Temporal Characteristics and Atmospheric Drivers of Onset and Termination of Soil Moisture Droughts in Europe

Understanding the drivers of droughts is essential to improve the prediction of these extreme events and develop potential early warning systems and mitigation strategies. Until now, many studies have examined general relationships between precipitation, soil moisture, and large-scale circulation patterns and the causes of individual drought events in Europe. However, there is still much to be understood, particularly about the climatology of the onset and termination (O&T) of droughts in this region. Therefore, here, we investigate the temporal characteristics and atmospheric drivers associated with O&T of soil moisture droughts in southern (SEU) and central Europe (CEU) on the seasonal scale during the period of 1981–2020. We also examine atmospheric mechanisms that intensify droughts from their onsets. 

Different drought indices (SPEI, surface, and root-zone soil moisture anomalies —SM) obtained from various data sources that are based on observations and land surface models (ERA5, ECAD, GLEAM-v3, and GLDAS-2) were used to calculate the duration and seasons of occurrence of O&T. The traditional clustering method, combined with the L-moment correlation analysis, were performed to detect atmospheric drivers in geopotential heights (GP) from ERA5. 

Our results show that the duration of O&T depends on the drought index: it is shorter in SPEI and surface SM but longer in deeper SM, which implies that atmospheric signals do not fully propagate to the deep soil layers. The season of occurrence of O&T depends not only on the drought index but also on the dataset and the region. The difference between the datasets is more pronounced in CEU, showing no common season of occurrence of O&T. In contrast, more O&Ts are likely to occur during the fall and winter in SEU, indicating the importance of precipitation variability during these wet seasons in initiating and terminating droughts. In terms of atmospheric drivers, frequent anticyclonic circulation patterns in central Europe linked with the northward shift and weakening of westerlies are more commonly detected during the onsets. Conversely, the opposite cyclonic circulation condition prevails during the termination periods. Strong increases in regional GP related to the initial development of dry conditions are observed during the onsets. However, during the intensification period, regional GP exhibits little variability with no statistically significant change in the mean values. Surface temperature (T) also changes little during O&T and the intensification period, which may indicate that, on a multi-seasonal time scale, the influence of temperature on droughts is less pronounced than those of the large-scale atmospheric drivers. However, the effect of T becomes significant in the summer. Anomalously high T intensifies the impacts of droughts during the warm seasons in ongoing multi-season droughts. A clear example of this is the multi-season 2015–2020 drought in CEU.