Atmospheric Circulation as a Factor Contributing to Increasing Drought Severity in Central Europe

Abstract

Long-lasting and severe droughts seriously threaten agriculture, ecosystems, and society. Summer 2018 in central Europe was characterized by unusually persistent heat and drought, causing substantial economic losses, and became a part of a several years long dry period observed across this region. This study assesses the magnitude of the recent drought within a long-term context and links the increased drought severity to changes in atmospheric circulation. Temporal variability of drought conditions since the late 19th century was analyzed at seven long-term stations distributed across the Czech Republic using the Palmer Drought Severity Index and the Standardized Precipitation Evaporation Index. The Palmer Z Index and a variation of the Standardized Precipitation Evaporation Index were used to study rapidly emerging short-term droughts and to link these episodes to atmospheric circulation. Changes in circulation were analyzed through circulation types calculated from flow strength, direction and vorticity in mean sea level pressure data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis for 1948-2018. Increasing drought severity across the Czech Republic with record-low values of the drought indices during 2015-2018 was found. The trend was distinctive in both vegetation (April-September) and cold (October-March) periods. The tendency toward more severe droughts in recent decades was linked to changes in frequency of dry and wet circulation types, highlighting the important role of atmospheric circulation in regional climate. It remains an open question whether the significantly increasing frequency of dry circulation types in the vegetation period is related to climate change, or rather represents multidecadal climate variability.

Soil drought and circulation types in a longitudinal transect over central Europe

“…It should be noted, however, that more recent studies have pointed out that complex processes over the Arctic present substantial challenges to robust signal detection (Francis et al ., 2017) and thus the links between Arctic Amplification and mid‐latitude atmospheric circulation remain uncertain (Coumou et al ., 2018). The observed increase of anticyclonic types in central Europe (Lhotka et al ., 2020) cannot, therefore, be reliably attributed to changes in the Arctic.…”

“…Because the method was originally developed to represent circulation patterns over the British Isles, an area characterized by larger pressure gradients compared to central Europe, slight modifications had to be made to its parameters in order to avoid a scarcity of directional types and a surplus of unclassified days (Plavcová and Kyselý, 2011). Although this adjustment was partly subjective, the resulting circulation types were clearly linked to daily maximum and minimum temperatures (Plavcová and Kyselý, 2012), precipitation (Plavcová et al ., 2014), and also heat waves (Lhotka et al ., 2018) and droughts (Lhotka et al ., 2020).…”

“…The same type of circulation classification as that employed herein was also used by Lhotka et al (2020) for the Czech Republic, with the classification centred on Bohemia, although meteorological drought indicators were used rather than soil-moisture percentiles and the period investigated was 1948-2018. As emerged in Trnka et al (2009), easterly airflow was disclosed as a drought driver, although accompanied by a southerly direction, not northerly as herein.…”

“…(2009) related changes in soil‐moisture availability to circulation types ( Grosswetterlagen , after Hess and Brezowsky, 1952), for the 1881–2005 period, followed by a study by Lhotka et al . (2020) that applied a new objective classification of circulation types for the 1881–2018 period. Hlavinka et al .…”

“…Again, there are only a few soil-moisture/drought studies related to the Czech Republic (further CR) and the Slovak Republic (SR). Trnka et al (2009) related changes in soil-moisture availability to circulation types (Grosswetterlagen, after Hess and Brezowsky, 1952), for the 1881-2005 period, followed by a study by Lhotka et al (2020) that applied a new objective classification of circulation types for the 1881-2018 period. Hlavinka et al (2011) presented the SoilClim model, which was further used for calculation of soil moisture.…”