It is shown that anomalous aerosol loading in late-spring leads to remarkable and large-scale variations in the monsoon evolution. Excessive aerosols in May lead to reduced cloud amount and precipitation, increased surface shortwave radiation, and to land-surface warming. However, the monsoon strengthens in June (and July).
The analysis suggests that the significant, large-scale aerosol influence on monsoon circulation and hydroclimate is mediated by the heating of the land-surface, pursuant to reduced cloudiness and precipitation in May. The finding of the significant role of the land-surface in the realization of the aerosol impact is somewhat novel, as only heating of the lower troposphere (from aerosol's absorption of solar radiation) and solar dimming effects on both land and oceans have hitherto been emphasized, mostly in context of long-term trends.
Our work indicates the potential importance of the aerosol "semi-direct" effect in generating interannual variability of the South Asian summer monsoon.
Previous studies, based mostly on climate models, are also discussed, in particular in terms of the potential of coupled models in correctly representing air-sea interactions in the Indian Ocean, which is a fundamental mechanism regulating the evolution of the monsoon. It is shown that, as far as regional air-sea-land interactions are concerned, coupled models still present several deficiencies which, in our opinion, suggest some caution in over relying on models in predicting the response to aerosol loading in that region.