2015: joint warmest year on record in Europe
Climate Indicator Bulletins (CIBs) are user-driven climate information products which provide simple, effective and timely knowledge abstractions from the large amount of observation and reanalyses data available in UERRA. The bulletins focus on user groups in sectors such as disaster prevention, health, energy, water resources, ecosystems, forestry, agriculture, transport, tourism and biodiversity at European, national and local levels.
This bulletin presents an overview of the 2015 temperature evolution in Europe based on a large number of measurements.
Last update: 25 January 2016.
Annual mean temperature
Figure 1: Graph of annual (Jan-Dec) averaged daily mean temperature averaged over Europe, with respect to the 1981-2010 climatology. Temperatures below normal are in blue, temperatures above normal are in red, with the 2015 value in green.
The annual temperature for 2015 ranks among the highest temperatures measured in Europe! The temperatures averaged over Europe for last year (2014) and 2015 are very similar, with 2015 being only very slightly warmer.
Surface air temperatures are measured at an extensive network of stations in Europe. These data are collected and aggregated into a European temperature. The annual mean values in this E-OBS dataset are presented in Figure 1 for the land area between between 35°-75°N and 25°W-45°E. The green bar represents data for the year 2015.
The grey bars in the panel indicate the estimated uncertainties which take into account the errors introduced by spatial interpolation over areas without observation stations, inhomogeneities in the temperature data that result from station relocations / changes in measurement instruments etc., and biases due to urbanisation, as documented in Van der Schrier et al., 2013 and Chrysanthou et al., 2014. The uncertainties tell us that although we are not 100% certain about the ranking of individual years, the overall positive trend since the 1980s is very pronounced.
A map (Figure 2) of the annual mean temperature for 2015 with respect to the 1981-2010 long-term average shows that most of Europe was warmer in 2015 than usual, with a pronounced west-east temperature contrast. Much of eastern Europe was exceptionally warm, warmer even than 2014 which (on average) is about as warm as 2015. Western European countries were on average cooler, but still show higher than usual temperatures with the exception of parts of Ireland where lower than average temperatures were observed.
The ranking of the 2015 European temperature as joint warmest on record is due to the exceptionally warm December 2015. Replacing the December 2015 with the 1981-2010 December climatology, would set 2015 back as the third warmest year (after 2007 and 2014). Below, some more information on the remarkable December 2015 is provided.
Figure 2: Map of anomalous annual mean temperature for 2015, with respect to the 1981-2010 climatology.
Seasonal temperature anomalies
The temperature anomalies (with respect to the 1981-2010 long-term average) show large contrasts over Europe in all seasons. Striking are the below-average temperatures in Spain and Portugal in winter, the cooler than usual summer in northern Europe and the persistent coldness in Ireland. Spring was particularly warm in Spain, where in mid-May unseasonably warm temperatures of 40°C were reached in many parts of Spain. Exceptionally hot conditions in Spain at the end of June, with 44°C reached in Cordoba and a new Madrid July record, spread north and east bringing heat waves to central Europe. Paris saw very hot weather with about 40°C; Germany broke an all-time record for hottest day of the year and Switzerland also had its second warmest week ever (behind the 2003 value). Although the warmth was very persistent in east Europe, the contrast with the rest of the continent ranked the summer of 2015 as only the eleventh warmest on record. During autumn, only November was warm throughout Europe, contributing to having in Autumn 2015 the smallest temperature contrasts in Europe. Overall, Autumn 2015 ranks as the third warmest on record.
Figure 3: Map of anomalous seasonal mean temperatures with respect to the 1981-2010 climatology. Winter values relate to Dec 2014 - Feb 2015.
Warm days and warm nights
Figures 4 and 5 show the percentages of warm days and warm nights for 2015, with respect to the long-term mean (1981-2010). A warm day or night is defined as a day where the maximum (TX) or minimum (TN) temperature exceeds the 90th percentile of the values from the 1981-2010 period. This threshold, relating to the warmest 10% of the days or nights, is calculated for each position on the map, relating it specifically to the local conditions. In summer 2015, the Scandinavian countries saw fewer warm days and warm nights, while the central and southern parts experienced up to 40% more warm days and nights than in a usual summer. The high frequency of warm days reflects the heat waves and associated record temperatures that were observed in parts of Europe. The high number of warm nights is relevant from the health perspective; high night-time temperatures have adverse effects on human health. The heat wave of early July observed over parts of northwestern Europe was estimated to have lead to a 26% higher mortality in Belgium and a strongly increased mortality in the Netherlands in this period.
