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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 26: 1133–1147 (2006) Published online 20 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1301 THE BEHAVIOR OF EXTREME COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING a S. VAVRUS,a, * J. E. WALSH,b W. L. CHAPMANc and D. PORTISc Center for Climatic Research, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, WI 53706, USA b International Arctic Research Center, University of Alaska-Fairbanks, 930 Koyukuk, Fairbanks, AK 99775, USA c Department of Atmospheric Sciences, 105 S. Gregory Street, University of Illinois, Urbana, IL 61801, USA Received 27 May 2005 Revised 22 November 2005 Accepted 23 November 2005 ABSTRACT Climate model output is used to analyze the behavior of extreme cold-air outbreaks (CAOs) under recent and future climatic conditions. The study uses daily output from seven GCMs run under late-twentieth century and projected twentyfirst century radiative conditions (SRES A1B greenhouse gas emission scenario). We define a CAO as an occurrence of two or more consecutive days during which the local mean daily surface air temperature is at least two standard deviations below the local wintertime mean temperature. In agreement with observations, the models generally simulate modern CAOs most frequently over western North America and Europe and least commonly over the Arctic. These favored regions for CAOs are located downstream from preferred locations of atmospheric blocking. Future projections indicate that CAOs – defined with respect to late-twentieth century climatic conditions – will decline in frequency by 50 to 100% in most of the Northern Hemisphere during the twenty-first century. Certain regions, however, show relatively small changes and others actually experience more CAOs in the future, due to atmospheric circulation changes and internal variability that counter the thermodynamic tendency from greenhouse forcing. These areas generally experience greater near-surface wind flow from the north or the continent during the twenty-first century and/or are especially prone to atmospheric blocking events. Simulated reductions in CAOs are smallest in western North America, the North Atlantic, and in southern regions of Europe and Asia. The Eurasian pattern is driven by a strong tendency for the models to produce sea-level pressure (SLP) increases in the vicinity of the Mediterranean Sea (intermodel mean of 3 hPa), causing greater advection of continental air from northern and central Asia, while the muted change over western North America is due to enhanced ridging along the west coast and the increased frequency of blocking events. The North Atlantic response is consistent with a slowdown of the thermohaline circulation, which either damps the warming regionally or results in a cooler mean climate in the vicinity of Greenland. Copyright 2006 Royal Meteorological Society. KEY WORDS: extreme events; cold-air outbreaks; IPCC; climate models; climate change; greenhouse warming 1. INTRODUCTION A cold-air outbreak (CAO) is an incursion of cold polar air into middle or lower latitudes, resulting in extreme negative anomalies of surface air temperature. Influxes of extremely cold air adversely impact large areas of the heavily populated middle latitudes during most winters. In addition to causing human suffering and fatalities, temperatures during severe cold outbreaks can harm vegetation and can fall below the thresholds for which buildings and other infrastructural components were designed. The incidence of extreme cold outbreaks is intertwined with climate variability in several ways, yet their precise relationship remains uncertain. First, the atmospheric circulation patterns conducive to the build-up and southward migration of cold airmasses show intra-seasonal to inter-decadal variability; which * Correspondence to: S. Vavrus, Center for Climatic Research, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, WI 53706, USA; e-mail: [email protected] Copyright 2006 Royal Meteorological Society 1134 S. VAVRUS ET AL. is a fundamental characteristic of climate. Second, climate changes, especially changes of mean temperature, can be expected to impact the intensity of CAOs. However, one of the apparent climatic paradoxes of the past several decades is that, although the northern continental regions (northwestern North America, northern Asia) have shown the greatest warming during winter, the incidence of extremely cold outbreaks has been at least as great in the 1980s and 1990s as in the earlier decades of the century (Figure 2 in Walsh et al., 2001). The warming pattern in northern continental regions is consistent with natural variability over seasonal to decadal timescales, but not inconsistent with model-derived projections of greenhouse warming. Another apparent climatic paradox is the finding of Thompson et al., 2002, (p. 1426) that extreme cold events are less common over the south-central United States during the warm phase of the ENSO cycle, even though mean winter temperatures are below-normal in this region during warm ENSO events. Similarly intriguing findings related to trends of temperature have been recently reported, apparently independently, by Michaels et