Assessing Subseasonal Predictability and Sources of Drought-Flood Abrupt Alternation Over Eastern China Using ECMWF S2S Ensemble Forecasts

Abstract

Drought–flood abrupt alternation (DFAA) events pose major challenges for subseasonal-to-seasonal (S2S) prediction owing to their rapid hydrological transitions and complex circulation controls. Using ECMWF S2S ensemble forecasts, this study explores the subseasonal predictability of three objectively identified DFAA events over eastern China during 2022–2024. Across the analyzed cases, we find that the forecast skill tends to be higher during the drought phase than during the subsequent transition and flood onset. This tendency is associated with persistent drought conditions being linked to slowly evolving, spatially coherent large-scale circulation anomalies and relatively greater ensemble consistency at subseasonal lead times. An important source of subseasonal predictability is linked to mid-tropospheric circulation anomalies, including Rossby wave trains and subtropical–midlatitude coupling, which provide coherent precursor signals approximately 10–30 days prior to the observed transitions. In contrast, forecast skill degrades notably during the transition period and subsequent flood onset, when short-term triggering processes—particularly tropical cyclone–related circulation—introduce substantial ensemble divergence and timing uncertainty. Among the cases analyzed, the monsoon-related event is associated with relatively higher and more stable subseasonal predictability, whereas the tropical cyclone–influenced event exhibits reduced forecast reliability. These results highlight the importance of large-scale circulation precursors for extended-range monitoring of DFAA events, while underscoring the intrinsic limitations of current S2S systems in predicting the precise timing and intensity of abrupt flooding onset.