PERSISTENT PLANETARY SCALE CIRCULATION PATTERNS AND THEIR RELATIONSHIP WITH COLD AIR OUTBREAK ACTIVITY OVER THE EASTERN UNITED STATES.

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 18: (1998) PERSISTENT PLANETARY SCALE CIRCULATION PATTERNS AND THEIR RELATIONSHIP WITH COLD AIR OUTBREAK ACTIVITY OVER THE EASTERN UNITED STATES. CHARLES E. KONRAD II* Department of Geography, Saunders Hall, CB 3220, Uni ersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina , USA Recei ed 16 July 1997 Re ised 15 February 1998 Accepted 3 March 1998 ABSTRACT Previous work has shown that the intensity of cold air outbreaks over the south-eastern US displays a stronger relationship with planetary scale circulation anomalies than with the synoptic scale anomalies that are directly associated with the outbreak. In this study, these planetary scale circulations are identified over three temporal scales and related to outbreak intensity. To carry this out, standardized 5, 10, and 15 day surface pressure and 500 mb heights over North America are used to identify a sample of 57 circulation periods. From this sample, the circulation anomalies identified on the three scales are related to the intensity of current and incipient (i.e. 1 2 weeks later) outbreak activity over the south-eastern US. Correlation maps are constructed to portray the patterns of correlation between the circulation anomalies and outbreak intensity. In order to investigate regional variability in the nature of the relationships, comparisons are made with outbreak activity over the north-eastern US. The majority of the circulation features show stronger relationships with outbreak intensity when the 10- and 15-day mean circulations are considered. This implies that persistent, lower frequency modes of circulation play a relatively greater role in controlling the intensity of cold air outbreaks over the eastern US Royal Meteorological Society. KEY WORDS: cold air outbreak; synoptic climatology; winter circulation; eastern United States 1. INTRODUCTION Circulations on a variety of scales contribute to the formation of a cold air outbreak over a given region. Synoptic scale circulations are most directly associated with the development of a cold air outbreak; however, these circulations are embedded in a planetary scale regime that may persist for a week or longer. As viewed from a synoptic scale perspective, cold air outbreaks are forced by the southward movement of cold air from a high latitude source region into the middle latitudes. The winds responsible for the southward advection of cold air are tied to the pressure gradient between a departing cyclone east or northeast of the outbreak region and an approaching anticyclone to the west or northwest of the region (Figure 1 provides an example). Various synoptic studies have associated surface cyclogenesis (Wendland, 1987) and anticyclogenesis (Dallavalle and Bosart, 1975, Colucci and Davenport, 1987) with the development of a cold air outbreak. In the case of strong or extreme outbreaks, however, Konrad and Colucci (1989) showed that cyclogenesis usually follows the invasion of cold air over eastern North America; Boyle (1986) noted a similar pattern over East Asia. These works and several synoptic climatological studies (Rogers and Rohli, 1991; Konrad, 1996) suggest that strong outbreaks are tied more to the strength of the upstream anticyclone than the downstream cyclone. * Correspondence to: konrad@geog.unc.edu Contract grant sponsor: National Science Foundation; Contract grant number: SES CCC /98/ $ Royal Meteorological Society