al. (2000) and Kalkstein et al. (1990). While both groups found that the coldest and driest airmasses over subarctic land areas have warmed significantly in recent decades, Michaels et al. contend that the Northern Hemisphere warming since the mid-1940s ‘is almost exclusively confined to the dry, cold anticyclones of Siberia and North America’. Since outbreaks of cold polar air into middle latitudes of North America have not lessened in the period since the 1940s, atmospheric dynamics and the large-scale circulation may play a key role in CAOs. Several other observations underscore the surprisingly ambiguous link between mean climate and extremes. The number of state-record minimum temperatures within the United States by decade shows no trend from the 1890s onward and is not significantly correlated with the national mean wintertime temperature over this period (Figure 1). This behavior of extreme events not following the mean temperature trend agrees with the analysis of Kunkel et al. (2002), who found no decrease in the number of extreme cold days in the United States since 1895, despite a mean wintertime warming of 1.3 K during this interval. Klein Tank et al. (2002) report an increase in cold-spell days over Europe during 1976–1999, in contrast to the regional warming trend and the pronounced global warming signal during this period. Wintertime sea-level pressure (SLP) over the CAO source region of northern Canada has decreased significantly since the 1950s (van Wijngaarden, 2005), suggestive of weaker and/or fewer CAOs; yet the frequency of North American outbreaks has not declined during this interval (Walsh et al., 2001). These findings provide impetus for identifying and understanding the dynamical mechanisms responsible for sporadic, short-lived CAOs that can occur somewhat independent of the state of the background climate. Among the atmospheric circulation patterns that have been associated with CAOs over North America are the Arctic Oscillation (AO) or its regional counterpart, the North Atlantic Oscillation (NAO); the Pacific-North American (PNA) anomaly pattern; and ENSO, as noted above. In the case of the AO, the negative phase (characterized by a weakened annular mode of the Northern Hemisphere circulation) is associated with increased cold outbreaks over much of the United States as well as parts of Europe and Asia (Thompson and Wallace, 2001; Jeong and Ho, 2005). This tendency is consistent with an amplified longwave pattern in the troposphere. The PNA teleconnection pattern has long been known to be associated with cold outbreaks over central and eastern North America through a build-up of an upper-air ridge over western North America. Surface anticyclogenesis over the northwestern part of the continent, and a northwest-to-southeast steering flow aloft facilitates the transport of cold airmasses toward the equator. The PNA pattern shows some correlation with ENSO and with the Quasi-Biennial Oscillation (Thompson et al., 2002). Cold outbreaks over Europe are associated with a breakdown of the NAO or its larger counterpart, the AO. Walsh et al. (2001) have shown that surface airmasses of Siberian origin migrate westward when the onshore flow from the Atlantic Ocean weakens during the negative phase of the NAO or AO. The correlation between wintertime surface air temperature and the AO extends well eastward into northern Asia, as shown by the correspondence between recent decadal-scale trends of the AO and surface air temperatures (Thompson and Wallace, 1998). The linkage to conditions over the southern part of the Asian landmass, such as the cold outbreak of the winter of 2002–2003, is less apparent, although it is likely that factors affecting the intensity of the Siberian anticyclone are related to such outbreaks. Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1135 12 1.5 10 1.2 8 0.9 6 0.6 4 0.3 2 0.0 0 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s Contiguous U. S. Winter Temperature (C) Number of State-record Minimum Temperatures COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING -0.3 Decade Figure 1. Number of state-record minimum temperatures by decade in the United States (bars) and the corresponding national wintertime decadal-mean temperature (dots). Data were obtained from the National Climatic Data Center (http://www.ncdc.noaa.gov) The linkage of cold outbreaks to climate, and the likelihood of climate changes over the coming decades to a century, gives climate models a potentially valuable role in providing (1) diagnostic evaluations of extreme cold outbreaks in terms of dynamical and physical processes, and (2) projections of future changes in the characteristics (i.e. frequency, intensity, geographical distribution) of cold outbreaks over the decade-to-century timescale. An example of the latter is the Hunt and Elliott (2004) recent evaluation of projected changes in CAO events simulated by the CSIRO model. In the present study, we extend this type of assessment to include an ensemble of seven models being used in the ongoing Fourth Assessment of the Intergovernmental Panel on Climate Change (IPCC). Our objectives are to (1) determine the models’ capabilities in simulating CAO events, (2) document the statistical characteristics of CAO events in the