Variability of flash droughts and their circulation drivers in Central Europe

Events with unusually rapid onset and intensification (usually called flash droughts) are emerging into the spotlight as dangerous subseasonal climatic phenomena. Even though they bear a potential for significant socio-environmental impacts, research of their spatiotemporal variability and meteorological drivers in Central Europe has been so far limited. This contribution implements and adapts established methods of flash drought detection and conducts their comprehensive spatiotemporal analysis based of gridded soil moisture data from the SoilClim model. Studied area consists of the Czech Republic, Slovakia and north part of Austria for the 1961–2021 period. The gridded flash drought results were divided into four clusters using Wards 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 study found that frequency of flash droughts was slightly decreasing in the winter half-year and slightly increasing in the summer half-year, with substantial amplification in April–June season. The increase was slower than in the case of seasonal droughts, being driven by 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 predominance of anticyclonic types and absence of cyclonic types, while the course of flash drought episodes was also connected to increased temperatures, often connected to warm airflow. Compared to soil drought in general, flash droughts in Central Europe were more tightly connected to circulation, particularly to anticyclonic patterns and southeastern airflow.