extreme cold outbreaks in the united states and europe ... · extreme cold events on a regional...

2642 VOLUME 14J O U R N A L O F C L I M A T E

q 2001 American Meteorological Society

Extreme Cold Outbreaks in the United States and Europe, 1948–99

JOHN E. WALSH, ADAM S. PHILLIPS,* DIANE H. PORTIS, AND WILLIAM L. CHAPMAN

Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois

(Manuscript received 14 June 2000, in final form 2 November 2000)

ABSTRACT

Reanalysis output for 1948–99 is used to evaluate the temporal distributions, the geographical origins, andthe atmospheric teleconnections associated with major cold outbreaks affecting heavily populated areas of middlelatitudes. The study focuses on three subregions of the United States and two subregions of Europe. The coldoutbreaks affecting the United States are more extreme than those affecting Europe, in terms of both the regionallyaveraged and the local minimum air temperatures. There is no apparent trend toward fewer extreme cold eventson either continent over the 1948–99 period, although a long station history suggests that such events may havebeen more frequent in the United States during the late 1800s and early 1900s. The trajectories of the coldestair masses are southward or southeastward over North America, but westward over Europe. Subsidence of severalhundred millibars is typical of the trajectories of the coldest air to reach the surface in the affected regions. Sealevel pressure anomalies evolve consistently with the trajectories over the 1–2 weeks prior to the extremeoutbreaks, and precursors of the cold events are apparent in coherent antecedent anomaly patterns. Negativevalues of the North Atlantic oscillation index and positive anomalies of Arctic sea level pressure are featurescommon to North American as well as European outbreaks. However, the strongest associated antecedent anom-alies of sea level pressure are generally shifted geographically relative to the nodal locations of the North Atlanticand Arctic oscillations.

1. Introduction

Extreme cold-air outbreaks affect larger numbers ofmidlatitude residents than other types of severe weathersuch as tornadoes and hurricanes. While concern aboutcold outbreaks runs counter to the popular notion ofglobal warming, population growth and more complextransportation systems make the midlatitudes increas-ingly vulnerable to extreme outbreaks of cold air (Witten1981). Moreover, the changes of atmospheric circulationthat may accompany future global warming are highlyuncertain, leaving open the possibility that the severityof cold outbreaks in middle latitudes may not decreaseas the global mean temperature increases. In the presentpaper, we address the frequencies, origins, and atmo-spheric teleconnections associated with cold air out-breaks affecting the United States and Europe. One ofour primary goals is to assess the linkage between thesesevere cold outbreaks and the large-scale hemisphericcirculation. A particular focus will be the preoutbreakevolution of the near-surface atmosphere in the Arctic

* Current affiliation: National Center for Atmospheric Research,Boulder, Colorado.

Corresponding author address: John E. Walsh, Department of At-mospheric Sciences, University of Illinois at Urbana–Champaign, 105S. Gregory Avenue, Urbana, IL 61801.E-mail: [email protected]

and subarctic. As a hemispheric heat sink, the northernhigh latitudes play a central role in the attainment ofthe extremely low temperatures that can affect the mid-dle latitudes.

2. Relation to previous work

While many studies have addressed cold surges as-sociated with the Asian monsoon (cf. Joung and Hitch-man 1982), the present study will focus on the coldestairmasses affecting the United States and Europe. Forthe United States, Wexler (1951) was among the firstto address in a systematic way the characteristics of coldor polar anticyclones, which result from a combinationof mass convergence, diabatic cooling, and cold air ad-vection. Individual outbreaks have been examined byWagner (1977), Quiroz (1984), and Mogil et al. (1984),among others. Composites of several anticyclones orig-inating in the northwestern Canada–Alaska region wereconstructed by Dallavalle and Bosart (1975), althoughtheir study emphasized the eventual fate of the anti-cyclones after their southward migration over the UnitedStates. Colucci and Davenport (1987) compiled a syn-optic climatology of rapid anticyclogenesis events overNorth America and the adjacent oceans during one year.Many of the cold-season anticyclogenesis events inwestern North America were followed by cold-air out-breaks, although none were in the ‘‘record-setting’’ cat-

15 JUNE 2001 2643W A L S H E T A L .

egory addressed in the present paper. In a diagnosis oftwo major 1977 outbreaks over North America, Konradand Colucci (1989) focused on cold advection, adiabaticwarming (subsidence) or cooling, and upstream cy-clones. Their results suggested that the intensity of thecold air outbreaks varies with the size of the area ofcold advection and with the timing of the associatedcyclogenesis. Colle and Mass (1995) examined cold airsurges east of the Rocky Mountains by diagnosing themomentum, energy, and vorticity budgets in a casestudy. A composite analysis included in that study point-ed to the importance of interactions between synopticsystems and the topography of the western UnitedStates. Colucci et al. (1999) used an ensemble of nu-merical model forecasts to infer that nonadiabatic pro-cesses appear to have played a key role in the intensi-fication of at least one extreme cold outbreak affectingthe United States in 1985. Colucci et al.’s results leftopen the possibility that radiative cooling may be en-hanced by the presence of low-level condensate, in-cluding clouds and clear-sky ice crystal precipitation(Curry 1983, 1987). Kalkstein et al. (1990) used syn-optic data from four stations in Alaska and northwesternCanada to infer that the coldest air masses in this sourceregion were somewhat less frequent and less cold in the1970s and 1980s than in earlier decades. This findingdoes not, however, preclude quite different temporalvariability of the extreme cold outbreaks that reach themidlatitude areas of North America.

