Alternation Between the Extreme Drought-Flood Event in the North China Plain in Summer 2024

In the summer of 2024, the North China Plain was subjected to a compound drought-flood extreme event, characterised by a prolonged and intense drought from April to June, followed by a severe flood in July. The extreme drought days in the plain are the fourth highest in June, while the precipitation is the highest in the recent half century. Based on daily observations of precipitation and combined meteorological drought data, we initially proposed a suitable approach to define the drought-flood abrupt alternation (DFAA) events in the plain and conducted an intra-seasonal evolution analysis. The alternation point is reached in early July. Subsequently, the severe flood rapidly replaced the drought due to the occurrence of record-breaking intense rainfall on six consecutive days from 4 to 9 July. The DFAA in 2024 is closely associated with the monsoon rain belt and circulations, particularly the western Pacific subtropical high (WPSH). In June, the WPSH was situated considerably further south than usual, thus maintaining the rain belt in southern China. In early July, the WPSH exhibits a sudden northward shift of approximately five latitudes, which enables the transportation of water vapour to the North China Plain from the south boundary of the plain. Concurrently, the transverse trough in the middle latitudes causes a strong low vortex and precludes the transportation of moisture to the regions north of the plain, thereby engendering persistent lower-level moisture convergence in the plain and prompting the formation of persistent intense rainfall and severe flooding.