Effects of Mountain Uplift on East Asian Summer Climate Investigated by a Coupled Atmosphere–Ocean GCM

To study the effects of progressive mountain uplift on East Asian summer climate, a series of coupled general circulation model (CGCM) experiments were performed. Eight different mountain heights were used: 0% (no mountain), 20%, 40%, 60%, 80%, 100% (control run), 120%, and 140%. The land–sea distribution is the same for all experiments and mountain heights are varied uniformly over the entire globe. Systematic changes in precipitation pattern and circulation fields as well as sea surface temperature (SST) appeared with progressive mountain uplift. In summertime, precipitation area moves inland on the Asian continent with mountain uplift, while the Pacific subtropical anticyclone and associated trade winds become stronger. The mountain uplift resulted in an SST increase over the western tropical Pacific and the Maritime Continent and an SST decrease over the western Indian Ocean and the central subtropical Pacific. There is a drastic change in the East Asian circulations with the threshold value at the 60% mountain height. With the mountain height below 60%, the southwesterly monsoon flow from the Indian Ocean becomes strong by uplift and transports moisture toward East Asia, forming the baiu rainband. With higher mountain heights, intensified subtropical trade winds transport moisture from the Pacific into the Asian continent. In order to investigate how the SST change affected the results presented herein, additional experiments were performed with the same experimental design but with the atmospheric GCM (AGCM). A comparison between CGCM and AGCM experiments revealed that major features such as a shift in precipitation inland and an appearance of the baiu rainband by higher orography were reproduced similarly in both the AGCM and the CGCM. However, there was a qualitatively as well as quantitatively different feature. The anticyclonic circulation anomalies in the lower troposphere, which appeared by mountain uplift in the tropical western Pacific in the CGCM associated with lowered SST, fed more moisture over East Asia and resulted in a stronger baiu rainband in the CGCM than that in the AGCM. An extent of the monsoon westerly flow is regulated by competition between the Pacific subtropical anticyclone and the southwest monsoon. The confluence zone was located near the Philippines throughout the mountain uplift in the AGCM, but it shifted backward to the west via mountain uplift in the CGCM associated with simulated SST changes. Overall the CGCM showed a larger sensitivity to mountain uplift than the AGCM due to the SST changes, thus warranting an examination of the importance of air–sea coupling and a need for the use of coupled models for such sensitivity studies.