The Benguela Upwelling System: Quantifying the Sensitivity to Resolution and Coastal Wind Representation in a Global Climate Model

Of all the major coastal upwelling systems in the World's ocean, the Benguela, located off south-west Africa, is the one which climate models find hardest to simulate well, with a common warm sea surface temperature (SST) bias. At issue are problems with stratocumulus clouds, the structure of the atmospheric jet, as well as ocean processes such as upwelling and horizontal currents. In contrast, regional and coastal high-resolution ocean models often exhibit a cool SST bias in the same region, possibly due to too vigorous upwelling. This presentation aims to understand why the results of these two approaches are so different, using the global Community Climate System Model (version 4) and the Regional Ocean Modelling System (ROMS) embedded in CCSM4. It is found that the structure of the wind stress and wind stress curl close to the coast associated with the eastern edge of the St. Helena anticyclone governs the CCSM4 and ROMS response. A broad wind stress curl field leads to southward currents and broad, slow upwelling, by Sverdrup dynamics and Ekman pumping respectively, which acts to worsen SST bias in CCSM4. However, wind stress curl confined to a narrow band near the coast results in northward currents and more localized and strong upwelling, acting to improve SST bias, in a sensitivity experiment using the ROMS-CCSM4 system. The results are discussed in terms of previous modelling studies and observations, and in terms of the large scale climate simulations.