Several studies during the past decade have addressedcold air outbreaks in the context of Florida citrus freez-es. Rogers and Rohli (1991) showed that such freezestend to occur in clusters, for example, the 1890s and1977–89. Rogers and Rohli showed that major freezeevents in Florida have tended to be associated with thepositive phase of the atmospheric Pacific–North Amer-ican (PNA) teleconnection pattern, as did Downton andMiller (1993). Downton and Miller also included theSouthern Oscillation index and the North Atlantic os-cillation index in a statistical comparison with monthlymean and monthly mean minimum temperatures in cen-tral Florida.

More recently, DeGaetano (1996) showed a tendencytoward fewer cold minimum temperature threshold ex-ceedences in the northeastern United States over the1959–93 period. This tendency appears to be part of abroader trend toward higher minimum temperaturesover the United States during the past half-century (Karlet al. 1991; Easterling et al. 2000). However, for themore recent period since 1975, Kunkel et al. (1999)report an absence of trends in fatalities and citrus dam-age due to cold waves in the United States.

For much of Europe, wintertime minimum tempera-tures have also been increasing more rapidly than themaximum temperatures (Brazdil et al. 1996; Heino etal. 1999). However, there have been suggestions thatthe occurrence of extremely cold winter conditions inGreat Britain may have increased during the late 1970s

and 1980s (Thompson 1987), and the 1990s have alsoseen occasional episodes of extreme cold (Brugge1991). Nevertheless, the overall tendency toward milderwinters appears in the temperature data for much ofEurope, and this tendency is at least partially attributableto the predominance of the positive phase of the NorthAtlantic oscillation (NAO) during the 1980s and 1990s(Hurrell and van Loon 1997; Thompson and Wallace1998). The fact that the NAO modulates the intensityof maritime airflow into much of Europe suggests thatthe NAO will play a role in the susceptibility of Europeto cold outbreaks, although the most appropriate time-scale for delineating this association has not been es-tablished. Furthermore, the NAO has been shown byThompson and Wallace to be part of a broader hemi-spheric pattern of atmospheric variability, the so-calledArctic oscillation. The extension of this mode to highlatitudes gives it a potential role in the evolution ofextreme air masses affecting North America as well asEurope.

The study described here will draw upon atmosphericfields for 1948–99 from an atmospheric reanalysis,which provides a consistent set of temperature and cir-culation fields for all middle and high-latitude areas.The availability of the reanalysis output permits 1) theidentification of extreme cold events on a region-widebasis, and 2) a systematic comparison of low-temper-ature extreme events in different regions and from dif-ferent decades of the past half-century. In addition topermitting an assessment of the temporal variability ofextreme cold events on a regional basis, the reanalysisoutput permits an evaluation of the atmospheric circu-lation anomalies that precede major cold air outbreaks.We are thus able to address the geographical origins,the evolution and the atmospheric teleconnections as-sociated with extreme cold events in different regions.Our focus on these aspects is intended to provide abroader perspective on the cold events than has emergedfrom the studies summarized above.

Section 3 is a summary of the relevant reanalysis data.In section 4, we discuss the procedure by which thecoldest events are identified for various regions in theUnited States and Europe. The temporal distribution ofthe extreme outbreaks in the various regions is sum-marized in section 5, while section 6 addresses the geo-graphical origins of the cold outbreaks in terms of tra-jectories, antecedent atmospheric circulation anomalies,and the teleconnection patterns associated with eventsaffecting the different geographical regions. While ear-lier studies of teleconnections (e.g., Downton and Miller1993) used monthly and/or seasonally averaged quan-tities to assess atmospheric linkages, our analysis insections 4–6 is based on daily data to capture better theextreme excursions that occur on shorter timescales. Inparticular, our diagnostic analysis is intended to identify1) the relevant timescale of the evolution of a majorcold outbreak and 2) the associated spatial character-istics at the various stages of evolution. In this respect,


FIG. 1. The three North American regions and two European regions for which extreme cold events were identified.

the present work extends the synoptic climatologicalstudy of Konrad (1996), who correlated cold-air out-breaks over the southeastern United States to the at-mospheric circulation over North America.

3. Data summary

The dataset used here was the gridded output of theatmospheric reanalysis performed by the National Cen-ters for Environmental Prediction–National Center forAtmospheric Research (NCEP–NCAR). The NCEP–NCAR 40-Year Reanalysis is described by Kalnay et al.(1996). The output used here consisted of 43-daily gridsprovided by the National Oceanic and Atmospheric Ad-ministration/Cooperative Institute for Research in En-vironmental Science (NOAA/CIRES) Climate Diagnos-tics Center, Boulder, Colorado (http://www.cdc.noaa.gov). The grids have a horizontal reso-lution of 2.58 lat 3 2.58 long. The reanalysis begins inJanuary 1948 and extends to the present; out study pe-riod ends in December 1999, resulting in a 52-yr periodof record. The fields used here are the surface air tem-peratures (from the lowest model level, s 5 0.995), thesea level pressure and the three-dimensional upper-airwinds (for the trajectory computations in section 5). Inthe cases of the surface air temperatures and sea levelpressures, the 43 -daily values were averaged into dailygrids. The four daily grids of wind for each day wereretained for the trajectory computations. We note thata second reanalysis, performed by the European Centrefor Medium-Range Weather Forecasts (ECMWF) is alsoavailable for years 1979–93. Although the shorter pe-riod of the ECMWF reanalysis limits its utility in a studysuch as the present one, we have compared the relevantoutputs from the two reanalyses. The comparison ofsurface air temperatures is summarized below.

While the temperatures, pressures and wind fields are

based on observational data assimilated directly into thereanalysis, other model-derived fields such as radiativefluxes, cloudiness, and sensible and latent heat fluxesare available in the reanalysis output archive. However,these model-derived variables have much larger uncer-tainties than the assimilated variables (e.g., Walsh andChapman 1998) and hence are not included in the pre-sent study.

