ACCESS NUMBER: 30857 

STUDY TITLE: Northeastern Gulf of Mexico Physical Oceanography Program: Eddy 

Monitoring and Remote Sensing 

REPORT TITLE: Surface Circulation and the Transport of the Loop Current into the 

Northeastern Gulf of Mexico 

 

CONTRACT NUMBER(S): 1435-01-97-CA-30857 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANING AREA: Northeastern Gulf of Mexico 

FISCAL YEARS OF FUNDING: 1997, 1998, 1999, 2000 

COMPLETETION DATE OF REPORT: December 2001 

COSTS: FY 1997: $159,488; FY 1988: $159,488; FY 1999: $159,488; FY 2000: 

$159,488 

PROJECT MANAGER: Frank Muller-Karger 

AFFILIATION: University of South Florida 

ADDRESS: 140 7th Ave. S., St. Petersburg, Florida 33701 

PRINCIPAL INVESTIGATOR: Frank Muller-Karger 

KEY WORDS: Northeastern Gulf of Mexico, NEGOM, Loop Current, Eddies, Upwelling, 

Physical Oceanography, Biological Oceanography, fronts, circulation, satellite imagery, 

ocean color, sea surface temperature (SST), Chlorophyll-a 

BACKGROUND: To properly manage shelf resources, we need to better understand 

processes in the ocean and be able to assess oceanic variability over large space and 

time scales. In particular, it is critical to understand the motion of waters within a region 

to assess the potential consequences of possible spills derived from oceanic, shelf, and 

coastal mining industries. This Cooperative Agreement between the University of South 

Florida (USF) and the Minerals Management Service (MMS) was established to jointly 

assess the value of merged satellite data and specific oceanographic field data in 

determining the circulation and biological productivity of waters within the Northeast Gulf 

of Mexico (NEGOM). The effort centered around collecting, processing, and merging 

historical and concurrent Infrared, Radar-Altimetry, and Ocean Color satellite data as 

well as selected in situ information. We used various national and international sensors 

based upon availability of the data, honoring restrictions in the use and distribution of 

such data. 
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OBJECTIVES: The primary goal of this work was to assess the utility of merging 

specific satellite and field data sets in quantifying the magnitude and variability of water 

movement in the Northeastern Gulf of Mexico. Specifically, we sought to document and 

to better understand the behavior of the Loop Current, eddies shed by the Loop Current, 

the circulation of waters over the shelf and along the coast of the NEGOM, and the 

interaction between offshore and shelf waters. We have developed a strategy to derive 

new products useful to MMS and other interested parties for monitoring the circulation 

of the region by combining several different data sets. With these, we updated historical 

statistics on major oceanographic features observed in the region. We published part of 

the results of these analyses in the peer-reviewed literature, and additional manuscripts 

were being prepared upon completion of this summary. 

DESCRIPTION: Satellite data used for this study were Advanced Very High Resolution 

Radiometer (AVHRR) Sea Surface Temperature (SST) distribution fields, Sea-Viewing 

Wide-Field-of-View Sensor (SeaWiFS) ocean color-derived pigment concentrations, and 

TOPEX and ERS Radar Altimetry Sea Surface Height (SSH) fields. The first two 

datasets were collected locally using an antenna located at the College of Marine 

Science, USF, in St. Petersburg, Florida. The product for the NEGOM area was 

mapped to 1x1 km2 and 2.8x4 1 km2 pixel resolution. Meanwhile, TOPEX and ERS 

data were collected and processed at the University of Colorado, Boulder. To quantify 

the magnitude and variability of water movement in the NEGOM, we merged Sea 

Surface Height fields, Sea Surface Temperature fields, and Ocean Color Pigment 

Products. We also merged satellite and field data products from companion operational 

projects sponsored by the MMS, specifically concurrent buoy track, and hydrographic 

and wind velocity data. 

SIGNIFICANT CONCLUSIONS: The AVHRR SST data provided substantial 

information on circulation patterns during the winter (October-May), when temperature 

gradients were strong. However, during summer (June-September), AVHRR data for 

the most part showed uniform sea surface temperature patterns over the NEGOM. We 

confirmed that the AVHRR provided information on the position of the Loop Current 

during summers after images were contrast-stretched. Also, significant upwelling 

events were seen during summer along the periphery of the NEGOM, specifically in 

coasts of the Big Bend region and in the region off Pinellas and Manatee Counties (near 

the mouth of Tampa Bay). During the period of May-July 1998, we observed unusual 

upwelling along the coasts of the Florida Panhandle in the northeastern Gulf of Mexico 

that led to 3-6 °C lower SST than normal. We also observed strong upwelling near the 

shelf break in the northeastern Gulf of Mexico after hurricanes Earl (September 2-4, 

