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STUDY TITLE: Current and Acoustical Measurements Over the Vertical Water Column 

In-Situ and PIES and Northeastern Gulf of Mexico Physical Oceanography Program: 

DeSoto Canyon Eddy Intrusion Study 

REPORT TITLE: Deepwater Observations in the Northern Gulf of Mexico from In-situ 

Current Meters and PIES, Final Report, Volume I: Executive Summary and Volume II: 

Technical Report 

CONTRACT NUMBER: 1435-01-00-CT-31132 and 1435-01-96-CT-30825 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1999, 2000, 2001, 2002, 2003 

COMPLETION DATE OF REPORT: August 2003 

COSTS: FY 1999: $799,774: FY 2000: $577,428: CUMULATIVE PROJECT COSTS: 

$2,924,36. 

PROJECT MANAGER: E. Waddell 

AFFILIATION: Science Applications International Corporation 

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

PRINCIPAL INVESTIGATORS: P. Hamilton, J. Singer, K. Donohue 

KEY WORDS: Gulf of Mexico, DeSoto Canyon, Loop Current, eddies, cyclones, 

current, topographic Rossby waves, sea surface height, circulation, field measurements, 

transport, Sigsbee Escarpment, slope. 

BACKGROUND: This study was conducted in a deepwater region of significant 

potential for oil and gas exploration and development. Prior industry current 

measurements showed that strong current speeds are both possible and occur over a 

vertical layer that extends from the bottom to a nominal depth of 1000m. The U.S. 

Department of the Interior requires an environmental characterization of these 

deepwater potential lease areas for making appropriate management decisions. The 

present report presents the results of a field measurement and synthesis study 

conducted by scientists from SAIC and the University of Rhode Island. 

OBJECTIVES: In-situ current measurements, hydrographic data, satellite altimetry and 

surface drifters were used to document and characterize dynamic conditions occurring 

in the upper (above 1000 m) and lower layer (below 1000 m) over a region in the vicinity 

of the Sigsbee Escarpment. These data were to be used to develop a characterization 

of the pattern of currents in the lower layer and possible linkages to 

conditions/processes in the upper layer of the water column. In addition, inverted echo 

sounders with pressure (PIES) were deployed for six months in conjunction with a fulldepth mooring to document full-depth temperature, salinity and velocity profiles. 
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DESCRIPTION: This study area is located on the far western flank of the DeSoto 

Canyon at the base of and extending up the Sigsbee Escarpment. Measurements were 

generally centered about 91°W, 27° 15’ N, in water depths that ranged from ≈1500 m on 

the top of the Escarpment to ≈2100 m away (southeast) from the base of the 

Escarpment. 

In August 1999, two years of field measurements were initiated with various 

combinations of joint funding by the MMS and BP. Initially, four moorings were 

deployed. Three were near-bottom moorings that extended approximately 400 m above 

the local bottom. Each short mooring had current meters at 10, 100 and 400 m above 

the bottom. One near bottom/short mooring was at the top of the Escarpment; the other 

two were near the base. Mooring separations were on the order of 20-30 km. A fulldepth mooring was deployed near the base of the Escarpment and contained a 

combination of Acoustic Doppler Current Profilers (ADCPs), thermistors, fixed-level 

current meters and conductivity/temperature sensors. These were distributed vertically 

so the resulting observations would document well the vertical differences in currents, 

temperature and salinity as they varied with time. Between 12 and 18 months, three 

moorings (two near bottom and one full depth) were maintained, all of which were 

located at the base of the Escarpment. During months 18-24, these three moorings 

were supplemented with an additional mooring on the Escarpment but close to one of 

the longer-term near-bottom moorings. In addition, three Inverted Echo Sounders with 

Pressure (PIES) were deployed at this time in a triangle about the full-depth mooring. 

Rotation, deployment and maintenance cruises were conducted semiannually. 

For this study, these data were supplemented with satellite altimetry obtained from the 

web site maintained by the University of Colorado (http://wwwccar.Colorado.EDU/~realtime/gom-real-time_vel/). These daily graphics provided a 

serially continuous characterization of most major dynamic features in the upper layers 

(above ≈ 1000 m) of the Gulf of Mexico. Trajectories for surface drifters (drogued at 50 

m below the water surface) within Eddy Millennium in the interval of January – July 

2001 were purchased from the Horizon Marine, Inc. archive. 

SIGNIFICANT CONCLUSIONS: Strong near-bottom currents varied significantly with 

time and due to position relative to the Escarpment. The lower layer (bottom to ≈1000 

m below the water surface) was strongly barotropic so only relatively weak shears 

occurred. The major variations in deep water currents appear to be driven by 

topographic Rossby waves that move through the study area in long duration wave 

trains that were separated by intervals of relative quiescence (noticeably lower current 

velocities). These trains of waves and associated higher velocities could persist for 

intervals ranging from two to six months. 

STUDY RESULTS: Good data return resulted in long, generally unbroken time series 

that provided a basis for an excellent documentation of dynamic conditions and 

processes within and proximate to the study area. Measurements provided excellent 

characterization of the two-layer structure of the current field in the deeper Gulf of 

Mexico. In this study area, the upper layer was strongly affected by Loop Current 

eddies, cyclonic eddies and cyclonic boundary eddies that occur on the Loop Current 

and on Loop Current eddies. The migrating eddy fields were a major cause of the time 
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varying velocity and temperature field. In the lower layer (depths > ≈ 1000 m), current 

shear was often relatively weak such that in 2000 m water depths, this lower layer 

moved almost as a “slab”. The higher speed events that occur in this lower layer were 

periodic (8-12 day periods are common), and these periodic velocity signals could 

persist over intervals ranging from two to six months, based on the two years of 

measurements taken in this study. The deep currents were such that the highest 

current speeds occurred near the bottom (bottom intensified). Highest measured 

currents were approximately 95 cm/s (≈ 2 knots) at 100 m above the bottom, and 75 

cm/s at 10 m above the local bottom. Currents above 50 cm/s were relatively common 

with the passage of trains of waves. If, as these data seem to show, these trains of 

periodic current variations are due to passage of topographic Rossby waves, their 

source region is probably relatively close to the measurement sites and at these short 

periods do not appear to propagate into the western Gulf basin. 

The preliminary examination of PIES and associated GEM methodology, showed this 

technology provided a very good method for estimating time series of vertical 

temperature and salinity profiles. These combine to estimate accurately the profile of 

geostrophic current vectors. Measured reference current velocities near the seabed 

were used to convert the shear profile to an absolute velocity profile that agreed well 

with the measured profile. Similarly, the PIES-based estimates of temperature and 

salinity time series agreed well with the measured values. 

STUDY PRODUCT(S): Hamilton, P., K. Donohue, J.J. Singer, and E. Waddell,. 

Deepwater Observations in the Northern Gulf of Mexico from In-situ Current Meters and 

PIES, Final Report, Volume I: Executive Summary. OCS Study MMS 2003-048. U.S. 

Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, LA. 21 pp. 

Hamilton, P., K. Donohue, J.J. Singer, and E. Waddell,. Deepwater Observations in the 

Northern Gulf of Mexico from In-situ Current Meters and PIES, Final Report, Volume II: 

Technical Report. OCS Study MMS 2003-049. U.S. Dept. of the Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 103 pp. 

Hamilton, P., and A. Lugo-Fernandez, 2001. Observations of high speed deep currents 

in the northern Gulf of Mexico. Geophys. Res. Lett., 28, 2867-2870.