4. Event identification and sensitivities

The extreme cold events were identified for fiveheavily populated regions, three in the United States(MW 5 Midwest, EC 5 East Coast, GC 5 Gulf Coast)and two in Europe (NE 5 Northern Europe, WE 5Western Europe). Figure 1 shows the boundaries ofthese regions as they have been defined here; the shapesof the boundaries are constrained by the 2.58 3 2.58resolution of the reanalysis output. The three regions inthe United States are east of the Rocky Mountains, sothey are all susceptible to cold polar air masses that areoften constrained on the west by the Rocky Mountainrange as they slide southeastward. Indeed, the three re-gions share some (but not all) of the same extremeevents. Of the two European regions, the mountains ofNorway are the major orographic feature of NE, whilethe Alps are the only major mountain range in WE. Thelatter region includes Great Britain as well as a largeportion of the North Sea, while NE includes the BalticSea. The MW, EC, and GC regions of the United Statesconsist almost entirely of land and are smaller than theEuropean regions. The respective regional areas are 1.773 106 km2 (MW), 1.07 3 106 km2 (EC), 0.99 3 106

km2 (GC), 3.47 3 106 km2 (NE), and 2.47 3 106 km2

(WE).The extreme cold events for each region were defined

on the basis of the daily surface air temperature anom-

15 JUNE 2001 2645W A L S H E T A L .

alies averaged over the region. All anomalies are relativeto the daily means (i.e., 52-yr average) for the calendardate and region. The daily means for a region wereobtained using the 52-yr averages of the daily valuesfor all grid points in the region. The spatial averagingwas performed by summing the gridpoint anomalies ina region and dividing by the number of grid points.Since the actual areas varied from region to region, weexamined the sensitivity to domain size by repeating theprocedure for one region in three different sizes. Thefinal selections of cases were virtually identical, al-though the relative ranks of the selected events did showsome changes.

Once a date was selected, the antecedent and sub-sequent 10 days were eliminated from the search foradditional extreme dates in the same region. In order toaddress the sensitivities of the selection procedure tothe duration of the extreme cold event, the procedurewas repeated after replacing each daily regional anom-aly by a 3-day centered average of the anomaly and bya 5-day centered average of the anomaly. The use ofthe temporally averaged anomalies gives slightly moreweight to events in which the extreme cold persisted inthe same region for more than one or two days.

Prior to discussing the temporal distribution of thecold events in each region, we note a concern about apossible inhomogeneity of the NCEP reanalysis becauseof the introduction of satellite data. While the intro-duction of satellite data should impact the upper-airfields more than the surface fields used here, we nev-ertheless split the reanalysis output into 1948–78 and1979–99 periods in order to address possible impactsof the introduction of satellite data. The calendar-monthmeans and standard deviations of the regional daily tem-peratures were found to show no systematic differencesbetween the pre- and post-1979 periods. For example,the monthly mean temperatures for 1948–78 (1979–99)in the EC region of the United States were 6.78C (7.08C)for November; 1.18C (1.18C) for December; 20.98C(20.98C) for January; 0.58C (0.78C) for February; and4.88C (5.08C) for March. The corresponding standarddeviations of the daily temperatures in the EC regionwere 5.28C (4.88C) for November; 5.68C (5.88C) forDecember; 6.18C (5.68C) for January; 5.58C (5.78C) forFebruary; and 5.18C (5.28C) for March. In no region ormonth did the mean temperatures of the two periodsdiffer by more than 18C, nor did any standard deviationof the daily temperatures differ by more than 18C.

Table 1a lists the dates with the largest temperatureanomalies in the five regions. It is apparent that thereis only partial overlap between adjacent regions, evenafter allowing for 1–2-day differences due to the mi-gration of the core of an air mass. The numbers of daysin common between adjacent regions of the UnitedStates are 4 of 10 (MW and EC), 5 of 10 (EC and GC),and 6 of 10 (MW and GC). The two European regions(NE and WE) have only 3 of 10 dates in common.

Table 1a shows that the regional temperature anom-

alies are larger in the United States than in the Europeanregions. This tendency is especially evident in the casesof the 8th–10th ranked anomalies, which are nearlytwice as large in the United States as in Europe. Theseresults imply that the North American regions are moresusceptible to large negative temperature anomalies(2158 to 2208C) than their European counterparts. Ta-ble 2 contains the actual temperatures for the MW andWE regions from the years 1979–93, which are commonto the NCEP and ECMWF reanalyses. The degree ofsimilarity of corresponding temperatures from the tworeanalyses implies that many of the ranks of the extremedates would shift slightly, but that the overall suite ofextreme events would remain quite similar, if based ona different reanalysis. In four of the five regions, thesingle most extreme date remains the same regardlessof the reanalysis. Although the corresponding temper-ature anomalies from the two reanalyses generally differby a degree or so, both reanalyses show that the extremedaily temperature anomalies are colder by 108–188C inthe United States than in Europe. The relative extremityof the North American regional temperatures is en-hanced by 1) the smaller areal fractions of water in theNorth American regions and 2) the smaller sizes of theNorth American regions. The differences in size do notchange the fact that the actual minimum temperatureswere more extreme locally in North America, as werethe maximum magnitudes of the local anomalies.

Tables 1b and 1c indicate how the suites of extremedates change if the selection is based on 3-day and 5-day averaged anomalies rather than single-day anoma-lies. The lengthening of the averaging period from 1–5 days often results in shifts of several ranks, especiallyin the lower portion of each list. In some cases, a dateis displaced from a list by another date that had fallenjust short of inclusion when an alternative averagingperiod was used. The lists from the 1-day and 5-dayaveraging procedures have the following numbers ofdates in common: 5 of 10 in MW, 8 of 10 in EC, 5 of10 in GC, 7 of 10 in NE, and 9 of 10 in WE.

