Seasonal cycle of sea surface temperature in the tropical Angolan Upwelling System

Abstract

Abstract. The Angolan shelf system represents a highly productive ecosystem. Throughout the year the sea surface is cooler near the coast than further offshore. The lowest sea surface temperature (SST), 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 identify 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 SST 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 suggest that the heat flux, due to turbulent mixing across the base of the mixed layer, is an important cooling term. This turbulent cooling, being 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.

Coastal trapped waves and tidal mixing control primary production in the tropical Angolan upwelling system

“…In contrast, during the upwelling season, we find denser water near the surface, resulting in a weaker stratification below the mixed layer. These differences in the upper ocean can be explained by surface heat and freshwater fluxes, as well as river runoff, resulting in warmer and fresher surface waters during the downwelling season compared to the upwelling season ( 9 , 13 , 32 , 47 ). Temperature differences are determined by the seasonal cycle of surface heat fluxes ( 32 ), while changes in salinity are mainly influenced by the advection of fresh water from the Congo River by the southward flowing Angola Current ( 47 ).…”

“…These differences in the upper ocean can be explained by surface heat and freshwater fluxes, as well as river runoff, resulting in warmer and fresher surface waters during the downwelling season compared to the upwelling season ( 9 , 13 , 32 , 47 ). Temperature differences are determined by the seasonal cycle of surface heat fluxes ( 32 ), while changes in salinity are mainly influenced by the advection of fresh water from the Congo River by the southward flowing Angola Current ( 47 ). The strength of the Angola Current, and thus the strength of the freshwater advection, is modulated by the different phases of the semi-annual CTWs propagating southward along the Angolan coast.…”

“…The primary energy source of vertical mixing in the tAUS is thought to be dissipation of internal tidal energy, where energy available for mixing on the shelf is nearly constant throughout the year (31). This is supported by observations, as seasonal differences in the turbulence levels from the available data in the tAUS were not identified (32). Therefore, we combine the ocean turbulence data collected during the six cruises on board of R/V Meteor with the seasonal gradients of nitrate from all available biogeochemical data.…”

“…The interaction of the barotropic tide with the continental slope leads to the generation of internal tides that partly propagate onto the continental shelf and dissipate in shallow water (31). Elevated dissipation rates of turbulent kinetic energy (TKE) are frequently observed in shallow waters on the Angolan shelf, which are responsible for the near-shore cooling of surface waters (31,32). The regionally confined enhanced mixing on the shelf was associated with breaking internal waves, resulting in the development of frontal structures that leads to the aggregation of small pelagic fish (44).…”

“…Further note that in the Peruvian upwelling system, where wind stress maxima and productivity maxima are also disjoint, the seasonal cycle of productivity is discussed to be related to the seasonality of the mixed layer depth with shallowest mixed layer depths during the maxima in productivity ( 10 , 12 ). This mechanism that involves vertical dilution of phytoplankton and light limitation during periods of deep mixed layer cannot explain the productivity in the tAUS because in contrast to the Peruvian upwelling system the mixed layer depth is deepest during the main productivity season ( 32 ).…”