Joint Influence of La Niña‐Like and North Atlantic Tripole Sea Surface Temperature Anomalies on the Drought‐to‐Flood Abrupt Alternation in the Middle‐Lower Reaches of Yangtze River in China

ABSTRACT When an area experiences drought and then floods within a short timeframe, the ‘Drought‐to‐Flood Abrupt Alternation (DtFAA)’ event occurs, which causes severe injuries and huge economic loss, and also greatly increases the risk of secondary geological disasters. Based on multiple observation and reanalysis datasets, this study analyses the characteristics of extreme DtFAA events in the middle‐lower reaches of the Yangtze River (MLYR) in China. The results show that a total of 8 extreme DtFAA events occurred in the MLYR during 1980–2022. The drought periods are mainly in spring, and the critical transition periods from drought to flood predominantly occur in June. Compared with drought periods, the significant increase in 500‐hPa geopotential height over the subtropical region and the pronounced decrease over the middle‐high latitudes of East Asia during the flood period constitute the key circulation factors responsible for the occurrence of DtFAA. In the preceding winter of DtFAA events, a pronounced La Niña‐like SSTA exists in the tropical Pacific, whereas in spring, a ‘+ − +’ tripole SSTA (sea surface temperature anomaly) appears from south to north in the North Atlantic. The winter La Niña‐like and the spring tripole SSTA are mutually independent, and both can induce the 500‐hPa circulation patterns that trigger DtFAA events. Among the 8 extreme DtFAA events, 5 events occur when the winter Niño3.4 index is below 0 and the spring North Atlantic Tripole index exceeds 0. In the other 3 years satisfying these conditions, the DtFAA also occurs in part of the MLYR. Therefore, the winter tropical Pacific La Niña‐like and the spring North Atlantic tripole SSTA jointly lead to the occurrence of DtFAA events in the MLYR. When they are met, the occurrence probability of DtFAA in the MLYR is extremely high.