Flash droughts in Central Europe and their circulation drivers

Flash droughts, defined as events with unusually rapid onset and intensification, are emerging into the spotlight as dangerous subseasonal climatic phenomena capable of causing substantial socioenvironmental impacts. However, research on their spatiotemporal variability and major drivers in Central Europe has been limited thus far. This study used gridded soil moisture data from the SoilClim model for the region consisting of the Czech Republic, Slovakia and northern part of Austria in the 1961-2021 period. Established methods of flash drought detection were implemented and adapted to conduct their comprehensive spatiotemporal analysis. The gridded flash drought results were divided into four clusters using the Ward's hierarchical agglomerative method. Individual flash drought episodes were delimited for each cluster, divided into three phases (onset, course, end) and investigated separately in terms of drivers, represented by three meteorological variables (precipitation, actual evapotranspiration, maximum temperature) and atmospheric circulation types based on the objective classification (derived from flow strength, direction and vorticity). The frequency of flash droughts slightly decreased in the winter half-year and slightly increased in the summer half-year, with substantial amplification in the April-June season. The increase was slower than in the case of seasonal droughts, being driven by the longer-term accumulation of water deficit. Circulation drivers exhibited much stronger and more direct influence in the summer half-year, particularly causing the onset of flash drought episodes during the predominance of anticyclonic types and absence of cyclonic types, while the course of flash drought episodes was also connected to increased temperatures and often connected to warm airflow.