North Atlantic Storm Track Variability and Its Association to the North Atlantic Oscillation and Climate Variability of Northern Europe

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JEFFREY C. ROGERS Department of Geography, Ohio State University, Columbus, Ohio (Manuscript received 8 January 1996, in ®nal form 4 November 1996)

ABSTRACT The primary mode of North Atlantic storm track variability is identi®ed using rotated principal component analysis (RPCA) on monthly ®elds of root-mean-squares of daily high-pass ®ltered (2±8-day periods) sea level pressures (SLP) for winters (December±February) 1900±92. It is examined in terms of its association with 1) monthly mean SLP ®elds, 2) regional low-frequency teleconnections, and 3) the seesaw in winter temperatures between Greenland and northern Europe. The principal storm track component is characterized by high synoptic variability preferring one of two areas at any given time. The northeastern Atlantic center (identi®ed by positive RPCA scores) is characterized by deep cyclones in the area extending from Iceland northeastward to the Nor- wegian and Barents Seas, whereas the Bay of Biscay center (negative scores) is linked to cyclone activity around that area and into the Mediterranean basin. Combined principal component analysis is used to link the high- frequency storm track pressure variability with that of lower frequencies (monthly mean pressures). In this, the primary storm track pattern is linked to large monthly mean SLP variations around the Bay of Biscay and near northern Scandinavia and the Barents Sea. This pattern does not suggest a strong storm track link to the North Atlantic Oscillation (NAO). Instead, the results presented indicate that the dominant mode of variability in the storm track is associated with low-frequency SLP anomalies in the extreme northeastern Atlantic. When the component scores reach their highest positive values, both the mean Atlantic subpolar low and subtropical high are unusually strong and displaced farther northeast than normal. The pressure ®eld intensi®es to the northeast and produces strong zonal ¯ow extending into Europe, bringing abnormally high surface air temperatures as far east as Siberia and below normal temperatures over Greenland and northern Africa (the ``Greenland below'' seesaw mode, GB). Besides this eastward extension of the mean pressure ®eld, anomalously high European winter temperatures can also be somewhat less frequently caused by mild return ¯ow around the Siberian high, which is displaced farther west than normal. In this situation the Icelandic low is in its normal Denmark Strait location and cyclones move along the more southerly storm track toward the Mediterranean basin, contributing to the synoptic forcing that helps develop the westward extended high. The NAO appears to be only indirectly linked to the European component of the GB mode of the winter surface air temperature seesaw.

1. Introduction tion and is linked to observed climatological and ocean- ographic variability (van Loon and Rogers 1978; Lamb Climate variability associated with changes in inten- and Peppler 1987; Moses et al. 1987; Mann and Drink- sity and location of storm tracks has not been studied water 1994; Hurrell 1995). There are, however, other to as great an extent as that associated with low-fre- Atlantic regional teleconnections, including the west At- quency components of the circulation such as standing lantic and east Atlantic patterns, identi®ed at the 500-mb waves (van Loon and Williams 1976; Shabbar et al. level (Wallace and Gutzler 1981); the NAO; east At- 1990), blocking ¯ows (Rex 1951; Namias 1964, 1978; lantic and Eurasian patterns at 700 mb (Barnston and Dickson and Namias 1976; Lejenas 1989), and atmo- Livezey 1987); and three additional sea level pressure spheric teleconnection patterns. The latter constitute a patterns beside the NAO (Rogers 1990). set of slowly varying circulation features that retain their It is well established that synoptic-scale eddy activity identities on monthly charts, each having speci®c spatial is largest downstream of the major stationary wave centers of action. In the lower troposphere over the troughs. For example, the amplitudes of the variance North Atlantic, the North Atlantic Oscillation (NAO) is statistics of bandpass ®ltered (2.5±6-day periods) typically regarded as the primary regional teleconnec- 500-mb heights are characterized by zonally elongated maxima extending across the western ocean basins from the east coasts of North America and Asia, representing areas of high temporal variability in geopotential heights Corresponding author address: Dr. Jeffrey C. Rogers, Depart- ment of Geography, Ohio State University, Columbus, OH 43210- and preferred trajectories of weather systems (Blackmon 1361. et al. 1977; Blackmon et al. 1984). Cai and Van den E-mail: [email protected] Dool (1991) demonstrate that storm tracks also occur

