The Changing Seasons: Anthropogenic Shifts in Flash Drought Patterns

Anthropogenic climate change has considerably increased the risk of flash droughts which are rapidly developing drought events that pose growing threats to both societies and ecosystems. Despite their rising prominence, the physical drivers and seasonal evolution of flash droughts in a warmer climate remain poorly understood, due to the lack of consensus on the detection framework and the challenges in forecasting these rapid onset, short duration, and spatially localized phenomena. Here, we use the Community Earth System Model large ensemble (CESM2-LE) simulations under the SSP3-7.0 scenario to investigate shifts in flash drought seasonality and associated physical mechanisms. The study reveals that the risk for flash droughts is expected to increase significantly across multiple months by the end of the 21st century, with July exhibiting the highest exposure for the Northern Hemisphere while March for the Southern Hemisphere. Furthermore, nearly half of the future flash drought events are projected to occur outside of their historical season, with approximately 29% shifting one month later and 28% one month earlier, indicating a significant rise in the emergence of high-impact events in a new season. The time of emergence for the shift in flash drought seasonality, defined by a signal-to-noise ratio (SNR) exceeding two, is projected to occur after the 2070s. This late emergence would indicate the impact of human-induced climate on Flash drought in the far future by the end of the 21^st century. Our analysis further reveals that while the overall risk of Flash drought increases, the SNR of flash drought occurrence declines in some regions, highlighting the growing challenge in detecting clear trends of flash drought due to interaction between soil moisture and future warming. Moreover, the research emphasizes the importance of moisture flux convergence (MFC) in the development of flash droughts, due to its interaction with water balance and the moisture in the water cycle through enhanced divergence of atmospheric moisture. The study emphasizes the need to integrate MFC dynamics and thermodynamics into drought early warning systems and seasonal forecasting frameworks. Understanding atmospheric precursors, particularly MFC, will be essential for enhancing resilience in a changing climate.