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.