and so are not considered significant.
48

V. Practical results: Comparing Drifter Results with Wind Forcing.

In order to make comparisons between observed currents and the winds 

that force them, it is usual practice in a scientific experiment to deploy 

meteorological buoys near the experiment to measure winds. In the work 

described here, such buoys from the NDBC were in place.

As a general rule, however, when we wish to use the results of 

experiments such as this in broad applications, we would expect to have 

available only the standard weather bureau products. In order to make our 

results from this study applicable to calculations under those circumstances, 

we have done a series of calculations to determine the correlations between 

the drifter currents and a standard meteorological product, the so-called 

NCEP (National Center for Environmental Prediction, NOAA) winds. These 

wind data are available over the Gulf of Mexico on a grid every 2.5° in 

latitude and longitude. For our purposes here, we interpolated to positions 

midway between two standard NCEP grid locations to provide daily wind 

information as close as possible to offshore locations near the drifter 

experiment.

It is well known that for motions in the 3-10 day “weather band” 

coastal currents preferentially move approximately along the isobaths. Figure 

V-1 shows the drifter motions for March 1996 data, in which the drifter tracks 

obviously follow the local bottom topography in the Big Bend region north of 

St. Petersburg. These motions are of course consistent with the along-isobath

coherence shown in Section II. For our analysis, therefore, we grouped the 

drifter data into two main sets appropriate to the bottom slope. One set is 

formed from data in the Big Bend region, between St. Petersburg and Cape 

San Blas, but using only data near the center of the region, avoiding the edges 

where the topography turns. The second data set is from the region between 

Cape San Blas and the western edge of the data shown in Figure V-1. The 

idea was to have the bottom topography be roughly similar for all the data in 

each group--that is, to group the data from the very wide shelf and the data 

from the narrow shelf region to the north separately. The data in each group 

were then further divided by depth. 

In a manner similar to that in the previous section, we first determined 

the angle of wind orientation that maximizes the correlation between the 

NCEP daily winds and the currents from drifters. Figure V-2 shows how the 

correlation function changes as a function of wind rotation, from the study 

region to the east, in the area between St. Petersburg and Cape San Blas. For 
49
50
51

these calculations we used a wind data set interpolated to 28.75° N, 83° W. 

Further explanation of the angular conventions is discussed in a later section.

Two results emerge from this figure. First, the correlations are 

moderately high – over 0.6 for both the longshore and the onshore 

components. Second, as one might have expected, the angle of rotation that 

gives the highest correlation for the longshore component is not the same 

angle that gives the highest correlation for the onshore component. 

Figure V-3 shows the scatter plot of the comparison between longshore 

winds and longshore drifter motions. A least-squares fit through these points 

gives a slope of 1.06 (using cm/sec for the surface currents, m/sec for the 

winds). The correlation is that found at the maximum in figure V-2. The 

dashed lines near the best fit show an estimate of the uncertainty of this 

coefficient (determined by a Bootstrap method). 

The intercept, -0.86, says that when the winds stop blowing there is a 

flow of .86 cm/sec to the northwest. For this time period. This value is 

slightly above the standard error of the mean, but should not be misconstrued 

as a generally applicable result. The winds in some other year are not likely 

to be the same as those for this experiment.

We performed similar calculations for the longshore motions and the 

onshore, or cross-shelf, motions, in both of the regions described earlier. The 

essential results are given in Tables V-I, V-II.

______________________________________________________________

Table V-I. Correlations, R, and optimum wind angles between NCEP winds 

and NEGOM surface currents from drifters in the region between St. 

