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Hydrol. Earth Syst. Sci., 21, 1397–1419, 2017 www.hydrol-earth-syst-sci.net/21/1397/2017/ doi:10.5194/hess-21-1397-2017 © Author(s) 2017. CC Attribution 3.0 License. The European 2015 drought from a climatological perspective Monica Ionita1,2, Lena M. Tallaksen3, Daniel G. Kingston4, James H. Stagge3, Gregor Laaha5, Henny A. J. Van Lanen6, Patrick Scholz1, Silvia M. Chelcea7, and Klaus Haslinger8 1Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany 2MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany 3Department of Geosciences, University of Oslo, Oslo, Norway 4Department of Geography, University of Otago, Otago, New Zealand 5University of Natural Resources and Life Sciences Vienna (BOKU), Institute of Applied Statistics and Computing, Vienna, Austria 6Hydrology and Quantitative Water Management Group, Wageningen University, Wageningen, the Netherlands 7National Institute of Hydrology and Water Management, Bucharest, Romania 8Central Institute for Meteorology and Geodynamics, Vienna, Austria Correspondence to: Monica Ionita ([email protected]) Received: 9 May 2016 – Discussion started: 19 May 2016 Revised: 27 January 2017 – Accepted: 16 February 2017 – Published: 8 March 2017 Abstract. The summer drought of 2015 affected a large por- positive geopotential height anomalies over Greenland and tion of continental Europe and was one of the most severe northern Canada. Simultaneously, the summer sea surface droughts in the region since summer 2003. The summer temperatures (SSTs) were characterized by large negative of 2015 was characterized by exceptionally high tempera- anomalies in the central North Atlantic Ocean and large pos- tures in many parts of central and eastern Europe, with daily itive anomalies in the Mediterranean basin. Composite anal- maximum temperatures 2 ◦C higher than the seasonal mean ysis shows that the western Mediterranean SST is strongly (1971–2000) over most of western Europe, and more than related to the occurrence of dry and hot summers over the 3 ◦C higher in the east. It was the hottest and climatologi- last 66 years (especially over the eastern part of Europe). cally driest summer over the 1950–2015 study period for an The lagged relationship between the Mediterranean SST and area stretching from the eastern Czech Republic to Ukraine. summer drought conditions established in this study can pro- For Europe, as a whole, it is among the six hottest and dri- vide valuable skill for the prediction of drought conditions est summers since 1950. High evapotranspiration rates com- over Europe on interannual to decadal timescales. bined with a lack of precipitation affected soil moisture and vegetation and led to record low river flows in several major rivers, even beyond the drought-hit region. The 2015 drought developed rather rapidly over the Iberian Peninsula, France, 1 Introduction southern Benelux and central Germany in May and reached peak intensity and spatial extent by August, affecting espe- Drought is part of the natural climate cycle that commonly cially the eastern part of Europe. Over the summer period, affects large areas and can last for several months or even there were four heat wave episodes, all associated with per- years. It is a complex phenomenon with wide-ranging en- sistent blocking events. Upper-level atmospheric circulation vironmental and socioeconomic impacts and is considered over Europe was characterized by positive 500 hPa geopoten- globally to be one of the costliest natural hazards (Wil- tial height anomalies flanked by a large negative anomaly to hite, 2000; EEA, 2010). In Europe, the overall losses due the north and west (i.e., over the central North Atlantic Ocean to drought over the period 1976–2006 have been estimated extending to northern Fennoscandia) and another center of at about EUR 100 billion (EC, 2007). Drought can affect all components of the hydrological cycle, from its origin as a Published by Copernicus Publications on behalf of the European Geosciences Union. 1398 M. Ionita et al.: The European 2015 drought from a climatological perspective deficit in precipitation, which combined with high evapo- et al., 2013). Persistent dry (wet) conditions are usually as- transpiration losses can lead to deficit in soil moisture and sociated with anticyclonic (cyclonic) circulation, while the subsequently can manifest itself as a hydrological drought, sea surface temperatures (SSTs) can play also an important i.e., deficits in streamflow and groundwater (Tallaksen and role via the interaction with large-scale climatic or oceanic Van Lanen, 2004). In cold regions, a temperature anomaly modes of variability (e.g., North Atlantic oscillation (NAO), may cause a similar winter drought to develop (Van Loon El Niño–Southern Oscillation (ENSO); Ionita et al., 2011, and Van Lanen, 2012). Consequently, drought can cause a 2015; Schubert et al., 2014). Altogether, when favorable wide