ACCESS NUMBER: 30660-19942 

STUDY TITLE: Interpretative Synthesis of LATEX Shelf and Slope Circulation Patterns 

from Satellite and In-situ Measurements 

REPORT TITLE: Wind and Eddy-related Circulation on the LA/TX Shelf and Slope 

Determined from Satellite and In-Situ Measurements: October 1993-August 1994 

CONTRACT NUMBER: 14-35-0001-30660-19942 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Northwestern 

FISCAL YEARS OF PROJECT FUNDING: 1996; 1997; 1998; 1999; 2000 

COMPLETION DATE OF REPORT: April 2001 

CUMULATIVE PROJECT COST: $18,762 

PROJECT MANAGER: N.D. Walker 

AFFILIATION: Coastal Studies Institute, Louisiana State University 

ADDRESS: Howe-Russell Geoscience Complex, Louisiana State University, Baton 

Rouge, LA 70803. 

PRINCIPAL INVESTIGATOR: N.D. Walker 

KEY WORDS: physical oceanography, currents, Louisiana/Texas shelf, Louisiana 

coastal current, upwelling, satellite oceanography, drifters, Loop Current eddies, NW 

Gulf of Mexico, shelf circulation, slope circulation. 

BACKGROUND: The Louisiana-Texas Shelf Physical Oceanography Program 

(LATEX), sponsored by the Minerals Management Service, was funded to improve the 

understanding of circulation and associated causal mechanisms on the Louisiana and 

Texas continental shelves. Circulation in this region is complex as it is influenced by 

several time-varying environmental factors including wind forcing, river discharges, and 

the location and intensity of detached Loop Current warm-core eddies. During the 

LATEX field measurement program many valuable datasets were collected which 

individually revealed some aspects of circulation on the continental shelf and slope of 

Texas and Louisiana from April 1992 to December 1994. In this project, an attempt was 

made to further the understanding of near-surface shelf and slope circulation processes 

in the northwest Gulf of Mexico (GOM) by synthesizing several datasets collected 

during the LATEX field measurement period. The datasets included in this analysis 

included NOAA AVHRR sea-surface temperature (SST) imagery, TOPEX and ERS-1 

sea-surface height (SSH) data, SCULP surface drifter data and LATEX-A current meter 

and temperature measurements. 
ACCESS NUMBER: 30660-19942 

OBJECTIVES: The goal of this research was to use selected high quality 

measurements obtained during the LATEX project from October 1993 through 

September 1994 to obtain a better understanding of synoptic-scale and seasonal-scale 

variations in surface and near-surface circulation on the Louisiana/Texas continental 

shelf and slope and the primary forcing mechanisms for the identified patterns. 

DESCRIPTION: The NOAA satellite SST image data provided regional synoptic views 

of SST distributions over the LATEX shelf and slope when skies were relatively cloudfree. Shelf and slope circulation features such as the effluent plumes of major rivers, 

offshelf flow features (squirts), and Loop Current rings were identifiable and trackable in 

relatively clear-sky imagery. The SSH information, in the form of 10-day synthesis 

maps, depicted the locations and intensities of warm-core rings and cold-core eddies in 

the study region, information which is weather-independent. The SCULP surface 

drifters provided knowledge on lagrangian surface circulation patterns. The satellite 

datasets provided crucial information on the location and movements of shelf and slope 

oceanic features with which the drifter trajectories could be more accurately interpreted. 

The LATEX-A current meter measurements provided time-series measurements 

simultaneously at key locations on the LATEX shelf, enabling a more thorough 

investigation of the impacts of wind forcing on inner shelf circulation. 

