Impact of Anomalous Aerosol Heating on the Onset of the Asian Summer Monsoon

This study examines the impact of anomalous differential generation of available potential energy by absorbing aerosols on the transition and early active phases of the south Asian summer monsoon. Aerosol direct and indirect effects represent radiative forcings which can modify tropospheric temperature profiles and extend cloud lifetimes. Recent studies have suggested that over monthly and seasonal time scales, these effects can lead to modified flow and rainfall regimes in the south Asian monsoon region. Of special interest is the covariance of heating and temperature prior to active monsoon onset. It can be shown that anomalous generation of available potential energy due to aerosol radiative effects can impact the cascade of energy from the Hadley circulation to the monsoon non-divergent flow. In order to quantify the potential impact of aerosol radiative forcing and the resulting changes in monsoon onset timing and intensity, an ensemble of WRF/CHEM regional weather and chemistry model forecasts are created for the south Asian summer monsoon region during May June and July. The forecasts with aerosol direct and indirect effects are compared to control forecasts without aerosols. The evolution of irrotational and non-divergent kinetic energy, generation of available potential energy and rainfall are presented. It is found that the ensemble with aerosol direct and indirect forcing contains on average a more intense and rainy onset over south India which is statistically significant compared to the ensemble variability. The more intense monsoon onset is related to a more intense Arabian Sea low level jet which is displaced to the south compared to the jet in forecasts without aerosol.