5. Temporal distribution of extreme events

Before addressing the decadal variations of the ex-treme events, several winters deserve mention becauseof their multiple events. The winter of 1962–63 pro-vided three of the top 10 entries for the Western Europeregion on each of the three lists (1-day, 3-day, and 5-day events) in Table 1. This winter is widely regardedas the coldest of the past half-century, and perhaps ofthe entire twentieth century, in the United Kingdom(e.g., Thompson 1987). Interestingly, this winter alsoappears at least once on each of the North Americanlists, and it even appears multiple times on the list ofthe 10 coldest single days in both the East Coast (EC)and Gulf Coast (GC) regions. The Northern Europeanregion, on the other hand, experienced 3 of its 10 coldestevents in January–February 1985. This same winter also


TABLE 1. Largest negative anomalies of regionally averaged temperatures based on (a) 1-, (b) 3-, and (c) 5-day running means. In (b) and(c), center dates of 3- and 5-day periods are listed.

MW EC GC NE WE

(a) 1-day criterion22 Dec 8924 Dec 8303 Feb 9615 Jan 7220 Jan 85

222.58C220.6219.9218.3218.1

25 Dec 8321 Jan 8519 Jan 9423 Dec 8917 Jan 77

219.08C218.8218.6218.3216.7

23 Dec 8925 Dec 8321 Jan 8513 Dec 6230 Jan 66

221.28C219.8219.6217.6217.3

10 Jan 8730 Dec 7803 Feb 6609 Feb 8521 Jan 85

215.48C214.6213.2211.3210.6

12 Jan 8702 Feb 5601 Feb 5407 Jan 8523 Dec 62

212.88C211.529.128.928.7

04 Feb 8913 Nov 8617 Nov 5929 Jan 6624 Jan 63

217.7217.7217.1216.8216.5

22 Feb 6304 Feb 9617 Feb 5813 Dec 6202 Mar 80

216.4216.4216.2216.2215.9

11 Jan 8202 Feb 5104 Feb 9624 Jan 6311 Jan 62

216.8216.5216.4215.7215.3

16 Dec 7806 Jan 5005 Jan 8514 Feb 7931 Jan 56

210.5210.3210.329.829.7

09 Feb 8605 Mar 7113 Jan 6307 Feb 9102 Feb 63

28.628.628.628.328.2

(b) 3-day criterion24 Dec 8322 Dec 8903 Feb 9604 Feb 8929 Jan 66

219.98C219.8218.3215.2215.1

23 Dec 8925 Dec 8304 Feb 9618 Feb 5820 Jan 94

216.58C215.6215.2215.2214.7

23 Dec 8925 Dec 8304 Feb 9611 Jan 6222 Jan 85

218.78C216.6214.3213.6213.6


214.58C213.4211.9210.629.8

12 Jan 8702 Feb 5601 Feb 5407 Jan 8505 Mar 71

211.38C210.228.928.728.4

11 Mar 9824 Jan 6303 Nov 9128 Feb 6217 Feb 58

214.7214.4214.3214.2214.0

18 Jan 7702 Mar 8013 Dec 6211 Jan 8221 Jan 85

214.6214.3213.6213.1212.7

13 Dec 6218 Jan 7709 Mar 9603 Feb 5117 Feb 58

213.4213.3212.6212.1212.1

06 Jan 8521 Jan 8516 Dec 7802 Mar 8730 Jan 56

29.629.429.128.728.7

24 Dec 6203 Feb 6313 Jan 6309 Feb 8607 Feb 91

28.328.027.727.627.4

(c) 5-day criterion24 Dec 8302 Feb 9621 Dec 8925 Jan 6305 Feb 89

217.98C217.3217.1213.2212.9


214.18C212.8212.7212.6212.2


215.18C213.3211.3211.3210.7


213.78C211.9210.029.829.6

13 Jan 8702 Feb 5607 Jan 8502 Feb 5402 Feb 63

29.98C28.428.228.127.9

31 Jan 5104 Mar 6009 Dec 7202 Feb 8517 Feb 58

212.7212.5212.2212.2212.2

18 Jan 7712 Dec 6201 Mar 8009 Jan 7022 Jan 70

212.0211.5211.1210.6210.6

18 Jan 7710 Mar 9613 Jan 8208 Jan 7005 Mar 60

210.6210.5210.3210.0210.0

06 Jan 8501 Mar 7121 Jan 8502 Mar 8710 Jan 68

28.928.328.128.027.9

25 Dec 6205 Mar 7130 Dec 9620 Jan 6309 Feb 86

27.727.427.327.326.8

TABLE 2. Comparison of temperatures from NCEP–NCAR and ECMWF reanalysis for major cold outbreaks during 1979–93.

MW region

Date TNCEP TECMWF

WE region

Date TNCEP TECMWF

22 Dec 8924 Dec 8320 Jan 8504 Feb 8913 Nov 86

225.28C223.7221.6220.3211.7

226.78C224.7224.5219.9214.5

12 Jan 8709 Feb 8607 Jan 8507 Feb 9102 Jan 79

210.28C25.626.125.225.1

29.28C26.027.225.623.9

appears on each of the single-day lists for the NorthAmerican regions. While these outstanding years sug-gest that extreme cold can affect both sides of the At-lantic in the same year, the North American lists do notshow the single-year ‘‘clusters’’ of events to the extentthat the European regions do. The implication is thatpersistent cold during a single winter may be less com-mon in the United States than in Europe, although itmust be noted that the sample of these exceptional sin-gle-year clusters in Europe is small.

