The propagation from atmospheric flash drought to soil flash drought and its changes in a warmer climate

Soil flash droughts (SFDs), characterized by a rapid decline in soil moisture during drought onset, occurred frequently in recent decades and raised great challenges to drought monitoring and forecasting. Similar to traditionally slow-developing droughts, SFDs could originate from atmospheric droughts, but whether there is a connection between atmospheric flash droughts (AFDs) and SFDs remains unexplored. In this study, we identified AFDs and SFDs using 15-day mean vapor pressure deficit (VPD) and 5-day mean soil moisture (SM) respectively, and examined their occurrence frequency and propagation relationships during the growing seasons over global vegetated lands. Results show that the frequency of AFDs displays minor regional differences while SFDs are more frequent in humid regions. The global mean fractions of AFDs that trigger SFDs and SFDs that follow AFDs are 15 % and 31 % respectively, with significant spatial variability. Semi-humid and humid regions show higher propagation relationships between AFDs and SFDs. Antecedent SM conditions play critical roles in the propagation from AFDs to SFDs. Medium antecedent SM conditions (∼52nd percentile) accompanied by significantly elevated evapotranspiration (ET) at the onset of AFDs favor the occurrence of SFDs. High (∼69th percentile) and low (∼26th percentile) SM conditions limit the propagation from AFDs to SFDs. In a warmer future, the occurrence of AFDs is projected to increase globally, with a mean increase rate of 52.7 ± 4.27 % under a moderate emission scenario. The SFDs are projected to increase by 15.4 ± 7.03 %, with a larger increase in semi-humid and humid regions. The fraction of SFDs that follow AFDs is projected to increase by 33.33 ± 2.97 %, indicating a stronger link between SFDs and AFDs in a warmer climate. These findings improve our knowledge in the complicated propagation relationship between AFDs and SFDs and imply the urgency for adapting to flash droughts under climate warming.