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

Abstract. Frequent record-high temperatures in recent years have led to a rise in both the frequency and the intensity of drought events while also altering the formation process of droughts. The worldwide occurrences of flash droughts during the past decade are evidence that droughts may evolve in an unusually rapid manner. Such a change in droughts challenges existing drought monitoring and early-warning systems typically designed for monthly scale purposes. This leads to a query regarding to what extent the formation process of droughts would be accelerated at 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 the drought onset (DDO; defined as the time period it takes for moisture to transition from a normal state to below-average condition) to measure how rapidly the drought develops. A two-stage experimental framework was then 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 d at 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 first 4 weeks (or 1 month) of the drought onset stage. High temperatures in the pre-drought and post-drought onset exhibited asymmetrical effects on DDO. In areas with a high normalized difference vegetation index (high-NDVI areas), pre-drought high temperatures presented a leading role in accelerating the formation of drought, while in low-NDVI areas, high temperatures after the drought onset were favourable for the ongoing development of drought. The findings in this study may supplement the process of understanding of drought in the future warming environment.