Download The behavior of extreme cold air outbreaks under greenhouse

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]
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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)
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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
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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
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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).
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(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
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(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)
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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
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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
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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
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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
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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
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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.
ACKNOWLEDGEMENTS
This project is supported by collaborative NSF grants ATM-0332099, ATM-0332081 and OPP-0327664. We
acknowledge the international modeling groups for providing their data for analysis, the Program for Climate
Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the JSC/CLIVAR
Working Group on Coupled Modeling (WGCM) and their Coupled Model Intercomparison Project (CMIP)
and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC WG1 TSU for
technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the
Office of Science, U.S. Department of Energy.
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