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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L14702, doi:10.1029/2007GL030196, 2007
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Causes of the northern high-latitude land surface winter
climate change
Jiping Liu,1 Judith A. Curry,1 Yongjiu Dai,2 and Radley Horton3
Received 29 March 2007; revised 18 May 2007; accepted 14 June 2007; published 18 July 2007.
[1] In the past two decades, the northern high-latitude
(>40°N, NHL) land surface winter climate has experienced
some of the most rapid changes on Earth, warming at almost
2.5 times the global average warming. Here we examine
impacts of two dominant climate modes - Arctic Oscillation
(AO) and Aleutian Low (AL) - on the winter NHL land
surface temperature for the period 1958 –2004, showing that
1) a combination of the AO and AL explains 70% of the
variance of the NHL land surface temperature variability,
including the cooling during the 1960s and 1970s and the
persistent warming thereafter; and 2) interactions between
the AO and AL tend to constrain impacts of the extremes
of the AO and AL and minimize spatiotemporal extremes in
the NHL climate. The amplification of the winter NHL land
surface warming relative to the global average warming can
be attributed to the combination of the AO and AL,
although the extent to which the greenhouse warming may
influence the AO and AL is uncertain. Citation: Liu, J., J. A.
Curry, Y. Dai, and R. Horton (2007), Causes of the northern highlatitude land surface winter climate change, Geophys. Res. Lett.,
34, L14702, doi:10.1029/2007GL030196.
1. Introduction
[2] In the past few decades, the annual average temperature over the NHL land surface has risen at almost twice
the rate of the global average, disrupting the region and
its people in many ways [e.g., Arctic Climate Impact
Assessment, 2004]. Based on the long-term station data
from the Global Historical Climatology Network and
Climatic Research Unit, annual average temperatures in
virtually all NHL land regions increased during 1966 –
2003, with trends exceeding 1 – 2°C/decade in northern
Eurasia and northwestern North America [Arctic Climate
Impact Assessment, 2004]. Likewise, surface temperature
derived from satellite thermal infrared measurements, which
covers the period 1981– 2001, exhibited statistically significant warming trends of 0.5°C/decade in NHL Eurasia, and
1.06°C/decade in NHL North America [Comiso, 2003].
Accompanying the warming, the NHL land surface climate
(i.e., precipitation, snow cover and vegetation) has been
undergoing some of the most rapid and severe changes
on Earth, which has been associated with wide-ranging
impacts on the environment, regional societies and econo1
School of Earth and Atmospheric Sciences, Georgia Institute of
Technology, Atlanta, Georgia, USA.
2
School of Geography, Beijing Normal University, Beijing, China.
3
Center for Climate Systems Research, Columbia University, New
York, New York, USA.
mies [Serreze et al., 2000; Arctic Climate Impact Assessment,
2004]. Because of these impacts and the role of the NHL land
regions in global climate, it is important to understand the
mechanisms responsible for the temperature variability over
the NHL land surface so as to develop a more reliable
predictive capability and understand more completely the
interaction of this region with the global climate system.
Some previous studies have shown associations between the
Northern Hemisphere extratropical circulation and NHL land
surface climate [e.g., Palecki and Leather, 1993; Thompson
et al., 2000; Honda et al., 2005]. Here we examine the roles of
two dominant patterns of atmospheric circulation variability
(the Arctic Oscillation and Aleutian Low) individually and
in combination in modulating the interannual-to-decadal
variability of the NHL land surface temperature.
[3] The cause of the Arctic amplification of greenhouse
warming has been traditionally attributed to the ice albedo
feedback mechanism. Other possible explanations have
included dynamical feedbacks associated with atmospheric
or oceanic poleward heat transport [Holland and Bitz, 2003;
Alexeev et al., 2005; Cai, 2006], and internal feedbacks
associated with polar processes [Overland et al., 2004].
Here we propose an alternative mechanism associated with
the variability of the combination of the Arctic Oscillation
and Aleutian Low.
2. Index and Data
[4] The Arctic Oscillation (AO) is defined as the leading
mode of Northern Hemisphere sea level pressure (SLP)
variability from the empirical orthogonal function (EOF)
analysis. A series of papers have shown that the AO exerts a
strong influence on the winter atmospheric circulation in the
northern mid- and high- latitudes [e.g., Thompson and
Wallace, 2000]. The AO imprints a distinctive signature
on winter temperature anomalies in the Arctic and Eurasian
Continent, and the upward trend in the AO from 1968 to
1997 accounts for half of the observed winter warming over
Eurasia [Thompson et al., 2000; Moritz et al., 2002].