1998) and Georges (September 25- October 1, 1998) suggesting that the shelf and 

slope may have enhanced upwelling associated with hurricanes. Using monthly mean 

AVHRR SST images, TOPEX/ERS SSH data, and current data derived from surface 

drifters deployed in 1996, the mean flow at the shelf break in the DeSoto Canyon region 

from January through July was from west to east, and in the west Florida Shelf region 

from north to south. An anticyclonic jet followed the curvature of the shelf off the Florida 

Panhandle shelf break. In August, flow reversed and the mean surface currents at the 

shelf break were from south to north-northwest off the west Florida Shelf, east to west in 
ACCESS NUMBER: 30857 

the DeSoto Canyon region, and offshore in the Mississippi Delta region. This gave the 

flow a cyclonic curvature. The flow reversal seen in 1996 occurred as a major eddy 

separated from the Loop Current, and it persisted into the fall. The combined AVHRR, 

altimeter, and drifter-derived monthly velocities indicate that there is a seasonal cycle in 

the flow over the west Florida shelf, with southward flow during the spring and 

northward flow during late summer. While some very strong currents were also 

observed in the northern and western part of the NEGOM, currents here in general were 

more erratic. The historical ocean color data obtained from the CZCS shows that the 

pigment concentration patterns are an effective tool for tracing small scale as well as 

large scale circulation patterns in the GOM. These patterns were very clearly outlined 

during summer months, and therefore the combination of AVHRR and CZCS/SeaWiFS 

was very useful for outlining the position of the Loop Current, eddies, and various 

instability waves visible along fronts in the region. The SeaWiFS ocean color data and 

in situ data collected during a series of 9 cruises showed that freshwater from the 

Mississippi River spread eastward in the summers of 1998, 1999, and 2000. In Spring 

and Winter, however, there was no significant eastward entrainment of the Mississippi 

River water. Upon merging the AVHRR SST data with the altimeter fields, we found 

good correlation between warm areas and elevated dynamic heights, and cool areas 

and low dynamic heights. Using AVHRR SST, CZCS, SeaWiFS, TOPEX/ERS SSH and 

in-situ data for 24-year period 1976-1999, we found that the Loop Current and 

associated warm water penetrated as far as 27.5 °N about 10 % of the time (i.e., about 

2 events every 3 years), and 28 °N about 5% of the time (i.e., 2 events every 5 years). 

No penetration to 29 °N was detected. 

STUDY RESULTS: For the period covered under this study (April 1997-December 

2000), approximately 10,560 AVHRR SST images of the Gulf of Mexico (GOM) were 

collected by the USF station. These images were stored in high (1x1 km2

) and low 

(2.8x4.1 km2) pixel resolution. Monthly mean SST for the Gulf of Mexico was computed 

for the period of 1993-2000. Approximately 2,400 SeaWiFS images (September 1997- 

December 2000) were collected over the Gulf of Mexico and monthly means of 

chlorophyll-a concentration computed. In addition, the historical Coastal Zone Color 

Scanner (CZCS) data were processed to pigment products and then averaged to derive 

a series of 92 monthly pigment concentration means. Near-real time and monthly mean 

SST and SeaWiFS images for the Gulf of Mexico can be accessed at: 

http://imars.marine.usf.edu (some areas of this site are password protected). 

A CD-ROM containing all of the historical altimetry data for the Gulf of Mexico from 

1993 through the end of 1999 was produced at the University of Colorado (Dr. R. 

Leben). The final version of the processed data included both sea surface anomaly 

relative to the 1993-1999 time period and an estimate of the total dynamic topography. 

A model mean was added to the sea surface height anomaly to produce an estimate of 

the total dynamic topography. The 1993-1999 mean from the GOM model hindcast 

simulation performed for the MMS Deepwater Reanalysis and Synthesis Project was 

used. These data are also available on line. Dr. Leben has developed a Loop Current 

Intrusion (LCI) index based on altimetry data, which was analogous to the El NiñoSouthern Oscillation (ENSO) indices. Several integral measures of Loop Current 
ACCESS NUMBER: 30857 

properties including estimates of Loop Current area, volume and circulation have been 

produced from the statistics of the historical data set. The Loop Current circulation, 

which was estimated by a line integral of the velocity along the 17-cm sea surface 

height contour, was a proposed index for continuous monitoring of the Loop Current 

penetration and shedding. The 17-cm contour was chosen by experience and success 

tracking the Loop Current and its eddies in altimetry and with a data assimilative model. 

