Low confidence in multi-decadal trends of wind-driven upwelling across the Benguela upwelling system due to internal climate variability

Abstract. 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 is strongly related to the South Atlantic Anticyclone that is believed to migrate poleward in response to anthropogenic global warming. Here, we investigate multi-decadal changes of the South Atlantic Anticyclone and their impacts on the cross-shore integral of wind-driven coastal upwelling, the wind stress curl-driven, and total upwelling across the Benguela Upwelling System. Even though the detailed structure of surface wind over the coastal zone matters for both local wind-driven coastal upwelling and wind stress curl-driven upwelling, we show that it is not of major importance for the total amount of upwelled water. We found a robust connection between the Anticyclone intensity and the integrated wind stress curl-driven and total upwelling. However, this connection for the wind-driven coastal upwelling is weak. With more signatures during austral winter, the upwelling in the equatorward portion of the Benguela Upwelling System 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, this trend features a small signal-to-noise ratio and can be obscured by internal climate variability. This view is further supported by a multi-decadal trend in the integrated coastal and wind-stress-curl-driven upwelling in several upwelling cells, which hardly depict any significant systematic changes.