Thursday, 14 January 2016: 11:00 AM
Room 357 ( New Orleans Ernest N. Morial Convention Center)
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.
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