Western Pacific subtropical high

The summertime rainfall in China and the East Asian monsoon activities are strongly influenced by the western Pacific subtropical high (WPSH). The WPSH, particularly its location changes on seasonal and interannual timescales are strongly affected by both the ENSO-related thermodynamics in the tropical western Pacific and the convection activities in this region. When the west Pacific is in its warmer episode during the developing phase of a La Niña event, the convections around the Philippine Sea intensify. These intensified convections cause the location of the WPSH to be more northern and eastern via the Hadley cell and PJ pattern (Nitta, 1987; Huang, 1992), leading to less summer rainfall in reaches of the Yangtze and Huaihe Rivers in China, and in regions of South Korea and Japan. When the west Pacific is in its colder episode during the El Niño phase, convections around Philippine Sea weaken, resulting in the location of WPSH to be more southern and western, and henceforth causing more summer rainfall even floods in reaches of Yangtze and Huaihe rivers, and regions of South Korea and Japan. Because of the importance of the convections around Philippine, some studies have focused on influences of these convective activities on the east-westward migration of the WPSH (cf. Huang et al., 2001), showing that those anomalous convections facilitate the formation of an anomalous anticyclonic circulation near Philippine Sea.

Chang et al. (1999) examined the relationships of the East Asian summer monsoon (EASM) variations on interannual timescales with the SSTA in tropical Pacific. It is found that if the positive SSTA are found in the eastern equatorial Pacific in the winter season before a wet EASM, the negative SSTA will be found there in the fall season following the wet EASM. In a year of a wet EASM, because of the positive feedback between Hadley and Walker cells, a Rossby wave is generated as a response of the atmosphere to both the cooling and the evaporation-wind feedbacks in the west Pacific. This Rossby wave strengthens the WPSH over South China Sea (SCS) and tropical western Pacific. The ridge of WPSH stretches westward with time from the winter through to the coming fall, inducing an anomalous anticyclone at 850hPa near the coasts of the southern part of China. This anomalous anticyclone plays an important role in Meiyu rainfall processes. Firstly, it prevent the front from moving south ward before and during the Meiyu period, causing the Meiyu front to locate steadily in reaches of Yangtze and Huaihe rivers for longer period of time. Secondly, it leads to intensifying the Meiyu front due to the stronger pressure gradients in the northwest flank of the WPSH. Thirdly, this anomalous anticyclone can intensify the air decent in South China Sea, inducing the SST to increase there and henceforth to facilitate more water vapor to be transported from SCS northward into the precipitation area.

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