An Evolution of the Asian Summer Monsoon Associated with Mountain Uplift-Simulation with the MRI Atmosphere-Ocean Coupled GCM-

Abstract

Using the MRI global atmosphere-ocean coupled general circulation model, we had six simulations with different mountain heights, i.e., 0% (M0), 20% (M2), 40% (M4), 60% (M6), 80% (M8), and 100% (M, control run) of the present global orography, respectively, to study climate changes due to progressive mountain uplift. The changes of the Asian summer monsoon, with progressive mountain uplift is studied in this paper. An active convection region extends with mountain uplift to form a moist climate in South and East Asia. Monsoon circulation such as low-level westerly, and upper-level anticyclonic circulation, is also enhanced with mountain uplift. The increase in precipitation, and the enhancement of southwesterly, in the later stages of the mountain uplift, appear only over India and the south and southeastern slope of the Tibetan Plateau. Over the coastal region of Southeast and East Asia, where the maximum precipitation appears in M0, precipitation decreases gradually with mountain uplift, and the southwesterly in the later stages becomes weaker. In the connection with these changes, surface heat flux changes remarkably over moist Asia in the earlier stages of mountain uplift, compared with that in the later stages. The intensity of the Indian, Southeast Asian, and East Asian monsoon was investigated with indices which are defined by area mean precipitation. The Indian monsoon becomes strong gradually with mountain uplift; particularly, in the later stages, the remarkable enhancement is found. The intensity of the South Asian monsoon is the strongest in M4. Thus, in the later stages of mountain uplift, that becomes weaker in association with the northwestward migration of the convective activity. Although the East Asian monsoon is enhanced gradually with mountain uplift, the enhancement in the earlier stages is larger than that in the later stages. In the equatorial Indian Ocean, SST also increases with mountain uplift, resulting in the increase in precipitation. The increase in SST results from the change of the ocean surface dynamics due to the enhanced monsoon circulation. This result could not be obtained if CGCM was not used in this study.