Dr. Leben also developed software for automatic tracking of Loop Current Eddy centers, 

as well as for tracing the perimeter of an eddy by using the 17-cm contour of SSH 

anomaly. 

This effort included extensive collaboration and participation in NEGOM cruises 

conducted by Texas A&M University. Flow-through data confirmed that freshwater from 

the Mississippi River spread eastward in the summers of 1998, 1999, and 2000. 

Mississippi water was seen near 27oN and 87oW, as indicated by low salinity (~23 psu), 

high Chlor-a (~2 mg/m3, compared with <0.1 mg/m3 for the adjacent oligotrophic ocean), 

and high colored dissolved organic matter (CDOM absorption at 400 nm is ~0.3 m-1, 

compared with <0.05 m-1 for the oligotrophic ocean). The ocean color satellite imagery 

(SeaWiFS) showed that the spatial patterns of eastward entrainment of this river plume 

were similar in the three summers. In some instances, the eastward entrainment was 

facilitated by an anticyclonic eddy located south of the Desoto Canyon area. In Spring 

and Winter, however, there was no significant eastward entrainment of the Mississippi 

River water. Correlation between CDOM and salinity was higher (r2 = -0.85 or higher) 

than the correlation between CDOM and chlorophyll-a (r2

 = 0.783 or less), suggesting 

that CDOM is a better tracer of low salinity associated with river plume waters. 

STUDY PRODUCTS: Melo Gonzalez, N., F.E. Müller-Karger, S. Cerdeira Estrada, R. 

Pérez de los Reyes, I. Victoria del Rio, P. Cardenas Perez, and I. Mitrani Arenal. 2000. 

Near-surface phytoplankton distribution in the western Intra-Americas Sea: The 

influence of El Niño and weather events. Journal of Geophysical Research. Vol. 105 

(C6):14,029-14,043. 

Müller-Karger, F.E., F. Vukovich, R. Leben, B. Nababan, C. Hu, and D. Myhre. 2001. 

Surface circulation and the transport of the Loop Current into the Northeastern Gulf of 

Mexico: Final Report. OCS Study MMS 2001-0102, U.S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, La. 54 pp. 

Muller-Karger F.E. 2000. The spring 1998 northeastern Gulf of Mexico (NEGOM) cold 

water event: remote sensing evidence for upwelling and for eastward advection of 

Mississippi water (or: how an errant Loop Current anticyclone took the NEGOM for a 

spin). Gulf Mexico Science, 18:55-56. 

Müller-Karger, F.E., F. Vukovich, R. Leben, B. Nababan, C. Hu, and D. Myhre. 2000. 

Remote Sensing study of upwelling in the northeastern Gulf of Mexico and of the effects 

of Hurricanes Earl and Georges. Annual Report: Year 2. U.S. Dept. of the Interior, 

Mineral Management Service, Gulf of Mexico OCS Region, New Orleans, La. OCS 

Study MMS 2000. 43 pp. 
ACCESS NUMBER: 30857 

Müller-Karger, F.E., F. Vukovich, R. Leben, B. Nababan, and D. Myhre. 1998. 

Northeastern Gulf of Mexico Physical Oceanography Program: Eddy Monitoring and 

Remote Sensing. Technical Report of the U.S. Department of the Interior, Minerals 

Management Service, Gulf of Mexico OCS Region. OCS Study MMS 98-0051. 41 pp. 

Hu , C., F.E. Muller-Karger, D.C. Biggs, K.L. Carder, B. Nababan, D. Nadeau, and J. 

Vanderbloemen. 2001 (In Press). Comparison of ship and satellite bio-optical 

measurements on the continental margin of the NE Gulf of Mexico. Int. J. Rem. 

Sensing. 

Hu, C., B. Nababan, D. C. Biggs, and F. E. Muller-Karger. 2000. Variability of biooptical properties at sampling stations in the NE Gulf of Mexico. AGU 2000 Fall 

Meeting. Eos, Transactions, American Geophysical Union, Vol. 81, No. 48. 

Hu. C., F.E. Muller-Karger, D.C. Biggs, B. Nababan, D. Nadeau. 2000. Spatial and 

Temporal Variations of Bio-optical Properties in the Northeastern Gulf of Mexico: Twoyear Observations from Ship and Satellite. 2000 Ocean Sciences Meeting. January 24-

28, San Antonio, Texas.