North–South Opposition of Drought–Flood Abrupt Alteration in the Middle and Lower Reaches of Yangtze River Basin

Abstract Drought–flood abrupt alternation (DFAA) events, characterized by their complex compound nature and profound impacts on ecosystems and human societies, have emerged as critical hydrometeorological risks in the middle and lower reaches of the Yangtze River basin (MLYRB), China. The concentrated and unstable characteristics of precipitation during the mei-yu season provide a typical climatic background for DFAA events. By using satellite precipitation data from 2001 to 2023, this study analyzes the spatiotemporal characteristics of DFAA occurrences and investigates how rain belt migration and mei-yu anomalies influence DFAA formation and regional differentiation. Results show that over 40% of annual DFAA events are concentrated in summer, mainly in eastern subregions with high precipitation variability. During the mei-yu season, northward migration of the rain belt reshapes the precipitation distribution and produces a distinct north–south opposition, with flood-to-drought (FTD) transitions prevailing in the south and drought-to-flood (DTF) events dominating the north. The average DFAA-affected area reaches 19.5% of the basin, with nearly equal shares of the two alternation types. Mechanistically, DFAA events are generally found to follow a chain-driven process initiated by anomalies of the western Pacific subtropical high (WPSH), which restructure moisture transport pathways and rain belt position, then lead to regional drought and flood anomalies, and finally trigger DFAA events. Meanwhile, a weakened South China Sea summer monsoon (SCSM) further enhances the north–south opposition. This study highlights the critical role of the rain belt movement and mei-yu variability in shaping DFAA patterns and provides insight into their physical drivers and regional differentiation. Significance Statement Drought–flood abrupt alternation (DFAA) events in the middle and lower Yangtze River basin pose significant challenges for regional water management and disaster preparedness. This study clarifies the spatiotemporal distribution of these events and examines how monsoons, moisture transport, and the western Pacific subtropical high (WPSH) interact during the mei-yu season to shift the rain belt and lead to sudden hydroclimatic extremes. By uncovering how these factors combine to drive severe DFAA events, this work enhances understanding of compound hydrometeorological hazards and provides insights to support early warning systems and climate resilience planning across East Asia and other monsoon-affected regions worldwide.