Accelerated soil moisture drought onset link to high temperatures and asymmetric responses associated with the hit timing

Abstract. Frequent record high temperatures in recent years have led to a rise in both the frequency and intensity of drought events, while also altered the formation process of drought. The worldwide occurrences of flash droughts during the past decade are the evidence that drought may evolve in an unusually rapid manner. Such a change of drought challenges existing drought monitoring and early warning systems typically designed for monthly-scale purposes. This leads to a query that to what extent the formation process of drought would be accelerated under high temperatures. Based on the reanalysis product, we identified historical drought events across China during 1950–2021, and defined a new drought characteristic, the duration of drought onset (DDO, defined as the time period taken for moisture transition from a normal state to below-average condition) to measure how rapid the drought develops. Then a two-stage experimental framework was designed to establish the relationship between DDO and antecedent soil moisture, precipitation, and air temperature, and to estimate the impacts of high temperatures with different warming scenarios and varied hitting time (pre-, and post-drought onset) on DDO. Results showed that DDO would shorten by 10~50 days under 35 °C (high temperatures in meteorology) in relative to that of annual mean temperature scenarios. Overall, the impacts of high temperatures were greatest during the week of drought onset. High temperatures of pre-drought and post-drought onset exhibited asymmetrical effects on DDO. In high NDVI areas, pre-drought high temperatures presented a leading role for accelerating the formation of drought, while in low NDVI areas, high temperatures after drought onset were favorable for the ongoing development of drought. Findings in this study may supplement the process understanding of drought in the future warming environment.