The western Pacific subtropical high (WPSH) plays a major role in Asian climate. Previous examination on changes in the WPSH found that the WPSH has strengthened and extended westward since 1980, contributing to the inter-decadal scale sift of eastern Asian climate. The present study suggests that this significant change of the WPSH is deducible from the atmosphere's response to the observed Indo-Pacific SST warming. Coordinated by a European Union Framework 6 project “understanding the dynamics of the coupled climate system” (DYNAMITE), five AGCMs were forced by a same idealized SST patterns representative of the Indian Ocean warming and cooling. The results of numerical experiments suggest that the westward extension of the WPSH in recent decades was caused by the increased monsoon condensational heating associated with a warming of the tropical Indo-Pacific Ocean. The warming of the Indo-Pacific Ocean has caused excessive rainfall over the South Asian monsoon domain in all AGCMs' response, indicating an intensified monsoon condensational heating. The response of the WPSH to the monsoon heating exhibits a westward extension in all of the five AGCMs' results. The westward displacement of the WPSH is 15.4° for multi-model ensemble, which is slightly larger than those captured by the average of ERA40 and NCEP/NCAR reanalysis. All models' circulation responses are consistent with the results of Gill (1980), supporting the monsoon diabatic heating mechanism proposed by Rodwell and Hoskins (1996). The summertime low-level equatorial flank of the WPSH to the east of the Asian summer monsoon is interpreted as a Kelvin response to the monsoon condensational heat source, while the intensified summertime poleward-flow along the western flank of the WPSH is in accord with Sverdrup vorticity balance. The climate impact of Indo-Pacific Ocean warming is also manifested in the remarkable South Asian anticyclone termed as “South Asian High” on the upper troposphere. The increased monsoon heating associated with the warming of the tropical Indo-Pacific Ocean has resulted in an expansion of the SAH, as evidenced in the observation.

Key Words: Western Pacific Subtropical High, Westward extension, Indo-Pacific Warming, AGCM