broader framework of daily temperature variations, (3) evaluate and compare the changes of CAO events projected for the middle-to-late twenty-first century by the seven models, and (4) relate the projected changes of CAO events to projected changes in the large-scale atmospheric circulation. In Section 2 we describe the models and our method of identifying CAO events. Section 3 describes (1) the models’ present-day simulations of CAOs, including the relation of CAOs to the higher moments of the daily temperature distributions, and (2) the changes of twenty-first century CAO frequency projected by the models, together with corresponding changes in the SLP fields. The discussion in Section 4 places the results into a broader context of modeling and diagnostic studies relevant to CAOs. 2. MODEL INFORMATION AND METHODOLOGY The GCMs used in this analysis are part of the IPCC’s data archive at Lawrence Livermore National Laboratory (https://esg.llnl.gov:8443/index.jsp). This collection of state-of-the-art models from international Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1136 S. VAVRUS ET AL. centers is being used for the IPCC’s 4th Assessment Report (AR4) and consists of GCMs from more than a dozen institutes. For our study, we have used seven models that provided a daily output of surface air temperature and SLP (Table I). Three time periods of approximately twenty years each were chosen for analysis: the late-twentieth century (average of years 1980–1999), the mid-twenty-first century (centered on year 2055), and the late twenty-first century (centered on 2090). The late-twentieth century interval is taken to be the modern or current climate and represents radiative forcing consistent with observed greenhouse gas concentrations, solar variability, and volcanic forcing. The twenty-first century intervals represent the simulated climate as the system is forced with rising levels of greenhouse gases, according to the SRES A1B scenario (IPCC, 2001). This emission scenario assumes a nearly linear increase in atmospheric CO2 concentration to 720 ppm by year 2100, along with a near-linear rise in N2 O concentration and an increase in CH4 that peaks in year 2050. There is no well-accepted definition of a CAO, and past studies have used various criteria (e.g. Boyle, 1986; Konrad, 1996; Walsh et al., 2001). In this study, we define a CAO as an event of at least two consecutive days during which the daily mean surface air temperature at a grid point is at least two standard deviations below the simulated mean wintertime (DJF) surface air temperature at that location. The standard deviation (σ ) used here is the average value of the 90 daily standard deviations of inter-annual surface air temperatures during the winter (1 December–28 February). This definition of a CAO possesses many desirable properties: it (1) identifies only extreme CAOs (2σ events) but still provides a reasonably large sample size, (2) filters out single-day extreme cold events that might be caused by local effects, such as strong radiational cooling over a fresh snow pack, (3) does not constrain the frequency of CAOs to be uniform spatially, as does a percentile threshold, (4) relates CAOs to meteorological variables (average temperature and variability), (5) elucidates asymmetries between the frequencies of extreme cold versus extreme warm events during winter, (6) has flexibility for assessing future CAOs, which can be defined either with respect to the current climate (late-twentieth century, as is done in this study) or to the future climate (by using the mean temperature and standard deviation of a twenty-first century time interval), and (7) preferentially selects the more severe cold waves during midwinter (when variability is largest) by using the seasonal mean of daily wintertime standard deviations, rather than using the standard deviation for each individual winter day. While alternative definitions could be chosen, the salient findings emerging from our analysis should apply regardless of the exact criteria used. Table I. Description of the GCMs used in this study Originating group, country of origin National Center for Atmospheric Research (NCAR), USA Geophysical Fluid Dynamics Laboratory (GFDL), USA Goddard Institute for Space Studies (GISS), USA Institut Pierre Simon Laplace (IPSL), France Center for Climate Systems Research, National Institute for Environmental Studies, and Frontier Research Center for Global Change (JAMSTEC), Japan Meteorological Research Institute (MRI), Japan National Center for Atmospheric Research (NCAR), USA Copyright 2006 Royal Meteorological Society Model Resolution Reference CCSM3 T85L26 GFDL-CM2.0 2.0° × 2.5° L24 Delworth et al., 2005 GISS-AOM 3.0° × 4.0° L12 Russell and Rind, 1995 IPSL-CM4 2.5° × 3.75° L19 – MIROC3.2 T42L20 – MRI-CGGM2.3.2 T42L30 Yukimoto et al., 2001 PCM T42L18 Washington et al., 2000 Collins et al., 2005 Int. J. Climatol. 