range of impacts affecting the environment, society phase conditions are met, both large-scale atmospheric as and economy, where impacts on agriculture and public wa- well as oceanic factors could act as precursors to dry (wet) ter supply are most frequently reported (Stahl et al., 2016). summers over Europe (Kingston et al., 2013, 2015; Ionita et Prolonged droughts with severe impacts, such as the ma- al., 2012, 2015). jor drought in 2003, have highlighted Europe’s vulnerability This paper summarizes a collaborative initiative of mem- to this natural hazard and alerted governments, stakeholders bers of UNESCO’s FRIEND-Water programme to perform and operational agencies to the disastrous effects droughts a pan-European assessment of the drought of 2015 from may have on the society and economy, including the need for a climatological point of view. In an accompanying paper mitigation measures (EEA, 2001, 2010; EC, 2012). (Laaha et al., 2016), a similar pan-European assessment of The year 2015 was characterized by one of the worst the drought from a hydrological perspective is performed droughts recorded in Europe, particularly in the central with a focus on streamflow records. Impacts and examples and eastern part of the continent (Van Lanen et al., 2016). of how the drought of 2015 was managed are described by The 2015 summer (June–July–August (JJA)) was the third- Van Lanen et al. (2016). The objectives of this paper are warmest summer in Europe since 1910 (after 2003 and as follows: (a) to characterize the temporal and spatial ex- 2010), and coincided with the warmest month (August 2015) tent of the summer 2015 drought event using both daily and and year (2015) on record, with a global temperature monthly climate data (surface level analyses); (b) to analyze anomaly of 0.9 ◦C above the 20th century average (NOAA, the key drivers of the event, with a special emphasis on the 2016). Record high temperatures were recorded at several role played by the SST and large-scale (atmospheric) circula- places in Europe in July and August 2015, including Kitzi- tion modes of variability (upper-level analyses); (c) to com- gen in Germany (40.3 ◦C), Catania in Italy (42.8 ◦C), Cór- pare the key characteristics of the 2015 drought with the 2003 doba in Spain (45.2 ◦C), Dobrichoviceˇ in the Czech Republic event and (d) to place the summer 2015 drought event into a (39.8 ◦C) and Yerevan in Armenia (40.9 ◦C). In some coun- long-term perspective. tries (e.g., Czech Republic), it was the second-driest summer The paper is organized as follows: Sect. 2 gives an intro- of the last 50 years, where only 2003 had lower rainfall (Van duction to the data and methods used. The results of surface Lanen et al., 2016). The drought was accompanied by ex- analysis of the summer drought of 2015 are shown in Sect. 3, treme low flows, especially over the central and eastern parts while the prevailing ocean–atmosphere conditions are pre- of Europe, and caused serious socioeconomic impacts in var- sented in Sect. 4. The similarities and differences with the ious water-related sectors (Van Lanen et al., 2016). summer event of 2003 and the long-term context of the 2015 The 2015 event was extreme, but not singular. In fact, the event are shown in Sect. 5. A detailed discussion is given in beginning of the 21st century has seen an increase in the fre- Sect. 6, while the concluding remarks and some recommen- quency of prolonged droughts and heat waves in Europe, in- dations are provided in Sect. 7. cluding the event of 2003 that affected large parts of cen- tral Europe. Other major events include the drought in 2008 over the Iberian Peninsula (Andreu et al., 2009), the drought 2 Data and methods and heat wave in Russia in 2010 (Barriopedro et al., 2011), 2.1 Climate, oceanic and hydrologic data and the spring drought of 2011 over western parts of Europe. As average global temperatures continue to rise, it is esti- Climate variables, i.e., daily time series of precipitation (P ), mated that droughts will further intensify in the 21st century, mean temperature (T ) and maximum temperature (Tx/, orig- particularly in certain seasons and regions, e.g., in south- inate from the E-OBS dataset (Haylock et al., 2008). E- ern and eastern Europe (Stagge et al., 2014; Spinoni et al., OBS is a daily gridded observational dataset for precipita- 2016), parts of North America, Central America, and south- tion, minimum, mean and maximum temperature and sea ern Africa (Seneviratne et al., 2012; Orlowsky and Senevi- level pressure in Europe. It is based on the European Cli- ratne, 2013; Prudhomme et al., 2013; Wanders et al., 2015). mate Assessment and dataset (ECA&D) station information. A better understanding of the spatial and temporal devel- The full version dataset covers the period 1 January 1950 un- opment of these major drought events, in particular their til 31 December 2015, in total 66 years.

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