[5] The intensity and location of the Aleutian low (AL), a
semi-permanent subpolar area of low pressure, is a primary
indicator and driver of the North Pacific winter climate
[Overland et al., 1999]. The formation of the AL is
associated with the large-scale low frequency mean flow
or stationary wave pattern, and the constant stream of
storms that track across the northern North Pacific Basin.
Here the AL index is defined as the principle component of
the leading EOF mode of SLP variability over the region
30°N –65°N, 160°E– 140°W, which is strongly correlated
with the index defined by the area-weighted SLP (r = 0.997
[Trenberth and Hurrell, 1994]).
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030196$05.00
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from the NOAA Climate Monitoring and Diagnostics
Laboratory (http://www.cdc.noaa.gov/ClimateIndices). We
confine the analysis for the period 1958 – 2004. Comparisons of the NCEP/NCAR reanalysis with station observations suggest high monthly correlations [e.g., Cullather and
Lynch, 2003], which gives us more confidence about the
NCEP/NCAR reanalysis in NHL.
3. Results and Discussion
Figure 1. Time series of (a) the standardized Arctic
Oscillation, (b) the standardized Aleutian Low (positive
values correspond to strong Aleutian low), (c) the combined
standardized AO and AL, (d) the NHL land SAT anomalies
(°C) and (e) the NASA GISS global SAT anomalies for the
period 1958 –2004.
[6] Here we analyze 47 years (1958 – 2004) of winter
average data (defined as December, January and February).
Specific data sets used include the NCEP/NCAR reanalysis
(National Centers for Environmental Prediction and the
National Center for Atmospheric Research [Kistler et al.,
1996]), the Arctic land surface air temperature compiled
from several sources (see http://climate.geog.udel.edu/
climate for details), and the NASA Goddard Institute
for Space Studies global surface air temperature analysis
[Hansen et al., 1999]. The AO and AL indices are obtained
[7] Figures 1a and 1b show the year-to-year variability of
the standardized wintertime AO and AL indices. The
correlation between the two time series is 0.35, suggesting
that the variability of the AO and AL tends to be out of
phase for the period 1958 –2004. However, the negative
correlation is moderate, only accounting for 12% of the
shared variance. For some periods (for example, from 1963
to 1968), the AO and AL were in phase.
[8] To show the spatial changes of the winter NHL
climate in response to changes in the amplitude and polarity
of the AO and AL for the period 1958– 2004, the sea level
pressure (SLP) and surface air temperature (SAT) anomalies
were regressed on the standardized AO and AL indices
(Figure 2). Consistent with previous studies [e.g., Thompson
and Wallace, 2000], the SLP pattern associated with the AO
show one sign over most of the Arctic Ocean and the
opposite sign over Europe/the North Atlantic and the North
Pacific. However, the SLP anomalies induced by the AO are
not statistically significant in the North Pacific. Rather,
statistically significant SLP expressions induced by the
AO are only found in the Arctic Ocean, northern Siberia
and Canada, and the North Atlantic and western Europe,
which resembles the North Atlantic Oscillation (NAO
[Hurrell, 1995]) pattern (Figure 2a). By contrast, the SLP
pattern induced by the AL shows well below-normal values
over the North Pacific and Alaska, indicating a deepening of
low pressure in this region, and small above-normal values
extending from the Arctic Ocean to the North Atlantic.
However, the AL only has statistically significant SLP
expression in the North Pacific (Figure 2b). Hence, the
AO and AL show different atmospheric circulation response
characteristics, implying that the wintertime AO and AL are
associated with nearly independent physical processes in
influencing the NHL climate.
[9] Consistent with the dynamic response in atmospheric
circulation, the SAT pattern associated with the AO has
statistically significant large positive anomalies over northern Eurasia and large negative anomalies over northeastern
Canada and west Greenland, with moderate positive anomalies over northeastern North America and moderate negative
anomalies over Alaska and far Eastern Siberia (Figure 2c).
By contrast, the SAT pattern connected with the AL shows a
statistically significant strong warming extending from
Alaska to central Canada (Figure 2d). Thus, the AO and
AL influence the NHL land surface climate substantially,
with the AO having greater impact over northern Eurasia,
and the AL having greater impact on western North
America.
[10] To clarify the individual roles of the AO and AL on
the winter NHL land surface climate, we remove the
interactions between the two modes by removing linearlyregressed impacts of the AL from the original SLP and SAT
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Figure 2. Regression coefficients of the SLP (mb, contour) and SAT (°C, shaded) on the standardized AO (a, c) and AL
(b, d) indices (The light shading in Figures 2a and 2b and the contour in Figures 2c and 2d represent correlation coefficients
that are significant at the 95% confidence level).
anomaly time series in each grid cell. Then, the residual
SLP and SAT are regressed onto the AO index after
removing the impacts of the AL. The residual SLP pattern
(Figure 3a) resembles the original SLP pattern (Figure 2a).