᭧1997 American Meteorological Society

Unauthenticated | Downloaded 09/26/21 02:59 AM UTC 1636 JOURNAL OF CLIMATE VOLUME 10 ahead of troughs associated with traveling low-fre- in the pure sense. Serreze's (1995) automated cyclone quency waves. These waves are associated with monthly detection and cyclone tracking algorithm represents an averaged circulation anomalies that are frequently as- improvement on trajectory methods and is used on sociated with modes of speci®c teleconnection patterns. twice-daily gridded National Meteorological Center Lau (1988) showed that teleconnection patterns found (NMC, now known as the National Centers for Envi- in monthly time averages are linked to storm tracks. For ronmental Prediction) analyses in order to examine cli- example, he linked large latitudinal/meridional displace- matological characteristics of cyclones and their trajec- ments in the easternmost or downstream portion of the tories. Lau (1988) identi®es storm track modes by ap- Atlantic 500-mb storm track to the east Atlantic tele- plying empirical orthogonal function (EOF) analysis of connection pattern, while baroclinic activity in the west- the monthly root-mean-square statistics of bandpass ernmost or upstream portion of the storm track is linked (2.5±6 day) ®ltered twice-daily 500-mb geopotential to the west Atlantic pattern. heights. The unique patterns in the variance statistics Relatively few papers have addressed how storm track represent a simple way to evaluate storm tracks, in- variability is associated with regional variability of cli- volving fewer arbitrary decisions than traditional tra- mate, as measured by monthly mean ®elds of surface jectory (manual) methods (Wallace et al. 1988), but cre- air temperature, precipitation, and pressure. This may ating proxies of the cyclone tracks (Anderson and Gyak- partly be due to dif®culty in applying cyclone frequency um 1989) that do not identify individual cyclone and and trajectory datasets, and interpreting results from anticyclone centers as might be required when diag- them, and due to an emphasis on monthly mean cir- nosing instantaneous weather conditions. Wallace et al. culation anomalies in studies of climate variability. The (1988) point out that both cyclones and anticyclones are recent development of automated cyclone tracking al- associated with the maxima in the bandpass ®ltered pres- gorithms (Serreze 1995) and the use of second-moment sure ®eld variances. Although the usage of ``storm statistics of pressures ®ltered over synoptic timescales tracks'' is somewhat misleading in describing the prod- (Lau 1988) has helped focus interest on the role of storm uct of variance methodologies, the isolated patterns have tracks in climate variability. For example, Rogers and been found to closely match those found by manual Mosley-Thompson (1995) link recent increases in storm methods. Hence, in this study, the phrase ``storm track'' activity in the Barents and Kara Seas to unusually mild refers to high-frequency ¯uctuations in the ®ltered pres- winters of the 1980s in Siberia, and Serreze et al. (1997) sure ®elds rather than to the trajectory of individual examine the synoptic characteristics associated with the cyclones. mean Icelandic low and recent atmospheric circulation changes. 2. Data and methodology The purpose of this paper is 1) to identify the primary mode of Atlantic storm track variability, 2) to show how Northern Hemisphere gridded daily and monthly it is related to low-frequency teleconnections, and 3) to mean sea level pressure (SLP) data are used. The data relate it to regional climate variability in northern Eu- are available at every 5Њ of latitude and longitude from rope and elsewhere. The paper argues for a new inter- 20Њ to 85ЊN for the period November 1899±March 1992. pretation of how atmospheric circulation variability is Gridded maps are available once daily for either 1300Z linked to a well-known regional climatic phenomenonÐ (from 1899 to 1939) or 1200 UTC and are available the seesaw in winter air temperatures between Green- twice daily (0000 and 1200 UTC) from 1955±56 land and northern Europe. The air temperature seesaw through 1959±60 and for all winters starting with 1962± was ®rst qualitatively linked to the North Atlantic Os- 63. Daily maps are missing from December 1944±De- cillation by van Loon and Rogers (1978, hereafter vLR), cember 1945, but monthly charts are available for cal- and the assumed linkage has often been repeated. The endar year 1945. Other than this 13-month period, miss- Atlantic storm track and its long-term variability is ex- ing daily data were replaced by pressure averages of the amined with a historical sea level pressure dataset ex- day prior and the day after. The different SLP data tending back to 1899. The analysis is performed (see sources are listed in Table 1 of Trenberth and Paolino section 2) so as to obtain both a spatial representation (1980), who extensively examine the monthly SLP data of the storm track pattern plus a long-term temporal for errors, inhomogeneities, trends, and discontinuities. index of the strength and polarity of the storm track. Monthly mean surface air temperature data on a 5Њ The storm tracks are also analyzed in conjunction with lat ϫ 10Њ long grid (Jones et al. 1991) for 1899±1990 composites based on monthly mean pressure ®elds, in are used. Monthly surface air temperatures are also used which the mean lows can generally be associated with for Oslo (59.9ЊN, 10.7ЊE), Norway, and Jakobshavn the site of most frequent cyclone passages and even most (69.2ЊN, 51.0ЊW) and Egedesminde (68.7ЊN, 52.8ЊW) frequent cyclogenesis and cyclolysis (Serreze et al. in western Greenland. Temperature departures at the 1997). latter two stations are combined to make a complete Storm tracks can be identi®ed by following low pres- western Greenland record, necessitated by a lack of data sure centers on synoptic charts and plotting their tra- at Jakobshavn after 1970. jectories on maps, thereby producing ``cyclone tracks'' Once-daily gridded SLP data, spanning the period 27

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FIG. 1. A 120-winter-month (1951±90) average of the rms of high-pass ®ltered daily sea level pressures (in mb). Climatological data sites mentioned in the text are represented by dots in western Greenland, at Oslo, the Azores, and at gridpoint 65ЊN, 20ЊW in Iceland.

