The 2018 northern European hydrological drought and its drivers in a historical perspective

Abstract

Abstract. In 2018, large parts of northern Europe were affected by an extreme drought. A better understanding of the characteristics and the large-scale atmospheric circulation driving such events is of high importance to enhance drought forecasting and mitigation. This paper examines the historical extremeness of the May–August 2018 meteorological situation and the accompanying meteorological and hydrological (streamflow and groundwater) drought. Further, it investigates the relationship between the large-scale atmospheric circulation and summer streamflow in the Nordic region. In May and July 2018, record-breaking temperatures were observed in large parts of northern Europe associated with blocking systems centred over Fennoscandia and sea surface temperature anomalies of more than 3 °C in the Baltic Sea (May, July) and the Barents Sea (July). Extreme meteorological drought, as indicated by the three-month standard precipitation index (SPI3) and precipitation-evapotranspiration index (SPEI3), was observed in May, and covered large parts of northern Europe by July. Streamflow drought in the Nordic region started to develop in June, and in July 68 % of the stations had record-low or near-record-low streamflow. Extreme streamflow conditions persisted in the southeastern part of the region throughout 2018. Many groundwater wells had record-low or near-record-low levels in July and August. However, extremeness in groundwater levels and (to a lesser degree) streamflow show a diverse spatial pattern. This points to the role of local terrestrial processes in controlling the hydrological response to meteorological conditions, including aquifer properties. Composite analysis of low summer streamflow and 500 mb geopotential height anomalies revealed a distinction between summer streamflow variability in western/northern Norway and the rest of the region. Low summer streamflow in western/northern Norway is related to high-pressure systems centred over the Norwegian Sea. In the rest of the Nordic region, low summer streamflow is associated with a high-pressure system over the North Sea and a low-pressure system over Greenland and Russia at similar latitudes, resembling the pattern of 2018. This study provides new insight into different hydro-meteorological aspects of the 2018 northern European drought, as well as identification of large-scale atmospheric circulation patterns associated with summer streamflow drought in the Nordic region.