|Figure 4: Map of percentages of days in summer which are classified as 'warm', with respect to the 1981-2010 climatology. Warm days relate to the days where the daily maximum temperatures exceed the thresholds related to the 10% warmest days, determined over the 1981-2010 period.||Figure 5: Map of percentages of nights in summer which are classified as 'warm', with respect to the 1981-2010 climatology. Warm nights relate to the days where the daily minimum temperatures exceed the thresholds related to the 10% warmest nights, determined over the 1981-2010 period.|
The relative sunshine duration anomaly for autumn with respect to the 1961-1990 climatology is presented in the map below. Much of Europe saw above average sunshine amounts. It is interesting that the autumn temperature contrast between relatively cool Portugal and relatively warm Spain is reflected in this sunshine duration map. Ireland, much of the UK, northern France, parts of Germany and especially Russia were cloudier than usual.
Figure 6: Map of relative sunshine duration anomaly for autumn 2015, with respect to the 1961-1990 climatology. (source: RCC-CM, based on SYNOP and CLIMAT stations)
Summer days and frost days
A summer day is defined as a day where the daily maximum temperature exceeds 25°C. Figure 7 shows the number of summer days for 2015 as a deviation from the 1981-2010 long-term average. The northern countries saw fewer summer days than usual during 2015, while most of southern Europe saw more summer days.
A frost day is a day where the daily minimum temperature drops below 0°C. This temperature is usually reached during the night-time. Figure 8 shows that especially western Europe, the area around the Baltic and parts of Central Europe saw a spectacularly low number of frost days.
Figure 7: Number of summer days during 2015 (daily maximum temperature exceeding 25°C) expressed as anomalies from the 1981-2010 climatology.
Figure 8: Number of frost days during 2015 (daily minimum temperature below 0°C) expressed as anomalies from the 1981-2010 climatology.
Strength of the westerlies
The North Atlantic Oscillation (NAO) index provides a measure of the strength of the westerly winds across Europe. Positive values indicate stronger than normal westerly conditions, and negative values indicate a reduced prevalence of westerly conditions. During the winter season, positive NAO values are typically associated with warmer than average conditions across northern Europe and cooler conditions across the Mediterranean region. During winter 2014/2015 the NAO station-based index reached an all time high positive value (Figure 9), indicating an exceptional frequency of westerly winds across Europe. The pattern of winter temperature anomalies described above reflects this state of the NAO, but the cooler than normal sea-surface temperatures in the North Atlantic (see below) somewhat tempered the effect that would typically be expected across western Europe.
Figure 9: The winter (December to February) station-pair NAO index from 1865 to 2015. The index is calculated from the difference in normalized mean sea-level pressure, between readings from Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland. (source: NCAR/UCAR)
The circulation pattern during December 2015
The exceptionally warm conditions experienced across most of western Europe during December 2015 (see the "Annual mean temperature" section above) have made it the warmest December since 1950 for large parts of Europe. Figure 10 shows the ranking of December 2015.
Figure 10: Top 5 ranking of December 2015 (1 means warmest December since 1950, 2 means second warmest December since 1950, etc)
The warm conditions were related to a very high frequency of westerly or south-westerly winds across the regions. The ranking of December 2015 for the areas in Europe are shown above. It shows that for large parts in Europe, 2015 was the warmest December since 1950. In Figure 11 we show the number of westerly/south-westerly days over the British Isles during that month. Although the index is focused on the British Isles, the values also correspond to conditions across much of northwestern Europe. The frequency of westerly/south-westerly days in December 2015 was only exceeded once before over the 1871-2015 period and is equal to the frequency during December 1949. The dominance of these winds in December 2015 was associated with a high pressure system located over the western Mediterranean and a low pressure over Iceland. In Figure 12 we show a map of the Sea Level Pressure averaged during the month. This provides information on the atmospheric circulation configuration leading to the westerly/south-westerly conditions. It shows a high-pressure system over south-central Europe, advecting warm air northwards towards the British Isles and mid-latitude Europe. This pattern contrasts with the average conditions during the previous 2014 December, where the southerly high-pressure was situated more to the west leading to a higher frequency of westerly winds compared to December 2015. In association with the upper-air steering mechanism demonstrated in Figure 12, the flooding experienced over northern England resulted from a high frequency of low-pressure systems crossing the North Atlantic and northern Europe. A further effect of this pressure configuration was the low ranking of temperatures seen in the figure above over the Eastern Mediterranean and in particular Turkey, which being on the eastern flank of the high pressure system experienced a north westerly air-flow.