However, the low pressure anomaly now extends from the
Arctic Ocean to the North Pacific, pushing the original high
pressure anomaly towards the Sea of Okhotsk. This results
in anomalous southwesterly winds, favoring more frequent
incursions of warm air masses from the North Pacific into
high-latitude western North America, unlike the original
eastward propagation of weather systems that favors more
frequent incursions of cold, polar air masses into Alaska.
Thus, without the influence of the AL, the AO makes the
warming over western and central North America much
stronger, and shifts the maximum warming center from
southeastern Canada to central Canada. Moreover, the temperature in eastern and central Siberia increases by 0.5°C
(Figure 3c) as compared to Figure 2c. Therefore, the existence of the AL and its interactions with the AO tend to make
the AO a more zonal mode in the North Pacific, reducing the
AO’s warming (cooling) impact on the NHL land surface for
the positive (negative) polarity of the AO.
[11] Similarly, the impacts of the AL with the AO
removed on the residual SLP and SAT with the AO removed
were also determined. Interestingly, without the impacts of
the AO, the central intensity of the AL becomes slightly
weaker, and the spatial coverage of the AL becomes slightly
smaller in the North Pacific (Figure 3b) as compared to
Figure 2b. However, the AL now extends further northward
and sets up a statistically significant broader low pressure
anomaly extending from the Canadian Archipelago to the
Kara and Barents Sea. This results in anomalous westerly
winds, favoring more frequent advection of warm air
masses from the northern Atlantic into northeastern Europe
and eastern Siberia, which increases the temperature over
eastern Siberia by 0.5– 1°C (also statistically significant,
Figure 3d) as compared to Figure 2d. More importantly, the
AL now has statistically significant SLP connections with
north Europe and the northern Atlantic. Specifically, with
the interactions between the AO and AL, there is no
correlation at all between the AL and NAO indices (r =
0.03), but after removing the impacts of the AO, there is a
moderate correlation between the AL and NAO indices (r =
0.26, significant at the 90% confidence level). The implication is that without the interference of the AO, it is easier
for the AL to set up teleconnections. To support this, the
winter 300hpa stream function was regressed on the AL
index with and without the impacts of the AO, respectively.
The result shows a more distinct wave number 3 pattern
once the AO is removed (Figure 3f), not only over the
northeastern Atlantic, but also over eastern and central
Siberia. Therefore, the existence of the AO and its interactions with the AL tend to make the AL a more local mode
rather than teleconnectivity (propagating along the wave
guide in the polar jet [Huth, 2006]), reducing the AL0s
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Figure 3. Same as Figure 2, except Figures 3a and 3c (Figures 3b and 3d) are regression coefficients of the residual SLP
and SAT with the AL removed (AO removed) on the standardized AO (AL) index with the AL (AO) removed; Regression
coefficients of the 300hpa stream function (m2/s 106, contour) on the AL index (e) with and (f) without the impacts of
the AO (The light shading are significant at the 95% confidence level).
warming (cooling) impact on the NHL land surface for the
positive (negative) polarity of the AL.
[12] Therefore, the positive (negative) AO would cause
more warming (cooling) if the AL did not have a tendency
to act against it, and vice versa for the AL. That is, the
coupling of the AO and AL tends to constrain the impacts of
the extremes of the AO and AL, leading to relatively smaller
temperature anomalies than would occur if the AO and AL
work as independent physical processes. Hence, the coupling of the AO and AL serves to minimize spatiotemporal
extremes in the winter NHL land surface temperature.
[13] During the period 1958 –2004, a winter warming
trend of 0.4 °C/decade is found over the NHL land surface
(significant at the 95% confidence level). Overlain on this
trend is considerable decadal variability. As shown in
Figure 1d, there was a dramatic cooling during the 1960s
and 1970s (the cooling over the NHL land surface is also
almost three times greater than the global average cooling
during this period), and a persistent warming thereafter.
Many studies suggest that this warming is connected with
the positive trend in the AO [Thompson et al., 2000; Moritz
et al., 2002]. However, the AO index has had a relatively
different, more episodic behavior over the 47-year period as
compared to the time series of the NHL land surface
temperature (Figure 1). In fact, the correlation analysis
(Table 1) reveals that the correlation between the AO and
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Table 1. Correlations Between NHL Land SAT and Indices for the
Periods 1958 – 2004 and 1960 – 1979 (the Cooling Period), and
NHL Land SAT Trends That are Linearly Congruent With Indices
for the Period 1958 – 2004
Index
1958 – 2004
1960 – 1979
1958-, °C/decade
AO
AL
AO + AL
0.48a
0.45a
0.83a
0.31
0.65a
0.85a
0.12
0.08
0.35
a
Correlations significant at the 99% confidence level (student’s t-test).
