Mechanistic insights into tropical circulation and hydroclimate responses to future forest cover change

Abstract. Afforestation and the prevention of deforestation are important climate mitigation strategies, alongside reductions in greenhouse gas emissions. However, the biogeophysical effects of potential future forest cover change on the atmospheric circulation and tropical hydroclimate remain uncertain. We address this research gap using future scenario simulations from seven multi-ensemble models participating in the Land Use Model Intercomparison Project (LUMIP). The largest scenario differences in afforestation and avoided deforestation are located in the tropics, leading to robust increases in local evapotranspiration and precipitation, but widespread decreases in net precipitation (precipitation minus evapotranspiration), especially over Africa. Our results suggest that two competing mechanisms shape the tropospheric circulation and net precipitation response over Africa: Not only do forests increase evaporation, but they also increase surface momentum fluxes, thereby slowing near-surface winds and reducing orographic net precipitation. Opposing this surface drag effect is an energetic effect due to increased net energy input to the atmosphere, which strengthens convection and increases net precipitation. While the surface drag effect dominates and leads to a net precipitation decrease over western and southeastern Africa, the energetic effect dominates and leads to a net precipitation increase over central Africa. This tropical hydroclimate response to the forest cover change is largely independent of the background climate under low- to medium-warming scenarios. Our findings contribute to an improved understanding of the mechanisms of forest cover impact on future hydroclimate changes in the tropics and highlight the importance of considering hydroclimatic feedbacks in the context of future afforestation strategies.