Seasonal cycle of sea surface temperature in the tropical Angolan upwelling system

Abstract. The Angolan shelf system represents a highly productive ecosystem. Throughout the year sea surface temperatures (SSTs) are cooler near the coast than further offshore. Lowest SSTs, the strongest cross-shore temperature gradient and maximum productivity occur in austral winter when seasonally prevailing upwelling favourable winds are weakest. Here, we investigate the seasonal mixed layer heat budget to analyse atmospheric and oceanic causes for heat content variability. By using different satellite and in-situ data, we derive monthly estimates of surface heat fluxes, mean horizontal advection and local heat content change. We calculate the heat budgets for the near coastal and offshore regions separately to explore processes that lead to the observed differences. The results show that the net surface heat flux warms the coastal ocean stronger than further offshore thus acting to damp spatial SST differences. Mean horizontal heat advection is dominated by meridional advection of warm water along the Angolan coast. However, its contribution to the heat budget is small. Ocean turbulence data suggests that the heat flux due to turbulent mixing across the base of the mixed layer is an important cooling term. This turbulent cooling that is strongest in shallow shelf regions is capable of explaining the observed negative cross-shore temperature gradient. The residuum of the mixed layer heat budget and uncertainties of budget terms are discussed.