Response of South Asian summer Monsoon to 21st century changes in greenhouse forcing and SST feedbacks

Our multi-ensemble member experiments using the NCAR CCSM3 over the period 1951-2099 indicate that global warming results in weakening of South Asian summer monsoon by weakening of meridional tropospheric temperature gradient, easterly-shear and upper-and-lower level monsoon circulation. Although similar anomalies in summer monsoon dynamics suppress summer monsoon precipitation at present, simulated weakening in South Asian summer monsoon dynamics in response to increases in greenhouse forcing does not lead to suppression of summer monsoon precipitation. In order to identify the mechanisms and/or biases responsible for such opposite responses of summer monsoon dynamics and summer monsoon precipitation, we have conducted two sets of multi-ensemble sensitivity experiments forced with anthropogenic increases in GHG+SST, SST-only and GHG-only. We find that SST feedbacks dominate the response of precipitation in the late 21st century, with changes in GHG forcing having little effect when applied without changes in SST. However, we find that CCSM SST biases create substantial biases in simulated summer precipitation. Thus, we are conducting a third set of experiments in which we prescribe bias-correct SSTs in the late 20th and 21st centuries, using a quantile-mapping technique. The SST-bias- correction experiments will help to understand both the relative role of changes in GHG and SST in dictating the response of South Asian summer monsoon to global warming, and the influence of SST biases on the simulated future climate change.