26: 1133–1147 (2006) COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING 1137 3. RESULTS 3.1. Simulation of modern CAOs The spatial pattern of extreme CAOs in the IPCC models and observations over the Northern Hemisphere is shown in Figure 2. The GCMs are generally accurate in their simulation of primary features, with a high pattern correlation with observations (r = 0.72) and the maximum number of days meeting the CAO criteria around 4 per winter. The two favored terrestrial regions for CAOs are in western North America, extending from the North Pacific into the upper Midwest, and in Europe to western Asia. The GCMs show a southward bias in maximum CAO frequency over eastern Europe and central Asia, although the eastern edge of this swath near Lake Baikal is accurately captured. The models also underestimate the frequency in the southeastern United States: mean simulated values range from 0.5 to 2 days versus 2 to 2.5 days in observations. This regional bias occurs in all the models and reflects the inability of GCMs to penetrate Arctic air masses far enough southeastward over North America. Minimal occurrences of CAOs are seen in the coldest parts of the Arctic, as well as over northern Africa. The models also produce a minimum over a wide stretch of the North Pacific and the central Atlantic; features present but less pronounced in observations. The agreement across the models can be assessed from the map of intermodel spread in CAO occurrence, expressed here as the standard deviation in mean CAO frequency among the seven GCMs (Figure 2(c)). This plot shows that the primary CAO centers simulated over western North America and Europe are a fairly robust feature (standard deviations <1 day/winter), while relatively poor agreement among models occurs in scattered pockets that are primarily in high latitudes and offset from the most frequent CAO occurrences. An interesting, yet poorly recognized, aspect of wintertime climate in the Northern Hemisphere is the non-Gaussian nature of the daily temperature frequency distribution at most locations (Figure 3). Across most of the domain poleward of 20 ° N (71%), the temperature distribution is negatively skewed (area-average of −0.23), meaning that cold extremes are more common than warm extremes. Some regions show highly negative skewness values around −1.0 or less, indicating a strong propensity for CAOs. The very high spatial correlation coefficient between observed CAO frequency and skewness (−0.84) suggests that a simple statistical index of skewness can serve as an accurate proxy for physical expressions of extreme weather. The generally skewed daily mean temperature distributions in the Northern Hemisphere can be related to our definition of an extreme CAO: if all locations had perfectly normal distributions, then the frequency of CAOs would be essentially the same everywhere (∼2.3% of winter days two or more standard deviations below the mean temperature). In general, the skewness pattern indicates that the favored CAO regions on land are outside of Arctic airmass source regions but close enough to be subjected to occasional incursions of polar air (Figures 2, 3). The tongue of high skewness and CAO frequency extending into the northern Great Plains is consistent with previous research showing that cold air surges often travel parallel to major mountain ranges (Colle and Mass, 1995; Garreaud, 2001). An important dynamical influence of the geographical CAO distribution appears to be the prevalence of atmospheric blocking events, whose anticyclones provide extended periods for the formation and propagation of polar air masses toward the equator. Wintertime blocking events in nature are most common over the North Pacific/Alaska region, northwestern Russia, and the North Atlantic, and they are known to be conducive to CAOs (Namias, 1950; Dole, 1986; Thompson and Wallace, 2001). Figure 4 shows the distribution of persistent strong anticylonic pressure anomalies during winter and illustrates that the models reproduce the first two favored blocking regions. The center around Alaska is consistent with the swath of maximum CAO occurrences from the Bering Sea into the northwestern United States (Figure 2), a region prone to CAOs due to the northerly flow that accompanies blocking events (Carrera et al., 2004). The secondary CAO maximum simulated over Europe appears to be associated with the other blocking center over northwestern Russia, whose circulation anomalies favor enhanced easterly flow of continental air from Asia into Europe. This flow pattern is consistent with the observed trajectory of coldest air during European CAOs and with the negative (reduced westerlies) phase of the NAO and its broader counterpart, the AO (Walsh et al., 2001; Thompson and Wallace, 2001). Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1138 S. VAVRUS ET AL. (a) (b) 0 0.5 1 1.5 0.4 0.6 2 2.5 3 3.5 4 (c) 0 0.2 0.8 1 1.2 1.4 1.6 Figure 2. (a) Simulated and (b) observed number of CAO days per winter during the late-twentieth century. Modeled values are the 7-GCM mean for years 1981–1999, and observations are for the 1948–2003 interval from NCEP-NCAR Reanalysis (Kalnay et al., 1996). The agreement among the models is represented by (c) the intermodel standard deviation (CAO days/winter) among the seven GCMs 3.2. Future changes in CAOs As the GCMs were driven by the SRES A1B greenhouse emission scenario from 2000 to 2100 (Section 2), they generally simulated fewer extreme cold days during the twenty-first century at most locations. The identification of future CAOs could be based either on the absolute temperature threshold of