Direct Radiative Effects of Anthropogenic Aerosols and Their Effects on the South Asian Winter Monsoon

The spatial-temporal distribution and direct radiative effect of three anthropogenic aerosols (sulfate, black carbon, and organic carbon) were simulated by using the regional climate model RegCM4.0 (Regional Climate Model verson 4.0) coupled with a chemistry module. Results from our investigation of the effect of aerosols on the South Asian winter monsoon show that the temporal and spatial changes in optical depth and surface short wave radiative forcing may be caused primarily by the sulfate aerosol. In the transition period between the South Asian summer and winter monsoons and during the prevailing South Asian winter monsoon, the distributions of negative short wave radiative forcing at the top of the atmosphere and at the surface are nearly consistent with the distribution of the aerosols. The absolute value of the intensity of the radiative forcing at the surface is much greater than its intensity at the top of the atmosphere. Correlation and composite analyses show that the aerosols' column burdens in the main distribution area of anthropogenic aerosols in South Asia is negatively correlated with the onset and intensity of the South Asian winter monsoon in the transition period between the South Asian summer and winter monsoons and during the prevailing South Asian winter monsoon. These effects may be attributed to the fact that aerosols absorb solar radiation, thereby inducing the changes in temperature and geopotential height.