November±4 March from 1899±1900 through 1991±92, the ®rst component, and are not examined further here. were high-pass ®ltered using a binomial ®lter with Components beyond number 2 had fewer similarities weights Ϫ0.0625, Ϫ0.25, ϩ0.625, Ϫ0.25, and among the three analyses. The ®rst component pattern Ϫ0.0625Ðthe weights associated with the low-pass bi- explained between 27% and 29% of the total dataset nomial ®lter of n ϭ 4 (1-4-6-4-1). The ®lter has max- variance in all three analyses. imum response in the 2±8-day periodicity range, typi- A combined principal component analysis (CPCA) cally associated with passage of synoptic systems. The was performed, with rotation, to examine the spatial rms of the high-pass ®ltered data are then obtained for interrelation between high-pass ®ltered SLP variability each winter month. A rotated principal component anal- and that of monthly mean SLP ®elds. CPCA helps an- ysis (RPCA) is performed on Atlantic monthly rms val- alyze the relationship within and among spatially and ues extending from 80ЊWto20ЊE and from 30Њ to 70ЊN, temporally large datasets (Bretherton et al. 1992). The following procedures outlined by Barnston and Livezey methodology was found to extract coupled patterns be- (1987) and Rogers (1990). The input data for the RPCA tween datasets somewhat more accurately than canon- are not areally weighted. ical correlation analysis and singular value decompo- RPCA was also performed on 1) monthly rms of sition (Bretherton et al. 1992), and Wallace et al. (1992) 1948±92 once-daily SLP data in order to compare the similarly ®nd CPCA comparable to singular value de- 93-yr analysis to potentially more accurate recent daily composition as a methodological tool. Monthly mean data, and 2) on the monthly rms of twice-daily SLP SLPs used in the analysis are from Rogers (1990), ex- data. The latter evaluation employs Blackmon's (1976) cept that they are updated to 1992 and exclude data for 31-weight bandpass ®lter designed for twice-daily data 1945, conforming with the rms dataset. Monthly pres- that emphasizes 2.5±6-day periods. The ®rst two com- sures are on a 20Њ long ϫ 5Њ lat, 13 ϫ 11 grid extending ponent patterns of the additional RPCA analyses closely from 160ЊE eastward to 40ЊE. The grid could not include replicate spatial patterns and temporal score variations Eurasia due to missing data before 1945. The CPCA is found in the 1899±1992 analysis. The ®rst component performed after standardizing the monthly data on both pattern and its relation to regional climate variability is grid sets by creating departures from normal and then the subject of this paper. The second component pattern dividing by the monthly standard deviations. in each analysis exhibits a center of maximum pressure variability west of Greenland, similar to the second cy- 3. The Atlantic storm track and SLP analyses clone track pattern described by Serreze (1995). The time series scores of this component are not signi®cantly a. Climatology of the rms high-pass ®ltered SLP correlated to monthly mean surface air temperatures Figure 1 shows the mean rms of high-pass ®ltered elsewhere in the North Atlantic region, unlike those of SLPs, averaged over 120 winter months from December

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FIG. 2. Time series of scores associated with the ®rst rotated principal component of the monthly rms ®elds of high-pass ®ltered sea level pressures. Monthly (thin solid line) and seasonal (thicker solid line) values are shown, including zero values for the missing data from December 1944 through 1945.

1950 through February 1990. The mean rms exceeds 6 RPCA storm track pattern. Differences in monthly mean mb over the northeastern United States, eastern Canada, rms values (Fig. 3a), obtained by subtracting the mean and into the west-central Atlantic, reaching a maximum rms distribution during negative cases from those of of 6.7 mb at grid point 45ЊN, 60ЊW near Nova Scotia. positive cases, have a spatial pattern very similar to that This maximum is very near that found by Blackmon et displayed in the rotated principal component loadings. al. (1977, their Fig. 2b) at sea level and by Lau (1988, The largest mean rms variations form a dipole (Fig. 3a) his Fig. 1) at 500 mb. The general con®guration of rms with centers in the extreme northeastern Atlantic and values above 4 mb is similar to that found in these earlier Norwegian Sea, where the net mean rms differences studies although the area in Fig. 1 extends farther north exceed 4 mb, and over the eastern Atlantic around Por- and east toward the Barents Sea than Lau's rms maxima. tugal. The mean rms differences between the two da- The 5-mb contour is oriented northwest±southeast over tasets are statistically signi®cant at the 95% con®dence the Canadian Plains east of the Rocky Mountains, and interval, based on a two-tail t-test, over large areas of in the Atlantic it extends to Iceland and eastward toward the northern Atlantic from 50ЊWto50ЊE and from 55ЊN the United Kingdom. The mean rms values in the sub- to 80ЊN, as well as over southern Siberia at 55Њ±60ЊN. tropical Atlantic are generally lower than 2 mb south The composite mean rms for the 32 largest positive of 30ЊN. The Paci®c basin rms maxima reaches 5.8 mb cases (Fig. 3b) exceeds 7 mb over Newfoundland and at 45ЊN, 170ЊE and does not extend as far northeastward Labrador, and in the area around Iceland, with values into high latitudes as in the Atlantic. over 5 mb from the East Greenland Sea northeastward to Novaya Zemlya. The rms maximum exceeds that of the long-term mean (Fig. 1) by over 2 mb in the north- b. The storm track pattern scores eastern Atlantic. The composite rms for the 27 negative The scores of the ®rst rotated principal component cases (Fig. 3c) exhibits a maximum near Newfoundland, are a time series of the primary mode of Atlantic month- extending from Maine to nearly the southern tip of ly rms ®elds of high-pass ®ltered SLPs. Monthly RPCA Greenland, and comparatively low values in the north- scores are standardized and have numerical values rang- eastern Atlantic, only reaching 3±4 mb. The main axis ing from Ϫ2.1 to ϩ3.3 (Fig. 2). Using an arbitrary cutoff of the rms maxima is oriented toward the Bay of Biscay score of Ϯ1.25, the 32 highest positive and 27 lowest and the Mediterranean basin. negative monthly scores are identi®ed and used to il- The rms values in the southern dipole center west of lustrate spatial changes associated with extremes of the Portugal (Fig. 3a) range from 2 to 3.5 mb in the positive