Strong south-westerlies were not the only consequence of this pressure pattern: Central Europe experienced several days of weak winds, fog and high concentrations of atmospheric pollutants.
Figure 11: The number of westerly/south-westerly days over the British Isles during December months since 1871. The data are provided by the Climatic Research Unit, University of East Anglia (https://crudata.uea.ac.uk/cru/data/lwt/).
Figure 12: Sea level pressure averaged over December 2015. The data were obtained from the NCEP/NCAR Reanalysis project (http://www.esrl.noaa.gov/psd/data/reanalysis/reanalysis.shtml).
Sea Surface Temperature
The west-east temperature difference in Europe for 2015 relates to cooler than usual sea surface temperatures in a large part of the central North Atlantic. The anomalous cooling most likely relates to enhanced air-sea heat exchange related to the stronger winds over the North Atlantic in the first few months of the year. Figure 13 shows the annual mean sea surface temperature of 2015 with respect to the long-term mean 1982-2010. This cold anomaly extended westward, surrounding Ireland and most of the UK and reaching the Portugese coast. Sea surface temperatures in the Mediterranean, the Black Sea and the Baltic Sea were (much) warmer than usual. North of Iceland, the northern North Atlantic and the White Sea (northeastern Russia) were also exceptionally warm, which relates to the low sea ice cover in Spring. Sea surface temperatures were particularly cold in the summer months June to August and the situation has been slowly returning to near-normal conditions since that period.
Figure 13: Sea Surface Temperature averaged over 2015, with respect to the 1982-2010 mean. (source: NOAA_OI_SST_V2 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA)
During 2015 a number of significant rainfall events occurred across certain regions of Europe, but examining the year's rainfall totals compared to normal values, 2015 was not remarkable from the European perspective. Figure 14 shows that the Norwegian coast was wetter than usual and that Scotland was drier than usual, but on the whole the total annual precipitation amount is on the dry side. However, Europe saw some strong variations in the number of rainy days (a day with precipitation of 1 mm or more). Figure 15 shows that Portugal and parts of Spain, Ireland, parts of France and Germany and especially eastern Europe saw many fewer rainy days than usual. The UK, the Netherlands and Scandinavia saw more rainy days than usual.
|Figure 14: Annual precipitation over 2015 (mm), as deviations from the long-term mean of 1981-2010.||Figure 15: Annual number of rainy days, as deviations from the long-term mean of 1981-2010.|
2015 in a long-term perspective
The number of frost days for 2015 is less than the long-term average for many parts of Europe (Figure 8). The lower than usual number of these days is in line with a general decreasing trend in frost days (Figure 16) where the trend is shown over the 1951-2015 period. However, this trend - a decrease of up to 9 days per decade - will only have added little to the very large deviations of up to 50 days from the long-term mean observed in 2015.
A similar conclusion exists for the number of summer days. A trend analysis (Figure 17) shows generally increasing trends with no significant trends over Scandinavia. The 2015 deviation from the long-term mean (Figure 7) shows a strong increase in these summer days for southern Europe, while northern Europe saw near-normal amounts of summer days or even fewer than usual.
The information presented in this bulletin, and particularly the comparison between the observed 2015 values against the trend values, demonstrates the significant year-to-year variability that exists in Europe's climate. While record European-wide temperature averages should not be expected for each successive year, or for every part of the European domain, the variability from year-to-year is superimposed on a long-term warming trend.
|Figure 16: Trend in the annual number of frost days over the period 1951-2015.||Figure 17: Trend in the annual number of summer days over the period 1951-2015.|
Authors: Else van den Besselaar, Richard Cornes, Christiana Photiadou, Gerard van der Schrier, Gé Verver, Albert Klein Tank, Antonello Squintu (KNMI, The Netherlands)
Please send your questions, remarks, suggestions to CIB-feedback.
The development of this Climate Indicator Bulletin is initiated by the WMO RA VI Regional Climate Centre Network (Europe and the Middle East).
This EURO4M Climate Indicator Bulletin is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.