Atmospheric Circulation Drives Dry‐to‐Wet Abrupt Alternation in the Lancang–Mekong River Basin

Abstract

The Lancang–Mekong River Basin (LMRB), a crucial transboundary region that sustains millions of people across Southeast Asia, faces growing threats from compound hydrological extremes under climate change. Among them, dry‐to‐wet abrupt alternation (DWAA) is particularly critical, intensifying risks to agriculture and water security. However, its spatiotemporal patterns and associated dry‐to‐wet circulation transitions remain poorly understood. This study analyzes DWAA events in the LMRB from 1961 to 2022, with a focus on identifying atmospheric circulation patterns linked to dry and wet extremes and characterizing their paired configurations during dry‐to‐wet shifts. Results show that DWAAs in the upstream region occur mainly in May and June, while those in the downstream region are most frequent from July onward. A decreasing trend in DWAA frequency is observed in the midstream region, contrasted by increasing occurrences downstream. Three circulation patterns are identified for dry extremes and four for wet extremes. Their complexity and diversity are largely controlled by the Western North Pacific Subtropical High (WNPSH), tropical cyclones (TCs), multi‐low‐pressure systems, and dual‐high‐pressure systems. DWAA formation reflects a coherent evolution of large‐scale circulation, where the weakening, displacement, or restructuring of dry‐phase anomalies rapidly shifts the basin toward moisture‐convergent conditions. In terms of DWAA‐driving configurations, TC‐ and WNPSH‐related patterns are the dominant drivers. Moreover, WNPSH‐coupled configurations have become more frequent, whereas those linked to multi‐low‐pressure systems have declined.