Figure 2 summarizes the decadal frequencies of theextreme cold outbreaks listed in Table 1. (For the con-struction of Fig. 2, each event in Table 1 is countedonce, but not more than once if its center date falls

within one week of a previously counted event.) It isapparent that there is no trend toward fewer extremecold events in the North American regions. The largestnumber of North American events occurred during the1980s and the 1960s, and the smallest number duringthe 1950s. The final decade, 1990–99, is tied with the1970s. The 1980s and 1960s also stand out as the de-cades with the most extreme cold events in Europe,although the 1990s emerge as the decade with the fewestoccurrences in Europe. Thus, despite the recent trendtoward higher mean minimum temperatures noted ear-lier, neither the North American nor the European areasexamined here have seen a corresponding trend in thefrequencies of extreme cold events.

15 JUNE 2001 2647W A L S H E T A L .

FIG. 2. Decadal distribution of the major cold events in (top) NorthAmerican regions and (bottom) European regions.

Our conclusion about the absence of trends may nev-ertheless be dependent on the period examined in thisstudy. The period from the 1930s to the 1950s is knownto have been relatively warm over the Northern Hemi-sphere land areas, as it followed a warming from thecentury’s early decades (IPCC 1996). Figure 3 showsthe coldest single-day events dating back to the late1800s at a single station (Urbana, IL: 40.18N, 88.28W)in the MW region. There is indeed an absence of ex-treme cold from the late 1930s to the early 1960s. How-ever, the late 1890s and the first few decades of the1900s are well populated by extreme cold days. Rogersand Rohli’s (1991) finding that more Florida citrusfreeze events occurred around the 1890s and after 1977is consistent with the Urbana record. The absence ofnotable trends in the frequency of severe cold eventswithin the past 2–3 decades as noted by Kunkel et al.(1999) is also supported by the temporal distributionsin Figs. 2 and 3. Hence it appears that the frequency ofextreme cold outbreaks over the United States may becharacterized by low-frequency (multidecadal) varia-tions, implying that trend assessments based on onlyseveral decades of data must be made with caution.Low-frequency variations in the severity of winters overwestern Europe (specifically, southern England) havealso been suggested by Selfe (1970).

6. Spatial characteristics and evolution of coldevents

The large temperature anomalies addressed here areassociated with cold polar air masses, which can rangeup to several thousand kilometers in scale. As an illus-tration of the scale of the cold air outbreaks, Fig. 4a isa composite of the temperature anomalies based on thefive highest-ranking single-day events in the MW regionof the United States (cf. Table 1a). Not surprisingly, thelargest anomalies are found over the midwestern portionof the United States; the magnitudes of the anomaliesexceed 208C in this area. When the single-day temper-atures are first normalized by the standard deviations ofthe daily temperatures and then composited, the com-posite anomalies exceed three standard deviations in theMW region, as shown in Fig. 4b. The negative anom-alies cover most of the United States and the southernhalf of Canada, indeed showing the several-thousand-kilometer scale characteristic of large air masses. Al-though the temperature anomalies of the airmasses con-tributing to Fig. 4 vary somewhat in size and locationof the largest anomaly, the area inside the two-standard-deviation contour of Fig. 4b covers half the contiguousUnited States as well as part of south-central Canada.Hence the air masses are large in scale and share muchof the same geographical coverage. The scales and nor-malized magnitudes of the composites for the other re-gions are similar, although the actual magnitudes of thecomposite anomalies for the European regions aresmaller. Thus the events identified here are spatiallyextensive features with anomalies exceeding severalstandard deviations over a substantial portion of theregion under consideration.

In order to determine the origins of the cold air re-sponsible for the large anomalies, back-trajectories wereevaluated using the three-dimensional winds in the re-analysis output. The origin of each back-trajectory wasa point 50 mb above the surface (s 5 0.95) at thelocation of the largest negative temperature anomaly onthe center date of the cold event in a particular region.The trajectories were computed using 1-h time steps.The hourly winds were interpolated linearly from the6-h reanalysis. For each 1-h increment, the horizontaland vertical velocities at the nearest grid point to theposition of the air parcel were used to move the parcelback to a location for the next (farther back in time) 1-h timestep. Each trajectory was plotted at 6-h intervalsextending back through the 12 days prior to the arrivalof the cold parcel at its destination 50 mb above thesurface in the appropriate region of the United Statesor Europe. Thus each trajectory plot consists of 48points, each of which differs horizontally and verticallyfrom its prior and subsequent positions according to thelocal winds at the corresponding time.

Figure 5 shows a sample of six North American tra-jectories, two for each of the three regions in the UnitedStates. The trajectories are for temporally distinct events


FIG. 3. Years of days with lowest daily mean temperatures at Urbana, IL. Daily temperatures are plotted on y axis, colder downward.

FIG. 4. (a) Surface air temperatures (8C) composited over five coldest days in MW region (Table 1a), and (b) corresponding normalizedvalues (units: std dev of daily temperature).

15 JUNE 2001 2649W A L S H E T A L .

FIG. 5. Trajectories (horizontal views in upper panels, vertical views in lower panels) of air reaching a point 50 mb above surface oncenter days of the following major North American cold events from Table 1c: (a) GC #2, 25 Dec 1983; (b) EC #2, 18 Feb 1958; (c) MW#5, 5 Feb 1989; (d) EC #5, 18 Jan 1994; (e) MW #2, 2 Feb 1996; (f) GC #1, 23 Dec 1989.

selected from the five most extreme events listed in theNorth American columns of Table 1c. It is apparent thatthe core of the cold air reaches the affected region fromthe north or northwest. The most common origins ofthe trajectories (including those not shown in Fig. 5)are the northern fringes of North America (e.g., Fig. 5f),the Arctic Ocean (e.g., Fig. 5a), northern Asia (e.g., Fig.5c), and the subpolar seas of the North Atlantic (e.g.,Fig. 5e). Many of the trajectories suggest stagnation orslow movement over northwestern Canada, where thesnow-covered terrestrial surface and minimal solar ra-diation are conducive to diabatic cooling of the low-level air. Figure 5 also shows that the air parcels undergosubsidence during their southward trajectory towardmiddle latitudes. The subsidence, which is consistentwith the proximity of a high pressure center at the core

of the cold air mass, typically results in 100–300 mbof descent, although Figs. 5a and 5d show that the de-scent can be considerably greater if the trajectory ex-tends back to the Asian side of the Arctic. Trajectoriessuch as the latter suggest that the parcels were in othersynoptic systems prior to their entrainment into the coldpolar air masses. The 100–300 mb magnitudes of sub-sidence typical of most trajectories indicate that the airnear the surface would have been even colder than itwas if it were not for the adiabatic warming associatedwith the subsidence.