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FIG. 3. Composites of monthly rms (in mb) for sets of months with extreme opposite modes of the ®rst principal component of Atlantic area rms ®elds of high-pass ®ltered sea level pressures, 1900±92. The three diagrams include the (a) net mean rms differences (mb) between the (b) composite positive mode cases and (c) the composite negative cases. mode cases (Fig. 3b), but they are 3 to 4.5 mb in the (1021 mb) and lies in the south-central North Atlantic negative mode cases (Fig. 3c). The net mean rms dif- near 25ЊN, 45ЊW (Fig. 4a). A trough of comparatively ferences around the dipole center (Fig. 3a) are statis- low pressure extends toward the Bay of Biscay and tically signi®cant with 95% con®dence from 0Њ to 30ЊW across southern Europe and the Mediterranean Sea. This and 30Њ to 45ЊN, due to small rms variability at these case corresponds primarily to that of Fig. 3c with com- latitudes in the cases that make up the composites. paratively higher than normal mean rms values over the Composite means of raw monthly SLPs were obtained east-central Atlantic, suggesting an active storm track after stratifying the RPCA scores into ®ve groups sep- toward Portugal and the Mediterranean and a weak sub- arated at points corresponding to numerical score values tropical high. of Ϫ1.0, zero, ϩ1.0, and ϩ2.0 (Fig. 4). This data strat- In months with the highest positive RPCA scores i®cation illustrates changes occurring in mean intensity (Fig. 4e), both the mean subpolar low and the subtrop- and spatial locations of Atlantic centers of action as ical high extend farther northeast of normal. The mean score values change. The composite mean for the set of subpolar low is 994 mb at 70ЊN10ЊE in the Norwegian± months with scores between zero and ϩ1.0 (Fig. 4c) Barents Sea area, with pressure under 996 mb as far resembles closely the long-term mean Atlantic SLP ®eld with a minimum of 996 mb over the Denmark Strait east as 50ЊE near Novaya Zemlya. The highest rms val- and a subtropical maximum near the Azores at 30ЊN, ues of high-pass ®ltered pressures occur (Fig. 3b) over 30ЊW with a central pressure of just over 1024 mb. Iceland and farther northeast suggesting that cyclone In months with scores lower than Ϫ1.0 (Fig. 4a), the activity proceeds northeastward into the Norwegian and subtropical high and Icelandic low are weaker than nor- Barents Seas and even farther east in these cases. The mal and shifted to the south and west of their mean subtropical high extends northeast of normal, well over positions (Fig. 4c). The mean low (1001 mb) extends the Mediterranean Basin, with a maximum pressure of over a large area southeast of Greenland and relatively 1028 mb. The mean SLP is 1008±1012 mb around the high pressure between 1010 and 1016 mb occurs across Bay of Biscay in Fig. 4a but it is about 1026 mb in Fig. the northeastern Atlantic and the Barents and Kara Seas 4e. Shading in Fig. 4a shows that SLPs are signi®cantly to Novaya Zemlya. The Azores high is relatively weak different between Figs. 4a and 4e in areas centered over

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FIG. 4. Composites of Atlantic mean sea level pressures (mb) for subset groups when the monthly scores (Fig. 2) of the ®rst principal component monthly rms of high-pass ®ltered sea level pressures are (a) lower than Ϫ1.0; (b) between Ϫ1.0 and zero; (c) between zero and ϩ1.0; (d) between ϩ1.0 and ϩ2.0; and (e) for cases higher than ϩ2.0. Lighter and darker shading represent areas where the differences in pressure are statistically signi®cant with 95% and 99% con®dence between different combinations of maps. Shading in (b) through (e) represents signi®cant differences with the preceding map while shading in (a) represents signi®cant differences between (a) and (e). southern Europe and northern Europe and the north- with extreme positive scores (Fig. 5a), daily pressures eastern Atlantic. are most frequent between 995 and 1000 mb (998 mb It is apparent from Fig. 4 that with increasingly pos- mean), and they are most frequent between 1005 and itive RPCA scores 1) the mean subpolar low intensi®es 1010 mb (1009 mb mean) in months with negative as it shifts to the northeast, 2) the subtropical high in- scores (Fig. 5b). The range of daily pressures is roughly tensi®es and migrates northeastward of its mean posi- the same in both sets of extreme cases. However, in tion, 3) the pressure gradient between the centers of months with the lowest negative scores (Fig. 5b) there action intensi®es as they shift northeastward, and 4) the is a greater frequency of occurrence of pressures be- storm track shifts from a northwest±southeast orienta- tween 1015 and 1025 mb than in the positive cases (Fig. tion (for low negative scores) to a southwest±northeast 5a) and far fewer pressures under 995 mb. orientation, extending deep into the high Arctic. The The day-to-day changes in SLP are examined during eastward shift in the subpolar and subtropical SLP ®elds different categories of storminess as de®ned in the in- is apparent in the eastward shifts in areas of statistically dex. During the 27 months of lowest scores, 20 pairs signi®cant pressure differences in Figs. 4b±d. of days show an SLP change that exceeds 20 mb from A further example of the nature of synoptic variability one day to the next (Fig. 5d), whereas daily pressure is presented in Fig. 5, showing the frequency distribu- changes of less than 6 mb account for over half of the tion of daily raw pressures and absolute daily pressure pairs of days. The daily absolute pressure change ex- differences at 65ЊN20ЊW, a grid point over Iceland with ceeds 20 mb on 203 pairs of days in positive months large differences in monthly rms values (Fig. 3a) and (Fig. 5c), about 21% of all cases. In summary, the daily mean pressures (Figs. 4a and 4e). During 32 months absolute pressure changes at 65ЊN, 20ЊW tend to be

