Coastal Boundary Conditions and the Baroclinic Structure of Wind-Driven Continental Shelf Currents

Abstract

The generation of continental shelf currents by wind forcing is investigated by analytical and numerical methods. The investigation is motivated by observations from the Coastal Ocean Dynamics Experiment. A central assumption is that the vertical structure of the response over the inshore half of the shelf is controlled by the vertical distribution of the turbulent stress. This suggests a two-layer model of the wind-driven circulation, in which the upper layer represents a surface wind-mixed layer, and the lower layer represents the remainder of the fluid. The response of this idealized dynamical model to wind forcing is examined and compared with observations in the 2?7-day period band. For the alongshore velocity gain relative to local wind stress, an onshore surface maximum and an offshore interior maximum are robustly reproduced by the model. These features are evidently related to a dynamical transition over the inner half of the shelf, in which the alongshore wind stress is balanced more by acceleration of near-surface alongshore flow and less by time-dependent Ekman transport as the coast is approached. This differs from a previous hypothesis, based on a linear model in which the turbulent stress was confined to infinitesimally thin surface and bottom boundary layers, which related the alongshore flow structure to the cross-shore profile of the alongshore wind amplitude. In the present model, the cross-shore velocity variances are roughly comparable to those observed over the onshore half of the shelf. This also contrasts with the previous model results, which underpredicted cross-shore velocity variances by more than an order of magnitude. However, the present agreement is probably fortuitous, as the enhanced lower-layer cross-shore flow is frictionally driven, and should probably be confined to a bottom boundary layer as it was in the previous model. The results demonstrate that the response of these models over the inner half of the shelf depends strongly on a poorly understood coastal boundary condition.