Extremely cool summers in Central and Eastern Euro

Authors: Robert Twardosz Urszula KossowskaCezak

Publish Date: 2014/09/11

Volume: 75, Issue: 2, Pages: 2013-2026

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Abstract

The paper reports on a study of extremely cool summers ECS a rare but significant phenomenon responsible for various adverse economic effects These include in particular effects on agriculture and the tourist industry The seasons their spatial extent and individual characteristics were identified in Central and Eastern Europe The research was based on average seasonal air temperatures June to August and the numbers of days with maximum temperatures 25 30 and 35 °C and minimum temperatures 20 °C as recorded at 59 weather stations during the period 1951–2010 An ECS was defined as having an average temperature at least 2 standard deviations t ≤ tav − 2σ lower than the average at a given station during the study period ECS occurred at only 34 of the 59 stations 58  of all stations one or two per station mainly in the northern part of the study area Six such seasons were recorded by at least 5  of all stations in 1962 1969 1976 1978 1984 and 1994 The average temperature of an ECS was typically 2–3 °C lower than the longterm average While some ECSs included an extremely cool month most such seasons 58  were defined by low temperatures persisting for long periods in all summer monthsThe occurrence of extremely warm or cool months and seasons is a known feature of the temperate climate zone in Europe where the weather is influenced by a highly variable atmospheric circulation Exceptionally warm months and seasons have attracted much research especially in recent years when they tended to occur more frequently than 30–40 years ago KossowskaCezak and Twardosz 2012a b Twardosz and KossowskaCezak 2012 2012a b This focus seems more than justified in view of the fact that such warm conditions cause adverse biometeorological conditions droughts and other phenomena such as forest firesThe situation is quite the opposite with extremely cool summers ECS and their presence in climatological research The studies available indicate a higher frequency of occurrence of such seasons in the nineteenth century and early twentieth centuries Gerstengarbe and Werner 1992 Filipiuk and Kaszewski 2000 Elizbarashvili et al 2007 Two particular seasons attracted particular attention ie the cool and wet summer of 1816 eg Piervitali et al 1997 Trigo et al 2009 and the summer of 1913 the coolest summer in Central Europe during the 1871–1990 period Filipiuk and Kaszewski 2000 Reasons leading to this relative neglect of ECS include their lower frequency of occurrence or the complete lack of such seasons such as has recently been the case and their less severe impacts on nature biometeorological conditions and the economy than those of extremely hot summers This does not however mean that ECS have no adverse effects Persistent low temperatures in summer are felt in agriculture in the form of smaller and delayed harvests of inferior quality produce as reported for example in the cold and wet summer of 1816 Trigo et al 2009 Tourism is affected particularly by a combination of wet and cold which not only diminishes comfort but also the safety of holidaymakers de Freitas 2003 Scott and Lemieux 2010 Cold summers in Central Europe are mostly a result of the advection of fresh maritime air from the Atlantic Ocean that is often accompanied by intensive rainfall KossowskaCezak 1997 Kuziemska 1975 Jaagus 2006 Twardosz et al 2011 This means that during these cool periods excessive rainfall is a greater threat to human life and activity than the low temperature itselfMany researchers have demonstrated Piervitali et al 1997 Slonosky et al 2001 Bardin 2007 Zveryaev 2007 Sidorenkov and Orlov 2008 Anisimov et al 2011 Twardosz et al 2011 that air temperature and precipitation depended on atmospheric circulation Cool summers in Central Europe coincide with a high frequency of cyclonic circulation from the west and north which is normally accompanied by greater cloud cover than in anticyclonic situations KossowskaCezak 1997 Twardosz et al 2011 In Eastern Europe summer cold waves are caused by similar types of circulation which are linked specifically to the socalled diving Scandinavian cyclones Elizbarashvili et al 2007 Isayev and Sherstyukov 2008The literature also mentions volcanic causes of cool summers such as in the case of the anomalously low temperatures in the summer of 1816 in Europe after the eruption of Tambora in 1815 Piervitali et al 1997 Trigo et al 2009 That exceptionally cold summer of 1816 was also associated with causes linked to circulationThe distribution of surface pressure field was similar to a dipole with a low over Europe and a high pressure over the North Atlantic and Iceland According to Piervitali et al 1997 this pattern could correspond to frequent blocking situations with a cold airflow coming from high latitudes towards the CentralSouthern regions of EuropeThe study is intended to find out the frequency and spatial coverage of extremely cool summer seasons in Central and Eastern Europe and the change of this frequency from the midtwentieth century in the wake of progressive climate warming The paper links in with earlier research by the authors KossowskaCezak and Twardosz 2012 2013 Twardosz and KossowskaCezak 2013a b on extremely hot summers

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