The importance of internal climate variability in the multi-decadal trend of the wind-driven upwelling on the west African coasts

Like other Eastern Boundary Upwelling Systems, the upwelling near the southwest African coasts is primarily alongshore-wind-driven, whereas it is controlled mainly by the wind stress curl farther offshore. The surface wind regime across the Benguela Upwelling System (BUS) is strongly related to the South Atlantic Anticyclone (SAA), which is believed to migrate poleward in response to anthropogenic global warming. Here, we investigate multi-decadal changes of the SAA and its impacts on the coastal Ekman transport as a primary driver of coastal upwelling and the wind-stress-curl-driven upwelling across the BUS by using the ERA-5 data sets. Our findings indicate that the SAA plays a significant role in the regional wind-driven upwelling with different impacts on the coastal Ekman transport and the offshore wind-stress-curl-driven upwelling. Further, the upwelling in the equatorward sector is significantly affected by the anticyclone intensity. In contrast, the poleward portion is also influenced by the meridional position of the anticyclone. The multi-decadal trend in the sea level pressure across the South Atlantic renders a considerable heterogeneity in space. However, the trend is broadly associated with a small signal-to-noise ratio, which can be attributed to internal climate variability. This view is further supported by the multi-decadal trend in coastal offshore transport and the wind-stress-curl-driven upwelling in multiple upwelling cells, which hardly depict any significant systematic changes.