Dynamical properties of a buoyancy‐driven coastal current

Abstract

The outflow of buoyant waters from major estuaries affects the dynamics of inner continental shelves profoundly as lateral density gradients force an alongshore current. Often the Coriolis force causes the outflow to remain trapped near the coast. We observed one such current, the Delaware Coastal Current, on the inner shelf near the Delaware Estuary on the eastern seaboard of the United States. The spatial variability along the shelf, however, suggests at least two dynamically distinct regions that we term source and plume regions. In the source region we find fronts, a current whose width scales well with the internal deformation radius, and a ratio of relative to planetary vorticity that reaches unity, that is, the Rossby number is O(1). As nonlinear inertial forces in the across-shelf momentum balance are weak, we suggest that such forces contribute to the along-shelf momentum balance only. Farther downstream in the plume region, we find much reduced lateral density gradients, a current much wider than the deformation radius, and relative vorticities that are much smaller than the planetary vortiCity. From our observations we compute nondimensional dynamical parameters, with which we discuss our observations. The Burger, Rossby, and Ekman numbers for the Delaware Coastal Current suggest that most models of buoyancy-driven coastal currents do not apply to this coastal flow. I. INTRODUCTIONThe lateral flux of buoyancy from estuaries into the coastal ocean constitutes a forcing agent influencing both circulation and mixing. The density differences between brackish estuarine and salty oceanic waters force a flow on the shelf. Density gradients induce pressure gradients which are often balanced by Coriolis forces. In the northern hemisphere the Coriolis force turns the estuarine outflow to the right looking seaward and traps the buoyant water on the inner shelves. Across-shelf mixing with ambient shelf water is thus reduced.Such currents distribute riverborne nutrients, larvae, sediments, sewage, toxic chemicals, and oil from accidental spills dominantly along the shore. This study will describe one such current, namely the Delaware Coastal Current. Woods and BeardsIcy[1988] studied barotropic estuarine outflow problems with a set of analytical, numerical, and laboratory experiments. The outflowing fluid enters a uniformly sloping shelf, where vortex tube stretching and friction determine its path. Their studies relate indirectly to the discharge of water from major rivers, since they isolate barotropic from baroclinic effects. For small Rossby numbers, they found that Csanady's [1978] arrested topographic wave dynamics resulted, whereas for moderate Rossby numbers they reproduced the one-layer results of BeardsIcy and Hart [1978]. Beardsley and Hart prescribed an outflow from a point source, while Woods and Beardsley [1988] modeled the shelf flow that was forced by an outflow from a finite gap in the coastline. In both studies estuarine waters turn to the right in the northern hemisphere. Most outflows, however, ...

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