Volcanic Aerosol Forcing Suppresses Tropical African Rainfall: Mechanisms and Modulation by a Warmer Climate

Abstract

Volcanic eruptions are among the most abrupt natural forcings of the climate system and can induce pronounced hydroclimatic disruptions, particularly in monsoon‐influenced regions. Using a 30‐member CESM ensemble, we examine how the 1815 Tambora‐size eruption would affect precipitation across tropical Africa under both preindustrial (Tam‐1850) and moderately warmed (Tam‐RCP4.5) background climates. The eruption produces substantial surface cooling (up to −1.2 K) and widespread wet‐season rainfall suppression (up to −0.94 mm/day, ∼29% of the climatological mean), with the strongest impacts over the Sudano‐Guinean West Africa and the Congo Basin region. Diagnostic analyses show that enhanced vertical moisture export associated with weakened large‐scale ascent is the dominant contributor to the precipitation decline, embedded within a broader reorganization of tropical circulation characterized by vertical moisture advection and a southward displacement of the Intertropical Convergence Zone. Additional drying arises from reduced surface evaporation and weakened low‐level moisture convergence linked to volcanic cooling of adjacent tropical oceans. Under the Tam‐RCP4.5 scenario, increased atmospheric moisture availability partially buffers wet‐season drying in the Sudano‐Guinean West Africa, whereas in the Congo Basin region, precipitation reduction intensifies due to persistent subsidence and enhanced vertical moisture advection. These precipitation responses signal potential hydropower perturbation and call for further studies incorporating runoff, reservoir operation, and energy system modeling.