ACCESS NUMBER: 30787 

STUDY TITLE: Northeastern Gulf of Mexico Inner Shelf Circulation Study (GM-95-01) 

REPORT TITLE: Northeastern Gulf of Mexico Inner Shelf Circulation Study 

CONTRACT NUMBER: 14-35-0001-30787 

SPONSORING OCS REGION: Gulf of Mexico Region 

APPLICABLE PLANNING AREA(S): Northeastern Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: FY 95 through FY 98 

COMPLETION DATE OF REPORT: February 2002 

COSTS: $ 1,696,258 

 FY 95: $1,240,000 

 FY 96: $56,447 

 FY 97: $199,811 

 FY 98: $200,000 

PROJECT MANAGER: Dr. Wilton Sturges 

AFFILIATION: The Florida State University 

ADDRESS: Department of Oceanography, Florida State University, Tallahassee, FL 32306 

PRINCIPAL INVESTIGATORS: W. Sturges, Florida State University; Pearn P. Niiler, Scripps 

Institution of Oceanography; R. H. Weisberg, University of South Florida 

KEY WORDS: northeastern Gulf of Mexico, ocean circulation, inner shelf, OSRA, skill assessment, process 

studies, drifting buoys, moored current meters 

BACKGROUND: With the addition of environmental assessment responsibilities for State waters, 

mandated by the Oil Pollution Act of 1990, the MMS is required to analyze oil-spill risks within the inner 

shelf zone (ISZ), the 3- to 10-mile-wide band of marine waters adjacent to the coast. The ISZ has significant 

differences in physical circulation from the open shelf, due to (1) overlap of the benthic boundary layer and 

surface mixed layer, (2) complex lateral boundary geometry, (3) wave energy focusing and concentration, 

and (4) wind stress shears associated with the marine boundary layer. A successful approach to 

environmental risk assessment in the ISZ, adopted by the MMS, is the use of actual field measurements 

(i.e., data, as opposed to model results), blended with regional information from all sources, to provide 

engineering estimates of long-shore currents. The study effort involves a series of current meter moorings in 

the northeastern Gulf of Mexico ISZ to develop an adequate database for these assessments. Results of the 

drifting buoy component of the study have been coming in on a regular basis. The current meter moorings 

have been deployed. 
ACCESS NUMBER: 30787 

OBJECTIVE: To advance our understanding of the nearshore circulation in the northeastern Gulf of Mexico 

and improve our capabilities of predicting oil-spill trajectories. The field measurements study proposed here 

will provide the data for ISZ oil-spill risk assessments mandated by the Oil Pollution Act of 1990. 

DESCRIPTION: This report presents the results of an experiment involving several hundred satellitetracked surface drifters used, together with an auxiliary series of moored current meters and 

meteorological buoys, to measure the nearshore 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 Institution of Oceanography. In addition to the surface drifter program, a program of moored 

current meters was carried out cooperatively by 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. 

SIGNIFICANT CONCLUSIONS: The 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 

approximately 2-week intervals, 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. Abstracts of the scientific publications that have resulted from these data refer to specific 

dynamical inferences drawn from these data. The 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 nearshore 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 at 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 venture. 

 

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 along-topography 

currents along isobaths is about 50 km in water depths between 10-30 m and 350 km in water depths of 

50-80 m. 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 100-m and 30-m 

depths 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 short-lived strong events in circulation. 

STUDY RESULTS: The measurements taken from the two different observation systems compare well. 

Also, if the wind blows at a certain speed in a given location, we can then compute the near-surface 

currents with a reasonable accuracy. The coefficients obtained are presented in the report. 
ACCESS NUMBER: 30787 

STUDY PRODUCTS: 

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. 

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

comparison. In preparation, draft complete. 

Hetland, R.D., Y. Hsueh, R.R. Leben, and P.P Niiler, 1999. A Loop Current-induced jet along the 

edge of the west Florida shelf. Geophys. Res. Letters, 26 (15), 2239-2242. 

Meyers, S.D., E.M. Siegel, and R.H. Weisberg, 2001. Observations of currents on the west Florida 

shelf beak. Geophys. Res. Letters, 28, 2037-2040. 

Ohlmann, J.C., and P.P. Niiler, 2001. A two-dimensional response to a tropical storm on the Gulf of 

Mexico shelf. J. Mar. Sys., 29, 87-99. 

Ohlmann, C. J., P. P. Niiler, C. A. Fox, and R. R. Leben, 2001. Eddy energy and shelf interactions in 

the Gulf of Mexico. J. Geophys. Res., 106 (C2), 2605-2620. 

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. 

Sturges, Wilton, and Robert Leben, 2000. Frequency of ring separations from the Loop Current in the 

Gulf of Mexico: a revised estimate. J. Phys. Ocean., 30, 1814-1819. 

Weisberg, R.H., B. Black, and Z, Li, 2000. An upwelling case study on Florida’s west coast, 

J. Geophys. Res., 105, 11459-11469. 

Yang, Huijun, Robert H. Weisberg, P. P. Niiler, W. Sturges, and W. Johnson, 1999. Lagrangian 

circulation and forbidden zone on the west Florida shelf, Cont. Shelf Res., 19, 9,1221-1245. 

P.I.’s affiliation may be different than that listed for Project Manager.