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STUDY TITLE: Exploratory Study of Deepwater Currents in the Gulf of Mexico 

REPORT TITLE: Exploratory Study of Deepwater Currents in the Gulf of Mexico, Final 

Report, Volume I: Executive Summary and Volume II: Technical Report 

CONTRACT NUMBER: 1435-01-02-CT-31152 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 2002, 2003, 2004, 2005, and 2006. 

COMPLETION DATE OF REPORT: December 2006 

COSTS: FY 2002: $297,938; FY 2003: $2,178,782; FY 2004: $1,266,797: FY 2005: 

$739,886 FY 2006: $608,478. CUMULATIVE PROJECT COSTS: $5,091,881 

PROJECT MANAGER: E. Waddell 

AFFILIATION: Science Applications International Corporation 

ADDRESS: 615 Oberlin Rd., Suite 100; Raleigh, NC 27605 

PRINCIPAL INVESTIGATORS: K. Donohue, P. Hamilton, K. Leaman, R. Leben, M. 

Prater, and D. R. Watts. 

KEY WORDS: Gulf of Mexico, Loop Current, Loop Current eddies, cyclones, current, 

PIES, Lagrangian, PALFOS, RAFOS, topographic Rossby waves, sea surface height, 

circulation, field measurements, transport, Sigsbee Escarpment, slope, two-layer flow. 

BACKGROUND: This study was planned and developed by the MMS to examine 

several key hypotheses and to evaluate methods of cost effective, broad regional 

documentation of important ocean processes in the deepwater of the northern Gulf of 

Mexico. 

OBJECTIVES: There were three stated program objectives. These are: 

• Increase deepwater current database and knowledge of the deep circulation in 

the Gulf of Mexico (GOM), 

• Make measurements sufficient to estimate parameters needed to design fullscale PO studies in deepwater regions of the GOM, and 

• Collect current data sufficient to test and/or evaluate the eight hypotheses 

relating to ocean physical processes in the study area. 

DESCRIPTION: This study area was in the mid-Gulf, approximately south of New 

Orleans. The east-west extent of this study area was between 88°W and 94°W. The 
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north-south extent was from the 1,000 m isobath in the north to the 3,000 m isobath or 

the limit of the US EEZ in the south. The year of field measurements were made 

nominally from March 2003 through April 2004. Actual moored arrays were placed 

between 88.5°W and 92.25°W and the 1,200 m isobath and the EEZ. Measurements 

were made with a variety of moored arrays and Lagrangian PALFOS and RAFOS 

drifters. In water measurements were supplemented by use of remote sensing, 

primarily sea surface heights and sea surface temperatures. 

Four full-depth moorings were used to measure currents and hydrographic 

characteristics over the complete water column at locations near the corners of the 

measurement domain. Two moorings each were above and below the Sigsbee 

Escarpment. Available to the study scientists were measurements made by two other 

MMS funded full depth moorings (made by LSU and CICESE) that were placed in or 

adjacent to the measurement area. Fifteen near-bottom mooring were also deployed; 

12 extended 500 m above the local bottom and had two current meters at 100 m and 

500 m. Three supplemental short moorings were also placed on three transects across 

the Sigsbee Escarpment to enhance resolving horizontal scales of motion in the vicinity 

of this significant bathymetric feature. These latter moorings extended 100 m above the 

bottom and had one current meter at 100 m. 

Observations from 26 PIES (Inverted Echo Sounders with Pressure) supplemented the 

data from the moored arrays. Using data reduction/processing schemes developed at 

the University of Rhode Island, accurate time series estimates of temperature, salinity 

and density profiles could be determined. Using density profiles, geostrophic velocities 

could be computed. These were then referenced to velocities measured at the fixed 

arrays. Thus, PIES and mooring placements were coordinated to optimize 

environmental and process descriptions and data analyses. PIES placements were 

also coordinated with groundtracks of various satellites with altimeters. The 

combination of pressure and density profiles was used to evaluate and enhance 

interpolation schemes applied to altimetry in the Gulf of Mexico. All mooring and PIES 

servicing was accomplished during the deployment, rotation and recovery cruises that 

occurred at six-month intervals. 

