Modification of sea surface temperature by chlorophyll concentration in the Atlantic upwelling systems

Abstract

AbstractThe influence of the chlorophyll on the upper Tropical Atlantic Ocean is investigated with long‐term (1979–2012) regional oceanic simulations with 1/4° horizontal resolution based on the NEMO3.6 model. The model solar radiation penetration scheme depends on the chlorophyll concentration. Simulations with time and spatially varying concentrations obtained from satellite ocean color observations are compared with a simulation forced with constant chlorophyll concentration of 0.05 mg m−3, representative of chlorophyll depleted waters. Results indicate that regions of the Tropical Atlantic with chlorophyll concentrations larger than in the reference simulation (i.e., [chl] > 0.05 mg m−3) get warmer at the surface, with the exception of the main upwelling regions where high chlorophyll concentrations are associated with a significant cooling of the sea surface (∼1°C in the Benguela upwelling). The analysis of the model heat balance shows that the biological differential heating causes negative temperature anomalies in subsurface source waters prior to their upwelling at the coast. The shallow mixed‐layer in the eastern equatorial and tropical Atlantic favors the persistence of these subsurface anomalies and may explain why the Benguela is particularly sensitive to the biological differential heating. In spite of the presence of high chlorophyll concentrations in the upwelling regions, both the larger amount of shortwave radiation captured in the surface layers and the modifications of the horizontal and vertical advection at the coast are found to play a secondary role in the SST change in the upwelling region.