Aerosol-induced effects on pre- and developed Indian summer monsoon and the corresponding onset and withdrawal

Abstract

Abstract The NCAR Community Earth System Model is used to study aerosol direct effect on Indian summer monsoon (ISM). We perform two sets of 30-year simulations subject to prescribed perpetual SST annual cycle: CTL and AERO. The two differ by anthropogenic emissions over Indian Peninsula that is included in AERO. We examine the differences in the onset and withdrawal between CTL and AERO. The results show that, relative to the ISM evolution in CTL, the transition of ISM from spring to fall in AERO exhibits a similar ensemble-mean onset date with larger spread and more abrupt change, and an earlier but more gradual withdrawal. The difference is related to aerosols-induced circulation changes in the pre-monsoon environment, including upward motion from the Ganges basin to Indian Peninsula, and strengthened anticyclonic circulation over the Arabia Sea and eastern Bay of Bengal. The changes in AERO-CTL for the developed monsoon period feature a northward shift of monsoon flow such that westerly within 10°-20°N is weaker but the tropical ITCZ (0°-10°N) and the local meridional circulation over northern India are stronger. The earlier monsoon withdrawal in AERO relative to CTL is attributed to the aerosol induced anticyclonic flow within 10°-25°N and cyclonic flow within 0°-10°N over eastern India and Bay of Bengal that resemble the transitional changes in the monsoon withdrawal. The anthropogenic aerosols also cause a 16% (32%) increase in concentration (AOD) by natural dust over India that is transported from the Middle East by aerosols-induced westerly wind in the pre-monsoon season.