Abstract
The zonal oscillation of the western Pacific subtropical high (WPSH) significantly influences the weather and climate over East Asia. This study investigates characteristics and mechanisms of the zonal variability of the WPSH on subseasonal time scales during summer by using a subseasonal WPSH (Sub-WPSH) index. Accompanied with the Sub-WPSH index, strong anticyclonic (cyclonic) anomalies are found over East Asia and coastal region south of 30°N on both 850 hPa and 500 hPa. During the positive period of the Sub-WPSH index, the WPSH extends more westward with enhanced precipitation over the Yangtze–Huaihe river basin and suppressed precipitation over the south of the Yangtze River in China. These precipitation anomalies can last for at least 1 week. While the subseasonal zonal variability of the WPSH is found to be closely associated with atmospheric teleconnections and local air- sea interaction, the mechanisms of the variability are different before and after mid-July (early and late summer). In both early and late summer, the East Asia/Pacific (EAP) wave train pattern affects the zonal shift of the WPSH by inducing a low-level anomalous anticyclonic/cyclonic circulation over the subtropical western Pacific, and this mechanism is stronger in late summer. In constrast, the influence of the Silk-Road pattern wave train is more important in the early summer. Meanwhile, in late summer, a stronger SST forcing on the atmosphere and a faster cycle of subseasonal variations of the WPSH are observed before the westward stretch of the WPSH, which could be related to the colder local SST anomalies. The westward stretch of the WPSH is accompanied by stronger anticyclonic anomalies in late summer.
Highlights
The western Pacific subtropical high (WPSH) is a basinscale anticyclone circulation system over the West Pacific in boreal summer (Tao and Chen 1987)
This study investigated characteristics and mechanisms of the zonal displacement of the WPSH on subseasonal time scales, by using daily reanalysis data from 1979 to 2015
A Sub-WPSH index, with a dominant period of 10–40 days, was defined by subseasonal anomalies of Z500 averaged over the core region (10–30°N, 110–140°E), which can well depict the zonal variations of the WPSH over its western edge
Summary
The western Pacific subtropical high (WPSH) is a basinscale anticyclone circulation system over the West Pacific in boreal summer (Tao and Chen 1987). The variability of the summer WPSH can be affected by air–sea interaction (Ren et al 2013; Qian and Shi 2017) and Rossby wave trains (Chen and Zhai 2015; Enomoto et al 2003; Yang et al 2013) Such wave trains include the East Asia/Pacific (EAP) teleconnection pattern in the meridional direction (Huang and Li 1987; Nitta 1987) in the middle troposphere, and the Silk-Road pattern (SRP) in the zonal direction (Lu et al 2002, Hsu and Lin 2007) in the upper troposphere. Yang et al (2013) noted that the eastward propagation of the SRP in the upper-level westerly jet could be the origin of the biweekly variations over East Asia in early summer Considering these two prevailing Rossby wave trains over the Western Pacific region, in this study we are interested in whether these wave trains have impacts on the zonal subseasonal variability of the WPSH.
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