Mean Currents Driven by Topographic Drag over the Continental Shelf and Slope

Abstract

A sequence of numerical simulations is described of wind-driven flow over irregular continental shelf topography. The model is barotropic, nonlinear, and forced by a periodic, spatially uniform alongshelf wind stress. The objective of the study is to determine whether topographic drag, known to be asymmetric for barotropic flow over the shelf, can generate substantial time-averaged alongshore currents in the presence of a fluctuating zero-mean wind stress. With realistic parameters, mean maximum alongshore currents of 0.05 to 7.0 cm s?1 are realized with flow in the direction of freely propagating shelf waves. The residual current strength is a strong function of wind stress period and bottom bump wavelength: larger forcing periods and shorter bump wavelengths enhance the time-mean circulation. Particle paths are generally observed to be chaotic, in contrast to the nearly cyclic behavior of the Eulerian velocity field. However, cross-shore particle dispersion is well correlated with the mean alongshore currents and may represent a testable observational signature of topographic drag effects. Model simulations using realistic spectra for both wind stress and bottom roughness yield a maximum flow of approximately 2.5 cm s?1. These results demonstrate that topographic drag asymmetries can lead to observable mean currents on continental shelves and may be a partial explanation for certain observed mean currents that run counter to mean alongshore winds.