Petersburg and Cape San Blas. The isobaths are assumed to be oriented 

toward 327° True, hence the onshore direction is 057°. Winds are 

interpolated to 28.75° N, 83° W. The coefficient “k” is the linear fit between 

wind speed (m/sec) and currents (cm/sec)

Depth Wind Angle Rmax k

 for Maximum R

Longshore/Onshore Longshore/Onshore Longshore/Onshore

10-20 m 340 045 0.68 0.63 1.06 0.56

20-30 m 330 020 0.66 0.62 1.19 0.58

30-50 m 315 045 0.56 0.58 1.25 0.63
52
53

______________________________________________________________

Table V-II. Correlations, R, and optimum wind angles between NCEP winds 

and NEGOM surface currents from drifters in the northern region, west of 

Cape San Blas. The isobaths are assumed to be oriented toward 305° True, 

hence the onshore direction is 035°. Winds are interpolated to 30.° N, 86.25°

W. The coefficient “k” is the linear fit between wind speed (m/sec) and 

currents (cm/sec)

Depth Wind Angle Rmax k

 for Maximum R

Longshore/Onshore Longshore/Onshore Longshore/Onshore

10-30 m 295 020 0.66 0.54 2.57 0.82

30-50 m 300 000 0.57 0.38 2.20 0.68

50-100 m 285 000 0.46 0.35 1.85 0.57

V.1 Longshore coefficients. 

Figure V-4 summarizes the results of the longshore comparisons. The 

x-axis is depth; a comparison plotted against distance offshore would show a 

somewhat different view, but for the present purpose the water depth is 

appropriate. In both the eastern and northern regions, the correlation between 

the winds and the drifter motions falls away smoothly with depth (or with 

distance offshore). The “k” values, which represent the ratio of current 

response to the wind forcing, show quite clearly that the speeds are higher, for 

the same wind speeds, by a factor of ~2 along the narrow shelf off PensacolaPanama City compared with the wide shelf in the east. This is a well-known 

result, in general, but the values shown here for surface current speeds are a 

new result.
54
55

V.2 Onshore coefficients.

Figure V-5 shows the equivalent plot for the Onshore comparisons. 

One gets the initial impression that if the x-axis were chosen to be distance 

offshore, a result much closer to a simple linear trend would appear. (This 

view is used to be consistent with Figure V-4.) 

The correlation values, R, in the eastern region remain similar to those 

of the longshore motions, but the correlations are substantially lower. One 

expects that on a broad shelf, as in the east, the inshore currents are isolated 

from the eddy motions imposed at the shelf break. In the north, however, the

region of our studies is not far from the shelf break, and offshore eddy 

motions affect the results here.

It may not be immediately obvious why these differences occur. 

Therefore, Figures V-6, V-7 show scatter plots for the longshore and onshore 

components of the deepest segment in the northern region. For the longshore

component, the drifter speeds reach 40 cm/sec. For the onshore component, 

the drifter speeds are half that or less. We know from the recent, magnificent 

satellite observations that the eddy swirling motions are, at least to first order, 

approximately horizontally homogeneous. Their effect (in our calculation) is 

therefore larger for the onshore motions. It is not that the drifters do not 

perform well, but that the motions are induced by effects other than the local 

winds. Figure V-6 shows that the correlation drops considerably as the 

currents are driven by the eddy motions in addition to the winds, even for the 

longshore flow. For the onshore motions, however, the effect is relatively

much larger.
56
57
58
59

V.3 The Angles of rotation.

Tables V-I, V-II also show the angles at which the highest correlations 

are found. 

Some explanation may be necessary here. For these calculations, in 

both cases we rotate the U wind component (originally to the east) to obtain 

the highest correlation for each pair. The “Longshore” plot shows the 

correlation between the longshore winds and the longshore currents. The 

“Onshore” plot shows the correlation between the onshore winds and the 

onshore velocity component. The angle of rotation shown on the figure 

however remains that of the rotation of the U wind component.

In some cases the two angles are not the same. That is, the angle of 

rotation that provides the highest correlation for the longshore flow is not the 

same as the angle of rotation that provides the highest correlation for the 

onshore flow. There are of course physical explanations for the differences 

between the two resulting angles. We know that, in general, longshore 

currents are driven by longshore winds, but the effect is indirect. The 

longshore flow results from the pressure gradient that arises from the 

longshore winds. Onshore motions, by contrast, require the effect of the 

surface Ekman layer more directly.