Figure 6 shows that subsidence also characterizes theEuropean trajectories. However, a fundamental differ-ence in the European trajectories is their indication ofsource regions to the east, particularly over northernAsia. The westward motion of the cold airmass cores


FIG. 6. As in Fig. 5, but for the following major European cold events: (a) NE #1, 9 Jan 1987; (b) NE #2, 30 Dec 1978; (c) NE #3, 4Feb 1966; (d) WE #2, 2 Feb 1956; (e) WE #3, 7 Jan 1985; (f) WE #4, 2 Feb 1954.

contrasts with the southward or southeastward move-ment of the cold air masses affecting the United States.In many of the European trajectories, there are indica-tions of stagnation over the western Siberian sector ofRussia, not unlike the stagnation of the North Americantrajectories over northwestern Canada. The westwardmigration of the European cold outbreaks suggests thatthe likelihood of such an outbreak is tied directly to theintensity of the eastward airflow reaching Europe fromthe North Atlantic. The NAO index is a natural measureof this intensity.

In order to examine more comprehensively the tem-poral evolution and spatial teleconnections associatedwith the cold outbreaks, we constructed compositeanomaly fields of sea level pressure (SLP) for varioustimes prior to the arrival of the core of the cold airmasses in their respective midlatitude regions. The in-

puts to the composite anomaly fields were the sea levelpressure fields from the 10 most extreme events listedin a particular column of Table 1c, for which the eventidentification was based on 5-day running mean tem-perature anomalies in a particular region. The pressureanomaly composites were constructed for the dates list-ed in Table 1c (‘‘Day-0’’), and for the dates 2 daysearlier (‘‘Day-2’’), 4 days earlier (‘‘Day-4’’), . . . , and12 days earlier (‘‘Day-12’’), thereby permitting exam-inations of the evolution of the SLP anomalies over the12 days prior to each region’s coldest days.

Figure 7 shows the composite SLP anomalies on Day-10, Day-6, Day-2, and Day-0 for the MW region of theUnited States. Ten days prior to the outbreak, there aremodest positive anomalies over the Arctic Ocean andnorthwestern North America, together with negativeanomalies over the eastern North Atlantic and the west-

15 JUNE 2001 2651W A L S H E T A L .

FIG. 7. SLP anomaly composites for (a) 10 days, (b) 6 days, (c) 2 days, and (d) 0 days prior to the dates ofthe 10 cold events in the Midwest (MW) region. Numbers below color bar denote the lower limit of each 2-mbrange of magnitudes. Unshaded areas have magnitudes smaller than 2 mb.

ern Bering Sea. This pattern intensifies by Day-6, as thepositive anomalies near the Alaska/Yukon border ap-proach 20 mb. The negative anomalies of up to 215mb offshore of southern Europe, in conjunction withpositive anomalies over the subpolar North Atlantic,result in a pattern that would project highly onto thenegative phase of the NAO and the Arctic oscillation.By Day-2, the positive anomalies over Alaska and west-ern Canada become the dominant feature, which spreadssoutheastward. The values of approximately 125 mb inthe Alaskan core on Day-2 represent impressive andhighly significant anomalies for a composite based on10 different cases selected on the basis of subsequenttemperatures thousands of km to the southeast. (The t-values of the Alaskan composite anomalies in Fig. 7c

exceed the 98% significance thresholds for a sample sizeof 10.) The pressure-temperature linkage is apparent inthe strength of the implied anomalies of northwesterlygradient winds on the forward side of the largest SLPanomalies. The airflow directly from the Arctic to theMidwestern United States is strikingly apparent onDay-0.

Figure 8 shows the corresponding sequence of SLPanomaly patterns for the EC region. (The patterns forthe EC and GC regions are sufficiently similar that thefollowing discussion applies to the GC as well as theEC region.) The outstanding feature of the Day-10 fieldis a strong signature of the negative phase of the NorthAtlantic and Arctic oscillations: composite anomalies of215 mb and 113 mb are found in the subtropical At-


FIG. 8. As in Fig. 7, but for the East Coast (EC) region.

lantic (offshore of Portugal) and the subpolar North At-lantic, respectively. The subtropical Atlantic feature ex-pands considerably by Day-6, and subsequently weak-ens, while the positive anomaly strengthens and movesthrough northwestern North America on Day-2. Astrong negative anomaly develops in the North Pacificby Day-6, resulting in a well-developed signature of theNorth Pacific oscillation (NPO). This signature transi-tions to a sea-level manifestation of the Pacific–NorthAmerican teleconnection by Day-2, when the negativeanomaly in the North Pacific and the positive anomalyover western Canada have magnitudes of 217 mb and119 mb, respectively. Figures 7 and 8 together implythat common features of the MW and EC outbreaks areantecedent negative NAO signatures, and a subsequentpositive NPO-to-PNA evolution.

