Wind-Forced Upwelling: The Role of the Surface Mixed Layer

Abstract

A numerical study is made of the dynamics of the circulation that arises from forcing by a steady, uniform alongshore wind over a uniform zonal shelf in the Southern Hemisphere. The results show that most of the upwelling is confined to the region inshore of the shelf break. Over the slope and within the region of wind forcing the circulation is found to be dominated by the growth of both an anticyclonic and a cyclonic eddy, an offshore flow, and downwelling. Two new mechanisms are identified as being responsible for these features of the upwelling circulation. First, the offshore advection of alongshore momentum (?0u?x) within the surface mixed layer (SML) is shown to accelerate the alongshore current near the eastern end of the region of wind forcing, leading to a sharpening of the cross-shelf gradient of sea level and an excess in the alongshore transport. This excess transport acts to raise sea level, leads to downwelling below the base of the SML, and causes the growth of the anticyclonic eddy. A scaling suggests that the advective term ?0u?x will be important provided that u?Lw(N/f)1/2/(fL2s) is of order one or larger. Here, u?, f, N, Lw, and Ls denote the surface friction velocity, Coriolis parameter, buoyancy frequency, fetch of wind forcing, and a scale for the shelf width. The magnitude of the advective term increases with near-surface stratification, since the SML depth will be smaller and the cross-shelf velocity u larger, and wind fetch and decreasing shelf width, since the alongshore shelf velocity ? will be larger. A second important effect is argued to result from the geostrophic cross-shelf flow associated with the alongshore gradients of density within the SML. The cross-shelf flows, largest near the edges of the region of wind forcing, act to raise (lower) sea level leading to an additional onshore (offshore) flow, vorticity production, and growth of the eddies.