Characteristics of Drought-Flood Abrupt Alternation and Atmospheric Circulation Causes in the Upper Reaches of the Hanjiang River During Late Summer to Early Autumn

To reveal the evolution law and climatic causes of drought-flood abrupt alternation during late summer to early autumn (June-September) in the upper reaches of the Hanjiang River, based on areal rainfall data and ERA5 reanalysis data, the Multi-scale Standardized Drought-Flood Abrupt Index (MSDFAI), which comprehensively considers the intensity and speed of drought-flood abrupt alternation, was adopted to identify drought-flood abrupt alternation events. Combined with atmospheric circulation, water vapor transport and other factors, the causes of such events were analyzed. The results show that a total of 13 drought-flood abrupt alternation events occurred in the upper reaches of the Hanjiang River from 1961 to 2025, including 7 cases of drought turning to flood and 6 cases of flood turning to drought. Since 2000, such events have occurred frequently, with July being the critical month for alternation. Further analysis indicates that in the years when drought turned to flood, the upper-level divergence at 200 hPa enhanced, the Western Pacific Subtropical High (WPSH) at 500 hPa extended westward and moved northward, and the low-level southwest airflow converged at 850 hPa from August to September; these three factors jointly promoted the increase of precipitation. Conversely, opposite circulation characteristics were observed in the years when flood turned to drought. In addition, the intensity of the Western Pacific water vapor channel regulated by the low-level wind field directly affects precipitation anomalies: water vapor transport significantly enhanced during the period of drought turning to flood, while it weakened during the period of flood turning to drought. The research results can provide scientific support for the accurate prediction of drought and flood disasters and the optimal allocation of water resources in the Hanjiang River Basin.