2 1210 C.E. KONRAD II The surface anticyclone cyclone couplet responsible for the advection of cold air at the surface is supported dynamically by a ridge trough system in the middle troposphere. In the stronger cold air outbreaks, the ridge trough system is quite pronounced providing north-westerly or northerly winds in the upper levels over the outbreak region (e.g. Figure 1). When this meridional circulation persists, short waves embedded in the larger scale flow move rapidly south-eastward (or southward) across the outbreak region. These short waves provide dynamic support for the repeated development and rapid southeastward movement of surface anticyclone cyclone couplets, thus, two or more outbreaks may be produced in close succession (Boyle, 1986; Konrad, 1989). The initiation of meridional flow aloft is linked to a hemispheric scale pattern of trough and ridge intensification that begins upstream of the incipient outbreak region and progresses rapidly downstream, coincident with the onset of the outbreak (Joung and Hitchman, 1982). The meridional circulations that force strong and frequent cold outbreak activity may also be related to persistent middle tropospheric ridging over the polar regions. This ridging is connected with the occurrence of 500 mb cyclones farther south than normal and hence stronger cold air outbreaks. In a sample of extreme outbreaks over eastern North America, Konrad (1989) noted that the polar ridging occasionally resulted from a splitting of the northern hemispheric circumpolar vortex by the northwardexpansion of warm-core ridges from the Pacific and Atlantic Oceans toward the North Pole. Interestingly, Umemoto (1985) identified a similar pattern in one of his East Asian cold outbreak types. Recent work (Konrad, 1996) suggests that the intensity of a cold air outbreak relates more strongly to low frequency circulation anomalies that persist for 5 days or more. However, it is not known what particular scales of circulation are most important. Perhaps most intriguing in the analysis of low frequency circulation anomalies is the potential role of feedback mechanisms in forcing more intense outbreaks. For example, persistent anomalies in the circulation (Dole and Gordon, 1983; Dole, 1989) are associated with the blocking features (Rex, 1950) which displace the normal tracks of cyclones. In the south-eastern US, extended periods of troughing and frequent outbreak activity encourage cyclones to develop farther south than usual, which provide heavier snows and more persistent snow cover (Namias, 1978). This in turn can contribute to further decreases in the air temperature (Namias, 1985; Leathers et al., 1995) and hence stronger outbreaks. Konrad (1996) provided some initial insights on the connection between scale of circulation and outbreak intensity by correlating the intensity of outbreaks over the south-eastern US with time filtered, standardized surface pressure and 500 mb heights over North America. The filtering, which emphasized variability less than and greater than 5 days, provided synoptic ( 5 days) and planetary ( 5 days) components of the flow. His work demonstrated that outbreak intensity is more strongly tied to planetary scale circulation anomalies. In particular, outbreak intensity was significantly correlated with the strength of four planetary scale circulation features during the 12 days preceding an outbreak: (i)500 mb ridging over western North America and the Arctic; (ii) 500 mb cyclone and troughing over the Great Lakes, (iii) surface anticyclone extending southeastward from western Canada to the Gulf of Mexico; and (iv) a 500 mb cyclone over eastern Canada (Figure 2 identifies the location of these features). During the early stages of the cold air outbreak (i.e days before), the surface anticyclone was most strongly tied to outbreak intensity. Immediately prior to the outbreak, 500 mb heights over the Great Lakes were most strongly related to outbreak intensity. The persistence of these circulation features is illustrated vividly by the observed 5- and 10-day mean fields associated with one of the strongest cold air outbreaks on record in the south-eastern US (Figure 1). This work (Konrad, 1996) raises three major questions: Firstly, does the strength of various circulation features show significant relationships with outbreak intensity? This is the converse of the question addressed by Konrad (1996), who tied outbreak intensity to the strength of the circulation features occurring during outbreak development. In other words, the study sample was restricted to periods in which outbreaks took place and thus neglected the strength of circulation features present during periods devoid of cold air outbreaks. Secondly, and perhaps most importantly, what component of the planetary scale circulation (beyond 5 days) is most important in dictating the intensity of outbreaks? Finally, are the relationships obtained between circulation features and outbreaks found in an adjacent region? These questions provide the motivation for this work.

3 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1211 In this study, lower and middle tropospheric circulations are defined on time scales of 5, 10, and 15 days over North America and related ia correlation maps to the strength of ongoing and incipient outbreaks (i.e. those occurring 1 2 weeks later) over the south-eastern US. Mean standardized surface Figure 1. Analyses of surface pressure (mb) and 500 mb height (Da) for the observed, 5-, and 10-day mean fields connected with an extreme cold air outbreak observed on 1200 h January Many stations in the south-eastern US reported all-time low temperatures for the month of January (Kousky, 1985) during this outbreak

