Does absorbing aerosol strengthen or weaken the South Asian Monsoon?

In this paper, the mechanisms and relative roles of solar dimming (SDM) effects vs. atmospheric heating effects by absorbing aerosols on the South Asian monsoon rainfall and circulation are investigated using an atmospheric general circulation model coupled to a mixed layer ocean, forced by prescribed global aerosol forcing. Comparison between experiments with and without aerosol forcing shows that equilibrium condition in the Asian monsoon atmosphere-land-ocean system is attained after approximately 30 years of integration, by which time the model sea surface temperatures (SST) in the Indian Ocean and western Pacific will have dropped by more than 1oC. In spite of the reduction in SST, rainfall is found to be increased in northern India and the Tibetan Plateau in late spring and early summer (May-June). This increase is attributed to the ''elevated-heat-pump" (EHP) effect, i.e., diabatic heating of the middle and upper troposphere over the southern Tibetan Plateau from atmospheric feedback, induced by absorbing aerosols (dust and black carbon) accumulated over the Indo-Gangetic Plain, and Himalayan foothills. In July-August, when the aerosol loading is substantially diminished, rainfall over all the Asian monsoon regions, except in the vicinity of the southern Tibetan Plateau and Himalayan foothills, is reduced. The overall reduction is due to a spin down of the large-scale monsoon circulation, stemming from the cooler earth surface and diminished large-scale thermal contrast between the Asian land mass and the adjacent oceans, induced by aerosol SDM. Results from long-term observations of aerosols and regional rainfall over South Asia, consistent with the above scenario will be presented.