The Role of Fine-Scale Winds in Upwelling and Coastal Circulation in the Southern Benguela Upwelling System

Eastern Boundary Upwelling Systems, driven by wind-induced Ekman transport, bring cold, nutrient-rich deep waters to the surface, making them hotspots of biological activity with significant economic, ecological, and social value. Also contributing to the upwelling of deep waters, is the Ekman pumping, which is created by cyclonic surface wind stress curl (WSC). WSC further influences local circulation and the formation of upwelling hotspots, such as those found downwind of prominent capes. Accurately representing these processes is critical for predicting future changes in upwelling and their impacts on marine productivity, in particular for the Southern Benguela Upwelling (SBU), a region characterised by a complex coastal geometry and orography. Using a fine resolution (1 km), curvilinear grid regional numerical model of the SBU, based on CROCO, this work highlights the sensitivity of upwelling processes to the fine scale spatial wind variability, by using two different wind products which differ by their resolution: ERA5 (~30 km) and WASA3 (~3 km). The lack of coastal wind drop-off in the CROCO-ERA5 simulation results in more intense nearshore and less intense offshore upwelling than observed in the CROCO-WASA3 simulation. These differences in the spatial structure of the coastal upwelling impact the coastal circulation. The inshore branch of the equatorward flowing Benguela Jet is shifted offshore in the CROCO-WASA3 run, while a poleward coastal jet emerges with intermittency. A better understanding of these upwelling and current structures could provide new insights into the generation of harmful algal blooms in the SBU.