Response of the Southern Benguela Upwelling System to Fine-scale Modifications of the Coastal Wind

Considering the amount of daily wind stress products available with increasingly finer horizontal scales, this study aims at highlighting the short-term response of the ocean to a simple change of momentum fluxes in a hydrodynamic model. Specifically, we use the last QuikSCAT-derived wind products processed at Ifremer (Institut Français de Recherche pour l’Exploitation de la Mer) to document and quantify the effect of small-scale wind stress structures on the Southern Benguela dynamics. We analyze the results of a regional model forced by wind stress fields derived from QuikSCAT observations. Two different horizontal resolutions are considered for the wind stress: QS25 and QS50, corresponding to native 25- and 50-km grids, respectively. The differences between both products highlight the primary importance of fine-scale momentum fluxes for both the structure and intensity of the wind- and wind curl-driven upwelling. Using QS25, we show that the coastal Ekman transport is reduced, leading to a warmer SST and a reduced oceanic coastal jet. QS25 finer wind stress curl patterns also favor the development of a stronger and shallower poleward undercurrent. The addition of a coastal wind correction to QS25 lets us investigate the possible implications of an imbalance between Ekman transport and Ekman pumping: a wind reduction in the coastal band often reduces the SST cooling, but the two mechanisms compensate each other when the characteristic length scales of the coastal upwelling and the orography-induced wind drop-off are similar.