The Congolese upwelling system (CUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea Surface Temperature minimum in the CUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular reproduces the signature of the surface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals a competition between warming by air-sea fluxes, dominated by the solar flux throughout the year, and cooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurface analysis shows that remotely-forced coastal trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed-layer and leads to the mixing-induced seasonal cooling in the Congolese upwelling system.

Stratospheric Aerosol Geoengineering (SAG) is proposed to offset global warming; the use of this approach can impact the hydrological cycle. We use simulations from Coupled Model Intercomparison Project (CMIP5) and Geoengineering Model Intercomparison Project (G3 simulation) to analyze the impacts of SAG on precipitation (P) and to determine its responsible causes in West Africa and Sahel region. CMIP5 Historical data are firstly validated, the results obtained are consistent with those of observations data (CMAP and GPCP). Under the Representative Concentration Pathway (RCP) scenario RCP4.5, a slight increase is found in West Africa Region (WAR) relative to present-day climate. The dynamic processes especially the monsoon shifts are responsible for this change of precipitation. Under RCP4.5, during the monsoon period, reductions in P are 0.86%, 0.80% related to the present-day climate in the Northern Sahel (NSA), Southern Sahel (SSA) respectively while P is increased by 1.04% in WAR. Under SAG, 3.71% of P change (decrease) was associated with a -3.51 value of efficacy in the West African Region (AR). Under G3, a significant decrease of P is found in the West African region. This decrease in monsoon precipitation is mainly explained by changes in dynamics, which leads to weakened monsoon circulation and a shift in the distribution of monsoon precipitation. This result suggests that SAG deployment to balancing all warming can be harmful to rainfall in WAR if the amount of SO2 to be injected in this tropical area is not taken into consideration.

Potential vorticity (PV) is a key parameter to analyze the generation and dynamics of oceanic mesoscale eddies. Adiabatic and diabatic processes can be involved in the generation of localized PV anomalies and vortices. However, PV is difficult to evaluate at mesoscale. In this study we argue that eddies created by diapycnal mixing or isopycnal advection of water-masses are associated with PV anomalies and significant isopycnal temperature/salinity anomalies (Ɵ’/S’). In contrast, eddies created by friction are associated with PV anomalies but with non-significant isopycnal Ɵ’/S’. Based on 18 years of satellite altimetry data and vertical Ɵ/S profiles from Argo floats, we analyze the isopycnal Ɵ’/S’ within new-born eddies in the tropical Atlantic Ocean (TAO) and discuss the possible mechanisms involved in their generation. Our results show that on density-coordinates system, both anticyclonic (AEs) and cyclonic (CEs) eddies can exhibit positive, negative or non-significant isopycnal Ɵ’/S’. Almost half of the sampled eddies do not have significant Ɵ’/S’ at their generation site, suggesting that frictional effects play a significant role in the generation of their PV anomalies. The other half of eddies, likely generated by diapycnal mixing or isopycnal advection, exhibits significant positive or negative anomalies with typical Ɵ’ of ±0.5°C. More than 70% of these significant eddies are subsurface-intensified, having their cores below the seasonal pycnocline. Refined analyses of the vertical structure of new-born eddies in three selected subregions of the TAO, show the dominance of cold (warm) subsurface AEs (CEs) likely due to isopycnal advection of large scale PV and temperature.