Changes in Sahel summer rainfall in a global warming climate: contrasting the mid-Pliocene and future regional hydrological cycles.

The evolution of Sahel summer rainfall in the context of global warming is a severe socio-economic concern because of its widespread influences on local agriculture, water resource management, food security, infrastructure planning, and ecosystems. Based on the mid-Pliocene simulations from the Pliocene Model Intercomparison Project Phase 2 and the historical simulations and shared socio-economic pathway 5–8.5 experiments from the Coupled Model Intercomparison Project phase 6, the present study contrasts the Sahel summer rainfall changes between the past mid-Pliocene and near future global warming climates. The results show that the Western African summer monsoon (WASM) circulation, closely linked with the Sahel summer rainfall change, tends to strengthen in both the past and future global warming climates, but the monsoonal circulation strengthening is much more intense in the past warm period than in the projected warm future. This causes that the multi-model ensemble (MME) mean increase ratio of Sahel summer rainfall in the past warming climate is about twice to three times larger than that in the future warming climate for the same increase of global mean surface temperature (the regional rainfall increase ratio in the MME mean: about 19.6% per one degree Celsius of global warming in the mid-Pliocene simulations versus about 7.7% per one degree Celsius of global warming in the SSP5-8.5 future projections). Such a striking discrepancy in the regional circulation and hydrological cycle changes is mainly attributed to a dramatically stronger warming over the Canadian Archipelago and Greenland during the mid-Pliocene warm period relative to the projected near future. The more significant northern high-latitude warming during the mid-Pliocene enhances the meridional temperature gradient between the extratropical and tropical regions, which could induce an excessive northward shift of the Intertropical Convergence Zone and a stronger WASM, and thus result in a more intense hydrological cycle around the Sahel region. Our results highlight that besides the global mean temperature increase, meridional warming patterns are also essential for the changes of WASM and regional hydrological cycle in a warmer world. Implications for projecting the regional monsoon and hydrological cycle changes at longer time scales than in the near future are discussed. [ABSTRACT FROM AUTHOR]