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and 5b, as well as those occurring between the extremes in monthly means in Figs. 4a and 4e, may be partly due to interannual changes in the background low-frequency (Ͼ8 days) circulation. The higher-frequency synoptic activity in a given month is superimposed upon a pre- vailing background low-frequency climatological mean monthly pressure ®eld. Figure 5 suggests that, while there may be a contribution by the background climatological mean pressure ®eld, the low mean pressures at 65ЊN, 20ЊW of the positive RPCA score months are accompanied by sizeable day-to-day pressure changes and many more daily pressures under 1000 mb. Con- versely, low score cases have smaller daily pressure changes with higher mean pressures, suggestive of large, slow moving anticyclones. The results of Fig. 5 are FIG.4.(Continued) consistent with the ®ndings of Serreze et al. (1997) that a considerable increase occurs in the number of cyclones in the Denmark Strait area when the mean Icelandic low higher (lower) when the mean monthly rms of high-pass is unusually deep. ®ltered SLPs are higher (lower) and when the monthly mean SLP is lower (higher). The distributions of Figs. 5a and 5b, and 5c and 5d, were statistically different c. Association to low-frequency teleconnections from each other with above 99% con®dence, estimated using the chi-square test. A combined principal component analysis (CPCA) The differences in daily pressures between Figs. 5a was used to determine the spatial interrelationships be-

FIG. 5. Frequencies of once-daily pressures at 65ЊN, 20ЊW during winter months when the ®rst principal component of monthly rms of high-pass ®ltered pressures has extreme (a) positive and (b) negative scores and frequencies of absolute day-to-day pressure changes at 65ЊN, 20ЊW during the same (c) positive and (d) negative months.

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maximum rms difference is not as large (about 3.6 mb), and 2) it is shifted slightly eastward over the Norwegian Sea at 65ЊN, 5ЊE. The monthly mean SLP difference ®eld in the CPCA (Fig. 6) exhibits a maxima over the Bay of Biscay and over northern Scandinavia and the Barents Sea, areas where the pressure variations between Figs. 4a and 4e are among the largest with high statistical signi®cance (Fig. 4a). Note that the pressure anomaly pattern indicates that cold northerly ¯ow would occur over western Greenland and milder maritime ¯ow over northern Europe. The spatial dipole pattern of SLP in Fig. 6 does not resemble that typically associated with the NAO in which centers occur near the Azores and the Denmark Strait near Iceland. Instead the southern center in Fig. 6 is near the Bay of Biscay and the FIG. 6. The mean sea level mean pressure differences (mb) oc- northern center is considerably northeast of Iceland. Ar- curring between the months with extreme positive and negative scores eas of statistical signi®cance in Figs. 4a and 6 are pri- of the ®rst rotated pattern of the combined principal component analysis (CPCA) of monthly Atlantic root-mean-squares of high-pass ®l- marily located farther east of the Azores and Iceland, tered SLP and monthly mean sea level pressure, 1900±92. Lighter areas where three other teleconnections identi®ed by and darker shading represent areas where the differences in pressure Rogers (1990) have centers of action. are statistically signi®cant with 95% and 99% con®dence.

d. The seesaw in winter surface air temperatures tween monthly high-pass ®ltered data and monthly mean SLPs, and to examine the degree to which the storm 1) STRATIFICATION OF SEESAW EVENTS USING THE track information can be linked to known atmospheric STORM TRACK SCORES teleconnections in monthly pressure ®elds. The CPCA Winter means of the storm track scores (Fig. 2) were produces a single time series of scores but eigenvector correlated to gridded hemispheric winter seasonal sur- loadings for both datasets. face air temperatures spanning 1900±90 (Jones et al. The correlation between scores of the monthly rms 1991). Statistically signi®cant coef®cients of correlation of high-pass ®ltered SLPs (Fig. 2) and the scores for (Fig. 7) have maximum positive values over Ireland, the the CPCA is r ϭϩ0.912 for n ϭ 275 months (signif- United Kingdom, and southern Scandinavia extending icance exceeding 99.9%). As such, the rms ®eld load- eastward into north-central Asia between 55Њ±75ЊN and ings (not shown) for the CPCA are similar to those of 70Њ±100ЊE. Positive correlations imply that an anoma- Fig. 3a with only two slight differences: 1) the mean lously northeastward extension of the storm track (Fig. 3b) is associated with higher than normal winter surface air temperatures over Europe and Eurasia (Rogers and Mosley-Thompson 1995) and below normal air temperatures over western Greenland, Baf®n Island, the Mediterranean Basin, and northern Africa. Conversely, Africa and the Mediterranean have above normal temperatures when the storm track scores are negative, occurring as storms migrate toward the Mediterranean basin and during which northern Europe and Eurasia have unusually cold winters. The winter air temperature seesaw between western Greenland and northern Europe can be identi®ed in Fig. 7. The seesaw is characterized by two temperature anomaly modes named ``Greenland below'' (GB) and ``Greenland above'' (GA), referring to the western Greenland temperature anomaly, and in which the northern Europe (represented by Oslo) temperature anomaly has the opposite sign. In keeping with vLR, monthly FIG. 7. Spatial distribution of coef®cients of correlation between seesaw extreme GB and GA events occur if the sign of RPCA scores of the Atlantic storm track eigenvector and gridded the western Greenland temperature anomaly is opposite winter mean air temperatures for land areas of the Northern Hemi- sphere, 1900±90 (from Jones et al. 1991). Correlation coef®cients of that at Oslo, with an absolute temperature anomaly dif- r ϭϮ0.32, r ϭϮ0.44, and r ϭϮ0.55 are signi®cant at the 95%, ference between them (Greenland minus Oslo) exceed- 99%, and 99.9% con®dence levels. ing 4ЊC.

