ACCESS NUMBER: 30633

STUDY TITLE: Louisiana-Texas Shelf Physical oceanography Program Eddy 

Circulation Study 

REPORT TITLE: Louisiana-Texas Shelf Physical Oceanography Program Eddy 

Circulation Study, Final Synthesis Report. Volume I: Technical Report and Volume II: 

Appendices 

CONTRACT NUMBER: 14-35-0001-30633 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Central and Western Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1991; 1992; 1993; 1994; 1995 

COMPLETION DATE OF REPORT: March 1996 

COSTS: FY 1997: $703,943; FY 1992: $673,356; FY 1993: $575,452; FY 1994: 

$237,026; FY 1995: $228,945 

CUMULATIVE PROJECT COST: $2,478,722 

PROGRAM MANAGER: Thomas J. Berger 

AFFILIATION: Science Applications International Corporation 

ADDRESS: 615 Oberlin Road, Suite 300, Raleigh, North Carolina 

PRINCIPAL INVESTIGATORS*: Peter Hamilton, Robert Leben, and George Born 

KEY WORDS: Physical Oceanography, Gulf of Mexico, Loop Current, Loop Current 

eddies, slope eddies, satellite altimetry, drifters, GEOSAT, climatology 

BACKGROUND: The present study is part of a sequence of programs designed to 

provide MMS with a basis for evaluating the potential environmental impacts of oil and 

gas exploration in the Gulf of Mexico region. This sequence began with the Gulf of 

Mexico Physical Oceanography Program, an approximately five-year (1983-1989) field 

effort to study processes throughout the Gulf with different areas/processes emphasized 

during each program year. Subsequently the MMS funded the Louisiana-Texas Shelf 

Physical Oceanography Program with field work during a three-year period from spring 

1992 through spring 1995. Two elements of this program focused on shelf circulation 

(LATEX A) and on the Mississippi River Plume//inner shelf processes (LATEX B), while 

the third element, this study, focused on eddy driven processed in deep water and the 

continental slope. 
ACCESS NUMBER: 30633

OBJECTIVES: Primary scientific objectives were to monitor and characterize three 

classes of mesoscale circulation patterns important in the open sea and slope waters of 

the northwestern Gulf of Mexico: (1) Loop Current Eddy shedding processes and eddy 

interactions with the slope; (2) smaller anticyclonic eddies found along the upper slope; 

and (3) “squirts and jets” - poorly understood shear zones usually located near the 

upper slope. Secondary support objectives were to provide efficient, centralized 

logistics support, including Service ARGOS liaison for drifting buoys, air deployed 

expendable bathythermograph (AXBT) and current profiler (AXCP) probes, and 

expendable bathythermograph (XBT) probes for a ship-of-opportunity program; and to 

disseminate information collected during this study to other investigators on a regular 

basis. 

DESCRIPTION: This study encompassed three years of field work from May, 1992, 

through April, 1995. Monitoring Loop Current eddies and mesoscale features was 

accomplished in 21 aerial surveys, with AXBTs and AXCPs, to examine the 

instantaneous hydrographic and velocity structures of selected features. Continuous 

monitoring of features was done using air and ship-deployed, satellite-tracked 

Lagrangian drifting buoys. Satellite altimetry was used to provide a historical 

perspective of conditions in the Gulf of Mexico from early 1985 through 1989. Through 

cooperative efforts with the University of Colorado, altimetry data from the ERS-1 and 

TOPEX/Poseidon satellites were available in near real-time during Years 2 and 3 of the 

Program as an aid in locating eddies for further study using AXBT and AXCP. 

SIGNIFICANT CONCLUSIONS: Satellite altimetry data, from various sources, which 

have been referenced to an accurate mean sea surface, such as is now available for 

the Gulf of Mexico, can be used to provide long term patterns and stable statistics of 

ocean circulation features. Statistics of eddy motions from the altimetry had good 

correlation with similar statistics from drifter tracks and survey data. Observations of 

interaction of two Loop Current eddies in the western Gulf of Mexico indicated mass 

conservation during merger, with energy for the merger supplied by a nearby cyclone of 

smaller size. Statistics of Loop Current eddy motions derived form survey data and 

from Lagrangian drifter tracks observed during May 1992 through April 1995 field period 

agrees with previous estimates, including those from the GEOSAT period. Smaller 

slope eddies were observed in detail for the first time as well as interactions amount 

these smaller features and Loop Current eddies and with the shelf circulation. The 

smaller features appear to play a major role in determining the circulation at the shelf 

break. Analyses indicate a rich variety of interactions among cyclones and anticyclones 

with a variety of impacts on circulation patterns and eddy persistence. 

STUDY RESULTS: Five Loop Current eddies out of six detected were tracked during 

the GEOSAT period and compared with survey data and Lagrangian drifter data. Loop 

Current eddy interactions in the western Gulf of Mexico were observed. Error analyses 

indicate that referencing the altimeter data to an accurate mean sea surface provides a 

set of stable statistics which compare well with similar statistics derived from 

hydrographic and Lagrangian drifter data. Eddy paths and separation periods derived 

from these data were consistent with similar estimates made by other investigators. 
ACCESS NUMBER: 30633

Observation of the merger of two Loop Current eddies in the western Gulf of Mexico 

showed that, in that instance, the merger conserved mass and circulation and that the 

energy required likely came from an adjacent smaller cyclone. This is in contrast to 

various earlier theories on the merger of two anticyclones. 

