Understanding the combined effects of global warming and anthropogenic aerosol forcing on the South Asian monsoon

There is growing evidence of the influence of northern hemisphere (NH) anthropogenic aerosols (AA) on the observed regional climate change over Asia in the recent few decades. While the radiative effects of AA can regionally offset the greenhouse gas (GHG) forcing, their combined impacts on the South Asian monsoon (SAM) are not adequately understood. The present study examines the individual and combined effects of GHG and AA forcing on the SAM precipitation response, based on numerical experiments conducted using the IITM Earth System Model version2 (IITM-ESMv2). We performed the following three sets of 50-year simulations based on the CMIP6 forcing (a) AER: where the atmospheric CO2 concentration is fixed to the pre-industrial level (i.e., year 1850) and AA levels correspond to the year 2005 (b) GHG: CO2 concentration corresponds to the year 2005 and AA levels correspond to the year 1850 (c) COMB: Both CO2 concentration and AA levels correspond to the year 2005. The three experiments are compared against the pre-industrial control (PICTL) run of the IITM-ESMv2. An intensification of SAM precipitation is noted in GHG relative to PICTL, whereas AER exhibits a decrease of SAM precipitation and weakening of monsoon circulation in response to the AA forcing. The results show that absorption of shortwave radiation above the atmospheric boundary layer over the Asian region creates surface radiation deficit and stabilizes the lower troposphere, leading to a slowdown of monsoonal winds, reduced evaporation over the Indian Ocean and decreased moisture convergence over South and Southeast Asia. A striking finding from our analysis is the reinforced suppression of SAM precipitation and organized monsoon convection in COMB, which results from an enhanced inter-hemispheric asymmetry in radiative forcing under the combined influence of the elevated CO2 and AA forcing. Furthermore, both the AER and COMB experiments reveal a widespread suppression of large-scale organized monsoon convective activity on sub-seasonal time-scales over South and Southeast Asia.