Overview of Aerosol and Monsoon Climate Interactions over Asia

As the most densely populated and the fastest developing region in the world, Asia has undergone rapid changes over the last two decades which has severe consequence on the atmospheric environment. The monsoon climate is driven by heat sources and sinks in the atmosphere stemming from the strong thermal contrast between land and ocean induced by the seasonally varying incoming solar radiation. Aerosols, by reflecting and absorbing solar radiation, reduce surface insolation and weaken the land-ocean thermal contrast, thus inhibiting the formation of monsoons. Locally, aerosol radiative effects alter the thermodynamical stability and convective potential of the lower atmosphere leading to reduced precipitation. The atmospheric thermodynamic state may also be altered by the aerosol microphysical effect. There exist multi-scale interactions between aerosols and the monsoon system from the location-specific alteration of single-column stability to the organization of mesoscale cloud clusters, and to the modification of monsoon circulation and precipitation. Here, we have analyzed the region-specific effects of aerosols by first dividing Asia into East Asia and South Asia. These two regions have different aerosol physical and chemical properties and different environmental factors. While many studies over East Asia have shown evidence of possible aerosol effects on the “north drought and south flood” summer monsoon precipitation trend, most studies over South Asia have suggested a link between aerosols and the weakening of convection in the atmosphere. Climate-scale numerical model simulations in Asia have indicated strong sensitivities to aerosol loading and properties. Aerosols effects on the Pacific storms associated with the winter monsoon over the Asia-Pacific rim are reviewed in this paper as well. This review attempts to lay out a common framework to better understand the climate of Asia and the roles that anthropogenic influences play. We have synthesized theoretical, observational, and modeling findings to provide an assessment of the current understanding, to underline uncertainties, challenges, and recommendations for future actions, and to unravel natural variability and anthropogenic effects in aerosol-monsoon interactions in Asia.