Changed Relationships Between the East Asian Summer Monsoon Circulations and the Summer Rainfall in Eastern China

In previous statistical forecast models, prediction of summer precipitation along the Yangtze River valley and in North China relies heavily on its close relationships with the western Pacific subtropical high (WPSH), the blocking high in higher latitudes, and the East Asian summer monsoon (EASM). These relationships were stable before the 1990s but have changed remarkably in the recent two decades. Before the 1990s, precipitation along the Yangtze River had a significant positive correlation with the intensity of the WPSH, but the correlation weakened rapidly after 1990, and the correlation between summer rainfall in North China and the WPSH also changed from weak negative to significantly positive. The changed relationships present a big challenge to the application of traditional statistical seasonal prediction models. Our study indicates that the change could be attributed to expansion of the WPSH after around 1990. Owing to global warming, increased sea surface temperatures in the western Pacific rendered the WPSH stronger and further westward. Under this condition, more moisture was transported from southern to northern China, leading to divergence and reduced (increased) rainfall over the Yangtze River (North China). On the other hand, when the WPSH was weaker, it stayed close to its climatological position (rather than more eastward), and the circulations showed an asymmetrical feature between the stronger and weaker WPSH cases owing to the decadal enhancement of the WPSH. Composite analysis reveals that the maximum difference in the moisture transport before and after 1990 appeared over the western Pacific. This asymmetric influence is possibly the reason why the previous relationships between monsoon circulations and summer rainfall have now changed.