American Warming Hole Is Not Unique; In Europe We Also Have Some

Global temperature has been rising continuously since 1960s and annual temperature in most regions follows this pattern. However there are regions with a stagnation or cooling of seasonal temperature. Such periods and areas of non-warming are termed ‘warming holes’ according to Pan et al. (2004). This term was for the first time used for the summer and fall cooling over Central and Southeastern United States in the second half of the 20^thcentury (Drijfhout et al. 2012; Wang et al. 2009). Signatures of warming holes were also detected in other regions of North America in various seasons: in Canada (Vincent et al. 2015) and in Mexico (Cavanaugh and Shen 2014).

In Europe the fall non-warming was mentioned in many resent papers (Klein Tank et al. 2005; Scherrer et al. 2012, and many others). A weak cooling was also detected in other European regions in other months (El Kenawy et al. 2012; Xoplaki et al. 2005). This suggests that the seasonal (or even monthly) scale is not sufficient to detect all warming holes and to describe and correctly localize the existing cooling. For this reason, a novel method of the trend detection in periods of a given length (10 to 90 days) sliding over year with a one day step was developed. This allows us to describe annual cycles of temperature trends at individual stations in full detail. The cluster analysis of annual cycles of trends uncovers the warming holes over European regions homogenous in space and time.

In our contribution, daily maximum (Tx) and minimum (Tn) temperature at 135 stations in Europe in the period 1961–2000 is analyzed. It turned out in Pokorná et al. (2017) that cooling trends of Tx and Tn exist in all days during the calendar year at least at several stations, however only in some months the non-warming is uniform in certain regions. The cluster analysis indicated 6 and 5 regions for Tx and Tn, respectively, with homogenous trends at stations belonging to one cluster and dissimilar behavior of trends between clusters. The animation of the maps with trends of 30-day sliding seasons with daily step allows one to observe that the European warming hole moves, expands, shifts and disappears during year. Starting in January, a week cooling exists in the Eastern Mediterranean and on the Black sea coast. The cooling of Tn expands to the western Mediterranean around mid-January and to western Europe at the beginning of February. At this time cooling of Tx and Tn also starts in Iceland and grows until its maximum at the beginning of March. In the rest of Europe, the warming hole falls back to southeastern Europe. Cooling expands again over whole Europe from the end of March until 10^th April when almost whole Central and southeastern and partly western Europe experience cooling. This warming hole lasts until the end of April and then it moves to northern and eastern Europe where it culminates around 19^th May, being more pronounced for Tn. Next cooling arises over the British Isles at the end of May, expands to Scandinavia and then to northwestern Europe in mid-June and to the Alps at the end of June. This cooling is notable mainly for Tx. The weak cooling remains in central and eastern Europe until the first decade of July; then it moves to eastern Europe at the beginning of August. Cooling rests there and gains strength to show its full power mainly for Tx in mid-September over whole Europe with exception of the most western and northern regions. Cooling of Tx weakens at the end of September but it extends to reach the largest size in mid-October. For Tn, rather a stagnation or weak cooling is detected during the same time. The warming hole retreats from central Europe to the east and really large cooling occurs in the second half of November over eastern and southeastern Europe. At the end of the year warming hole weakens and falls back to southeastern Europe.

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