资源环境科技发展态势分析平台

The record‐long 2020 Meiyu season since 1961 caused severe floods over the Yangtze and Huaihe River valleys (YHRV). Why the Meiyu duration doubled in 2020 remains a puzzle. We show that the long‐lasting Meiyu can be divided into three stages: advanced‐onset, strong‐persisting, and delayed‐withdrawal. The advanced‐onset was associated with an extremely negative‐phase East Atlantic/West Russia teleconnection. The strong‐persisting was attributed to a positive‐phase Pacific–Japan (PJ) pattern sustained by La Niña's rapid development. The delayed‐withdrawal was related to the combined effect of a positive PJ pattern and a mid‐troposphere “two ridge–one trough” pattern over Asia. Two subseasonal forecasting systems predicted the positive rainfall anomalies over the YHRV in the first two stages, but not the third stage, which may be associated with poor prediction of the two ridge–one trough pattern. These results highlight the role of subseasonal evolution in extreme climate events.