The Relation of Wind-Driven Coastal and Offshore Upwelling in the Benguela Upwelling System

Abstract

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind stress curl. Here, concurrently measured wind fields and satellite-derived chlorophyll-a concentration, along with a state-of-the-art ocean model simulation, spanning 2008–18 are used to investigate the connection between coastal and offshore physical drivers of the Benguela upwelling system (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model is fairly consistent across the entire BUS and is closely followed by the chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of submesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and are mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest that this relationship is far more complex than is currently thought and seems to be sensitive to climate changes, with short-and far-reaching consequences for this vulnerable marine ecosystem.