SIGNIFICANT CONCLUSIONS: Circulation on the inner and mid-shelf regions of the 

Louisiana/Texas continental shelf is controlled primarily by wind forcing. Down-coast 

flow (from Louisiana to Texas and southward along the Texas coast) prevails during 

much of autumn, winter and spring due to the prevalence of westward wind stress. The 

orientation of flow is generally along the isobaths and, therefore, changes from 

westward flow along the Louisiana shelf to southward flow along the Texas shelf when 

currents are down-coast. Velocities at the surface and down to about 10m are strongest 

when winds blow towards the southwest along the Louisiana coast and towards the 

south along the Texas coast. This wind stress situation occurs frequently with the 

intrusion of strong high pressure systems in the wake of cold-front passages. Downcoast current velocities are considerably stronger along the Texas coast as a result of 

the constriction in the continental shelf, moving from Louisiana to Texas. The summer 

up-coast flow regime is regionally extensive across the Louisiana/Texas shelf. There 

was no evidence of a surface convergence zone in summer between the up-coast flow 

regime along the south Texas coast and a down-coast flow regime from Louisiana. In 

fact, the data show that at 10 m depth up-coast flow (to north and east) occurred first 

along the northern Texas and central Louisiana shelf. Comparison of drifter data with 

sub-surface mooring data clearly demonstrate that the surface flow responds within 

hours to wind events whereas the sub-surface flow can exhibit lag times of days to 

weeks, especially along the south Texas coast. The up-coast flow regime of summer is 

characterized by coastal upwelling along much of the southern Texas coastline. 

Northward wind stress along the Texas coast results in a coastally extensive band of 

cool waters, 20-30 km wide, extending from Mexico to Matagorda Bay. These cool 

waters contrast with the warm surface waters typical of the rest of the Louisiana/Texas 

shelf. During June 1994, upwelling was stronger than normal and coastal upwelling 
ACCESS NUMBER: 30660-19942 

was detected along the entire Texas coast from 26oN to Galveston.The satellite SST 

imagery reveals that the cool coastal waters move towards the northeast with the 

general up-coast shelf flow along the Texas coast. Warm core eddies on the slope 

typically advect the cool upwelled waters seaward into the deeper GOM. Water 

temperatures below 25oC are commonly experienced along the Texas coast during 

June and July and sometimes August. The severity of the cooling and the longevity of 

upwelling are controlled primarily by the intensity and longevity of the northward wind 

stress. The most long-lived and pronounced up-coast flow events at both sites occurred 

in late June/early July and in late July. 

STUDY RESULTS: The synthesis of clear sky satellite SST data and SSH data 

enabled a better evaluation of the circulation features that influenced the movement of 

SCULP drifters during autumn and winter 1993/94. The proximity of three warm core 

rings and several cold core eddies to the Louisiana/Texas shelf edge impacted upon 

circulation of the drifters. The movement of these features over time altered surface 

circulation processes on the shelf. Of particular interest was the identification of a 

narrow off-shelf flow regime southeast from Matagorda Bay. This flow regime observed 

primarily in November and December 1993 was termed the Matagorda squirt. 

Approximately 40% of the drifters released in October and November 1993 experienced 

off-shelf advection within this squirt feature. In the satellite SST data, the feature was 

revealed as a streamer of relatively cool water extending between the 30m to 200 m 

isobath. The feature formed between a warm core ring (Eddy V/W) to the south and a 

cold core eddy on the shelf to the north. Daily-averaged velocities of the drifters 

averaged 39 cm/s and ranged from 19 to 59 cm/s in water depths between 50 and 1000 

m (Table 2). Subsequent drifter movements around the warm core ring were slightly 

higher, averaging 45 cm/s with maximum velocities reaching 75 cm/s. During 

December 1993, only 30% of the drifters were advected into this feature and by January 

the feature had disappeared. The SSH data demonstrated that the warm core ring 

moved away from the shelf, removing the main source of the forcing for this flow 

regime. This interesting case study reveals the large impact that warm core rings can 

have on shelf circulation, even away from the shelf edge. 

STUDY PRODUCTS: Walker, N.D. 2001. Wind and eddy-related circulation on the 

Louisiana/Texas shelf and slope determined from satellite and in-situ measurements: 

October 1993-August 1994, OCS Study MMS 2001-025. U.S. Dept. of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 74 pp.