Changes in coastal upwelling in the northern Gulf of Guinea under Stratospheric Aerosol Injection

This study aims to assess the impact of Stratospheric Aerosol Injection (SAI) on the coastal upwelling in the northern Gulf of Guinea based, on upwelling index computation and using the Community Earth System Model from the Geoengineering Large Ensemble (GLENS) project. GLENS project targets not only maintaining the global temperature but also the interhemispheric and equator-to-pole temperature gradient at their 2020 values by preventing part of the solar radiation from reaching the Earth's surface under a RCP8.5 scenario. The results show that along the coast of the northern Gulf of Guinea main upwelling cells are to the east of Cape Palmas and Cape Three Points, and that upwelling is most intense in the Ghana region compared to Côte d'Ivoire region. It is also found that Ekman transport associated with geostrophic flow can explain a large part of the intensity of the coastal upwelling in the northern Gulf of Guinea. Geostrophic flow towards the coast reduces upwelling intensity, especially in the Ghana region. In the context of global warming, boreal summer upwelling intensity decreases all along the coast by 6% (with 2% in the Côte d'Ivoire region and a more significant decrease of 10% in the Ghana region). This decrease in upwelling intensity is linked to the intensification of geostrophic flow towards the coast. Under SAI, coastal upwelling intensity is still decreased by 5% along the coast (with 3% in the Côte d'Ivoire region and 6% in the Ghana region), but this decrease is relatively weak compared to global warming. This increase in upwelling intensity compared with climate change, especially in Ghana, is associated with a 50% reduction in the effect of geostrophic flow limitation with respect to global warming. During the minor upwelling season, the upwelling intensity increases, due to Ekman transport, both under the climate change and SAI.