4 1212 C.E. KONRAD II Figure 2. Summary of the circulation features that were tied to the intensity of outbreaks by Konrad (1996). The solid and dashed lines depict circulation at the 500 mb level and surface, respectively. The dotted lines denote the boundaries of the south-eastern and north-eastern US as defined by the LFM grid pressure and 500 mb heights are calculated over 5, 10, and 15 days and used to define these circulations. A similar analysis is carried out for the north-eastern US to see if the relationships uncovered for the south-eastern US are applicable in an adjacent region. 2. METHODOLOGY Cold air outbreaks and the circulation features associated with their development were analyzed during the cold season (1 December 1 March) from 1979 through In order to carry out the analyses, gridded ( km), twice-daily synoptic data from a subset of the initialized fields of the limited fine mesh (LFM) model over portions of the North American land-mass and adjacent waters (i.e. area shown in map of Figure 1) provided by the National Center for Atmospheric Research (NCAR) were accessed. Since the initialized LFM data were interpolated from surface and radiosonde observations, the fields over the extreme northern portions of the area, where a coarser station network is found, may be subject to greater error. The northern (southern) edge of the extracted LFM field spans 110 (45 ) of longitude; thus, variations in the diurnal insolation cycle exert some influence on the temperature field. The surface temperature and pressure as well as 500 mb height fields were extracted from the LFM dataset. To control for diurnal and interseasonal variations, these fields were standardized by subtracting the field mean and dividing by the field standard deviation. The field means and standard deviations were calculated for each map time (0000 and 1200 h UTC) of the cold season by considering observed values for the 12-year study period in a 10-day window centered on each day. For example, the mean temperature field for 1200 h UTC 15 January was arrived at by averaging the observed fields for all 1200 h UTC map times occurring between 11 and 19 January during the 12-year study period. It was assumed that the computed 12-year means and S.D. s did not depart greatly from those that could be obtained from a much longer time period of analysis. Circulation patterns were identified on three scales in this study by constructing 5-, 10-, and 15-day means in the standardized surface pressure and 500 mb height fields over the North American LFM grid. A systematic sample of circulation patterns was collected from the time series of surface pressure and 500 mb height at all grid points over the study domain; the sample consists of 57 nonoverlapping circulation periods which fall within the 12 cold seasons of the study ( ). Five circulation periods were identified for each cold season from 1981 onward. Only four (five) circulation periods were identified for

5 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1213 the ( ) cold season due to incomplete data. Mean circulations of 5-, 10-, and 15-days were computed within the 15-day span of each period as shown by the example in Figure 3. Each circulation period was tied to the intensity of cold air outbreaks over the south-eastern and north-eastern US. These outbreaks can be defined in a number of different ways. In previous studies (Boyle, 1986; Konrad, 1989) cold air outbreaks were defined on the synoptic scale in an objective manner by simply identifying temperature minima or marked temperature drops in a time series over a period of 5 7 days. Using this definition each outbreak was essentially tied to a synoptic scale disturbance. Outbreaks may be defined over longer temporal scales, but much subjectivity is required in specifying the beginning or end of an outbreak (Konrad, 1996). Given this subjectivity and the prior use of a synoptic definition, a cold air outbreak was defined in this study on a synoptic scale by identifying the coldest three contiguous map-times over a region during a 5-day period. The intensity of the outbreak was defined as the mean temperature anomaly for these three coldest map-times. By using a three-map-time mean temperature anomaly, transient radiative influences on regional surface temperature, such as cloud cover and wind (Konrad, 1996), are reduced. Time series of the standardized surface temperature were computed over the south-eastern and north-eastern US domains (see Figure 2) from the gridded LFM data. For each period, the 5-, 10-, and 15-day mean circulations were linked with a cold air outbreak occurring concurrently (i.e. during a 5-day span centered on the midpoint of the circulation period) and with an incipient outbreak occurring between 7.5 and 12.5 days after the circulation period midpoint (Figure 3). Incipient outbreaks are considered so that the circulations controlling the development of an outbreak may be analyzed. It has been shown by Konrad, (1996) that circulations existing 12 days or more before the onset of an outbreak may be significantly related to outbreak intensity. Correlation analysis was used to identify the strength of the relationships between atmospheric circulation anomalies, identified on the three time scales, and the intensity of ongoing and incipient outbreaks. Using the sample of 57 circulation periods, the standardized surface pressure and 500 mb heights at each grid point on the North American LFM mesh were correlated with outbreak intensity. These point correlations were combined to form a map depicting spatial patterns of correlation between the strength of the circulation anomaly and outbreak intensity. Figure 3. Example of two circulation periods sampled during a hypothetical cold season. Mean circulations are computed over periods of 5-, 10- and 15-days and related to the intensity of current and incipient outbreak activity during the indicated 5-day time spans