Figure V-2 shows that the highest correlation for longshore drifter 

motions are found with winds blowing toward ~340° (rotated ~110° counterclockwise from East (we use here the oceanographic convention to describe 

directions). For the calculations in the eastern region we determined that the 

isobaths ran along ~ 327° True (rotated ~ 123° ccw from east). Thus, the 

wind direction for the highest correlation is about 10° to the right of the 

isobaths. We might have assumed a priori that the optimum wind direction 

would be slightly to the left of the isobaths. Whether this anomalous result is 

from a poor choice of bottom topography direction, from uncertainties in the 

NCEP winds, or merely random errors, we cannot tell. However, a finding 

that is within 10°, which is the resolution of our calculation, is probably as 

good as we might expect. The essential result is that the drifters, at the sea 

surface, go almost directly downwind. This is in keeping with the standard 

coastal, longshore-flow expectation.
60

And perhaps we should emphasize that the close agreement between 

these two directions is a near-shore, shallow water result. In open water away 

from the coast we would expect the near-surface Ekman veering to be at least 

35° or more. Careful measurements at depths as shallow as 5-10 m in deep 

water show veering angles as large as 65-75° (Price et al., 1980).

For the onshore motions, however, the highest correlation (from Figure 

V-2) is found at an angle of ~130° to ~140° ccw from east, for the U 

component. Thus the result, for the onshore component, is that winds 

blowing toward a true direction toward 045° have the highest correlation with 

onshore drifter motions. The direction normal to the isobaths here, which is 

the direction of onshore flow, is ~ 057° T; thus the onshore flow is found to 

be ~12° to the right of the onshore wind component. Based on the result of 

the longshore calculations, we would expect that this value is uncertain by 

~10°. If the onshore current component is found to be ~10° to 20° to the right 

of the wind, this is in keeping with the expected veering to the right in the 

upper Ekman layer. In Ekman’s classic result, he showed that in shallow 

water the angle of veering would be less than in deep water.
61

Section VI. An annotated bibliography of published papers, 

reports, and theses from this program.

He, R. and R.H. Weisberg, 2001a. Circulation and temperature budget on 

the west Florida shelf during the 1999 spring transition. Cont. Shelf Res., in 

press.

We have used the present version of our POM-based model (which 

extends from west of the Mississippi River to the Florida Keys) to simulate 

the currents and temperature/salinity variations during the spring 1999 

seasonal transition in response to local shelf-wide forcing by winds, surface 

heat flux, and river inflows. We first compare the model results with in situ 

data before diagnosing the model evolution. The paper supports the 

hypothesis of Weisberg et al. (1996) on the role of buoyancy forcing in the 

seasonal transitions, and it demonstrates how local forcing independent of 

the Loop current can account for much of the shelf circulation, in particular 

the "Green River" phenomenon south of Cape San Blas and the advection of 

Mississippi River water to the west Florida shelf.

He, R. and R.H. Weisberg, 2001b. Tides on the west Florida shelf, a model 

and in situ data comparison. In preparation, draft complete.

Here we force a regional model with a global tide model to describe 

the sea level and currents for the (barotropic) M2, S2, O1, and K1 

constituents. We compare sea level along the coast from Mississippi to 

southwest Florida, and we compare velocity hodograph ellipses where 

available. Tidal residuals are also estimated to assess the potential for tidal 

rectification. A future extension of this work will be to incorporate all 

available data and to consider the effects of stratification.

Hetland, R.D., Y. Hsueh, R.R. Leben, and P.P Niiler, 1999: A Loop Currentinduced jet along the edge of the West Florida shelf. Geophysical Research 

Letters, 26 (15), 2239-2242.

