Assessment of Global Warming and Aerosol Impacts on the Asian Monsoon

Abstract:

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 are assessed based on the 20^th 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, rainfall generally increases over the Asian monsoon land and adjacent oceanic regions, accompanied by local strengthening of the regional meridional cell . However, the monsoon strength as measured by the large-scale wind shear actually weakens due to increased atmospheric moist static stability. The monsoon response to GHG-only forcing in the historical run is quite similar to the baseline. On the other hand, the aerosol effects as inferred from the difference of the all-forcing and the GHG-only runs, strongly contribute to further weakening of the Asian monsoon through an enhancement of the stability effect, in conjunction with aerosol induced reduction in 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 mainly through SST changes, 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.