SIMULATED CHANGES IN THE ASIAN SUMMER MONSOON AT TIMES OF INCREASED ATMOSPHERIC CO2

Possible changes in the Asian summer monsoon due to increased atmosphe ric CO2 are investigated by an MRI global coupled atmosphere-ocean gen eral circulation model. The summer (June-August) monsoon rainfall in I ndia increases significantly with global warming. On the other hand, t he monsoon wind shear index, defined as the difference between 850 hPa and 200 hPa zonal winds over the northern Indian Ocean, decreases. At 850 hPa, the westerly wind shifts northward and intensifies from the Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warme r air leads to larger moisture flux convergence, contributing to the i ncreased rainfall. Therefore, the monsoon wind shear index is not a go od indicator for identifying any change of monsoon accompanying global warming. In contrast to the increased rainfall in India, change in ra infall is little over China where soil moisture becomes drier at times of increased CO2. It is also noted that the northern Eurasian contine nt becomes wetter in the increased-CO2 climate. The magnitude of the i nterannual variability of the Asian summer monsoon rainfall becomes la rger in the CO2 experiment than in the control experiment, particularl y in the later stage of the experiment after CO2 doubling. However it should be noted that the interdecadal variation of this interannual va riability is also large both in the control and the CO2 experiments.