The pressure imposed upon the West Florida Shelf by the Gulf of 

Mexico Loop Current is found to give rise to a southward-flowing jet along 

the shelf edge. The pressure-induced jet is simulated by a general circulation 
62

model of the Gulf. However, the physics causing formation of the jet are 

well represented by a simple continental shelf model incorporating an 

idealized geometry for the West Florida Shelf. The pressure response over 

the shelf, in an analogy to conductive heat transfer in a rod, is limited by the 

"insulating" effect of the steep topography of the West Florida Escarpment, 

which hinders the onshore spreading of the pressure influence. The shelfedge jet is apparent in the trajectories of satellite-tracked surface drifters 

deployed from February 1996 through March 1997. Evidence for the 

requisite pressure distribution over the escarpment is provided by 

measurements of sea level from the satellite altimeter aboard the 

TOPEX/POSEIDON satellite.

Meyers, S.D., E.M. Siegel, and R.H. Weisberg, 2001. Observations of 

currents on the west Florida shelf beak. Geophys. Res. Lettrs. 28, 2037-

2040.

Here the authors describe observations of currents in the vicinity of 

the shelf break to the west of Tampa Bay. The currents there are highly 

variable, and the effects of the Loop Current and its eddies are sporadic. 

Spring and summer of 1998 exhibited large stratification over the innershelf, even right up to the shoreline. In situ data shows that the circulation is 

very sensitive to stratification. By separating the surface and bottom Ekman 

layers, stratification in increases across-shelf transports.

Ohlmann, C. J., P. P. Niiler, C. A. Fox and R. R. Leben, 2001: Eddy energy 

and shelf interactions in the Gulf of Mexico. Journal of Geophysical 

Research, 106 (C2), 2605-2620.

Sea-surface height anomaly data from satellite are continuously available 

for the entire Gulf of Mexico. Surface current velocities derived from these 

remotely sensed data are compared with surface velocities from drifting 

buoys. The comparison shows that satellite altimetry does an excellent job 

resolving Gulf eddies over the shelf-rise (depths between ~200 and 2000 m) 

if the proper length scale is used. Correlations between altimeter and drifter 

derived velocities are statistically significant (r > 0.5) when the surface slope 

is computed over 125 km, indicating that remotely sensed sea-surface height 

anomaly data can be used to aid the understanding of circulation over the 

shelf-rise. Velocity variance over the shelf-rise from the altimetry data 
63

shows regions of pronounced eddy energy south of the Mississippi outflow, 

south of the Texas-Louisiana shelf, and in the northwest and northeast 

corners of the Gulf. These are the same locations where surface drifters are 

most likely to cross the shelf-rise, suggesting that Gulf eddies promote 

cross-shore flows. This is clearly exemplified by both warm and cold 

eddies. Finally, the contribution of Gulf eddies and wind stresses to changes 

in the mean circulation are compared. Results indicate that the eddygenerated vorticity flux to the mean flow is greater than the contribution 

from the surface wind stress curl, especially in the region of the Loop 

current and along the shelf-rise base in the western Gulf. Future modeling 

efforts must not neglect the role of eddies in driving Gulf circulation over 

the shelf-rise.

Ohlmann, J.C. and P.P. Niiler, 2001: A two-dimensional response to a 

tropical storm on the Gulf of Mexico shelf. Journal of Marine Systems, 29, 

87-99.

