Long-Term Variability of Southern African Hydroclimate Strongly Modulated by Asian Anthropogenic Aerosols, with Implications for Regional Ecosystems

Observations show a significant increase in austral summer (December–February, DJF) precipitation over Madagascar and a dipole over southern Africa since the mid-twentieth century, with implications for unique and biodiversity-rich ecosystems in a recognized global biodiversity hotspot. Yet the physical drivers of these long-term changes remain unclear. Over the same period, rapidly increasing anthropogenic aerosol emissions from Asia substantially altered hemispheric energy distributions and are known to influence remote hydroclimate through large-scale atmospheric circulation adjustments. However, their impacts on African rainfall have not been systematically assessed. We addressed this knowledge gap using historical simulations from Coupled Model Intercomparison Project phase 6 (CMIP6) models and idealized single-forcing experiments from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results suggest that Asian anthropogenic aerosol emissions played a key role in the observed increase in austral summer precipitation over Madagascar and southern Africa from 1930 to 2000 alongside the influence of internal variability. Increased sulfate aerosol emissions over Asia led to regional surface cooling and strengthened interhemispheric temperature and sea-level pressure gradients. This caused a southward shift of the Intertropical Convergence Zone (ITCZ) and the associated Hadley circulation, which resulted in enhanced moisture convergence and increased precipitation over Madagascar. In contrast, after 2000, rapid reductions in Asian aerosol emissions reversed the circulation response and contributed to declining precipitation over Madagascar and southern Africa. Applying this physical framework to near-future scenarios from the Regional Aerosol Model Intercomparison Project (RAMIP) further indicates that aerosol emission reductions will continue to drive substantial hydroclimatic adjustments. These precipitation changes from 2000 to 2020 are accompanied by increased vapor pressure deficit (VPD) and reduced leaf area index (LAI) over Madagascar and southern Africa, consistent with increased vegetation water stress. Taken together, our findings highlight how remote anthropogenic aerosol forcing can influence southern African hydroclimate and moisture-sensitive forests, underscoring the broader current and near-future implications for forests and terrestrial ecosystems in the region.