Winter 2010 in Europe: a Cold Extreme in a Warming Climate J

GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L20704, doi:10.1029/2010GL044613, 2010 Winter 2010 in Europe: A cold extreme in a warming climate J. Cattiaux,1 R. Vautard,1 C. Cassou,2 P. Yiou,1 V. Masson‐Delmotte,1 and F. Codron3 Received 7 July 2010; revised 25 August 2010; accepted 27 August 2010; published 22 October 2010. [1] The winter of 2009/2010 was characterized by record [3] This paper focuses on European cold temperatures of persistence of the negative phase of the North‐Atlantic winter 2010. The European wintertime climate is mostly Oscillation (NAO) which caused several severe cold spells driven by atmospheric dynamics over the North‐Atlantic – over Northern and Western Europe. This somehow unusual European (NAE) area [Walker, 1924], characterized by a winter with respect to the most recent ones arose baroclinic instability of the westerly jet stream which gen- concurrently with public debate on climate change, during erates planetary waves traveling from North‐Eastern America and after the Copenhagen climate negotiations. We show to the European continent [e.g., Charney, 1947]. The unstable however that the cold European temperature anomaly of nature of the jet also triggers quasi‐stationary circulation winter 2010 was (i) not extreme relative to winters of the patterns of larger scale, often referred to as “weather past six decades, and (ii) warmer than expected from its regimes”, which can persist from a few days to a few weeks record‐breaking seasonal circulation indices such as NAO [Legras and Ghil, 1985; Reinhold and Pierrehumbert, 1982; or blocking frequency. Daily flow‐analogues of winter Vautard, 1990]. The seasonal‐to‐decadal variability in 2010, taken in past winters, were associated with much European temperatures has often been considered as driven colder temperatures. The winter 2010 thus provides a by the frequency of occurrence of each regime [Philipp consistent picture of a regional cold event mitigated by et al., 2007; Vautard and Yiou, 2009]. For instance the long‐term climate warming. Citation: Cattiaux, J., R. Vautard, positive (negative) phase of the North Atlantic Oscillation C. Cassou, P. Yiou, V. Masson‐Delmotte, and F. Codron (2010), (NAO) is generally associated with rather warm (cold) Winter 2010 in Europe: A cold extreme in a warming climate, temperatures [e.g., Hurrell, 1995], while the persistence of a Geophys. Res. Lett., 37, L20704, doi:10.1029/2010GL044613. high‐pressure system over Northern Europe or the British Isles, often referred to as “European blocking” conditions, 1. Introduction leads to cold and dry weather over Western Europe [e.g., Yiou and Nogaj, 2004]. – – [2] The period December 2009 January 2010 February [4] Our aim is to investigate which weather regimes were 2010 (hereafter winter 2010) was punctuated by series of associated to the cold winter 2010, and their interplay with cold weather events and unusual snow accumulation in the temperature anomaly. In particular we use the “flow‐ several Northern Hemisphere countries (see http://www. analogues” approach developed by Yiou et al. [2007] to ncdc.noaa.gov/sotc/). In Europe, three successive cold out- analyze this temperature anomaly based on past relation- breaks and unusual persistence of snow cover were observed ships between atmospheric circulation and temperatures. (see http://www.knmi.nl/cms/content/79165), which shaped the public perception of an exceptionally intense winter. 2. Data and Methods Several states of Eastern United States also recorded their snowiest winter ever [Seager et al., 2010] while a few cities [5] The daily atmospheric dynamics is analyzed through witnessed record‐breaking cold daily temperatures [Wang re‐analyses of geopotential height at 500hPa (Z500) pro- et al., 2010]. These weather events occurred during and vided by National Centers for Environmental Prediction and after intense media activity covering international climate National Center for Atmospheric Research (NCEP‐NCAR) negotiations in Copenhagen, and raised up questioning about [Kistler et al., 2001] over the period 1948–2010 (http:// global warming. A global perspective nevertheless high- www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis. lights that winter 2010 was marked by a mean warm anomaly html). Anomalies are computed by removing at each grid at global scale, especially over Greenland, Canada, North‐ point the 1961–1990 daily climatology. Africa and Middle East (see http://data.giss.nasa.gov/gistemp/ [6] Several indices are used for the statistical analysis of maps/). Understanding and improving the predictability of winter 2010 seasonal dynamics: (i) the seasonal NAO index such mid‐latitude cold spells is a key societal issue, since their (NAOi) defined as the difference between Azores and Ice- fate in both frequency and intensity in a warming climate land normalized surface pressures [Jones et al., 1998; directly impacts sectors of energy demand, transport disruption Osborn, 2006] (computed from monthly values downloaded and social emergency protection systems. from http://www.cru.uea.ac.uk/∼timo/datapages/naoi.htm), (ii) the frequency of blocking days throughout the winter season as defined by Tibaldi and Molteni [1990] (computed over 30° rcW(±10°)), (iii) the winter Seasonal Dynamics 1LSCE, UMR 8212, IPSL, CEA‐CNRS‐UVSQ, Gif‐sur‐Yvette, France. Index (SDI) defined by Cattiaux et al. [2010] and based on 2Cerfacs, CNRS, Toulouse, France. optimal correlation between European atmospheric circu- 3LMD, IPSL, CNRS‐Ecole Polytechnique‐ENS‐UPMC, Paris, lation and temperatures, and (iv) the seasonal frequency France. of daily occurrences of North‐Atlantic “weather regimes” obtained by Cassou [2008] from a “k‐means” clustering Copyright 2010 by the American Geophysical Union. 