Abstract:In June 2024, rainfall in eastern China exhibited a distinct spatial distribution characterized by “droughts in northern China and floods in southern China”. Notably, the rainfall in the area south of Yangtze River exceeded the historic record since 1961. Both the results of the statistical analysis and the case study have demonstrated that the anomalous southerly moisture transport on the western side of the Western Pacific subtropical high (WPSH) played a significant function. However, the general circulation to the west of 110°E showed different characteristics from those in historical events. The eastern boundary of the North Africa high (NAH) exhibited a notable eastward and southward extension, connecting the western boundary of the WPSH over the southern tropical Indian Ocean. This resulted in the formation of an “inverted Ω” circulation pattern, a phenomenon that has never been observed previously. In addition to the El Nino, the record-breaking warm sea surface temperature (SST) in the western tropical Indian Ocean was another significant impact factor for the abnormal rainfall in the area south of the Yangtze River in early summer. From January to February 2024, the SST anomaly over the west part (20°S-20°N, 40°-75°E) of tropical Indian Ocean, exceeded 1℃ for the first time. The thermal effect of the underlying surface caused the rise in air temperature below 600 hPa over the domain. The thermal expansion of the air column led to an increase in geopotential height over the eastern boundary of the NAH, which in turn contributed to an intensification of the north wind component. It can be concluded that the formation of the “inverted Ω” circulation pattern was a result of the eastward extension of the NAH and westward extension of the WPSH, respectively. This resulted in the development of a strong cyclonic anomaly circulation over the equatorial Indian Ocean. The anomalous southerly wind compoments on its eastern side converged enhancing the transport of southwest moisture flow from the Bay of Bengal to the area south of Yangtze River, resulting in abnormal rainfall in the latter. Statistical analysis from historical observations have also verified this conclusion.