Changed relationships between the East Asian summer monsoon circulations and the summer rainfall in eastern China

Hui Gao,Weijing Li,Wei Jiang

doi:10.1007/s13351-014-4327-5

Abstract

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.

Highlights

China is located in the Asian-Australian monsoon region
The results indicate that the summer rainfall from South to North China is most directly influenced by the East Asian summer monsoon (EASM)
After the 1990s, the positive correlation weakened rapidly, especially in the 2010s when the CCs decreased to only about 0.1. This means that the western Pacific subtropical high (WPSH) had less direct impacts on the summer rainfall along the Yangtze River valley in this period, and this relationship became less important for forecasting

Summary

Introduction

China is located in the Asian-Australian monsoon region. Its weather and climate are greatly affected by monsoon systems, especially during boreal summer (June, July, and August). Before the 1990s, precipitation along the Yangtze River had a significant positive correlation with the intensity of the WPSH (Fig. 2a), especially over 110◦–140◦E This relationship is consistent with previous findings and has been applied in operational statistical climate prediction models (Zhao, 1999; Chen and Zhao, 2000). After the 1990s, the positive correlation weakened rapidly, especially in the 2010s when the CCs decreased to only about 0.1 This means that the WPSH had less direct impacts on the summer rainfall along the Yangtze River valley in this period, and this relationship became less important for forecasting. For the precipitation in North China (black curves), the close negative relationship with the Ural high during 1960–1990 weakened and became weak positive This result supports the same conclusion on the changed relationship between rainfall and WPSH/EASM

Possible explanation

Conclusions and discussion