a CAO from the late-twentieth century runs (i.e. two or more standard deviations colder than the mean surface air temperature during 1980–1999) or on a new threshold using the mean temperature and standard deviation of the future climate. Although both approaches provide insight into the behavior of future extreme weather, we have adopted the former definition as a stricter comparison with contemporary CAOs to address how these extreme events from the modern perspective may change through 2100. We describe below how these events are projected to change in the future by focusing on their time-averaged behavior during two decades within the middle and end of the twenty-first century. By the middle of the current century (approximately 2045–2065), the GCMs simulate significantly reduced CAO frequencies at most locations (76% decrease averaged over the Northern Hemisphere), but there are also distinct exceptions to this trend (Figure 5). Over North America, MIROC and IPSL project the largest Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 1139 +0.2 +0.4 +0.6 Figure 3. Skewness of daily-mean wintertime (DJF) surface air temperature from the NCEP-NCAR Reanalysis for years 1948–2003. Regions with large negative (positive) skewness tend to have a high (low) frequency of CAOs decreases of any models, with at least 50% reductions throughout the continent. By contrast, CAOs become more common in CCSM and GFDL across parts of western North America, especially in the CCSM, which produces increases of up to 60% in parts of the western United States. The response in Eurasia is generally less model-dependent and ranges regionally from nearly no change to a disappearance of CAOs (area-wise average decrease of 74%). Three models (GFDL, GISS, and IPSL) produce an area of noticeably greater CAO frequency over the North Atlantic, resulting in increased outbreaks in Iceland, the British Isles, and extreme northwestern Europe. These regional changes in CAOs can generally be linked to associated changes in mean atmospheric circulation, as implied by the spatial distribution of SLP anomalies (Figure 5). The models differ greatly on the response of SLP in the North Pacific–Gulf of Alaska region, varying from a 5-hPa increase in CCSM and GFDL to a 5-hPa decrease in IPSL. The rise of pressure in CCSM and GFDL is consistent with the surprising increase in CAO frequency over western North America, as the circulation change represents enhanced continental flow from Canada. By contrast, the enhanced maritime flow from the Pacific in IPSL coincides with considerably reduced CAO frequencies of 60 to almost 100%. We note, however, that these SLP changes can represent decadal variability rather than a monotonic trend due to greenhouse forcing, particularly in the North Pacific, where air pressure varies strongly on this timescale (Graham, 1994; Bond and Harrison, 2000). In Eurasia the major response is a SLP increase of up to 4 hPa over the greater Mediterranean region, a robust pattern produced by all the models. This pressure change helps to explain the tempered reduction in simulated CAOs in most models from the Mediterranean eastward, where decreases of under 40% are Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1140 S. VAVRUS ET AL. (a) (b) 0 0.3 0.6 0.9 1.2 1.5 Figure 4. Geographical distribution of persistent strong anticyclonic pressure anomalies, given by the occurrences per winter of three or more consecutive days in which the SLP is at least two standard deviations above the local, seasonal mean. Shown are the 7-GCM means for the (a) late-twentieth century and (b) late twenty-first century common but isolated regions show increases. In contrast to the highly variable North Pacific SLP anomaly, this increase in pressure over Western Europe changes much more monotonically during the twenty-first century and is even more pronounced by the end of the simulation (see following text). By the end of the twenty-first century (final two decades), the GCMs predict very large decreases in CAO frequency of more than 50% at most locations and 85% averaged across the Northern Hemisphere (Figure 6). However, these projected changes vary strongly across models, ranging from the near disappearance of CAOs in MIROC to widespread increases over the North Atlantic and Asia in GISS. Despite these variations among GCMs, there are certain common threads that are helpful in assessing the patterns. The most prominent feature on land is the relatively small projected changes in CAOs over central Asia, roughly from the Black Sea eastward to China. Virtually all the models generate this response, either as a relatively small decrease in CAOs or as an outright increase (GISS). Another noteworthy terrestrial response is the smaller reduction in CAO frequency over western North America than over the eastern part of the continent. CCSM, GFDL, and PCM agree on this pattern, while GISS, GFDL, and MRI extend this muted response into Mexico. An interesting aspect of these results is the tendency for the reductions in CAOs to be smallest where mean Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1141 COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING Pct. Change in CAO Days (CCSM) -100 -80 -60 -40 -20 0 -5 Pct. Change in CAO Days (GISS) -100 -80 -60 -40 -20 0 -3 0 -5 0 1 3 5 -3 -1 1 3 -5 -3 -1 1 3 -100 -80 -60 -40 -20 5 -100 -80 -60 -40 -20 -3 -1 1 3 0 Pct. Change in CAO Days (MIROC) 5 -100 -80 -60 -40 -20 Change in SLP (IPSL) -5 0 Change in SLP (GFDL) -5 0 -3 -1 1 3 5 Change in SLP (MRI) Pct. Change in CAO Days (MRI) Change in SLP (PCM) Pct. Change in CAO Days (IPSL) -100 -80 -60 -40 -20 -1 Change in SLP (GISS) Pct. Change in CAO Days (PCM) -100 -80 -60 -40 -20 Pct. Change in CAO Days (GFDL) Change in SLP (CCSM) -5 -3 -1 1 3 5 Change in SLP (MIROC) -5 -3 -1 1 3 5 Pct. Change in CAO Days over Land 5 CCSM GFDL GISS MRI PCM MIROC IPSL mean Northern Hemisphere -68 -72 -72 -74 -86 -84 -77 -76 North America -51 -58 -77 -73 -81 -88 -88 -74 Europe -67 -80 -73 -64 -83 -86 -68 -74 Asia -75 -75 -68 -77 -90 -77 -71 -76 Figure 5. Simulated percentage change in the number of CAO days/winter (left columns) and wintertime SLP (hPa, right columns) in the mid-twenty-first century (2045–2065). Contours bound regions where the change in CAO frequency does not represent a statistically significant decrease at the 90% confidence limit. Regions with less than 0.5 CAO day/winter in the late-twentieth century simulations are masked out. Greater CAO frequency is denoted in dark red, regardless of the percentage increase. The table summarizes the changes in CAO frequency over land for the entire Northern Hemisphere domain (20° –90 ° N), North America (20° –70 ° N, 165° –60 ° W), Europe (35° –70 ° N, 10 ° W–60 ° E), and Asia (20° –75 ° N, 60° –180 ° E) Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1142 S. VAVRUS ET AL. Change in SLP (CCSM) Pct. Change in CAO Days (CCSM) -100 -80 -60 -40 -20 0 Pct. Change in CAO Days (GISS) -5 -100 -80 -60 -40 -20 0 Pct. Change in CAO Days (PCM) -5 -100 -80 -60 -40 -20 0 -3 -1 1 3 5 Change in SLP (GISS) -5 Pct. Change in CAO Days (IPSL) Change in SLP (GFDL) Pct. Change in CAO Days (GFDL) -100 -80 -60 -40 -20 -5 0 -3 -1 1 3 5 Change in SLP (PCM) -3 -1 1 3 5 Change in SLP (IPSL) -100 -80 -60 -40 -20 0 -5 -1 1 -3 -1 1 -5 0 -3 -1 1 Pct. Change in CAO Days over Land North Northern Hemisphere America Europe -100 -80 -60 -40 -20 0 -5 -3 -1 1 3 5 CCSM GFDL GISS MRI PCM MIROC IPSL mean -83 -86 -73 -85 -83 -97 -91 -85 3 5 3 5 Change in SLP (MIROC) Pct. Change in CAO Days (MIROC) -100 -80 -60 -40 -20 -3 Change in SLP (MRI) Pct. Change in CAO Days (MRI) -79 -84 -85 -85 -86 -98 -93 -87 -74 -93 -76 -83 -77 -99 -88 -84 3 5 Asia -87 -80 -61 -85 -84 -95 -94 -84 Figure 6. As in Figure 5 but for the late twenty-first century (2080–2099) Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING 1143 temperatures are relatively warm, such as in central Asia and western North America, compared with colder locations over northern Eurasia and northeastern North America, where the intermodel mean decrease in CAO frequency exceeds 90%. The most noticeable change over the oceans occurs in GISS over the North Atlantic, where an extensive increase in CAOs occurs in conjunction with a remarkable mean wintertime cooling of up to 3 K. This regional decrease in temperature is consistent with the response of the GISS GCM reported by Russell and Rind (1999) who attributed it to a weakening of the thermohaline circulation brought on by enhanced atmospheric moisture transport under greenhouse forcing. The primary implication of this localized cooling for our study is that CAOs become more numerous than during the late-twentieth century over Iceland, Scotland, and Ireland in this model. As with the mid-twenty-first century response, the aforementioned regional variations in projected CAO changes during the end of the century can largely be explained by associated changes in mean atmospheric circulation (Figure 6). The most prominent SLP change affecting CAOs is the anticyclonic anomaly centered around the Mediterranean–Caspian Sea region, which is present in every model. This pressure build-up, which averages 3 hPa among the models, helps to mitigate dynamically the reduction of CAOs east of the Mediterranean Sea by promoting enhanced northerly flow of continental air masses into central Asia. A similar offsetting mechanism occurs in the PCM from the anticyclonic anomaly centered over northern Eurasia. The relatively smaller CAO changes over western versus eastern North America also appear to be sensitive to mean circulation changes, namely the tendency for SLP increases along the west coast. This pressure rise may be coupled to pressure falls over the central Arctic and/or the North Pacific, where the Aleutian Low deepens in most of the GCMs. The western North American SLP increase in CCSM, GISS, PCM, and GFDL causes greater mean northerly flow across the western half of the continent in these models and thus helps to explain the smaller decrease in CAO occurrence in this region compared with eastern North America. 