Interannual variations of westward extension area of western Pacific subtropical high and its relationship with precipitation in East Asia

The western Pacific subtropical high (WPSH) is viewed as an important part of the East Asian summer monsoon system and its western boundary location significantly affects regional weather and climate. Based on charac-teristics of the WPSH western boundary in boreal summer, this study proposes a new index of WPSH, the westward extension area (WEA) index, defined as the area enclosed by the 587 dagpm contour in the region (10 degrees-50 degrees N, 105 degrees-130 degrees E), which can reasonably describe the area variations of WPSH when approaching and landing coastal regions. Compared with the canonical area index of WPSH, the WEA index can more effectively represent the WPSH-associated atmospheric circulation, water vapor transport, and convective activities in the boreal-summer East Asia-northwestern Pacific. A big WEA index significantly indicates flood in the middle reaches of Yangtze River basin and drought in South China. The WEA of WPSH has a significant connection with precipitation in Maritime Continent, due to the convection-circulation interaction in Philippine Sea. There, the WPSH-related sea surface temperature (SST) cooling and rainfall deficiency can excite Rossby waves, which induces the anomalous anticyclone development and the westward stretching and intensifying of WPSH that in turn further reduces rainfall in Philippine Sea. Moreover, the WEA index is significantly modulated by the preceding El Nin similar to o-type and tropical Indian Ocean SST anomalies, which may provide a highly predictable in-dicator of the continental precipitation in East Asia.