Characterizing Soil Moisture Drought Onset and Termination in Europe

Abstract. Despite significant advances in understanding the mechanisms and drivers of droughts, climatological characteristics of soil moisture drought onset and termination (DO&T) in Europe remain relatively under-explored. Therefore, this study investigates the temporal characteristics of soil moisture DO&T and their connections to large-scale circulation patterns in Europe during 1980–2020. We analyze the duration of transition to DO&T, their seasonality, and atmospheric circulation patterns associated with each drought phase. The regions of study are central (CEU) and Mediterranean (MED) Europe, using soil moisture datasets from ERA5-Land, GLEAM version 3, SoMo.ml, Noah-LSM, and a simulation from the Community Land Model version 5 (CLM-TRENDY).

Our findings indicate that the duration of transition to DO&T depends on the definition of the transition period and the soil moisture dataset used. When the transition period is defined based on precipitation, the onset is longer than the termination. When the comparison is performed among the datasets, GLEAM shows the longer transition to DO&T, in contrast to ERA5-Land, which exhibits a shorter transition to DO&T. These discrepancies indicate that the selection of definitions and datasets can influence drought-related decision-making. Furthermore, we observed shifts in drought development speed during the study period: onset has become faster while termination has become slower in the eastern Mediterranean region.

In terms of seasonality, onset occurs more frequently during the wet seasons – summer and autumn in CEU and autumn and winter in MED – while termination is less seasonally constrained and shows more discrepancies among the datasets. Lastly, the effects of North Atlantic Oscillation (NAO), Scandinavian, East-Atlantic, and East Atlantic-West Russian (EA-WR) patterns on DO&T are assessed. Different patterns can serve as early warnings of droughts depending on the seasons and regions. For instance, it can be confirmed that NAO is important for the Iberian Peninsula, while EA-WR influences DO&T for all regions during winter. Overall, this characterization of DO\&T can provide a reference for evaluating potential future changes in drought characteristics, which are expected to be altered by global warming.