Role of mid-latitude intraseasonal oscillations in triggering flash droughts in the Yangtze River Basin

The 2022 mega flash drought over the Yangtze River Basin (YRB), which occurred without sufficient early warning, substantially affected water supply and hydropower generation. This raises a concern about exploring the precursors of flash droughts to facilitate prediction at subseasonal time scale. Previous studies have indicated that flash droughts are directly affected by insufficient precipitation and corresponding circulation anomalies. However, how intraseasonal oscillations (ISOs) provide possible precursors and affect the drought onset remains unclear. Based on the analysis of flash droughts over the YRB during 1961-2022, we find that 38.9 % and 16.7 % of them are influenced by 10-20-day and 20-60-day mid-latitude ISOs, respectively. Although 10-20-day ISOs trigger more flash droughts, 20-60-day ISOs are more favorable for accelerating the onset speed of flash droughts. Specifically, the 10-20-day ISOs trigger eastward Rossby waves, promoting the eastward shift of the South Asian High and enhancing subsidence over the YRB, thereby contributing to flash drought onset. The 20-60-day ISOs are associated with the stagnation of a Circumglobal Teleconnection, and facilitating the superposition of the South Asian High and the Western Pacific Subtropical High, accelerating flash droughts onset. Although the impact of different timescales of ISOs on flash droughts varies, their combined effects are a critical factor in triggering mega flash droughts over the YRB. Our study suggests transitions from wet phases to dry phases of ISOs are critical precursors for flash drought early warning.