Physical processes in the upwelling regions of the tropical Atlantic

Abstract

Abstract. In this paper, we review observational and modelling results on the upwelling in the inner tropical Atlantic. We focus on the physical processes that drive the seasonal variability of surface cooling and upward nutrient flux required to explain the seasonality of primary productivity. We separately consider the equatorial upwelling system, the northern coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. For the equatorial regime, we discuss the forcing of upwelling velocity and turbulent mixing as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is concentrated along northern boundary and is driven by both, local and remote forcing. The particular role of the Guinea Current, nonlinearity and the shape of the coastline are emphasized. For the tropical Angolan upwelling, we show that this system is not wind-driven, but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios.

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 ).…”

“…Along with understanding the drivers of the tropical upwelling system, it is of great socioeconomic interest to predict their variability. The coastal tropical upwelling system in the Pacific and Atlantic Ocean exhibits a pronounced seasonal cycle, with important interannual variations superimposed ( 11 – 13 ). Note that state-of-the-art climate models have difficulties to realistically represent eastern boundary upwelling regions and their variability ( 14 – 16 ).…”

“…Thus, productivity peaks about 1 month after the passage of the upwelling CTWs. Previous studies have discussed whether phytoplankton blooms are forced by CTWs, highlighting that productivity and the occurrence of upwelling CTWs are in phase ( 9 , 13 , 29 – 31 ). However, the control on productivity by the upwelling CTWs is still not resolved ( 13 ).…”

“…Previous studies have discussed whether phytoplankton blooms are forced by CTWs, highlighting that productivity and the occurrence of upwelling CTWs are in phase ( 9 , 13 , 29 – 31 ). However, the control on productivity by the upwelling CTWs is still not resolved ( 13 ). Note that the wind-driven upwelling in the tAUS is in general weak and peaks in May and can thus not explain the seasonal cycle in NPP (Fig.…”

“…In the tAUS, the role of vertical mixing, as a mechanism for upward nitrate supply, has been discussed on the basis of the investigated turbulent heat flux ( 13 , 29 ). The primary energy source for turbulence on the shelf of the tAUS is thought to be the dissipation of internal tidal energy ( 31 ).…”