Physical and Biological Processes in the Northern Gulf of Mexico Determined from Remote Sensing
Nan D. Walker
The Mississippi River is the largest river in North America draining 40% of the continental U.S. and discharging 18,400 m3 s-1 and 210 million tons of sediment annually to the northern Gulf of Mexico. Approximately 70% of the flow enters the Gulf through the bird-foot delta and the remaining 30% is carried down the Atchafalaya River. These river discharges have a large impact on physical and biological processes in the northern Gulf of Mexico. Available discharge data show that at least 70% of the combined Mississippi/Atchafalaya River flow is discharged onto the Louisiana shelf west of the bird-foot delta.
The river discharges entering the Gulf of Mexico can be readily detected in satellite imagery by their high reflectance, due to backscattering from suspended sediments, and their low temperatures. Satellite measurements from the NOAA AVHRR have been used extensively to investigate circulation processes and the fate of river waters in the northern Gulf of Mexico. Studies have shown that the buoyant plume of the Mississippi and Atchafalaya Rivers respond within hours to changes in wind forcing. The surface geometry of the Mississippi sediment plume is predictable from measurements of wind speed, direction and river discharge. East winds prevail with a frequency of about 62% throughout the year, forcing a westward flow of river water along the Louisiana coast and further downstream. Circulation is complex around the bird-foot delta as a result of the protrusion of land 70 km or so into the Gulf. During northeast wind conditions, most of the river discharge east of the bird-foot delta flow southward and then westward to join the flows from South Pass and Southwest Pass. West of the delta, these combined effluents turn northward where the flow splits, with a portion of the flow forming a clockwise gyre within the Louisiana Bight and a separate portion of the flow continuing westward along the Louisiana coast. During east wind periods, it is estimated that 60-80% of the discharge from the bird-foot delta moves westward. During west winds, the flow of river water around the bird-foot delta reverses and most of the freshwater moves eastward and southward. From October to March, strong west winds interrupt the prevailing east winds every 5-7 days with the passage of winter storms. During summer, relatively weak southwest winds can occur for several weeks to months reversing the direction of river waters for an extended period of time.
Satellite imagery of surface temperature variability have shown that offshelf flows, i.e. squirts and jets, are advected off the continental shelf in association with warm core eddies and warm core / cold core eddy pairs, resident on the continental slope. Research has shown that the initial impetus for the offshelf flow is wind forcing. These injections of shelf water from the Louisiana and Texas shelves into the deeper Gulf vary greatly in spatial structure (100-350 km long, 10-100 km wide), surface area (300-39,000 km2), and longevity (1-8 weeks). Surface velocities of 30-100 cm/s have been measured within these features. Research using ocean color sensors has shown that the shelf waters enhance the chlorophyll-a concentrations of the deeper Gulf. With the launch of the SeaWiFS ocean color sensor in mid-1997, we have gained the ability to detect river waters at greater distances from their source regions. Normalized water leaving radiances from the six visible-band channels of SeaWiFS provide the potential to detect suspended sediments, yellow substances and phytoplankton blooms (resulting from the nutrient enrichment of shelf waters). Efforts are underway by several research groups to obtain "sea-truth" measurements that will enable a more quantitative assessment of in-water biological constituents using SeaWiFS image data and the physical-biological interactions associated with Mississippi River effluents in the northern Gulf of Mexico.
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