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Stratospheric Aerosol Geoengineering (SAG), which involves injecting sulfur dioxide (SO2) into the stratosphere, has been proposed as a potential climate intervention strategy to mitigate global warming. In this study, we assess how SAG could affect chlorophyll concentrations in the Congolese Upwelling System (CUS), and identify the key processes responsible for these changes, using data from the Community Earth System Model version 2, specifically the SSP5-8.5 and Geo SSP5-8.5 datasets. The model reproduces chlorophyll concentrations at both the surface and subsurface, although it tends to underestimate their magnitude compared to observations. Under climate change (RCP8.5), compared to current climate, chlorophyll concentrations are projected to decrease throughout the year, mainly due to a reduction in diatoms, the dominant chlorophyll phytoplankton group in the region. Under SAG, a net increase in chlorophyll concentration is observed all year-round, except in September-October, largely driven by an increase in diatoms. The results reveal that, under global warming, the decrease in chlorophyll concentration is mainly linked to strong stratification observed below 20 m of the mixed layer, which prevents nitrate supply to the euphotic layer and consequently reduces biological activity that should increase chlorophyll. It should be noted that changes in meridional advection driven by changes in the meridional chlorophyll gradient also contribute to this decrease in chlorophyll in March-April and June. Under SAG, the increase in chlorophyll concentration seen is mainly associated with a decrease in stratification, which permits an increase in the supply of nutrients (nitrate) to the euphotic layer and thus high biological activity. Finally, the decrease in chlorophyll noted in August-October is caused by changes in meridional advection resulting from changes in the meridional current.