Investigating the Different Pathways of South Asian Summer Monsoon Response to Black Carbon Aerosols

Abstract

Black carbon (BC) aerosol is a short-lived climate forcer. BC aerosols are known to perturb the earth’s energy balance by interacting with radiation, modifying the cloud properties, and thereby changing the atmospheric heating pattern associated with various phenomena including the South Asian Summer Monsoon (SASM) system. However, many facets of BC in modulating the SASM have been debated. In this study, we investigate the relative impacts of an increases in local versus remote and global BC emissions on the SASM precipitation. We further the investigate the fast and slow responses of the SASM system to increases in BC emissions. We use a variant of the state-of-the-art Community Earth System Model (CESM1.2) and conduct a series of numerical experiments to understand the BC effects on the SASM system. Our results show that increased global BC emissions results in significant surface cooling across the south Asian region (especially over north India) and surface warming over the Tibetian Plateau, northern China and Mongolia region. Increased BC emissions also result in upper atmospheric warming in mid latitudes and a northward shift of the subtropical westerly jet stream. South-westerly winds over the Arabian Sea gets stronger with increases in both local and remote BC emissions. Increases in global BC emissions increases the seasonal mean SASM precipitation almost across South Asia except over the eastern Himalayan region and central peninsular India. While decreases in precipitation over peninsular India are found to be largely due to increases local BC emissions but increases in precipitation over northwest India and changes in precipitation in all other parts of India are mostly due to increases in remote BC emissions. Our results further show that unlike the combined effect of increases in all aerosols, SASM system response to increases in BC aerosols alone is dominated by the fast responses as compared to the slow responses induced through SST change.