Quarterdeck Magazine Online -- Texas A&M Oceanography

Texas A&M University
Department of Oceanography

Summer 1999 - Vol. 7, No. 1

Density differences move shelf water

Modeling buoyancy-driven flow due to
river discharge on the Texas-Louisiana shelf

by Yao-Tsai Lo

When fresh water from a river flows onto a continental shelf region occupied by saltier and denser oceanic water, potential energy may drive thermohaline currents, or vertical circulations caused by changes in density. Often the dynamic structure of the spreading low-salinity water over the shelf is referred to as a river plume.
Studies of the temporal and spatial structures of a river plume are of considerable interest not only because of the plumes' influences on the physical processes of the shelf circulation but also because of their close relationship to coastal ecosystem and environmental pollution problems.
In particular, a vast amount of land-drained materials, suspended organic materials, industrial wastes, and sewage brought onto the continental shelves through river discharge may significantly affect fishery production and water quality. Therefore, it is important to have a general understanding of the coastal ocean circulation as well as to trace and to predict the pathways and the distributions of river-borne dissolved materials or pollutants in the coastal ocean.
The Texas-Louisiana shelf is one of the primary fishery grounds in the nation and also has many oil activities offshore. The coastal circulation over the inner shelf is significantly affected by the freshwater discharged from the Mississippi-Atchafalaya River system, the largest in the United States. These two rivers add approximately 300 cubic kilometers of freshwater onto the shelf annually. Usually, high freshwater runoff from both rivers occurs in the spring period, with springtime average transport of about 20,000 and 10,000 m3s-1 for the Mississippi and Atchafalaya Rivers, respectively.
The principal objective of my research was to study the formation and evolution of the Mississippi-Atchafalaya River plumes on the Texas-Louisiana shelf by means of an existing numerical model. In oceanography, the ocean motions are governed by the incompressible Navier-Stokes equations.
To study the development and evolution of the Mississippi-Atchafalaya River plume, numerical models provide a convenient method for solving the time-dependent and fully nonlinear momentum equations.
First, I studied the river plume, which is essentially the response to the effect of the buoyancy anomaly induced by the river inflows. The model result showed that freshened bulges form near the river mouths, and the surface flow turns anticyclonically (clockwise) within the bulges in the northern hemisphere due to the Coriolis force (the effect of the earth's rotation). A coastal current, leaking from the bulge, is developed along the coast in a narrow zone (60 kilometers) toward the west. Next, the effect of wind on the existing river plume was considered. The result of my study showed that the structure of the existing river plume is significantly modified by the surface wind stress. Particularly, the low-salinity water moves across the shelf by the eastward component of wind.

Yao-Tsai Lo graduated with a Ph.D. in physical oceanography in May 1999. He plans to continue his postdoctoral studies on the Kuroshio Current at National Taiwan Ocean University.

Figure:
Each year, the Atchafalaya and Mississippi rivers add about 300 cubic kilometers of fresh water onto the Texas-Louisiana continental shelf. This figure is a model of annual mean river discharge, shown in light gray. Surface currents are shown as small arrows pointing in the direction of the current.

http://oceanography.tamu.edu/Quarterdeck/1999/1/lo.html
© Copyright 999, Department of Oceanography, Texas A&M University.

Send Quarterdeck inquiries to quarterdeck@ocean.tamu.edu.