6 1214 C.E. KONRAD II 3. RESULTS The correlation maps presented in this work provide relationships between the strength of the circulation anomalies over North America and the intensity of cold air outbreaks over the south-eastern and the north-eastern US. The intensity of the cold air outbreak is defined by a negative temperature anomaly; therefore, positive correlations provide a relationship between negative circulation anomalies (i.e. lower than average 500 mb height and surface pressure) and outbreak intensity. On the other hand, negative correlations portray a connection between positive circulation anomalies and outbreak intensity. In a strict statistical sense, the calculated significance level of the correlations (Figure 4) is valid only for a single correlation. The numerous correlations on the map provide an example of statistical multiplicity (see Katz and Brown, 1991). This is especially the case because the field data are spatially autocorrelated. Therefore, the reported significance level should be used only as a guide to the statistical significance of the multiple correlations. In the first part of this section, correlation maps are described for outbreak activity in the south-eastern US. In the second part, comparisons are made with the correlation maps associated with outbreak activity over the north-eastern US. And in the last part, the long range prediction of cold air outbreaks is examined briefly Cold air outbreaks o er the south-eastern US The correlation maps associated with the 5-, 10-, and 15-day mean surface pressure fields (Figure 4) show a broad region of negative correlations over the interior of the continent. These correlations identify a connection between the strength of a continental anticyclone and outbreak intensity. This connection is clearly the strongest for incipient outbreaks; it ties a strong anticyclone over central North America with the occurrence of strong outbreaks over the south-eastern US 1 2 weeks later. In comparing the scales of circulation, as represented by the 5-, 10-, and 15-day mean fields, correlations increase with increasing scale, that is, the most persistent anticyclonic circulation (i.e. 15-day mean field) relates most strongly to outbreak intensity. The statistical significance of this correlation increase is not known, however. Additionally, the positive trend is only evident in the incipient outbreak maps. Also noteworthy is the presence of two negative centers in the correlation map associated with current outbreaks. These centers, which may be tied to two anticyclones, are most pronounced in the 5-day mean field. Examination of the synoptic charts for some of the strong outbreaks reveals that the southern center is associated with the current outbreak and the northern center to a reinforcing surge of cold air (i.e. incipient outbreak) that will affect the south-eastern US over the next several days. A region of positive correlations is located along and east of the Atlantic coastline and indicates a relationship between negative pressure anomalies and outbreak intensity. The strongest correlations are found over northeastern North America, although the positive correlation center on the incipient outbreak maps is located slightly farther inland over Labrador. The positive correlation maximum is weaker than the negative correlation maximum (i.e. continental anticyclone) to the west for all three scales of circulation. This suggests that the strength of the anticyclone plays a more significant role than the cyclone in forcing current and incipient outbreaks. The differences in the strength of the correlation maxima are greatest in the 15-day mean fields. The correlation maps associated with the standardized 500 mb heights (Figure 5) reveal a broad region of positive correlations over south-eastern North America. This relationship obviously relates to the control of lower tropospheric temperature on the 500 mb height (i.e. colder temperatures force troughing and lower 500 mb heights). It follows, therefore, that the correlations between the 500 mb height and outbreak intensity are markedly higher (i.e points) in connection with current outbreak activity. The strength of the positive correlations generally increases with the scale of circulation. These increases are most notable in comparing correlations in the 5- and 10-day mean fields of the current outbreak map. Negative correlations are found over the northern and western portions of the continent and relate the strength of 500 mb ridging in these regions with outbreak intensity over the south-eastern US. In

