Role of anthropogenic aerosols in modulating wet and dry extremes of Indian summer monsoon rainfall

Extreme rainfall events in India, causing floods and droughts, damage lives and livelihoods and thereby significantly impact agriculture production, natural and constructed landscapes, water resources, and the economy. This part of the world is also a global hotspot for air pollution due to the increase of anthropogenic aerosols since the late 20th century. Aerosols influence the radiation budget and cloud microphysical and dynamical processes, thus influencing monsoon rainfall patterns and trends. However, temporal modulations in monsoon rainfall over the Indian subcontinent, characterized by wet and dry spells, are yet to be understood in the context of the role of enhanced anthropogenic aerosol emissions. To address this gap, in this study, we examine the link between increased aerosol levels and dry and wet spell characteristics of the Indian summer monsoon of the recent era (2001-2025), using observations and model simulations (ECHAM-HAM), made with a regionally representative Indian emission inventory.

We find aerosol-induced drying of both wet and dry rainfall extremes, in the recent period, over the Indian core monsoon region, using Indian Meteorological Department (IMD)’s rainfall and satellite-derived Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) datasets. In the recent era, aerosol enhancements correlate with increasing dry spells but decreasing wet spells, as well as, decreasing rainfall intensity in both wet and dry spells. Model simulations reveal aerosol-induced stabilization and reduction in convective potential energy, inhibiting upward moisture transport. There is also a cloud microphysical effect, reducing cloud drop size and inhibiting rainout. This study illustrates how high aerosol pollution levels over India can lead to rainfall deficits, affecting the region's water supplies and exacerbating climate risks.