Surface current data from drifting buoys and remotely sensed wind data 

recorded over the continental shelf in the northeastern Gulf of Mexico 

during the passage of tropical storm Josephine in October 1996 are 

examined. Drifter data show the existence of a strong surface jet (velocities 

reaching 1 m s-1) that moves up the west Florida shelf and westward along 

the Louisiana-Texas shelf and lasts for nearly one week. The coastal jet 

occurs during an intense synoptic-scale wind event where wind speeds reach 

15 m s-1. A simple force balance and statistical analysis are performed to 

assess the role of strong wind forcing. The primary balance shows an 

Ekman-type current. The role of local acceleration is greatest when winds 

are directed along bathymetry. A simple two-dimensional strongly forced 

shelf response model developed from the linear steady-state momentum 

equations also indicates larger along-shore currents due to both Ekman-type 

forcing by cross-shore winds and a cross-shore pressure gradient arising 

from conservation of mass. Model parameters fit empirically are within 

15% of theoretical values. The simple model explains 30 and 46% of the 

variance in the observed along-shore and cross-shore surface currents, 

respectively.
64

Sturges, Wilton and Robert Leben, 2000. Frequency of Ring Separations 

from the Loop Current in the Gulf of Mexico: a revised estimate. Jour. 

Phys. Ocean, 30, 1814-1819.

The offshore forcing of the west Florida Shelf is controlled primarily by 

the Loop Current and its rings. In this paper we examined the frequency of 

variability of the Loop Current and the rate at which it sheds rings on the 

basis of all the ring separation data since the late 1970s.

Sturges, Wilton, and B.G. Hong, 2001. Gulf Stream transport variability at 

periods of decades. Jour. Phys. Ocean., 31, 324-332.

In many studies of the variability of the Loop current, the basic transport 

of the Florida Current/Gulf stream system is taken as if it had constant 

transport. This study examines the longer-period variability of the Florida 

current. We find that the decadal scale fluctuations are larger than those of 

the annual cycle. The method uses model output on the offshore side of the 

Gulf Stream and sea level fluctuations at the coast. The results are 

compared with observed transport and the agreement is surprisingly good.

Walsh, J.J. K.D. Haddad, D.A. Dieterle, R.H. Weisberg, Z. Li, H. Yang, F.E. 

Muller-Karger, C.A. Heil, and W.P. Bissett, 2001. A numerical analysis of 

the landfall of 1979 red tide of Gymnodinium breve along the west coast of 

Florida - the fall/winter upwelling mode. Cont. Shelf Res., in press.

This paper is an attempt at combining physical and ecological models on 

the West Florida Shelf. Here we attempt to simulate the evolution of the 

1979 red tide, providing some insights into the joint workings of the biology 

and physics that culminate in near shore red tide effects.

Weisberg, R.H., B. Black, Z. Li, 2000. An upwelling case study on 

Florida’s west coast, J. Geophys. Res., 105, 11459-11469.

This paper discusses a specific case study of an upwelling event 

recorded both in satellite AVHRR imagery and in situ data. While 

upwelling (and downwelling) occur regularly on continental shelves with the 

passage of each weather front, rarely do all the necessary ingredients 

coincide to make the upwelling response visible in satellite imagery. In this 

case, the winds were light for several days prior to a strong, upwelling 

favorable wind event, and we had in situ velocity measurements for 
65

comparison. Through a combination of data and model analyses we 

demonstrate the Ekman-geostrophic spin up route of the inner shelf. Using 

both constant density and stratified runs (stratification estimated from 

velocity shear by thermal wind) we describe the relative effects and explain 

the finding of maximum upwelling just offshore and to the south of Tampa 

Bay. We also explain other regions of local upwelling maxima south of 

Cape San Blas.

Weisberg, R.H., Z. Li, and F.E. Muller-Karger, 2001. West Florida shelf 

response to local wind forcing: April 1998. J. Geophys. Res., in press.

Here, we describe in situ data and a numerical model simulation for 

the month of April 1998 and, buoyed by the fidelity between the two, use the 

model to analyze the dynamics. A new result is found. The data show a 

rectification of the inner-shelf responses to synoptic wind forcing wherein 

upwelling favorable winds produce disproportionately larger responses in 

both sea level and currents than downwelling favorable winds. Stratification 

accounts for the rectification, and this is most readily understood in terms of 

the streamwise component of vorticity. For downwelling favorable winds, 

the buoyancy torque due to isopycnals bending into the sloping bottom 

opposes the tendency by planetary vorticity tilting due to the vertically 

sheared coastal jet. This thermal wind effect negates the need for large 

relative vorticity dissipation by the across-shelf flow in the bottom Ekman 

layer. The opposite occurs for upwelling favorable winds. Buoyancy torque 

adds constructively with planetary vorticity tilting requiring larger 

dissipation of relative vorticity by the bottom Ekman layer. By enhancing 

(upwelling) or suppressing (downwelling) the bottom Ekman layer the entire 

response is reduced or increased, respectively. 

