Responses of South and East Asian summer monsoons to different land-sea temperature increases under a warming scenario

We used outputs from climate models that participated in the fourth assessment (AR4) of the Intergovernmental Panel on Climate Change (IPCC) to evaluate the responses of the South Asian summer monsoon (SASM) and the East Asian summer monsoon (EASM) circulations to different warming over land and ocean under a medium warming scenario, SRES A1B. Our results suggest that, even though near-surface warming over the Tibetan Plateau (TP) is greater than that over the tropical Indian Ocean (TIO) and the northwestern Pacific (NWP), the upper-tropospheric land-sea thermal contrasts between the TP and the TIO (TP-TIO) and between the TP and the NWP (TP-NWP) will decrease. At interdecadal and longer time scales, the change in the SASM circulation is consistent with the TP-TIO upper-troposphere thermal contrast. Conversely, the change in the EASM circulation is consistent with the TP-NWP lower-troposphere thermal contrast. However, at the interannual time scale, both changes in the EASM and SASM are significantly correlated with the upper-troposphere thermal contrast. Further analyses suggest that increases in moisture and changes in cloud cover induced by global warming may cause amplified upper-tropospheric warming over the TP and the oceans resulting in inconsistent changes in the vertical temperature distribution over these regions. Because the warming over the TIO and NWP is greater than that over the TP, the TP-TIO meridional and TP-NWP zonal thermal contrasts will both decrease. However, at the lower layer, the difference in thermal capacity between land and sea result in a larger thermal effect in the near-surface region of the TP than those over the surrounding oceans. We showed that a range of factors that affect thermal conditions will likely cause changes in the Asian monsoon across a range of time scales under a warming scenario. These changes reflect differences in the influence of the greenhouse effect and natural variability.