Flash Drought in the South of Yangtze River and the Potential Impact of North Atlantic Sea Surface Temperature

Flash droughts (FDs), a type of drought with rapid onset, occurred in growing season have damaging impacts on crops, ecosystem and hence livelihoods. However, the associated physical mechanism were not well understood, which prohibits a skillful early warning. Based on the reanalysis data for 1961-2022, we diagnosed the characteristics, antecedent meteorological conditions and large-scale atmospheric circulation of FDs over the region south of Yangtze River (SYR), a hotspot region of FDs. The FDs tend to occur during the periods before (i.e., from April to mid-June) and after (i.e., early July-September) Meiyu season (B-Meiyu and A-Meiyu periods). Large precipitation deficits and rapid elevated temperature caused by anomalous subsidence and moisture divergence are responsible for the onset of FDs during the two periods. The North Atlantic tripole-like sea surface temperature anomaly (SSTA) modes have potential impact on the anomalous circulations via modulating Rossby wave trains, but with different locations among the two periods. During B-Meiyu period, the SSTA mode is characterized by positive SSTA over subtropical North Atlantic and negative SSTAs over the tropical and mid-latitude ocean, triggering a "+ - + - + -" circulation pattern over North Atlantic-Eurasia, which reinforces the East Asian trough (EAT). Anomalous northeasterly winds dominate the SYR suppressing moisture transport from the tropics. The SSTA mode locates farther to the north during A-Meiyu period, inducing a "+ - + - +" wave train over mid-latitude North Atlantic-Eurasia, leading to the strengthening of the western Pacific subtropical high. The high-pressure anomalies control the SYR, resulting in anomalous subsidence and hence negative precipitation anomalies. In recent years, flash drought, a type of rapid-onset short-term drought, has received extensive attention worldwide. However, the large-scale atmospheric circulations and potential drivers of flash droughts have not been well understood. This study investigated the characteristics and possible physical mechanism associated with flash droughts in growing season over the region south of the Yangtze River (SYR), a hotspot region of flash droughts. We found that flash droughts were more likely to occur during April-mid June and early July-September, that are periods before and after Meiyu season (i.e., B-Meiyu and A-Meiyu). Preceding high temperature and low precipitation and thus high evaporative demand anomalies, caused by local anomalous subsidence and moisture divergence, contribute to the rapid onset of flash droughts. However, the accumulation time scales and magnitude of the anomalies during A-Meiyu period are larger than that during B-Meiyu period. The local atmospheric anomalies are associated with anomalous Rossby wave train over Eurasia originated from North Atlantic, which are induced by the North Atlantic tripole-like sea surface temperature anomaly (SSTA) patterns. However, the locations of the tripole SSTA patterns are different among the two periods, leading to different atmospheric circulation anomalies over Eurasia. Flash droughts (FDs) in region south of Yangtze River tend to occur during the periods before (B-Meiyu) and after Meiyu season (A-Meiyu) Precipitation and evaporative demand anomalies caused by anomalous subsidence and moisture divergence contribute to the rapid onset of FDs These anomalies are linked to Rossby wave modulated by North Atlantic SST anomaly modes, but with different locations among the two periods