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

• Published in: Journal of climate 2015-12, Vol.28 (23), p.9409-9432
• Main Authors: Small, R. Justin, Curchitser, Enrique, Hedstrom, Katherine, Kauffman, Brian, Large, William G.
• Format: Article
• Language: English
• Subjects: Advection; Atmosphere; Atmospheric models; Atmospherics; Bias; Biogeochemistry; Brackish; Climate; Climate models; Climate system; Climatology; Coastal currents; Coastal jets; Coastal upwelling; Coastal winds; Coastal zone; Coasts; Global climate; Global climate models; Marine; Meteorology; Modelling; Ocean circulation; Ocean circulation models; Ocean currents; Ocean models; Oceans; Offshore; Offshore structures; Precipitation; Representations; Resolution; Salinity; Sea surface; Sea surface temperature; Sensitivity; Surface temperature; Upwelling; Upwelling water; Wind; Wind stress; Wind stress curl; Wind structure
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli-d-15-0192.1

Of all the major coastal upwelling systems in the world’s oceans, the Benguela, located off southwest Africa, is the one that climate models find hardest to simulate well. This paper investigates the sensitivity of upwelling processes, and of sea surface temperature (SST), in this region to resolution of the climate model and to the offshore wind structure. The Community Climate System Model (version 4) is used here, together with the Regional Ocean Modeling System. The main result is that a realistic wind stress curl at the eastern boundary,anda high-resolution ocean model, are required to well simulate the Benguela upwelling system. When the wind stress curl is too broad (as with a 1° atmosphere model or coarser), a Sverdrup balance prevails at the eastern boundary, implying southward ocean transport extending as far as 30°S and warm advection. Higher atmosphere resolution, up to 0.5°, does bring the atmospheric jet closer to the coast, but there can be too strong a wind stress curl. The most realistic representation of the upwelling system is found by adjusting the 0.5° atmosphere model wind structure near the coast toward observations, while using an eddy-resolving ocean model. A similar adjustment applied to a 1° ocean model did not show such improvement. Finally, the remote equatorial Atlantic response to restoring SST in a broad region offshore of Benguela is substantial; however, there is not a large response to correcting SST in the narrow coastal upwelling zone alone.