NHL land SAT is 0.49 for the period 1958 – 2004, only
accounting for 24% of the shared variance. And, the
correlation decreases to 0.31 for the cooling period (during
the 1960s and 1970s). Thus, the AO explains only part of
the NHL land surface temperature variability. Similarly, the
correlation between the AL and NHL land SAT is 0.45 for
the period 1958 – 2004, only accounting for 21% of the
shared variance. However, for the cooling period (during the
1960s and 1970s), the AL had a larger correlation (r = 0.65)
with the NHL land SAT than did the AO (r = 0.31). This
analysis suggests that although the AO has received much
more attention than the AL in the last decade, the two
modes have comparable effects on the winter NHL land
surface temperature.
[14] To what extent can the interannual-to-decadal variability of the winter NHL land surface temperature be
explained by the combination of the AO and AL? As a
first step attempt, we simply add the two standardized AO
and AL time series together (Figure 1d). Surprisingly, the
new combined time series have much higher correlation (r =
0.83) with the NHL land SAT than either the AO or AL
alone. This new index (AO + AL) explains almost 70% of
the shared variance, which is a factor of three greater than
the shared variance with either the AO or AL alone.
Moreover, the correlation is robust for the cooling period
during the 1960s and 1970s (r = 0.85) that have proved
difficult to explain using the AO alone. Thus, the combination of the AO and AL accounts for much of the
variability of the winter NHL land surface temperature.
[15] The Arctic Climate Impacts Assessment summarized: ‘‘over the past 50 years, it is probable (66 – 90%
confidence) that Arctic amplification of greenhouse warming has occurred’’. And there is agreement among climate
models that the NHL warms much more than the rest of the
globe when subject to increasing levels of greenhouse gases
in the atmosphere [Arctic Climate Impact Assessment,
2004]. Our analysis suggests that the NHL land surface
does show an amplified rate of warming (0.4°C/decade) as
compared to the global average warming (0.16°C/decade)
for the period 1958 – 2004 (an amplification factor of 2.5).
We further calculate the NHL land SAT trends that are
linearly congruent with the AO, AL and AO + AL indices.
Surprisingly, the combination of the AO and AL can
account for 0.35°C/decade warming over the NHL land
surface, which is a factor of three greater than that explained
by either the AO and AL alone (Table 1). The ratio of the
NHL land SAT trend induced by AO + AL (0.35°C/decade)
and the global SAT trend (0.16°C/decade) is 2.2, which is
close to the observed amplification factor of 2.5. Thus, the
combination of the AO and AL contributes to the amplification of the winter NHL land surface warming relative to
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the global average warming, although the extent to which
the greenhouse warming may influence the AO and AL is
uncertain.
[16] It has been speculated that the greenhouse forcing
may favor the positive mode of the AO that also favors sea
ice losses [Shindell et al., 1999; Moritz et al., 2002]. Several
possible mechanisms that may enhance the AO in the future
have been reviewed by Gillett et al. [2003]. Although, there
is no consensus on how the AO will respond to increased
greenhouse gases in the future and presently the AO has
swung to the near-neutral or negative phase [Overland and
Wang, 2005], many coupled global climate models suggest
a more positive AO as climate warms [Fyfe et al., 1999;
Teng et al., 2006] Meanwhile, the AL may become deeper
in the future, since sea ice in the North Pacific may decrease
as climate warms. A more positive AO and a deeper AL
would be expected to produce increased warming of the
NHL land surface. However, if the interactions between the
two modes break down, even more warming could occur,
owing to the offsetting tendencies of the two modes that
reduce spatiotemporal extremes in the NHL climate. To the
extent that these modes are predictable, our finding can be
of practical use in applications that involve assessing future
winter NHL land surface warming. It would be also
worthwhile to extend the analysis backwards in time to
provide a better understanding of the variability of the
combination of the AO and AL.
[17] Acknowledgments. This research was supported by DOE
Atmospheric Radiation Measurement Program and NSFC under grant
40225013.
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J. A. Curry and J. Liu, School of Earth and Atmospheric Sciences,
Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, USA.
([email protected])
Y. Dai, School of Geography, Beijing Normal University, Beijing,
100875, China.
R. Horton, Center for Climate Systems Research, Columbia University,
New York, NY 10025, USA.
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