Disentangling the influence of local and remote anthropogenic aerosols on South Asian monsoon daily rainfall characteristics

Wet and dry periods within the South Asian summer monsoon season can have acute societal impacts. Recent studies have identified changes in daily rainfall characteristics of the monsoon, but the underlying causes are poorly understood. In particular, although the dominant role of anthropogenic aerosols in shaping historical changes in seasonal-mean monsoon rainfall has been documented, their influence on daily-scale rainfall remains unconstrained. Using an ensemble of single-forcing climate simulations, we find that anthropogenic aerosols have a stronger influence on late-twentieth century changes in the frequency of wet events, dry events and rainless days, compared with other climate forcings. We also investigate the role of aerosol-cloud interactions (indirect effects) in the total aerosol response, and the contribution of aerosols emitted from South Asia versus from remote sources. Based on additional simulations with the GFDL-CM3 climate model, we find that the simulated aerosol response over South Asia is largely associated with aerosol-indirect effects. In addition, local aerosols suppress wet-event frequency and enhance dry-event frequency over eastern-central India, where increases in aerosol loading are the largest. Remote aerosols cause a north-south dipole pattern of change in mean rainfall over India and fewer rainless days over western India. However, the overall spatial response of South Asian rainfall characteristics to total aerosol forcing is substantially influenced by thecombined non-linear climate response to local and remote aerosols. Together, our results suggest that understanding the influence of different aerosol emissions trajectories on the regional climate dynamics is critical for effective climate-risk management in this populated, vulnerable region.