Yang, Huijun, Robert H. Weisberg, P. P. Niiler, W. Sturges and W. 

Johnson, 1999: Lagrangian circulation and forbidden zone on the West 

Florida Shelf, Continental Shelf Research, 19, 9,1221-1245.

This paper presents some recent results of drifters released on the 

West Florida Shelf during 1996-1997 and compares these results to the 

numerical model results of the wind-driven circulation. Using satellite 

tracked surface drifters during the one year period from February 1996 to 

February 1997, a drifter free region. referred to as the "forbidden zone," is 

found over the southern portion of the West Florida Shelf. This finding is 
66

consistent with historical drift bottle data and with a recent numerical model 

study of the West Florida Shelf circulation response to climatological wind 

forcing. Direct drifter simulations by numerical model during March 1996 

show a good agreement with both the in situ ADCP current observation and 

drifter observation. Three mechanisms are proposed for the observed 

Lagrangian features. The primarily dynamic mechanism is the along-shore 

wind forcing, which induces a coastal jet. This jet tends to leave the coast 

and the bottom, giving rise to offshore transports onshore and near-surface 

offshore transports. The second is the convergent coastal geometry and 

bottom topography for the southward flow in central shelf near Tampa Bay 

that enforces the coastal jet and the bottom and near-surface transport. The 

last is kinematic, due simply to the short along-shore Lagrangian excursion, 

driven by the typical synoptic weather systems. Thus near-surface shelf 

waters over the north may not reach the southern coast of the West Florida. 

The implication is that surface hazards, such as an oil spill that may occur 

outside of the southern West-Florida shelf, may not greatly impact the 

southern coastal region except Florida Keys. However the biological and 

chemical patches over the north that may occur in the water column such as 

red tides still can easily reach the southern coastal region through the 

subsurface and bottom waters. 

MS Thesis

1) Siegel, E.M. (1999). Currents observed across the west Florida 

continental shelf. Master of Science Thesis, Department of Marine Science, 

University of South Florida, St. Petersburg, FL 33701, April 1999, 154pp.

This Thesis incorporates results from the MMS funded observations 

with other observations collected under separate funding in an attempt to 

provide an overview on how the currents vary across the west Florida shelf.

Technical Reports

FitzHugh, Gary, C.C. Koenig, Felicia C. Coleman, and W. Sturges, Dec. 

2000. Spawning-to-nursery connection for gag: Tests of mechanisms to 

explain seagrass recruitment patterns. Final report, Nat. Marine Fisheries 

Service. 30 pp + 20 Figs.
67

This is a study of the recruitment of Gag Grouper on the west Florida 

Shelf. The data concerning wind-driven offshore surface motions were 

determined from the drifter results in the MMS-sponsored studies of this 

report.

Jones, Ken, Jianguo Wang, and W. Sturges, 1998. Observed Currents 

during Year 1 of the MMS Northeast Gulf of Mexico Program. Unpublished 

report, Dept. of Oceanography, Florida State Univ., Tallahassee, FL, 32306, 

January 1998.

This reports describes the basic data results from the current meter 

observations taken during Year 1 of the study with plots of all results.

Weisberg, R.H., J.R. Parrish, E.M. Siegel, B.D. Black, J.C. Donovan and 

R.D. Cole (1998). Northeast Gulf of Mexico water velocity observations: A 

report on data collected from a subsurface moored acoustic Doppler current 

profiler at 28 50'N, 84 50'W, February 1996-April 1997.

