Characterizing Onsets and Terminations of Soil Moisture Droughts in Europe

Determining the exact timing of the onset and termination of soil moisture droughts is complex due to their multifaceted nature and slow development. Nonetheless, understanding when droughts have commenced and ended and identifying circulation patterns involved during their life cycle is crucial to enhancing early prediction systems and preparedness for such events. Still, the onsets and terminations (O&T) of soil moisture droughts have not been fully explored in Europe. Thus, this study investigates the climatological characteristics, namely, duration, preferred seasons and associated circulation, of O&T of soil moisture droughts in the Mediterranean (MED) and central Europe (CEU). To this end, we use five soil moisture datasets developed using different approaches: ERA5-Land, Noah-LSM, CLM-TRENDY, GLEAM, and SoMo.ml. 

Our findings indicate that the duration of O&T depends mainly on the dataset. ERA5-Land exhibits shorter and GLEAM longer duration of O&T across Europe. The other datasets fall within that range. This disparity appears to be linked to how each dataset outputs evapotranspiration, thereby affecting the resulting water balance in the soil levels. Despite the differences in duration, all datasets consistently indicate the same preferred seasons for O&T: onsets occur more frequently during the wet seasons, specifically summer in CEU and winter in MED, and terminations tend to occur during the driest seasons. This temporal consistency suggests similar variability and the same drivers of O&T across datasets. 

Focusing on large-scale drivers of droughts, we found that onsets occur with more frequent positive North Atlantic Oscillation (NAO). Positive NAO drives persistent large-scale anticyclonic pressure patterns during onsets, supporting further progression into droughts. Interestingly, positive NAO conditions are not pronounced during the mature phase of droughts. Terminations do not necessarily require opposite negative NAO conditions. They are characterized by a weaker pressure field and increased precipitation over the continent. These findings on circulation emphasize the important role of the NAO in initiating dry periods, thus indicating that it can serve as an early warning for droughts.