Heat waves and drought are phenomena associated with large negative impacts on society and the environment. Their common features include increasing frequency and intensity in recent decades in many regions of Europe, as well as the interconnectedness of the factors that contribute to their development. In this study, we compare the long-term changes in the characteristics of major heat waves and severe drought in Central Europe (during the vegetation period (April-September)), examine the temporal and causal relationships between the two phenomena, their spatial and temporal scales, and the connection with atmospheric circulation. Based on E-OBS (daily temperature and precipitation) data since 1950 for the vegetation period (April-September), drought is characterized by the difference between potential evapotranspiration and precipitation, while heat waves are defined as periods of large positive anomalies of daily temperatures from the mean annual cycle affecting a large area. Heat waves occurring in early/late vegetation period are referred as warm spells due to their lower absolute temperatures and possibly different circulation mechanisms. We use the Jenkinson classification of daily sea level pressure fields from the NCEP/NCAR reanalysis to describe the atmospheric circulation. Circulation types with significantly increased frequency during periods of major heat waves/warm spells and droughts are identified, and changes in their occurrence are studied. We also focus on differences between the early (April-June) and late (July-September) parts of the vegetation period. The analysis contributes to a better understanding of the interrelationships between drought, heat waves, atmospheric circulation and other driving mechanisms during 1950–2019 in Central Europe.
<p>Currently, there are changes in the hydroclimatic system, with most of Europe affected by droughts. Recent reconstructions on historical precipitation and temperature fields can be used for determination of impacts of meteorological, hydrological and agricultural droughts. Those reconstructions are available for European continent in gridded form (Casty et al.,2007). Aridity index, defined as a fraction of potential evapotranspiration and precipitation, can be used for characterization of humid &#8211; wet -- and arid &#8211; dry -- regions. It represents the ratio between energy availability and water availability. This study deals with conditional probabilities of transitions from arid to humid environment and vice versa. The aridity index was used to determine the transitions annual basis for the European continent for the period 1766 - 2015. The probabilities were calculated for each year, and for 10-year, 20-year and 30-year periods. It is shown that the recent droughts followed the drying of substantial part of Europe starting in 2014 (Hanel et al., 2018). The changes are most pronounced in Northern and Central Europe.</p><p>&#160;</p><p>references:</p><p>Casty C., Raible Ch. C., Stocker T. F., Wanner H., Luterbacher J., 2007: A European pattern climatology 1766-2000. Climate Dynamics 29. 791-805.</p><p>Hanel, M., Rakovec, O., Markonis, Y., M&#225;ca, P., Samaniego, L., Kysel&#253;, J., Kumar, R., 2018: Revisiting the recent European droughts in a long-term perspective.&#160;Scientific Report&#160;8, 9499.</p><p>&#160;</p>
<p>We study trends in drought across the central latitude strip of Europe (defined as the region 47.5&#8211;52.5 &#176;N and 2.5&#176;W&#8211;32.5&#176;E) during 1950&#8211;2019, and their links to atmospheric circulation. Drought characteristics are based on difference between potential evapotranspiration and precipitation in E&#8211;OBS data, and atmospheric circulation is characterized in terms of circulation types classified using daily sea level pressure patterns from the NCEP&#8211;NCAR reanalysis. Circulation types supporting drought in vegetation season (April&#8211;September) are identified, and we analyse changes in their occurrence since 1950, seasonal changes, and the connection with drought trends in individual European regions. We find that while in the early vegetation season, drought develops mainly in Central Europe, in the late vegetation season the most pronounced trends are shifted towards west. The circulation types supporting drought depend on regions and seasons, especially for directional types. The largest increase of the dry circulation types is observed in both seasons in Central Europe, and contributes to the pronounced drying.</p>
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