Occurrence and drivers of wintertime temperature extremes in Northern Europe during 1979-2016

Applying the daily ERA-interim reanalysis data from 1979 to 2016, we found that widespread cold (warm) wintertime extreme events in Northern Europe occurred most frequently in winter 1984-1985 (2006-2007). These events often persisted for multiple days, and their primary drivers were the pattern of atmospheric large-scale circulation, the direction of surface wind and the downward longwave radiation. Widespread cold extremes were favoured by the Scandinavian Pattern and Ural Blocking, associated with advection of continental air-masses from the east, clear skies and negative anomalies in downward longwave radiation. In the case of widespread warm extremes, a centre of low pressure was typically located over the Barents Sea and a centre of high pressure over Central Europe, which caused south-westerly winds to dominate over Northern Europe, bringing warm, cloudy air masses to Northern Europe. Applying Self-Organizing Maps, we found out that thermodynamic processes explained 80% (64%) of the decreasing (increasing) trend in the occurrence of extreme cold (warm) events. The trends were due to a combined effect of climate warming and internal variability of the system. Changes in cases with a high-pressure centre over Iceland were important for the decreased occurrence of cold extremes over Northern Europe, with contribution from increasing downward long-wave radiation and south-westerly winds. The largest contribution to the increased occurrence of widespread warm extremes originated from warming and increased occurrence of the Icelandic low.