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