The primary differences between the MW and EC(also GC) composites are 1) the stronger negative anom-alies over the oceans, and hence stronger NAO and NPOsignatures, in the EC (GC) composites, and 2) the moresouthward migration of the positive SLP anomaly in theEC and GC cases. The EC and GC cases may be con-sidered more dynamic in the sense that the positiveanomaly remains over northwestern North Americaeven at Day-0 in the MW case, while the anomaly centerclearly detaches and moves southeastward by Day-0 tothe contiguous United States in the EC and GC cases.This southeastward movement of the positive SLPanomalies is consistent with advection of the colder airinto the more eastern (EC) and southern (GC) sectorsof the country. Figures 8c and 8d also contain negativeSLP anomalies over eastern North America, implying

15 JUNE 2001 2653W A L S H E T A L .

FIG. 9. As in Fig. 7, but for the Northern European (NE) region.

coastal cyclogenesis downstream of the cold air masses.This type of cyclogenesis was noted by Konrad andColucci (1989) in their study of two cold outbreaks ofJanuary 1977.

The SLP anomaly patterns from which the Europeanoutbreaks evolve are very different from the patternsassociated with the North American outbreaks, as onemight expect from the earlier discussion of trajectories.Figure 9 shows that the Northern European events arepreceded at Day-10 and Day-6 by a pattern of positiveSLP anomalies over the Arctic and generally negativeanomalies in the middle latitudes of the North Atlanticand Eurasia. This pattern evolves by Day-2 into a strongvariant of the negative phase of the North Atlantic os-cillation, with anomalies of 122 mb and 216 mb overthe Norwegian–Barents Seas and the central North At-

lantic, respectively. While this couplet clearly impliesa major reduction of the normal westerly winds reachingEurope, its projection onto standard indices of the NAO(e.g., the Lisbon–Azores 2 Iceland pressure difference)would not be extremely strong because the anomalycenters are displaced considerably westward from theLisbon–Azores region and northeastward from Iceland.The displacement of the positive anomaly center to thenorthern Scandinavian coastal region optimizes the east-ward influx of cold Siberian air to Scandinavia and theremainder of northern Europe.

While the antecedent patterns for Western Europe(Fig. 10) show some similarity to those for NorthernEurope, there are several differences. First, the primarylobe of negative anomalies in middle latitudes developsconsiderably farther to the east, over the Mediterranean


FIG. 10. As in Fig. 7, but for the Western European (WE) region.

region, rather than over the central North Atlantic as inthe NE case. Second, the subpolar lobe of positiveanomalies develops earlier, becomes stronger, and iscentered farther south (between Iceland and Britain) inthe WE case than in the NE case. The North Atlanticdipole at Day-2 and Day-0 in Fig. 10 would projectstrongly and negatively onto the standard NAO indices,especially because the composite SLP anomaly centersnear Iceland have values exceeding 126 mb at Day-2and 124 mb at Day-0. Although the NAO signature isstrong in this case, again consistent with an extremeretardation (and even a reversal) of westerly airflow intoEurope, the positive SLP anomalies do not extend intothe Arctic Ocean. Hence, the projection onto the Arcticoscillation index of Thompson and Wallace (1998) willbe much weaker than the projection onto the NAO.

In order to address the linkages to the NAO and thecentral Arctic more quantitatively, indices of the NAOand Arctic sea level pressure were composited over the10 leading outbreaks (Table 1c) affecting each of theregions. The NAO index used here is based on nodalpoints that migrate seasonally to maximize the subpo-lar–subtropical correlation (Portis et al. 2001). (Thenodal locations for the calendar months November–March are listed in Table 3.) Figure 11 shows the com-posite values of the NAO for each region’s events andfor the 12 days preceding the events. For all regions,the NAO is systematically negative in the period leadingup to (and including) the cold outbreaks. The NAOvalues for the EC and GC regions of North Americaare more negative than 21.0 (standard deviations) formuch of the antecedent week; the composite mean value

15 JUNE 2001 2655W A L S H E T A L .

TABLE 3. Monthly nodal locations of the North Atlantic oscillationindex for Fig. 11.

Subpolar low Subtropical high

NovDecJanFebMar

558N, 358W608N, 208W658N, 208W608N, 358W608N, 508W

258N, 47.58W308N, 308W308N, 208W258N, 37.58W258N, 408W

FIG. 11. Composite value of the NAO index during and prior to 10 coldest outbreaks in (a) MW region, (b) EC region, (c) GC region,(d) NE region, and (e) WE region. Composite values are plotted as functions of number of days (0, 1, 2, . . . , 12) prior to coldest days ofevents in Table 1c.

reaches—1.8 for the GC events at a lead time of sixdays. The negative values are consistent with the com-posite fields in Fig. 8 and with the positive correlationsbetween winter-averaged values of the NAO index andtemperatures in Florida (Downton and Miller 1993). Forthe European regions, the composite index values aregenerally between 20.25 and 21.00, although the val-ues decrease to 21.8 for region WE over the few daysimmediately preceding the events. In this context, wereiterate that the centers of the SLP anomaly patternsmost directly linked to the European outbreaks (Figs.9–10) are generally displaced from the nodal points ofthe NAO, regardless of the definition of the NAO index.Nevertheless, it is apparent from Fig. 11 that cold out-

breaks over both continents are favored by the negativephase of the NAO.