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2) SLP ASSOCIATED WITH GB AND POSITIVE STORM TRACK ANOMALIES Mean SLP composites are created for the 30 GB cases when the storm track scores are greater than ϩ0.5 and for 24 cases when the scores were negative, ignoring nine ``overlap'' cases with scores between zero and ϩ0.5. The positive score months (Fig. 9a) are characterized by broad subpolar low pressure with centers west of Iceland and over the Norwegian Sea. Pressures under 1000 mb extend to Novaya Zemlya and the isobars across the double low extend zonally into Europe. The Atlantic subtropical high, as measured by the 1024-mb isobar, extends farther northeast than usual, and strong maritime westerly ¯ow extends well into Europe and Asia. The entire pattern is typical of high RPCA scores in Figs. 4d and 4e. A deep low also occurs near Iceland in GB cases with negative scores (Fig. 9b), but the isobars to the east generally lie parallel to the Scandinavian coast. Com- paratively high pressure covers the Barents and Kara Seas. The Siberian anticyclone, as measured by its 1020-mb isobar, spreads much farther north and west in Fig. 9b than it did in Fig. 9a, while the Atlantic subtropical anticyclone is displaced west, over the mid- ocean basin. Mean SLP differences between these modes (Fig. 9c) consist of a dipole with centers over eastern Europe and southwest of Ireland with a strong pressure gradient between the two centers lying across much of Scandi- navia and northern Europe. The eastern European dipole FIG. 8. Frequencies of RPCA storm track scores, at increments of 0.5, during individual winter months 1900±92 when the Greenland (Fig. 9c) is a center of anomalous high pressure in the above (GA) and Greenland below (GB) modes occur because of the GB±negative score months (Fig. 9b), and the ¯ow seesaw in winter air temperatures between Greenland and northern around this anticyclonic anomaly produces an anoma- Europe. lous southeasterly ¯ow over northern Europe (Fig. 9c) in conjunction with the above-normal surface air tem- The storm track scores are negative in 55 of 67 GA peratures. This mild southeasterly return ¯ow on the winter months since 1899 (Fig. 8a), with scores most time-averaged charts, such as Fig. 9b, occurs during frequently falling between Ϫ0.5 and Ϫ1.5. Cyclone ac- periods of westward extension of the Siberian anticy- tivity is concentrated near southern Greenland in these clone. The westward extension of the Siberian anticy- cases, and much higher mean pressures occur over clone is a well-known synoptic feature among mete- northern Europe (Figs. 4a and 4b). On the other hand, orologists in southern Europe. Makorgiannis et al. the storm track index values have a wider distribution (1981) obtained mean SLPs, 500-mb heights, and 1000± across the 63 GB events with 24 negative cases and 500-mb thicknesses for 20 winter cases when the Si- only 39 positive (Fig. 8b). The two distributions differ berian high was displaced to the west and found 1) the signi®cantly from each other with 95% con®dence. His- westward extension develops due to negative vorticity tograms such as these (Fig. 8) were constructed indi- advection aloft (it is not entirely due to radiational cool- vidually for temperature anomalies greater than absolute ing), 2) cyclogenesis in the Bay of Biscay and Medi- 4ЊC at Greenland and at Oslo, ignoring the seesaw cri- terranean basin often accompanies synoptic develop- teria, and were then strati®ed by storm track scores. The ment leading to a westward extended Siberian high, and tendency for a broader distribution of storm track scores 3) much of western Europe, and particularly north- across positive temperature anomalies is very predom- western Europe, undergoes signi®cant warm air advec- inant at Oslo (not shown), more so than is shown in tion during the westward extension of the anticyclone. Fig. 8b, but it occurs to a much lesser extent for the Note that 2) is consistent with the synoptic development negative temperature anomalies at Greenland. The re- found here for negative score cases, whereas 3) is con- sults suggest that, like GA cases, about 40% of GB cases sistent with above-normal temperatures in northern Eu- have large synoptic activity along the southern dipole rope. The northern European dipole center in Fig. 9c is storm track. in the same location where positive correlation coef®-

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FIG. 9. Mean sea level pressures (mb) when the Greenland below seesaw mode occurs and the RPCA storm track scores are (a) positive and (b) negative. (c) The net pressure differences, (b) minus (a), between those sets of cases. Mean sea level pressures are also shown (d) for the Greenland above seesaw cases that occur with negative RPCA storm track scores and for (e) the net pressure differences for the sets of cases, (d) minus (b). Lighter and darker shading in (c) and (e) represent areas where the differences in pressure are statistically signi®cant with 95% and 99% con®dence. cients of Fig. 7 are somewhat lower than at other points the sense of the subpolar low over the Denmark Strait, between Ireland and eastern Siberia, suggesting that in seems to play little direct role in above-normal winter this area another mechanism beside strong zonal ¯ow air temperatures over northern Europe. (and positive RPCA scores) is linked to higher than There is virtually no difference in the pressure dis- normal winter air temperatures. tribution (Fig. 9c) over the western Greenland seesaw The dipole centers in Fig. 9c are not the standard center between the two cases. For the GB cases, the centers of action of the NAO, having positions similar deep Icelandic low over the Denmark Strait brings to those from the CPCA in Fig. 6. Pressure differences northerly or northeasterly ¯ow across western Green- over Iceland and the Denmark Strait are not even sta- land and the Davis Strait for both the positive (Fig. 9a) tistically signi®cant. In a climatic context, the results and negative (Fig. 9b) cases. suggest that two separate synoptic settings and time- mean ¯ow patterns are linked to mild winter months in 3) SLP ASSOCIATED WITH GA AND NEGATIVE the northern European segment of the winter temperature seesaw. The maritime ¯ow producing mild condi- STORM TRACK ANOMALIES tions at Oslo in Fig. 9a is conditional on the extension Composite mean pressures are also obtained during of the low pressure into the Norwegian and Barents Seas the 55 Greenland above (GA) winter months, when the and extending into northern Europe. In Fig. 9b, the ¯ow storm track scores are negative (Fig. 9d). The mean is more southeasterly because of the westward extension Icelandic low is weak and displaced south of Greenland of the Siberian high: the impact on abnormally high with a trough of low pressure extending northwestward temperatures in Europe may primarily be due to the over the Davis Strait. The 1008-, 1012-, and 1016-mb Siberian anticyclone extension. The Icelandic low, in isobars imply southeasterly ¯ow and a trough over west-