4. DISCUSSION Our most important finding is that although CAOs (with respect to late-twentieth century standards) should become less common during this century, these extreme cold events will not disappear, despite pronounced increases in mean wintertime temperature. The models even project increased CAO frequency in certain regions through the 2090s. This result is consistent with that of Hunt and Elliott (2004), whose transient greenhouse simulation using the CSIRO GCM produced very strong CAOs over North America through the 2060s and over Eurasia through the 2080s, even though their model was driven by a more extreme CO2 forcing (SRES A2 scenario) than the A1B case we used. Hunt and Elliott noted that CAOs could exist in an increasingly greenhouse-forced climate due to the continued presence of intense polar anticyclones that advect Arctic air masses into middle latitudes. Our study of several GCM runs underscores the potential for atmospheric dynamics to mitigate the thermodynamic tendencies from greenhouse warming. We have demonstrated that changes in the mean circulation can account for many of the changes in extreme patterns. The relationship between climatic means and extremes is not obvious, as noted by Hegerl et al. (2004), who reported that two different GCMs forced by rising CO2 indicated changes in temperature extremes significantly different from changes in seasonal means over a large fraction (39–66%) of model grid points. A similar inconsistency occurs in our study, with respect to the magnitudes of greenhouse forcing, mean temperature changes, and decreases in CAO frequency. The radiative forcing in scenario A1B from the late 20th to mid-twenty-first century (+3.4 W m−2 ) is 69% as large as that between the late 20th to the late twenty-first century (+4.9 W m−2 ) (IPCC, 2001). The corresponding mean wintertime temperature increase, averaged across Northern Hemisphere land (20° –90 ° N) through the mid-twenty-first century (3.1 K) and the late twenty-first century (4.5 K), displays exactly the same 69% proportionality. However, the corresponding hemispheric-wide CAO reductions by the mid-twenty-first century are 89% as large as those at the end of the century (see tables in Figures 5 and 6), indicating that CAOs respond much more strongly during the first half of the twenty-first century. If the trend in CAO reductions up to the 2050s had persisted throughout the twenty-first century–in the same near-linear manner as the trends in greenhouse forcing and Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1144 S. VAVRUS ET AL. mean winter temperature–then CAOs would have disappeared during the 2070s. Instead, the hemisphericaveraged frequency of these extreme cold events dropped only slightly as the climate continued to warm considerably during the late twenty-first century. The major circulation features that shape future CAO behavior – enhanced ridging centered over southern Europe and strong decadal variability of SLP in the North Pacific – need to be considered when interpreting actual trends in CAOs during this century. In addition, the inherent tendency for blocking events to develop over the North Pacific–Alaska region not only accounts for the abundance of CAOs over northwestern North America in the present climate, but their increased prevalence in the future also appears to mitigate the reduction of CAOs as the climate warms (Figures 2, 4). Over western North America in the CCSM and GFDL models, a striking similarity exists between the preferred regions for CAOs in the late-twentieth century (Figure 7) and the locations with the smallest changes in CAO frequency during the middle and late twenty-first century (Figures 5, 6). There is a strong resemblance in each model’s simulated anomaly patterns between the middle and late twenty-first century, suggesting that the future behavior of CAOs in this region may be regulated largely by blocking events, in addition to influences by mean circulation changes. Furthermore, these two GCMs, which notably have the highest resolution of the 7 models, seem to show the topographic signature of the Rocky Mountains in both the modern and future CAO simulations. To the extent that high resolution provides additional model credibility, the results of these two GCMs suggest that natural variability in the form of short-term blocking events and decadal-scale oscillations over the North Pacific may provide a physical basis for expecting smaller changes in CAOs in western than in eastern North America. The robust intermodel mean SLP increase of 3 hPa over the Mediterranean region, which strongly affects CAO changes in Eurasia, may be in part a response to the common tendency of models to simulate decreased air pressure over the Arctic in greenhouse simulations (IPCC, 2001). The intermodel mean change in Arctic (70° –90 ° N) wintertime SLP is −2 hPa from the late 20th to the late twenty-first century, and all but one GCM produces a pressure fall during this period. The central Arctic response is important because of its known linkage to mid-latitude SLP anomalies through the NAO and AO. In many of the SLP anomaly maps in Figures 5 and 6, there appears to be an association between the evacuation of atmospheric mass in Polar (a) (b) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Figure 7. Simulated number of CAO days/winter during the late-twentieth century in the (a) CCSM and (b) GFDL models Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) COLD AIR OUTBREAKS UNDER GREENHOUSE WARMING 1145 Regions and the accumulation in middle latitudes. However, the details of exactly where the pressure rises and the associated effects on CAOs vary among models, as some GCMs produce a strongly AO-like pattern (GFDL, MIROC), while another (PCM) simulates large SLP decreases in the Arctic without the attendant Atlantic anticyclonic anomaly, which instead is situated over the Eurasian land mass. The drop in Arctic SLP may also be important for explaining the very large reduction in CAOs over eastern North America, where, in most models, outbreaks almost disappear by the late twenty-first century. Our analysis of the atmospheric component of the CCSM (Community Atmosphere Model, CAM3) under modern conditions (Vavrus et al., 2005) has revealed that very large positive Arctic SLP anomalies are more necessary for the generation of CAOs in the eastern half of the United States than on the western side. In the week following an exceptional build-up of Arctic pressure during winter (normalized SLP anomaly of at least 1.5 standard deviations), a simulated CAO occurred in the western United States 70% of the time (compared with 47% in an average week) versus 85% of the time (34% in an average week) in the eastern U. S. Furthermore, extensive CAOs (>50% aerial coverage) were preceded by a positive Arctic-wide SLP anomaly during the week prior to the outbreak only 47% of the time in the western half of the country (average normalized pressure anomaly of +0.22) versus 93% of the time in the eastern United States (average normalized pressure anomaly of +1.07). These findings are consistent with observational studies (Walsh et al., 2001), which show antecedent Arctic SLP anomalies of 12 to 18 hPa during the week preceding a CAO in the eastern United States. 5. CONCLUSIONS We have analyzed a suite of seven GCM simulations from the IPCC model archive to investigate the behavior of extreme CAOs in the present climate and in the future across the Northern Hemisphere. The projected climatic response is forced by sustained increases in greenhouse gas concentrations, based on the SRES A1B emission scenario through the year 2100. The climate models reproduce the primary features of modern CAOs, with respect to location and magnitude. Maximum frequencies of about four CAO days/winter are simulated over western North America and Europe, while minimal occurrences of less than one day/winter exist over the Arctic, northern Africa, and parts of the North Pacific. The geographic pattern of CAO frequency is strongly correlated with the skewness of the daily temperature frequency distribution, such that favored locations for CAOs experience highly negative skewness. Projected changes in twenty-first century CAOs also vary strongly by region. Reductions in CAOs over land are generally smallest in western North America and in southern regions of Europe and Asia. These areas are subject to offsetting dynamical factors, which can be grouped into two categories: changes in mean circulation and the effects of internal variability. The muted CAO changes over parts of Eurasia are mainly caused by the first mechanism, namely the tendency for anomalous high pressure to develop in the vicinity of the Mediterranean Sea and to promote enhanced continental flow from the Eurasian Arctic. The relatively small CAO reductions over western North America are attributable to a combination of anomalous anticyclonic flow downstream from higher pressure in the North Pacific and the increased prevalence of blocking events that form over the North Pacific/Alaska region and drive cold air masses toward the equator. Our study leads to several cautionary notes. First, because we have used only one ensemble member for each GCM’s simulation of the twentieth and twenty-first centuries, there is a possibility that our results are biased by model-generated internal variability. Although utilizing more ensemble runs per model would increase confidence in our conclusions (by helping to filter out natural decadal-scale climate oscillations), not all of the GCMs had multiple realizations available. Furthermore, averaging a model’s ensemble simulations may hide the potential for internal oscillations to produce an unlikely interval of climatic behavior that could occur from the single realization of future climate that nature will actually generate. Second, we note that the smallest projected changes in CAO frequency generally occur where the mean climate is relatively mild and where these extreme events can thus be most damaging. For example, many of the greatest economic losses from modern CAOs within the United States are borne by Florida citrus growers, while in Asia the recent CAO during the winter of 2002–2003 caused over 1000 fatalities in India and Bangladesh (Rogers and Rohli, 1991; http://www.ncdc.noaa.gov/oa/climate/research/2003/jan/hazards.html#Winter). Finally, our Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1133–1147 (2006) 1146 S. VAVRUS ET AL. investigation underscores that the relationship between climatic means and extremes is not obvious. The lack of correlation between recent variations in cold extremes and the observed mean warming trend is consistent with the model results, indicating that severe CAOs even by current standards are likely to continue throughout the twenty-first century and that surprises, such as regional increases in CAOs, may still occur amid an overall greenhouse-warmed world. 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