Figure 12 is a similar summary of the antecedentArctic pressure anomalies, defined here as the averageof the gridded SLP anomalies over the region polewardof 708N. This polar cap includes the Arctic Ocean andthe northern portions of the North Atlantic subpolarseas. The plots for each region in Fig. 12 show thatpositive SLP anomalies in the central Arctic precede thecold outbreaks. Of the three North American regions,the EC shows the largest SLP anomalies (6–8 mb fromDay-8 through Day-0), while the MW region shows thesmallest anomalies, although the composite SLP anom-aly for MW does reach 15.5 mb at Day-2. The plotsin Fig. 12 for the two European regions show opposingtemporal tendencies, consistent with the compositefields in Figs. 9–10. As the lead time for the NorthernEuropean region decreases from 12 to 0 days, the ArcticSLP anomaly increases from nearly zero to approxi-mately 10 mb. The anomalies for the more southern WEregion decrease from positive values of 5–6 mb at leadtimes of 8–12 days to values close to zero in the severaldays immediately prior to (and including) the major coldevents. Thus the characteristics of the large-scale SLP


FIG. 12. As in Fig. 11, but for composite values of anomalies of sea level pressure (SLP) averaged over the central Arctic (708–908N).

anomaly pattern include positive pressure anomalies inthe Arctic and negative NAO indices. While these twofeatures imply consistent contributions to a negativeArctic oscillation index, the composite fields in Figs.7–10 indicate that the relevant features in the NorthAtlantic and the Arctic often do not strengthen in phaseand are often displaced geographically from the AOnodes of Thompson and Wallace (1998). Hence the AOindex may not be the optimum indicator of an imminentcold outbreak over the 0–12 day timescale addressedhere. The AO’s linkage to temperature anomalies overlonger timescales may well be stronger.

7. Conclusions

This assessment of major cold outbreaks over the pasthalf-century leads to several conclusions, particularlywith regard to differences between the North Americanand European cold events.

R The cold outbreaks affecting the United States aremore extreme than those affecting Europe, in termsof both the actual air temperatures and the anomaliesof air temperature.

R There is no apparent trend toward fewer extreme coldevents on either continent, at least over the period

since 1948. Longer records remain to be investigatedin this context.

R The trajectories of the coldest air are southward orsoutheastward over North America, but generallywestward over Europe. Subsidence of several hundredmillibars is typical of most trajectories of the coldestair to reach the surface in each region.

R Sea level pressure anomalies evolve consistently withthe temperatures over the 1–2 weeks prior to the ex-treme cold outbreaks. Outbreaks affecting the East andGulf Coasts of the United States appear to result fromcold air ‘‘pools’’ that break off from high-latitude airmasses; the Midwestern outbreaks appear to be moreclosely tied to the advection of cold air on the easternside of air masses remaining in more northerly lo-cations. Outbreaks over Western Europe and NorthernEurope are distinguished by subtle locational differ-ences of large anomalies of sea level pressure, butoutbreaks affecting both regions involve the retarda-tion and/or reversal of the North Atlantic westerliesimpinging upon Europe.

R Negative values of the NAO index and positive anom-alies of Arctic sea level pressures are features commonto North American as well as European outbreaks.However, the strongest SLP anomaly features asso-

15 JUNE 2001 2657W A L S H E T A L .

ciated with the outbreaks are generally shifted geo-graphically relative to the nodal locations of the NAOand the Arctic oscillation.

There is a need to investigate further the linkage be-tween the extreme outbreaks and the North Atlantic andArctic oscillations by examining longer timescales andlow-frequency variations of these (and other) telecon-nection features. In this study, we have focused on theNorth Atlantic and Arctic associations because previousstudies have emphasized associations between NorthAmerican cold outbreaks and Pacific features (e.g., Rog-ers and Rohli 1991; Downton and Miller 1993).

An additional need is the extension of the trend anal-ysis to include the decades prior to the start of theNCEP–NCAR reanalysis in 1948. The networks of his-torical stations in the United States and Europe are good(e.g., the Historical Climatology Network of the UnitedStates). The single-station results reported here suggestthat extreme outbreaks may have been relatively fre-quent in the late 1800s and early 1900s, but the spatialrepresentativeness of data from one station is open toquestion. Any such extension back beyond the 1940swill not have the benefit of the three-dimensional windfields used in the present study.

Finally, there is a need for a diagnosis of the extremeairmass formation mechanisms in terms of the under-lying physics (e.g., surface fluxes) and dynamics, par-ticularly in the context of the frequent stagnation of thetrajectories over northern land areas. The relative im-portance of thermodynamic and dynamic processes inthe formation of extreme air masses is unclear. Colucciet al.’s (1999) assessment of cooling mechanisms innumerical weather prediction models suggests that adi-abatic and diabatic processes can both contribute to thecooling to extreme temperatures. Because the reanaly-sis-derived surface fluxes are subject to large uncer-tainties at present, we have refrained from the inclusionof surface energetics in this study. As reanalysis-derivedproducts improve, such studies will be feasible. Onemay also hypothesize that 1) an absence of cloudinessplays a role in the formation of the coldest air masses(Walsh and Chapman 1998), and 2) extensive snow cov-er favors the southward penetration of extremely coldair, in accordance with the results of Ellis and Leathers(1998). With regard to 1), Curry’s (1983, 1987) modelresults indicate that low cloudiness and clear-sky ice-crystal precipitation can enhance near-surface coolingin high latitudes during winter, even to the extent thatthe diabatic cooling leads to the intensification of thesurface anticyclone. Controlled experiments are neededfor a more general characterization of the importanceof this mechanism, as well as for tests of other hypoth-eses, and for case studies of some of the extreme coldevents identified in this study. Such experiments mayalso provide a means to investigate possible feedbacksthat may contribute to the clustering of extreme cold

outbreaks in certain winters, for example, the wintersof 1962–63 and 1984–85.

Acknowledgments. This work was supported by theNational Science Foundation’s Climate Dynamics Pro-gram through Grant ATM-9905924. Access to the re-analysis output was provided by the NOAA/CIRES Cli-mate Diagnostics Center at the University of Colorado.We thank two anonymous reviewers for their helpfulcomments on the original manuscript.

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