Case studies of the response and spatial distribution of wind-driven upwelling off the coast of Africa: 29–34° south

Abstract

The spatial distribution of wind-driven upwelling along the west coast of Africa (29–34°S) in the early summer is described using intermittent and sequential aerial survey observations, supported by surface-based time series. Three semi-permanent upwelling centers, at 29–31, 33 and 34°S were examined for response to local topographic and meteorological forcing, both before and after the active phase of upwelling. Differing scales, intensities and response lags in wind-driven coastal upwelling were documented through the analysis of selected aerial survey data sets. The upwelling area centered at 30°S was found to be of greater spatial extent than the upwelling plumes associated with the capes at 33 and 34°S. Variations in the depth of the marine atmospheric boundary layer affected the structure of equatorwards winds and upwelling activity off the capes. When the air temperature inversion was relatively low, wind stress was enhanced seawards of the capes, while equatorwards, a wind shadow reduced offshore Ekman transport and upwelling. The case studies highlight persistent mesoscale features in the wind and sea surface temperature fields which are formed by the relationship between coastal upwelling, wind forcing and local topography.