On the impact of wind curls on coastal currents

Abstract

Studies of upwelling and coastally-trapped wave theory, as developed over the past thirty years, have largely neglected effects of cross-shelf variation in wind stress and the resulting wind stress curl. However, recent satellite-based observations (QuikSCAT) of global wind stress patterns show significant and persistent wind stress curls extending well offshore in some coastal regions including the Benguela System. Motivated by this example, we use a relatively simple analytical model to investigate explicitly the impact of cross-shelf variation in wind stress on the structure of the coastal currents. The model is based on the linear Boussinesq equations of a stratified, flat bottomed coastal ocean on a f-plane (southern hemisphere), bounded by a straight vertical wall. The model includes a wind mixed layer and a linear friction rate. The model equations are solved using the method of Green's functions. There are two mechanisms imposing divergencies of the Ekman transport, (1) coastal inhibition and (2) wind stress curl. In the first case the coastal flows are affected significantly by Kelvin waves, due to the waveguide properties of boundaries. In the second case, the wind stress curl generates vertical motion and hence horizontal pressure gradients, where the associated geostrophic flows are limited by friction only. As a result, complex flow patterns with counter-currents can emerge. In order to highlight the role of wind stress curls, the responses of the coastal ocean to different cross-shore variations of the alongshore wind stress are compared with the baseline case of no wind curl.