Supplementary Material: Section 3.2

1 Supplementary material: Section 3.2 2 3 3.2 Geographical distribution of record high Tmax:record low Tmin ratios 4 5 While it is difficult to find a general overall geographic pattern in the Tmin and Tmax ratios, it 6 is seen in Figure 5a-c that there is a general increasing gradient for the most recent decade 7 from the southern to the northern latitudes along the three south-north transects, a rising 8 trend from west to east along Transect 4, and no clear trends along the Mediterranean 9 segment (Transect 5, which is to be expected from a cluster of stations that are located 10 within a broadly similar climatic zone). 11 12 Interestingly, ratios exceeding 5:1 or 6:1 can be found both in the Mediterranean segment 13 (Lisbon: 9.8; Malaga: 6.2; Rome: 5.4) and in the northern latitudes (St Petersburg: 7.9; 14 Murmansk: 7.6; Soedankyla: 7.1), and of course in the Arctic (14.6 in Bear Island and 25.0 in 15 Hopen). In the rest of Europe, values of these ratios are generally lower in the 2000s, 16 ranging from about 2:1 to 5:1. 17 18 While essentially empirically-based, an attempt will be made in the next paragraphs to put 19 forth some hypotheses that can help explain, through some plausible physical mechanisms, 20 why the upper and lower extremes behave in a similar manner in the warmest and coldest 21 reaches of the continent. 22 23 The high ratios close to or beyond the 60th parallel seem to be linked to the strong warming 24 that has been observed in the Arctic over the past two decades. A further explanation is 25 provided by Figure 11, which illustrates the temporal evolution of the 10% and 90% quantiles 26 of minimum temperatures. These are identified on the graph at Tmin Q10 and Tmin Q90, 27 respectively; the quantiles are defined as the lower and upper 10% tails of the annual 28 temperature distribution for each of the years from 1951 to 2013) at Hopen, Norway. As for 29 the other high-latitude sites used in this study, the rise in the cold extreme of Tmin over the 30 60-year period occurs at a rate three times greater than for the 90% quantile, representing an 31 effective warming of the cold tail of the Tmin distribution of 6°C during the 63 years since 32 1951, compared to just 1.8°C in the warm tail of the Tmin PDF. 33 34 ** Insert Figure S1 near here ** 35 36 The end-result of this progressive and rapid reduction of the coldest temperatures is that the 37 number of freezing days per year has also declined at Hopen, from an average 216 days per 38 year in the 1960s to 176 days in the 2000s. While not quite as impressive, the figures for St 39 Petersburg, Murmansk and Tromsoe show a decline of 20-25 days for the same period. In all 40 cases, the significant reduction in the number of freezing days can influence the duration of 41 snow cover in these regions, with a corresponding positive feedback effect on atmospheric 42 temperatures. 43 44 Conducting an analysis of the seasonal record ratios for three locations provided in Table 1 45 (where the grey shading highlights the strongest seasonal records for each location), it is 46 seen that Arctic summers and Northern European springs are those exhibiting the largest 47 ratios – thereby providing additional credence to the hypothesis that shorter snow seasons 48 could be one mechanism that helps explain the current dominance of Tmax record highs 49 over Tmin record lows. The very short summer season in Arctic Ocean region is amplified by 50 the loss of the coldest tails of the temperature PDF, while further south, in northern 51 Scandinavia and north-west Russia, warmer springs also contribute to a shorter freezing 52 season and therefore an enhancement of temperatures. 53 54 ** Insert Table S1 near here ** 55 56 In the Mediterranean zone, where stations such as Lisbon, Malaga and Rome also exhibit 57 strong ratios in the most recent decade, an analysis of the behavior of the 10% and 90% 58 quantiles of Tmin shows that the trends are in significant contrast to what takes place in the 59 northerly latitudes. As seen in Figure 12 for Rome, the upper quantile of the temperature 60 PDF increases much more rapidly than the lower quantile, by a factor of 4.4 for this particular 61 location. Since 1951, the 90% quantile has increased by an average of 2.8°C as compared to 62 only 0.6°C for the 10% quantile. Furthermore, a count of the number of dry days along 63 Transect 5 shows that days without rain have increased from an average of 255 days per 64 year in the 1950s to 285 days, i.e., an extra month of dry conditions over 60 years. A rapidly- 65 warming upper tail of the temperature PDF combined with declining precipitation may result 66 in enhanced evaporation of ground-water, leading to stronger positive feedbacks on 67 atmospheric temperatures. 68 69 ** Insert Figure S2 near here ** 70 71 Seasonal record ratios for three locations located along Mediterranean Transect 5, given in 72 Table 2, show the season in which the ratios are the largest is in all cases the summer, 73 which is consistent with the hypothesis that enhanced summer dryness, possibly already 74 beginning in the preceding spring, could be an explanatory mechanism leading to the high 75 ratios observed in the 2000s. 76 77 ** Insert Table S2 near here ** 78 79 80 81 Figure and table legends for Supplementary Material 82 83 Figure S1: Time series of the 10% and 90% quantiles of Tmin at Hopen, Norway 84 85 Figure S2: As for Figure S1, except for Rome, Italy 86 87 Table S1: Ratios of Tmax record highs:Tmin record lows on a seasonal basis for selected 88 stations in the northerly latitudes, in the decade 2000-2010. Grey shading highlights the 89 strongest seasonal ratios for each location. 90 91 Table S2: As Table 1, except for selected locations in the Mediterranean zone. 92 93