Direct Radiative Effects of Absorbing Aerosols on Indian Summer Monsoon

Recent studies presented that direct radiative forcing of anthropogenic aerosols, particularly absorbing aerosols, can affect pre-monsoon seasonal climate and hydrology. Such effect can be implemented through perturbation to the large-scale atmospheric stability by persistent aerosol forcing. Due to the population growth and industrialization, anthropogenic aerosols have markedly increased over South Asia. Using a coupled climate model, we have further analyzed the direct radiative effects of aerosols on South Asian monsoon system, especially the aerosol effects on the onset, distribution, and strength of Indian summer monsoon rainfall. With the model, several sets of 30 to 100 year-long simulations were carried out. Paired simulations including absorbing aerosols' direct radiative effects respectively with atmospheric heating, surface cooling and the total heat forcing are designed to isolate climate responses to absorbing aerosols. As relatively coarse spatial model resolution and simulation bias being considered, a new estimation method for Indian summer monsoon onset was proposed and compared with observational data. When included the total heat forcing, our model simulated the monsoon with a broader period of the onset period making the onset less predictable. The monsoon rainfall with low intensity (8 mm/day) was increased by about 0.5 mm/day and the rainfall with high intensity (10 mm/day) appeared less frequently. Forcing due to aerosols at the right place and time can alter the behavior of the monsoon system.