This data report provides a description of the mooring and instruments 

used, and gives graphical presentations of the data collected.

Web Sites

http://gulf.ocean.fsu.edu This site has been maintained primarily for the 

drifter experiment. Weekly and monthly maps of the drifter motions (Feb 

’95 – March ’96), prepared by W. Johnson, have been kept on this web site. 

Access to a variety of other data, including the current meter data, are also 

available there.

http://ocg6.marine.usf.edu provides information on research performed by 

the USF Ocean Circulation Group. Graphical access is provided for all of 

the data being collected. 
68

VII. References

Davis, R. E., Drifter observations of coastal surface currents during CODE: 

The method and descriptive view, J. Geophys. Res., 90, 4741-4755, 

1985.

FitzHugh, Gary, C.C. Koenig, Felicia C. Coleman, and W. Sturges, Dec. 

2000. Spawning-to-nursery connection for gag: Tests of 

mechanisms to explain seagrass recruitment patterns. Final report, 

Nat. Marine Fisheries Service. 30 pp + 20 Figs.

Herring, J., M. Inoue, G.L. Mellor, C.N.K. Mooers. P.P. Niiler, R.C. 

Patchen, R.C.Perez, F.M. Vukovich and W.J. Wiseman, Jr.. 2001: 

Coastal ocean modeling in the Gulf of Mexico (submitted for 

publication to Progress in Oceanography)

Hetland, R.D., Y. Hsueh, R.R. Leben, and P.P Niiler, A Loop Currentinduced jet along the edge of the West Florida shelf. Geophysical 

Research Letters, 26 (15), 2239-2242, 1999.

Jones, Ken, Jianguo Wang, and W. Sturges, 1998. Observed Currents 

during Year 1 of the MMS Northeast Gulf of Mexico Program. 

Unpublished report, Dept. of Oceanography, Florida State Univ., 

Tallahassee, FL, 32306, January 1998. 

Koblinsky, C.J. and P.P. Niiler, 1977: Direct measurements of circulation 

of the West Florida continental shelf. Oregon State University, 

School of Oceanography, Ref. 79-13. 

Niiler, P.P., 1976: Observations of low-frequency currents on the West 

Florida continental shelf. Memories Societe Royale des Sciences do 

Liege, 6 (10), 331-358.

Niiler, P., W. Johnson and N. Baturin, 1997: Surface Current and 

Lagrangian-drift Program, SCULP-I. Unpublished manuscript, 

Scripps Inst. of Oceanography, LaJolla, CA.

Ohlmann, J. C. and P. P. Niiler, A two-dimensional response to a tropical 

storm on the Gulf of Mexico shelf. Journal of Marine Systems, 29, 

87-99, 2001.
69

Ohlmann, C. J., P. P. Niiler, C. A. Fox and R. R. Leben, Eddy energy and 

shelf interactions in the Gulf of Mexico. Journal of Geophysical 

Research, 106 (C2), 2605-2620, 2001.

Price, James F., R. A. Weller, and R.R. Schudlich. Wind-driven ocean 

currents and Ekman Transport. Science, 238, 1534-1538.

Sturges, Wilton and Robert Leben, 2000. Frequency of Ring Separations 

from the Loop Current in the Gulf of Mexico: a revised estimate. 

Jour. Phys. Ocean, 30, 1814-1819.

Sturges, Wilton, and B.G. Hong, 2001. Gulf Stream transport variability at 

periods of decades. Jour. Phys. Ocean., 31, 324-332.

Van Meurs, P., The importance of spatial variabilities on the decay of nearinertial mixed layer currents: theory, observations and modeling, 

Ph.D. thesis, University of California, San Diego, 180 pp., 1995.

Yang, H., R. H. Weisberg, P. P. Niiler, W. Sturges and W. Johnson, 1999: 

Lagrangian circulation and forbidden zone on the West Florida 

Shelf, Continental Shelf Research, 19,1221-1245.
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