Dry and Wet Alternation Events in China during 1961–2020

Consecutive dry and wet extremes have drawn increasing attention due to their compounded social and environmental impacts, compared with isolated events. This study investigates dry–wet alternation events across China from 1961 to 2020 using the monthly Standardized Precipitation Index (SPI). Two transition types are identified: dry-to-wet (DTW) and wet-to-dry (WTD) events. DTWs and WTDs occur most frequently in eastern China, especially over the Haihe River basin and the southern Yellow River basin. On average, DTWs exhibit longer transition durations than WTDs. Both event types have significantly increased in frequency and shortened in transition time across North China, Southwest China, and the lower reaches of the Yangtze River basin, indicating an elevated risk of abrupt hydroclimatic transitions. Autumn is the most active season for both DTW and WTD events. Over recent decades, their high-frequency regions have expanded northward, accompanied by shorter transition durations since the 2010s. Moisture budget analysis indicates that DTWs are primarily influenced by anomalous vertical moisture advection, while WTDs are driven by enhanced meridional moisture transport, especially in northern basins. These contrasting mechanisms highlight the growing instability of regional hydroclimatic conditions and emphasize the need for improved prediction and management of abrupt dry–wet transitions under a changing climate.