Leading mode and physical dynamics of spring-to-summer rainfall evolution in eastern China

Eastern China experiences substantial precipitation variability, primarily driven by the East Asian monsoon system, which is characterized by the stepwise northward progression of rainfall belt. The movement of the rainfall belt has significant socio-economic implications, necessitating precise forecasting to mitigate the risks associated with extreme weather events. This study employs Seasonal Empirical Orthogonal Function (S-EOF) analysis to examine precipitation variations, focusing on the transition of rainfall belt from spring (April-May) to summer (June-July). The results reveal that the northward shift of rainfall belt during the spring-to-summer period is strongly linked to variations in the East Asian Summer Monsoon activity. The leading mode exhibits a center of maximum rainfall in South China (SC) during spring, shifting to the middle and lower reaches of the Yangtze River basin (MLYZB) in summer, which emphasizes the spatial progression of rainfall patterns between these regions. In positive phase years, enhanced precipitation in SC during spring is related to increased moisture transported by an anomalous anticyclonic circulation over the western North Pacific (WNP). Subsequently, during summer, the enhanced rainfall moves to MLYZB along with the northward migration of the anomalous anticyclone in WNP. During negative phase years, precipitation markedly reducing in the two regions, mainly due to an anomalous cyclonic circulation over the WNP obstructs the influx of moisture from the Pacific. In summer, a cyclonic circulation over the South China Sea redirects moisture from the Indian Ocean to SC, resulting in reduced precipitation in the MLYZB. These large-scale atmospheric circulation patterns also indicate that the dominant transition of rainfall from spring to summer in eastern China can be associated with the monsoonal dynamics in the Bay of Bengal during spring. In particular, anomalous Bay of Bengal Summer Monsoon (BOBSM) activity triggers atmospheric convective heating and amplifies soil moisture anomalies in the Indochina Peninsula, thereby influencing and modulating rainfall patterns over eastern China. To further elucidate the mechanisms underlying this influence, numerical experiments are conducted to investigate the detailed processes through which BOBSM impacts the seasonal transition of rainfall in eastern China. In conclusion, this study can offer significant theoretical insights that enhance precipitation forecasting and inform extreme weather analysis.