Increased aerosol scattering drives recent monsoon rainfall decrease over northern India

Abstract

The climate effects of atmospheric aerosols remain uncertain. Part of the uncertainty arises from the fact that scattering and absorbing aerosols have distinct or even opposite effects, and thus their relative fraction is critical in determining the overall aerosol climate effect. Here, combining observations and global model simulations, we demonstrate that changes in the fraction of scattering and absorbing aerosols play the major role in driving the monsoon precipitation decrease over northern India, especially the Gangetic Basin, since the 1980s. Increased aerosol scattering, or decreased aerosol absorption, manifested as a significant increase of aerosol single scattering albedo (SSA), causes strong cooling in the lower atmosphere, suppressing vertical convection and thus reducing precipitation. Further analysis of the Couple Model Intercomparison Project Phase 6 multi-model-mean historical simulation shows that failing to capture the SSA increase over northern India is likely an important source of the simulated precipitation trend bias in this area.