7 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1215 Figure 4. Correlation patterns between the strength of standardized 5-, 10-, and 15-day mean surface pressure anomalies over North America and the intensity of incipient and current cold air outbreaks over the south-eastern US. Positive and negative correlations (10** 2) are indicated by solid and dashed lines, respectively. Correlations exceeding 0.27 are significant at the.05 level or higher comparing the six maps, the strongest negative correlations are associated with incipient outbreak activity. The correlation maxima for incipient activity is centered over the polar regions and implies that high latitude ridging plays the strongest role in the circulation changes that lead to a strong outbreak over the

8 1216 C.E. KONRAD II south-eastern US 1 2 weeks later. The 15-day mean fields provide the strongest correlations and indicate that the persistence of high latitude ridging is related to outbreak intensity. In contrast, the strongest negative correlations associated with current outbreak activity are found over south-western North America. The strength of these correlations, however, decreases slightly as more persistent ridging Figure 5. Same as Figure 4 except for 500 mb height anomalies

9 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1217 circulations are considered. This implies that transient ridging over the western US, as inferred from the 5-day mean 500 mb heights, plays an increased role as the outbreak occurs over the south-eastern US. It should be emphasized, however, that these transients are not necessarily more important than low frequency planetary scale waves in controlling outbreaks. Since the 5-day mean fields contain waves with periods spanning 5 days to a season, significant correlations with these fields simply indicate a relationship between some combination of these wave types and outbreak intensity Comparison with cold air outbreaks o er the northeastern US The surface pressure correlation maps for the two regions (Figures 4 and 6) display a similar pattern, with positive correlations over the eastern fringes of the continent and negative correlations over the interior and the west on most of the maps. Not surprisingly, the locations of correlation maxima for the northeastern outbreaks are displaced slightly from the south-eastern outbreak maxima (approximately 5 latitude farther north). Although the patterns for the two regions are similar, marked differences are noted in the strength of the correlations. Most evident is the stronger negative correlation over the interior in connection with southeast incipient outbreak activity and with northeast current activity. This implies that the strength of the continental anticyclone is relatively more important in the development of a south-eastern outbreak (1 2 weeks later). For the northeast, the strength of the anticyclone is tied more strongly to an existing outbreak. Outbreaks in both regions show increases in the strength of the negative correlations as the length of the averaging period increases. In the north-eastern outbreaks, negative pressure anomalies along the East Coast at all three scales display stronger connections with current outbreak intensity. The strongest relationship is found for the 5-day mean fields and implies that transient cyclone activity is a significant factor in controlling the intensity of an outbreak in progress. In fact, the correlation is stronger than that associated with the upstream positive pressure anomalies. In examining incipient outbreak activity, coastal negative pressure anomalies show a relatively weak connection with northeastern outbreaks, suggesting that antecedent cyclone activity plays a lesser role in the circulations that lead to strong outbreaks. This does not appear to be the case for the southeast, as persistent cyclone activity on a 10-day time scale shows a moderate correlation with incipient outbreak intensity. The correlation maps associated with the 500 mb height anomalies for the two regions (Figures 5 and 7) also display similar patterns with positive (negative) correlations noted over the eastern (western and northern) portions of the continent. Northeastern outbreak intensity also shows a strong connection with the strength of troughing over eastern North America. Unlike the south-eastern outbreaks, however, height anomalies in the northeastern outbreaks are more strongly tied to the 5-day mean field than the 10- and 15-day mean fields. This suggests that transient waves in the 500 mb field play a greater role in controlling outbreak intensity over the northeast as compared to the southeast. However, as discussed earlier, these results do not imply that these transients are more important than the low frequency fluctuations in controlling outbreaks. Connections between the positive 500 mb height anomalies (i.e. ridging) over the western and northern portions of the continent and outbreak intensity show some intriguing differences in comparing the two regions. Over the northeast, 500 mb ridging relates most strongly to the intensity of current outbreak activity; over the southeast 500 mb ridging is more strongly connected with incipient outbreak intensity. Furthermore, the strength of the correlations increases with the length of the averaging period for current outbreak activity in the northeast and incipient activity in the southeast Long range prediction of cold air outbreaks The connection identified in this work between persistent circulations and incipient outbreak activity may be exploited to improve the long range prediction of cold air outbreaks. The 15-day mean surface pressure anomaly over western Canada correlates most strongly with outbreak intensity for the southeastern and north-eastern US regions 1 2 weeks later. To get a sense of how well these anomalies predict impending outbreaks, 10 of the strongest anticyclonic events over western Canada, as inferred using the