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to those of Lau's (1988) 500-mb analysis that identi®ed four Atlantic storm track modes. Lau's ®rst mode (`À1'') was a dipole pattern with centers located about 5Њ south and 10Њ±20Њ west of the dipole identi®ed in this study (Fig. 3a), and represented ``northward or southward migration of the storm tracks from their time mean position'' (Lau 1988). In this study, the large latitudinal divergence in the storm tracks is especially no- ticeable over the central and eastern portion of the ocean basin and over Europe, with either a southwest±northeast oriented track to the Barents Sea or a northwest± southeast oriented track toward the Mediterranean basin. Lau also identi®ed an `À2'' pattern, representing an in situ strengthening or weakening of eddy activity over the Labrador region rms maximum in high-pass ®ltered FIG.9.(Continued) pressures (Fig. 1). The changing strength of eddy activity is not identi®ed in a separate storm track pattern in this study, appearing to be part of the apparent in- ern Greenland, a situation often accompanying GA creases in eddy activity as the storm track becomes more west-coastal above-normal winter air temperatures active to the northeast (Figs. 4d and 4e). (Rogers 1985). A trough over the Norwegian Sea is very This study shows that the GB mode of the winter air weak and high pressure extends westward over much temperature seesaw can be explained by two separate of Europe. sea level circulation patterns. Europe has mild winters The pressure differences obtained by subtracting GB 1) when the storm track and the mean subpolar low (Fig. 9b) from GA (Fig. 9d), when the RPCA scores extend into the Norwegian and Barents Seas bringing are negative, is shown in Fig. 9e. This pattern appears strong maritime zonal ¯ow far into northern Europe similar to the NAO, with centers near Iceland and the (Fig. 9a), and 2) when the storm track does not extend Azores, and areas of statistical signi®cance over the beyond Iceland, and the mean low lies over the Denmark Denmark Strait and central Atlantic. The pattern cor- Strait with isobars parallel to the Scandinavian coast relation between Fig. 9e and that of the winter NAO (Fig. 9b). The latter case is the less common, but in (Fig. 2a in Rogers 1990) is r ϭ 0.865 across 76 grid these winters the NAO centers of action are near their points common to both ®gures. Pressure differences of normal ocean basin positions (Fig. 9b) and a westward- 18 mb occur in the Denmark Strait and 8 mb near 35ЊN, extended Siberian anticyclone assists in producing a 25ЊW. The elongated maximum of 6±8 mb extending strong southeasterly GB-event ¯ow into northern Eu- northeastward of the Black Sea is the net result of the rope. The ®rst case, with strong European zonal cir- westward-extended Siberian anticyclone in Fig. 9b and culation, has long been considered (vLR and others) the its absence in Fig. 9d. Figure 9e suggests that the tra- NAO-based cause of GB events. This is indeed the more ditional NAO, with centers near Iceland and the Azores, frequent mechanism for GB events (Fig. 8), but it is is embedded in its entirety in the realm of negative and brought about by the strong northeastward-extended weakly positive (Ͻϩ1.0) scores in the storm track in- storm track with a deep trough in the Norwegian Sea dex. The highest positive scores (Ͼϩ1.0; Figs. 4d, 4e, occurring in conjunction with northeastward movement and 9a) are instead cases when the storm track intensi®es of the subtropical high. This case of maritime ¯ow is to the north and shifts farther east, having no single arguably linked to a non-NAO eastward extension of teleconnection clearly linked to it. The implied geo- Atlantic cyclone activity. strophic ¯ow variations around the Denmark Strait cen- The question of whether the NAO has an atmospheric ter indicate a stronger northerly (southerly) ¯ow in GB circulation and climatic imprint extending well into Eu- (GA) over Greenland. The SLP anomalies illustrated in rope hinges on whether the Atlantic subpolar low is Fig. 9e represent the GA cases and cold ¯ow over Eu- really `Ìcelandic'' when there is a Norwegian or Barents rope would originate in the northeasterly ¯ow across Sea pressure minima characterized by either a single the Barents Sea and into Scandinavia. mean low or a deep extension of the primary low near Iceland. On the long-term climatological charts, the win- 4. Discussion tertime Icelandic low almost always appears over the Denmark Strait with a trough extending to the northeast. This study has shown the characteristics of sea level The strength of the trough is at issue, re¯ecting the North Atlantic storm track variability, along with its amount of eddy activity occurring in the extreme north- association to teleconnection patterns found on monthly eastern Atlantic. This study has linked high rms scores mean charts and to surface air temperature variability (Ͼϩ1) with 1) high rms variability in the extreme north- over northern Europe. The procedures used are similar eastern Atlantic (Fig. 3b), 2) deep mean low pressure