Five Loop Current eddies and several smaller slope cyclones and anticyclones were 

observed during May 1992 through April 1995 field period. Data from the three-year 

field program provided detailed tracks for four of the five Loop Current eddies observed 

and provided the first detailed data on Loop Current eddy interactions with smaller 

cyclones and anticyclones as well as interactions among the smaller scale features. 

Observations from simultaneous hydrographic surveys, drifter tracts, AXCP profiles and 

current meter records revealed the rich detail and complexity of the eddy field in the 

central and western Gulf of Mexico. These observations suggest that small scale eddy 

interactions on the upper slope are a prime mechanism for the movement of water onto 

and off the shelf, and are the cause of the lack of along shelf coherence among current 

meter records at even relatively short length scales. 

STUDY PRODUCTS: Berger, T.J., P. Hamilton, J.J. Singer, R.R. Leben, G.H. Born, 

and C.A. Fox. 1996. Louisiana/Texas Shelf Physical Oceanography Eddy Circulation 

Study Final Synthesis Report. Vol. I: Technical Report. OCS Study MMS 96-0051. 

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

Region, New Orleans, LA. 319 pp. 

Berger, T.J., P. Hamilton, J.J. Singer, R.R. Leben, G.H. Born, and C.A. Fox. 1996. 

Louisiana/Texas Shelf Physical Oceanography Eddy Circulation Study Final Synthesis 

Report. Vol. II: Appendices. OCS Study MMS 96-0052. U.S. Department of the 

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

151 pp. 

Biggs, D.C., G.S. Fargion, P. Hamilton, and R. R. Leben. 1996. Cleavage of a Gulf of 

Mexico Loop Current Eddy by a Deep Water Cyclone, J. Geophys. Res., 101:20, 629-

20, 641. 

SAIC. 1995. Louisiana/Texas Shelf Physical Oceanography Program: Eddy Circulation 

Study, Annual Report: Year 2. OCS Study MMS 95-0027. U.S. Department of the 

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

63 pp. 

SAIC. 1994. Louisiana/Texas Shelf Physical Oceanography Program: Eddy Circulation 

Study, Annual Report: Year 1. OCS Study MMS 95-0027. U.S. Department of the 

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

53 pp. 

Hamilton, P., D.C. Biggs and G.S. Fargion. 1995. Secondary Eddies on the LATEX 

Slope from Drifter and Hydrographic Data. Proceedings: Fifteenth Annual Gulf of 

Mexico Information Transfer Meeting. New Orleans, LA. December 12-14, 1995. 
ACCESS NUMBER: 30633

Leben, R.R., and G.H. Born. 1995. An Updated Climatology of Loop Current Eddies 

from GEOSAT Altimetry. Proceedings: Fifteenth Annual Gulf of Mexico Information 

Transfer Meeting. New Orleans, LA. December 12-14, 1995. 

Berger, T.J., K. Schaudt, J. Feeney and P. Coholan. 1994. Loop Current Eddy 

Shedding Cycle. EOS, Trans. AGU. Suppl. 75(15):212. 

Leben, R.R., G.H. Born and P. Hamilton. 1994. TOPEX/POSEIDON and ERS-1 

Altimeter Observation of Loop Current Eddies U, V, W, and X and Associated Cyclones. 

EOS, Trans AGU. Suppl. 75(16):212. 

Hamilton, P., G. Fargion and D. Biggs. 1994. Surveys of Eddies U, V, W, and X in the 

Central Gulf of Mexico with XBTs and ARGOS drifters. EOS, Trans. AGU Suppl 

75(16):212. 

Berger, T.J. 1993. Preliminary Chronology of Loop Current Eddies, 1981-1983. 

Proceedings: Thirteenth Annual Gulf of Mexico Information Transfer Meeting. New 

Orleans, LA. December 14-17, 1993. OCS Study MMS 94-0061. 

Hamilton, P., D.C. Biggs and G.S. Fargion. 1993. Survey of Eddies U, V, and w with 

Expendable Probes and Drifters. Proceedings: Thirteenth Annual Gulf of Mexico 

Information Transfer Meeting. New Orleans, LA. December 14-17, 1993. OCS Study 

MMS 94-0061. 

Leben, R.R., and G.H. Born. 1993. TOPEX/POSEIDON and ERS-1 altimeter data 

analysis in the Gulf of Mexico: Verification of a Climatological Altimetric Mean 

Reference Surface. Proceedings: Thirteenth Annual Gulf of Mexico Information 

Transfer Meeting. New Orleans, LA. 1993. OCS Study MMS 94-0061. pp. 408-413. 

*P.I.'s affiliation may be different than that listed for Program Managers.