Freshwater Input Influence on Sea Surface Temperature Mean State and Variability in the Southeastern Tropical Atlantic
The Southeastern Tropical Atlantic (SETA) is an eastern boundary upwelling region and one of the most productive marine ecosystems in the world. SST interannual variability within the SETA is dominated by Benguela Niños (Ninãs), which are extreme warm (cold) events that have severe consequences for local communities impacting rainfall, flooding, and fisheries. This thesis highlights the role of the previously overlooked mechanism of freshwater input, mainly from the Congo River, for both the SST mean-state and interannual variability in the SETA. To achieve this, a diverse array of datasets is utilized, including satellite measurements, ship-based in-situ observations, reanalysis products, and model simulation outputs. The presence of low salinity waters from the Congo discharge increases the mean state SST in the SETA coastal fringe by up to 0.9ºC from south of the Congo River mouth to 17ºS. North of the River mouth up to about 4ºS, this input significantly reduces the mean state SST by more than 1ºC. This impact is linked to a halosteric effect, which modifies the sea surface height gradient and alters geostrophic currents. In addition, anomalous freshwater input can boost Benguela Niño/Niña events by altering the water column’s stratification and consequently changing the turbulent heat loss magnitude. Anomalously high (low) salinity waters within the SETA are linked to the occurrence of positive (negative) Indian Ocean Dipole events via precipitation over the Congo basin and enhanced (reduced) river discharge, acting to amplify warm (cold) extreme events. Low salinity waters during the warm 1995 Benguela Niño generated nearly 3 times less turbulent fluxes when compared to the cold, high-salinity, 1997 Benguela Niña event. This thesis draws attention to the impacts of the up-until-now unnoticed mechanism of freshwater input on SSTs and coastal ocean surface dynamics in the SETA.
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