Low confidence in multi-decadal trends of wind-driven upwelling across the Benguela Upwelling System

Abstract

Abstract. Like other eastern boundary upwelling systems, in the Benguela Upwelling System, the upwelling along the coastline is primarily alongshore-wind-driven. In contrast, it is mainly driven 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. Using the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis for 1979–2021, we investigate multi-decadal changes in the South Atlantic Anticyclone and their impacts on coastal upwelling driven by alongshore winds, wind-stress-curl-driven upwelling within the coastal zone and total upwelling as the sum of both across the Benguela Upwelling System. Even though the detailed structure of surface wind over the coastal zone matters for both alongshore-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 zonally integrated wind-stress-curl-driven and total upwelling. However, such connection for the alongshore-wind-driven coastal upwelling is weak. 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. In general, the impacts of the anticyclone on the local upwelling are more robust during the austral winter. 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 interannual to decadal climate variability. This view is further supported by the coastal and wind-stress-curl-driven upwelling in several upwelling cells showing hardly any significant multi-decadal trends.