Assessment of Aerosol V. Greenhouse Warming Impacts on Asian Monsoon Precipitation and Climate (Invited Presentation)

In this study, we investigate the responses of the Indian monsoon to greenhouse gas (GHG) warming and to aerosol forcing. A baseline for global warming response is established from analysis of the multi-model mean (MMM) of 33 CMIP5 models based on a 140-year integration of 1% per year CO2 experiment. The relative roles of GHG warming and aerosol forcing on Asian monsoon precipitation are assessed based on the 20th century historical runs, under a) all-forcing including GHG and aerosols, and b) GHG only. Results show that as a baseline response to CO2 warming, precipitation generally increases over the Asian monsoon land and adjacent oceanic regions. However, the monsoon strength as measured by the large-scale wind shear actually weakens due to increased atmospheric static stability. The monsoon response to GHG-only forcing in the historical run is similar to the baseline. On the other hand, as inferred from the difference of the all-forcing and the GHG-only runs, aerosols through solar dimming (SDM) suppress monsoon precipitation, causing a further weakening of the Asian monsoon. A scale analysis of precipitation shows that under a hypothetical GHG-only forcing in the past century, the “effective precipitation efficiency” (EPE) would have to be strongly reduced in order to achieve water balance between dynamics and thermodynamics. Under all-forcing (including aerosol), the reduction in EPE is much smaller. Here, the weaker circulation needed for water balance can be achieved via the aerosol semi-direct effect in increased atmospheric stability, and differential aerosol solar dimming in lessening the GHG induced land-sea temperature difference between Eurasia and the surrounding oceans.

Comparing model results with observations, we conclude that while CMIP5 model results have provided useful information for better understanding of large-scale remote forcing aspects of aerosol and GHG impacts, the models show little skill in simulating the long-term effects of regional climate change in Asian monsoon regions due to the coarse model resolution and poor representation of regional feedback processes.