3 Does aerosol weaken or strengthen the Asian monsoon?

Abstract

Abstract In this paper, the relative roles of solar dimming (SDM) effects vs. atmospheric heating effects by absorbing aerosols on 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 is attained in the Asian monsoon atmosphere-land-ocean system 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 1°C. 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., heating of the middle and upper troposphere induced by absorbing aerosols (dust and black carbon) piling over the southern slopes of the Himalayas, coupled with feedback with the deep convection and the large-scale circulation. In July–August, when the aerosol loading is substantially diminished, rainfall over all the Asian monsoon regions, except over and in the vicinity of the Tibetan Plateau, 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 land–sea thermal contrast induced by aerosol SDM. The results are supported by preliminary observations of increased loading of absorbing aerosols over the Indo-Gangetic Basin, and enhanced monsoon rainfall in May–June over northern India during the last two decades.