0094‐8276/10/2010GL044613 L20704 1of6 L20704 CATTIAUX ET AL.: COLD WINTER 2010 IN EUROPE L20704 Figure 1. (a) Geopotential height at 500mb (Z500) anomaly averaged over winter 2010 above 20°N (from NCEP). Units: m. Labeled gray contours indicate standard deviations levels. (b) Normalized 1824–2010 time series (bars) and spline‐smoothing (line) of NAOi. Winter 2010 is indicated by the blue dashed line. (c) Normalized 1949–2010 time series of NAOi (bars), SDI (gray) and North‐Atlantic blocking days frequency (black). Winter 2010 values are indicated by dashed lines. Levels are inversed for blockings, see right axis. NAOi differ from Figures 1b to 1c since normalizing periods differ. method computed in the Empirical Orthogonal Functions flow‐analogue days. Using rather Euclidean distance or phase space [Michelangeli et al., 1995] over the NAE linear correlation for Z500 ranking, five or twenty flow‐ domain and the period 1974–2007. analogues, and the mean in analog temperature computation [7] Temperature data are from the European Climate does not change our results in a significant manner. Assessment and Dataset (ECA&D) project [Klein‐Tank et al., 2002], which provides daily minimum, maximum and mean temperatures (respectively Tmin, Tmax and Tavg) 3. Extreme Persistence of Daily NAO− Conditions at European stations (http://eca.knmi.nl/dailydata/). The data During Winter 2010 set is geographically homogenized by selecting the stations [9] Winter 2010 is characterized by an exceptional on the basis of (i) the simultaneous availability of Tavg, Northern Hemisphere mean atmospheric circulation [Wang Tmin and Tmax data, (ii) the availability of more than 80% et al., 2010]. The Z500 anomaly exhibits a strong zonal of daily values between 1 January 1948 and 28 February hemispheric pattern, with anomalously high (low) pressures 2010, (iii) a selection of only one station per grid cell of over the pole (mid‐latitudes) (Figure 1a). Such a structure 0.75° × 0.5° size. This method is similar to that used in corresponds to a negative phase of the Arctic Oscillation (AO) previous studies [Vautard and Yiou, 2009; Yiou et al., 2007], [Thompson and Wallace, 1998]. In particular L’Heureux and retains here 230 stations over Europe. Anomalies are et al. [2010] highlighted that the negative AO of December computed by removing at each station the 1961–1990 daily 2009 was a record. climatology, and in order to better quantify their amplitude, [10] Wintertime European temperatures are largely con- normalized anomalies relative to the mean and the standard trolled by the NAO [e.g., Hurrell, 1995; van Loon and deviation (s) of the 1949–2010 anomalies distribution are Rogers, 1978], which can be interpreted, even if controver- used. sial, as the regional signature of the AO [e.g., Ambaum et al., [8] The “flow‐analogues” method, used in section 5 to 2001]. Over the NAE domain, the winter 2010 hemispheric estimate daily temperatures observed during similar flow zonal structure is associated with an extremely negative conditions in past winters, was developed by Yiou et al. phase of the NAO, even constituting a record of NAOi since [2007] and consists as follows: for each day of winter winter 1824, almost 3s below average (Figure 1b). Negative 2010, ten flow‐analogues are selected among winters 1949 NAO events usually favor the development and persistence to 2009 in a 30‐day window centered on this given day. The of North‐Atlantic atmospheric blockings [e.g., Shabbar et al., selection is made on the basis of maximizing the Spear- 2001]. This is the case of winter 2010 which has the 2nd man’s correlation of daily Z500 NAE maps. Then, for each highest blocking frequency since 1949 (Figure 1c, freq = station, the daily “analog” temperature anomaly is defined 33%), close to the record of the winter 1963 (freq = 34%). as the median of daily temperature anomalies of the ten Moreover the winter 2010 SDI value is the 6th lowest 2of6 L20704 CATTIAUX ET AL.: COLD WINTER 2010 IN EUROPE L20704 Figure 2. (a–d) Winter weather regimes as computed from Z500 by [Cassou, 2008]: NAO+, Scandinavian Blocking, NAO− and Atlantic Ridge. Units: m. Percentages represent each regime frequency over the computation period (1974– 2007). (e–h) Frequencies of occurrence of each regime over 1958–2010. Units: days/90 (left axis) and s (right axis).

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