A Climatological and Synoptic Analysis of Winter Cold Spells over the Balkan Peninsula

An extensive climatological and synoptic analysis of the winter cold spells that occurred in the Balkan Peninsula over a 59-year study period (1961-2019) is the aim of the present study. Winter cold spells (WCSPs hereafter) are defined as periods of at least three consecutive days when the daily minimum temperature is below the 5% of the empirical winter distributions. This diagnostic index is used to detect the occurrence of cold events during the study period, while the duration, frequency, and intensity of these extreme climate events are further analyzed. Moreover, in order to investigate the relation of the WCSPs with the atmospheric circulation, two daily circulation type calendars, derived from an advanced automatic flexible classification, were utilized. The automatic daily circulation type calendars were used, aiming at identifying the atmospheric conditions that prevail before or during WCSPs. The climatological analysis showed that the spatial distribution of the extreme minimum temperatures in the Balkan Peninsula presents a positive gradient from north to south, whereas coastal areas present more moderate minimum temperatures than inland areas of the region. In terms of WCSPs, the winter of 1962-1963 was the one with the longest WCSPs, for most of the stations under study. In general, a decreasing trend in the frequency of WCSP occurrence has been found towards the end of the study period. The circulation type investigation revealed that, during WCSPs in the Balkan region, the associated circulation at the 500 hPa is the Cne (cyclonic northeastern) and at the 1000 hPa is the Anw (anticyclonic northwest).