Simulating the influence of doubled CO2 on the water budget over West Africa using RegCM4.7

This paper simulates the responses of water budget components to doubled CO2 (2 × 378 ppm) concentration in the atmosphere with atmospheric and oceanic surface warming of 2°C. Simulations employed version 4.7 of the Regional Climate Model of the International Centre for Theoretical Physics (ICTP). Two six-year experiments were each repeated twice with the same model physics and parameterizations. The control experiment held the CO2 concentration at 378 ppm (no warming), while the other experiment specified doubled CO2 concentration and warming. The results showed a positive response (60-100% increase) to doubled CO2 for precipitation, runoff, and storage terms in Sierra Leone, Burkina Faso, Guinea Bissau, and the ocean area between 3 and 13°N. However, there was a negative response (up to 60%) for northern Senegal, southern Mali, and northern Nigeria. The reductions in water fluxes were observed mostly on the leeward side of the highlands. Evapotranspiration showed a negative response (1-20%) to doubled CO2 on the land north of 20°N. Burkina Faso and southern Mali responded oppositely to doubled CO2, despite their spatial proximity.