Unauthenticated | Downloaded 09/26/21 02:59 AM UTC 1646 JOURNAL OF CLIMATE VOLUME 10 over the Norwegian and Barents Seas at the expense of with high component scores are associated with a rel- a separate low over the Denmark Strait (Figs. 4d and atively high number of days with pressure under 1000 4e), and 3) strong zonal ¯ow into Europe linked to mb (Fig. 5), and with comparatively large interdiurnal above-normal surface air temperatures as far east as pressure changes (over 20 mb in 24 h) and relatively Siberia (Fig. 7; see also Rogers and Mosley-Thompson low monthly mean pressures. 1995). The storm track component scores (Fig. 2) vary some- Finally, comparison is made between the results of what randomly through the twentieth century, unlike the this study and the cyclone trajectories obtained in the NAO index (Rogers 1984), and are signi®cantly posi- extremes of other SLP low-frequency teleconnections tively correlated to winter air temperatures over a region described in Rogers (1990). The cyclone tracks in the from Europe to Siberia. These correlations also show extremes of the NAO (Rogers 1990; his Figs. 8a and the seesaw in winter air temperatures between Green- 8b) are characterized by large latitudinal differences land and northern Europe (Fig. 7). The Greenland above over the central Atlantic. The NAO positive mode has mode of the winter temperature seesaw primarily occurs maximum cyclone frequency near the Denmark Strait in months with negative storm track scores (Fig. 8), with few cyclones occurring east of Iceland, and the indicating that the storm track is oriented toward the cyclones have a path toward the Bay of Biscay in the Mediterranean basin with high pressure over the north- NAO negative mode but they do not enter the Medi- eastern Atlantic Arctic. The GB mode of the temperature terranean basin. Each of the other three Atlantic sector seesaw typically occurs in months with positive storm sea level teleconnections (Rogers 1990; Figs. 8c±h) track scores, but in 40% of the cases the monthly scores have one polarity mode characterized by a pronounced are negative. With positive scores (Fig. 9a), mild Eu- cyclone frequency maximum to the east or northeast of ropean winters (GB) are due to strong maritime ¯ow Iceland (along with a northeastward-extended mean sub- associated with the northeastward-extended storm track polar low), while in the other phase there is a tendency and anomalous low pressure in the Norwegian Sea. With for cyclones to penetrate into the Mediterranean. The negative scores (Fig. 9b), the Icelandic low is con®ned results of this and the earlier study suggest that while to the Denmark Strait, and mild southerly or south- the NAO is linked to the latitudinal variability in the easterly ¯ow occurs east of the low assisted predomi- central Atlantic, the bulk of the variability in the storm nantly by a westward extension of the mean Siberian anticyclone (Makorgiannis et al. 1981). The strong mar- track is in the easternmost portion of the basin and is itime ¯ow in positive score cases has long been asso- associated with low-frequency changes in SLP that are ciated with the NAO but it is argued here (section 4) con®ned to that region. The NAO also seems con®ned that this is not so. The differences in mean SLP between to negative scores (GA cases; Figs. 4a and 9d) and those the sets of GB months with differing score polarities near zero or slightly positive (GB cases; Figs. 4c and (Fig. 9c) are not signi®cant near Iceland and the Den- 9b). mark Strait, the key northern center of the NAO. Instead the strong zonal case (Fig. 9a) represents the situation 5. Conclusions in which the storm track extends far into the northeastern Atlantic with both the mean subpolar low and Azores The primary mode of North Atlantic sea level storm high extending far to the northeast of their seasonal track variability is identi®ed using rotated principal mean positions (Figs. 4d and 4e). It is suggested that component analysis on monthly rms ®elds of high-pass the NAO may be closely linked to the latitudinal aspects ®ltered (2±8 day periods) sea level pressures for the of storm track variability in the central Atlantic, but the winters 1899±1900 through 1991±92. The primary com- low-frequency teleconnections that are linked to the pre- ponent of variability is a dipole pattern with centers in dominant mode of the storm track variability in the the extreme northeastern Atlantic and west of Portugal North Atlantic are in the far northeastern Atlantic. indicating that the storm track in a given month is either very active in the extreme northeastern Atlantic Arctic, Acknowledgments. This work is supported by the or it is oriented toward the Mediterranean basin. These NOAA Of®ce of Global Programs-Atlantic Climate storm tracks correspond respectively to positive and Change Program under Grants NA36GP0236 and negative mean score values in the time series of the NA56GP0213. I thank Mark Serreze of CIRES and the principal component. As the component scores become anonymous reviewers for valuable comments that im- increasingly positive, there is a substantial northeast- proved this paper. Chung-Chieh Wang, of the Atmo- ward extension of the storm track and the subpolar low spheric Science Program at The Ohio State University, intensi®es in the monthly mean ®elds over the Nor- assisted in producing the diagrams. This is Byrd Polar wegian and Barents Seas. The mean subtropical high Research Center contribution no. 1002. simultaneously intensi®es and moves northeastward, spreading over the Mediterranean basin. The westerlies REFERENCES strengthen in response to the intensi®cation and move- Anderson, J. R., and J. R. Gyakum, 1989: A diagnostic study of ment of the centers of action and spread far into Europe, Paci®c basin circulation regimes as determined from extratrop- generally bringing above-normal temperatures. Months ical cyclone tracks. Mon. Wea. Rev., 117, 2672±2686.

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