Delay of the First Transition of Asian Summer Monsoon under Global Warming Condition

We evaluate reproducibility of seasonal evolution of the thermal fields associated with the first transition of Asian summer monsoon (ASM) in atmosphere-ocean coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project phase 3 (CMIP3). Many CGCMs reproduce seasonal evolution of the thermal fields related to the first transition of ASM well, though the degree of reproducibility differs to some extent. Based on this evaluation, weighted multi-model ensembles are calculated, and the future projections of the ASM onset from the viewpoint of lower-tropospheric westerlies are conducted. The onset dates over the Bay of Bengal, the Indochina Peninsula and the South China Sea are projected to delay by 5 to 10 days in the end of the 21st century under the A1B scenario of the Special Report on Emission Scenarios (SRES), compared to those in the end of the 20th century. This change might be related with delay of the reversal of upper-tropospheric meridional thermal gradient between over the Eurasian Continent and the north Indian Ocean.