Relative contributions of atmospheric circulation and evaporation to the faster onset of flash droughts

Flash droughts are characterized by their abrupt onset and rapid intensification. Identifying and quantifying the factors that trigger and accelerate the onset of flash droughts is crucial for establishing reliable early warning systems, and thus alleviate their detrimental impacts on agriculture, ecosystems, and water resources. Recent findings indicate that the combined influence of soil moisture depletion and atmospheric aridity contributes to the faster onset of flash droughts. However, the understanding of the dynamic and thermodynamic processes that expedite this rapid onset still remains limited. In this study, we utilized a drought index derived by a state-of-the-art generation of land evaporation model (the fourth version of the Global Land Evaporation Amsterdam Model; GLEAM v4.0), to investigate the spatial distribution and trends in flash drought onset speeds from 1980 to 2023. Our goal was to quantify the relative contributions of atmospheric circulation and evaporation to the trends in onset speed. Our results reveal that the transition from evaporation being energy-limited to water-limited serves as a sufficient condition for flash drought onset in humid and semi-humid regions. The onset speed of flash droughts has exhibited a significant increase since 1980, with intensified atmospheric circulation identified as a key driver for the increasing onset rates. Additionally, elevated evaporation, resulting from increased soil moisture and evaporative demand in the preceding period, emerges as the primary thermodynamic and dynamic factor expediting flash drought onset. This study enhances our understanding of the dynamic and thermodynamic drivers underlying flash droughts, contributing to the advancement of the flash drought onset mechanism. Moreover, the insights gained from this research provide valuable information for predicting flash droughts and developing strategies for effective mitigation.