Impacts of reducing scattering and absorbing aerosols on the temporal extent and intensity of South and East Asian summer monsoon

Abstract. The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian and East Asian summer monsoon (SASM and EASM) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO₂. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT), absorbing (ABS) aerosols, or decreases in both. For SCT, the combination of the early transition of land-sea thermal contrast and sea level pressure gradient during the pre-monsoon season together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian High (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land-sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850-hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone, despite of the inherent nonlinearity of the atmospheric systems. Our findings suggest that emission controls that target e.g. emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions.