The European 2015 drought from a climatological perspective

Abstract

Abstract. The summer drought of 2015 affected a large portion of continental Europe and was one of the most severe droughts in the region since summer 2003. The summer was characterized by exceptionally high temperatures in many parts of central and eastern Europe, with daily maximum temperatures 2 °C warmer than the seasonal mean (1971–2000) over most of western Europe, and more than 3 °C warmer in the east. High evapotranspiration rates combined with a lack of precipitation affected the soil moisture content and vegetation stress and led to record low river flows in several major European rivers. This paper analyses the European summer drought of 2015 from a climatological perspective, including its origin, spatial and temporal development, and how it compares with the 2003 event. It discusses the main contributing factors controlling the occurrence and persistence of the event: temperature and precipitation anomalies, blocking episodes and sea surface temperatures (SST). Central Europe experienced during the summer of 2015 widespread areas of negative Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI) values, present from May onwards. Similar to the summer drought of 2003, the upper level atmospheric circulation over Europe was characterized by a positive 500 hPa geopotential height anomalies flanked by a large negative anomaly to the north and west (i.e. over the central North Atlantic Ocean extending to northern Fennoscandia) and another center of positive geopotential height anomalies over Greenland and northern Canada. Simultaneously, the summer SST was characterized by large negative anomalies in the central North Atlantic Ocean and large positive anomalies in the Mediterranean basin. The SST dipole-like anomalies between the central Atlantic and the Mediterranean Sea suggest a possible "atmospheric teleconnection" between the two regions, which in turn may affect the drought conditions over Europe. In an accompanying paper, the hydrological perspective of the summer 2015 drought is presented. Together, these two papers summarize a collaborative initiative of members of UNESCO’s FRIEND-Water program to perform a timely Pan-European assessment of the 2015 summer drought.

Can tree-ring density data reflect summer temperature extremes and associated circulation patterns over Fennoscandia?

“…the NAO and/or El Niño-Southern Oscillation (ENSO), Ionita et al 2012Ionita et al , 2015Schubert et al 2014). When favorable phase conditions are met, both large-scale atmospheric as well as oceanic factors could act as precursors for extreme summers over Europe (Ionita et al 2016). Thus, anticyclonic anomalies favor high SSTs around Fennoscandia through enhanced downward net radiative fluxes, but the high SSTs are also likely to play a role in enhancing TX90p by reducing the baroclinicity and thus reinforcing the anticyclonic circulation Shukla 2007, 2011a).…”

Vb Cyclones Synchronized With the Arctic‐/North Atlantic Oscillation

“…Vb cyclogenesis is strongly linked to the upper level atmospheric dynamics interacting with major orography in a low‐level baroclinic environment in the WM (e.g., Maheras et al, ; Trigo et al, ). The increase of WM sea surface temperatures of about +1 °C after 1990 (Ionita et al, ; their Figure 10d) appears as a plausible explanation for a sustained Vb cyclone occurrence rate. Vb cyclones do not occur fully randomly according to a Poisson point process, that is, they arrive either clustered or more regularly at times. Clustering is very prominent in the case of Vb‐GoG cyclones and is particularly strong in summer for this type.…”

“…Mode 4 events are not only most frequent in Autumn (about 42%) but also 53% of the (September‐October‐November)‐mode 4 events occurred during the last 17 of the 57 years. This may be due to the strong increase of WM sea surface temperatures of about +1 °C after 1990 (Ionita et al, ; their Figure 10d), apart from possible decadal background variations of the LAC, leading to a higher frequency of Vb‐mode 4 cyclones during autumn in recent years.…”

Climate signals in carbon and oxygen isotope ratios ofPinus cembratree‐ring cellulose from the Călimani Mountains, Romania

“…Because, δ 18 O records from Swiss stone pine reflects very well the drought conditions at European scale, it is fair to argue that δ 18 O can partially reflect also the prevailing large‐scale circulation (e.g., Rossby waves, atmospheric blocking) and the variability of the North Atlantic Ocean SST (Ionita et al ., , ; Schubert et al ., ; Kingston et al ., ; Spinoni et al ., ). High values of δ 18 O are associated with a high‐pressure system over southern and central Europe and the Mediterranean Sea, and with a low‐pressure over the northern Atlantic Ocean, northern Europe and Russia, which are linked to Rossby‐wave oscillations (Ionita et al ., , ; Van Lanen et al ., ) (Figure a). This pattern favours the advection of dry and warm air from the northern part of Africa toward the south‐eastern part of Europe (including the study site).…”