Six PALFOS floats were deployed. These had a residence depth of 1,000 m and a 

profiling interval of ten days. Each had a RAFOS receiver on board so that trajectories 

at the residence depth were recorded and transmitted via ARGOS along with the salinity 

and temperature profiles taken with each surfacing. Thirty RAFOS floats were ballasted 

to reside and move with currents at 1,000, 1,500, 2,000, 2,500 and 3,000 meters below 

the water surface. Approximately six months into the observations six additional 

RAFOS floats were deployed to help resolve lower-layer events. In April and May, 

RAFOS floats surfaced and transmitted via ARGOS the cumulative trajectories at each 

deployed depth. 

SIGNIFICANT CONCLUSIONS: As deployed, the combination of fixed position current 

measurements, PIES, Lagrangian drifters and satellite remote sensing combined to 

provide a cost-effective database that supported documentation and resolution of 

significant dynamic ocean processes and resulting circulation patterns. Horizontal 
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spacing of measurement sites was sufficient to resolve most key patterns. The 

expected two-layer flow patterns were again present and well documented using the 

range of deployed instruments. In the lower layer, an almost continuous sequence of 

eddy and wave-like motions resided in or moved through the measurement domain. In 

the upper layer, the Loop Current that impinged on the measurement domain for 

significant portions of the observations was a major factor affecting upper-layer currents 

and circulation. Strong Loop Current eddies separated close to the study region and 

moved west and southwest through the array. Boundary features/eddies on the Loop 

Current and Loop Current eddies also had a significant influence on circulation patterns 

in the upper layer. It is a key result that the multivariate data set produced for this study 

was able to document and reconstruct many of the characteristics of these dynamic 

features. The Sigsbee Escarpment was a barrier to lower-layer currents. Lower-layer 

conditions on the upper side of the Sigsbee were generally and consistently less 

vigorous than observed at the base of the Escarpment. 

STUDY RESULTS: Excellent data return from the varied observational methodologies 

over the one-year field effort combined to support a previously unrealizable 

reconstruction and quantitative mapping of ocean dynamics in the study area. Loop 

Current and associated eddies, both anticyclonic and cyclonic, dominated the upperlayer circulation. As an initial effort to provide a dynamical basis for the resulting 

features a shielded vortex instability was proposed. Energetic inertial oscillations were 

observed in the core of Eddy Sargassum. Inertial oscillations were also observed in 

response to several hurricanes and tropical storms that moved through or near the 

study area. 

Deep (lower layer) energetic currents were typically associated with eddies in the deep 

pressure fields. These deep eddies may be interpreted as a field of dispersive 

topographic Rossby waves (TRWs). The dispersive nature of the bottom circulations as 

shown by model results and float trajectories confirm the wavelike character of the deep 

flows. The Sigsbee Escarpment differentially affected the behavior of cyclonic and 

anticyclonic eddies that encountered this relatively steep bathymetric feature. 

Generally, flows below the Escarpment were consistently more vigorous that those 

above the Escarpment. Using a series of sequential upper and lower layer conditions, 

three possible linkages between the two layers are hypothesized. 

Comparison of satellite altimetry and comparable PIES data showed a good correlation 

with the lowest correlation above the Escarpment. Utility of altimetry for the weak 

surface signal of TRWs can be resolved with altimetry, although some continued 

consideration of shorter period fluctuations may help resolve this question. 

Deep drifting floats showed evidence of strong east-west exchange in the central Gulf, 

but significantly different once the float moved to the east or west of this central region. 

There appeared to be relatively little exchange between the eastern and western Gulf 

basins. Floats that moved to the western basin tended to converge closer to the slope 

and align with the local bathymetry. Further to the west, some floats were moved by 

deep eddies into the deeper portion of the basis away from the slope. 
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STUDY PRODUCTS: Donohue, K., P. Hamilton, K. Leaman, R. Leben, M. Prater, 

D.R.Watts and E. Waddell, 2006. Executive Summary: Exploratory Study of Deepwater 

Currents in the Gulf of Mexico. U.S. Department of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2006-073, 76 

pp. 

Donohue, K., P. Hamilton, K. Leaman, R. Leben, M. Prater, D.R.Watts and E. Waddell, 

2006. Technical Report: Exploratory Study of Deepwater Currents in the Gulf of 

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

OCS Region, New Orleans, LA. OCS Study MMS 2006-074, 399 pp. 

*P.I.’s affiliation may be different than that listed for the Project Managers.