10 1218 C.E. KONRAD II Figure 6. Same as Figure 4 except for cold air outbreaks over the north-eastern US 15-day mean fields, were identified and related to any strong outbreaks during a 15-day period following the peak of the event (i.e. the day during which the 15-day mean surface pressure anomaly was the strongest). Strong outbreaks were defined simply as the ten outbreaks with the coldest surface temperatures over the south-eastern and north-eastern US during the 12 year study period. Six out of ten strong

11 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1219 anticyclonic events were followed by a strong outbreak in the south-eastern US. On average, these occurred approximately 7 days after the peak of the anticyclonic event. Seven out of ten strong anticyclonic events were followed by a strong outbreak in the north-eastern US; these outbreaks were observed, on average, approximately 6 days after the peak of the anticyclonic event. These results are Figure 7. Same as Figure 4 except for 500 mb height anomalies and cold air outbreaks over the north-eastern US

12 1220 C.E. KONRAD II quite impressive given that there was only about a 15% chance of randomly selecting a 15 day period containing a strong outbreak. 4. DISCUSSION AND CONCLUSIONS In this study circulation patterns on three temporal scales over North America were related to the intensity of cold air outbreaks over the south-eastern US; these relationships were then compared with those identified over the north-eastern US. In considering incipient outbreak activity, the strength of the surface anticyclonic circulation over central Canada correlated most strongly with outbreak intensity, especially for south-eastern outbreaks. The 15-day mean fields displayed the strongest relationships with incipient outbreak intensity. This suggests that persistent anticyclonic circulations over central Canada may be associated with stronger outbreaks over the south-eastern and north-eastern US 1 2 weeks later. The persistent anticyclonic circulation was tied to both incipient and current outbreaks over the north eastern US; however, the strength of this circulation bears the strongest correlation with the intensity of current outbreaks in this region. Negative surface pressure anomalies over eastern Canada were significantly related to outbreak intensity in both regions, but the correlations were generally not as strong as those associated with the positive surface pressure anomalies found upstream over central Canada. High latitude ridging at the 500 mb level, which is often found immediately upstream of surface anticyclones, displays a moderate relationship with incipient outbreak intensity, but only over the south-eastern US. Not surprisingly, negative 500 mb height anomalies over and immediately north of the outbreak region bear the strongest relationship with current outbreak intensity. This relationship is the strongest for the 10-day mean fields in the south-eastern outbreaks and for the 5-day mean fields in the north-eastern outbreaks. Konrad (1996) established the importance of planetary scale circulation features in outbreak development over transient synoptic scale features; his study sample, however, was restricted to periods in which outbreak activity took place and thus precluded sampling of circulation features when there was no outbreak activity. Also, his work did not establish the scales of circulation beyond 5 days that relate to outbreak intensity. In this study circulation patterns that persisted well beyond 5 days (i.e days and beyond), were shown to be relatively more important in the development of stronger cold air outbreaks. There were some regions in which the more transient 5-day mean circulations exhibited stronger correlations with outbreak intensity, but this implies only that transient fluctuations in the circulation over these regions have a more marked influence; longer wave periods make up the 5-day mean field as well and thus display some relationship with outbreak intensity. The 15-day mean circulations that show the strongest connections with outbreak intensity can be decomposed into lower frequency components (i.e. those that vary on monthly to seasonal time scales). These circulations, such as the Pacific-North American (PNA) teleconnection pattern (Wallace and Gutzler, 1981), may bear significant relationships with the intensity of cold air outbreaks defined on the synoptic scale. Leathers et al. (1991) and others have identified strong connections between the positive phase of the PNA and below normal monthly mean temperatures over the south-eastern US. The pattern of positive 500 mb height anomalies over north-western Canada and negative anomalies over the eastern US, which has been tied to strong outbreak activity in this study, corresponds strongly to the positive phase of the PNA pattern. Essentially, the results of this study provide relationships between a large scale atmospheric forcing and a synoptic scale variable. The existence of these relationships demonstrates the potential for downscaling work (Hewitson and Crane, 1996), which involves the derivation of transfer functions that translate large scale circulation anomalies to information on regional scale phenomenon such as cold air outbreaks. Much of this downscaling work involves global climate model (GCM) output that has a monthly temporal resolution; therefore, ongoing work is aimed at identifying the connections between circulations on a monthly scale (e.g. PNA teleconnection pattern) and cold air outbreak activity.

