Research on Relationship between Various Indexes of the Western North Pacific Subtropical High and Summer Precipitation in Eastern China

Based on the reanalysis data and monthly precipitation data, in this study we investigate the variabilities of the western North Pacific subtropical high indexes (WPSHI) and discuss the relationship between WPSHI and summer precipitation in eastern China, together with the related circulation changes. Results show that WPSHI can be divided into three categories, namely, absolute intensity index (ASI), relative intensity index (RLI), and north-south index (NSI). The upward trend in ASI is associated with the expansion of the area covered by 5870 gpm isolines. The downward trend in RLI accords well with the shrink of the 0 gpm isoline of the eddy geopotential height (EGH). The lack of variations in NSI indicates that the ridge of WPSH oscillates around 25°N. Depicted by circulation anomalies and precipitation patterns, ASI cannot reflect the variation of local relative vorticity of WPSH, but it has close relationship with EAP (East Asia–Pacific) pattern. In this case, ASI has the best correlation with the precipitation in the Yangtze River basin. In the years when the ASI is high, north wind anomalies favor less rainfall in northern China, while the water vapor convergence strengthens the precipitation along the Yangtze River, and vice versa. RLI can better describe the variation of the local relative vorticity of WPSH, but it has a weaker relationship with the EAP pattern and the precipitation in eastern China. In the years when the RLI is high, precipitation center appears in the middle reaches of the Yangtze River. Conversely, drought occurs in the north of Yangtze River with anomalous north winds over China. NSI can describe both the variation of local relative vorticity of WPSH and the EAP pattern and has a high correlation with precipitations in southern and northern China. In the years when the NSI is high, enhanced vapor flux leads to flood in northern China and drought along the Yangtze River. When the NSI is low, the precipitation is trapped in southern China, as resulting from a reversed circulation pattern.