The Response of the South Asian Summer Monsoon to Temporal and Spatial Variations in Absorbing Aerosol Radiative Forcing

Previous studies on the response of the South Asian summer monsoon to the direct radiative forcing caused by anthropogenic absorbing aerosols have emphasized the role of premonsoonal aerosol forcing. This study examines the roles of aerosol forcing in both preand postonset periods using the Community Earth SystemModel, version 1.0.4, with the CommunityAtmosphereModel, version 4. Simulations were perturbed bymodel-derived radiative forcing applied (i) only during the premonsoonal period (May–June), (ii) only during themonsoonal period (July– August), and (iii) throughout both periods. Soil water storage is found to retain the effects of premonsoonal forcing into succeedingmonths, resulting in monsoonal central India drying.Monsoonal forcing is found to dry all of India through local responses. Large-scale responses, such as the meridional rotation of monsoon jet during June and its weakening during July–August, are significant only when aerosol forcing is present throughout both premonsoonal and monsoonal periods. Monsoon responses to premonsoonal forcing by the model-derived ‘‘realistic’’ distribution versus a uniformwide-area distributionwere compared. Both simulations exhibit central India drying in June. June precipitation over northwestern India (increase) and southwestern India (decrease) is significantly changed under realistic but not under wide-area forcing. Finally, the same aerosol forcing is found to dry or moisten the July–August period following the warm or cool phase of the simulations’ ENSO-like internal variability. The selection of years used for analysis may affect the precipitation response obtained, but the overall effect seems to be an increase in rainfall variance over northwest and southwest India.