13 PLANETARY CIRCULATION AND COLD AIR OUTBREAKS 1221 ACKNOWLEDGEMENTS The author wishes to thank the two anonymous reviewers for their thoughtful and thorough reviews of this paper. The synoptic data for this project was supported by the National Science Foundation under Grant SES REFERENCES Boyle, J.S Comparison of the synoptic conditions in midlatitudes accompanying cold surges over eastern Asia for the months of December 1974 and 1978, Part I: Monthly mean fields and individual events, Mon. Wea. Re., 114, Colucci, S.J. and Davenport, J.C Rapid surface anticyclogenesis: Synoptic climatology and attendant large scale circulation changes, Mon. Wea. Re., 115, Dallavalle, J.P. and Bosart, L.F A synoptic investigation of anticyclogenesis accompanying North American polar outbreaks, Mon. Wea. Re., 103, Dole, R.M. and Gordon, N.D Persistent anomalies of the extratropical wintertime circulation: Geographical distribution and regional persistence, Mon. Wea. Re., 111, Dole, R.M Life cycles of persistent anomalies: Part I: Evolution of 500 mb height fields, Mon. Wea. Re., 117, Hewitson, B.C. and Crane, R.G Climate downscaling: techniques and application, Climate Res., 7, Joung, C.H. and Hitchman, M.H On the role of successive downstream development in East Asian polar air outbreaks, Mon. Wea. Re., 116, Katz, R.W. and Brown, B.G The problem of multiplicity in research on teleconnections, Int. J. Climatol., 11, Konrad, C.E. and Colucci, S.J An examination of extreme cold air outbreaks over eastern North America, Mon. Wea. Re., 117, Konrad, C.E An Examination of Extreme Cold Air Outbreaks o er Eastern North America, Unpublished M.Sc. Thesis, University of Virginia, 129 pp. Konrad, C.E Relationships between the intensity of cold air outbreaks and the evolution of synoptic and planetary scale features over North America, Mon. Wea. Re., 124, Kousky, V.E Seasonal climate summary: The global climate for December 1984-February 1985: A case of intraseasonal oscillations, Mon. Wea. Re., 113, Leathers, D.J., Yarnal, B. and Palecki, M.A., The Pacific/North American teleconnection pattern and United States climate. Part I: Regional temperature and precipitation associations, J. Climate, 4, Leathers, D.J., Ellis, A.W. and Robinson, D.A Characteristics of temperature depressions associated with snow cover across the northeast United States, J. Appl. Meteorol., 34, Namias, J Multiple causes of the North American abnormal winter , Mon. Wea. Re., 106, Namias, J Some empirical evidence for the influence of snow cover on temperature and precipitation, Mon. Wea. Re., 113, Rex, D.F Blocking action in the middle tropospheric westerlies and its effect on regional climate: I. An aerological study of blocking action, Tellus, 2, Rogers, J.C. and Rohli, R.V Florida citrus freezes and polar anticyclones in the Great Plains, J. Climate, 4, Umemoto, T Synoptic Study of the Large-Scale Cold Outbreaks in East Asia, Reprints from Geographical Reports of Tokyo Metropolitan University, Wallace, J.M. and Gutzler, D.S Teleconnections in the geopotential height field during the Northern Hemisphere winter, Mon. Wea. Re., 109, Wendland, W.M Prominent November coldwaves in the North Central States since 1901, Bull. Am. Meteorol. Soc., 68,