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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L02504, doi:10.1029/2007GL032023, 2008

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ArticleEvolution of Arctic sea ice concentration trends and the role ofatmospheric circulation forcing, 1979–2007
Clara Deser1 and Haiyan Teng1
Received 13 September 2007; revised 31 October 2007; accepted 13 December 2007; published 22 January 2008.

[1] The retreat of Arctic sea ice in recent decades is apre-eminent signal of climate change. What role has theatmospheric circulation played in driving the sea ice decline?
To address this question, we document the evolution of
Arctic sea ice concentration trends during the period January
1979 – April 2007 in light of changing atmosphericcirculation conditions, in particular an upward trend in thewintertime Northern Annular Mode during the first half ofthe record and a downward trend during the second half. The
results indicate that concurrent atmospheric circulation
trends contribute to forcing winter and summer sea ice
concentration trends in many parts of the marginal ice zone
during both periods. However, there is also an emerging
signal of overall Arctic sea ice decline since 1979 in both
winter and summer that is not directly attributable to a trend
in the overlying atmospheric circulation. Citation: Deser, C.,
and H. Teng (2008), Evolution of Arctic sea ice concentration
trends and the role of atmospheric circulation forcing, 1979 – 2007,
Geophys. Res. Lett., 35, L02504, doi:10.1029/2007GL032023.

1. Introduction[2] The accelerating retreat of Arctic sea ice in recent
decades, evident in all months of the year, is one of the mostdramatic signals of climate change worldwide. The physicalmechanisms underlying the Arctic sea ice decline are notfully understood, but include dynamical processes related tochanges in winds and ocean currents, and thermodynamic
processes involving changes in air temperature, radiative
and turbulent energy fluxes, and ocean heat storage [Serreze
et al., 2007; Francis and Hunter, 2006, 2007]. A better
understanding of these mechanisms and their relationship toincreasing greenhouse gas concentrations is an important
step for assessing future predictions of Arctic climate change.
[3] What role has the atmospheric circulation played indriving the Arctic sea ice retreat? The consensus of numerous
studies [e.g., Deser et al., 2000; Rigor et al., 2002; Hu et al.,
2002; Rothrock and Zhang, 2005; Stroeve et al., 2007;
Serreze and Francis, 2006; Ukita et al., 2007] is that Arctic
sea ice declines from the 1960s to the early 1990s were
driven in part by a trend in the dominant pattern ofwintertime atmospheric circulation variability over thehigh-latitude northern hemisphere known variously as the‘‘Arctic Oscillation’’, ‘‘North Atlantic Oscillation’’, or‘‘Northern Annular Mode’’ (collectively referred to hereafteras the ‘‘NAM’’). In particular, the anomalous cyclonic windcirculation associated with the upward trend in the winter1

Climate and Global Dynamics Division, NCAR, Boulder, Colorado,USA.
Copyright 2008 by the American Geophysical Union.
0094-8276/08/2007GL032023$05.00

NAM flushed old, thick ice out of the Arctic via Fram Strait,causing the winter ice pack to thin which in turn preconditioned the summer ice pack for enhanced melt. However,
the trend in the NAM has reversed sign since the early
1990s yet Arctic sea ice has continued to decline [Overland
and Wang, 2005; Comiso, 2006; Serreze and Francis, 2006;
Maslanik et al., 2007]. This has led to speculation that the
Arctic climate system has reached a ‘‘tipping point’’ whereby
strong positive feedback mechanisms such as those associated with ice albedo and open water formation efficiency areaccelerating the thinning and retreat of Arctic sea ice [e.g.,
Lindsay and Zhang, 2005; Holland et al., 2006]. These
positive feedback mechanisms leave the ice pack more
vulnerable to forcing from other processes, natural andanthropogenic. For example, enhanced downward longwave
radiation associated with increases in air temperature, water
vapor and cloudiness over the Arctic Ocean [Francis andHunter, 2006] has become a dominant factor driving summer sea ice extent declines since the mid-to-late 1990s.
There is also evidence that the winter atmospheric circulation has continued to impact the winter sea ice distribution
since the mid-1990s [Comiso, 2006; Maslanik et al., 2007;
Francis and Hunter, 2007].
[4] The purpose of this study is to revisit the issue ofArctic sea ice trends since 1979 in the context of evolving
atmospheric circulation conditions, in particular trends in
the NAM. How have the pattern and magnitude of winterand summer Arctic sea ice concentration trends changed
from the first half (1979 – 1993) of the record when the trend
in the NAM was positive to the second half of the record(1993– 2007) when the trend in the NAM was negative?
What is the nature of the association between trends in
Arctic sea ice concentration and atmospheric circulation forthe period 1979 – 2007 as a whole?

2. Data and Methods[5] Monthly sea ice concentrations (SIC) on a 25 km 
25 km grid for the period January 1979 – April 2007 wereobtained from the National Snow and Ice Data Center(NSIDC). These data are derived from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) and
Defense Meteorological Satellite Program (DMSP) -F8,
-F11 and -F13 Special Sensor Microwave/Imager (SSM/I)
radiances using the NASA team algorithm [Cavalieri et al.,1999]. Monthly sea level pressure (SLP) data are from the
NCEP/NCAR Reanalysis Project [Kalnay et al., 1996] on a
2.5°  2.5° latitude grid.
[6] Arctic sea ice undergoes a large seasonal cycle, withmaximum extent in February – March and minimum extent
in August – September [Comiso, 2006]. Because SIC trends
are highly persistent from February – March to April, andL02504

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Figure 1. (left) Long-term mean sea ice concentrations (%) for (a) winter (February – April) and (b) summer (August –October) based on the period January 1979 – April 2007. The white ellipse around the North Pole indicates missing data.
(right) (c) – (e) Winter and(f) – (h) summer sea ice concentration trends (% per decade) during 1979 – 1993 (Figures 1c and1f), 1993– 2007 (Figures 1d and 1g) and 1979 – 2007 (Figure 1e and 1h). Note that 2006 is the last year of data in summer.

from August –September to October (not shown), we havedefined winter and summer as February – April and August –October, respectively. Figure 1 (left) shows the long-termmean winter and summer SIC distributions based on the
period January 1979 – April 2007. In winter, long-termmean SIC values between 10% and 90%, indicative of thelocation of the marginal ice zone, are found in the Labrador
Sea, the Greenland and Barents Seas, the Bering Sea, andthe Sea of Okhotsk. In summer, the marginal ice zone
retreats northward to coastal regions of the Arctic Ocean
and the Canadian Archipelago.

3. Results
3.1. SIC Trends[7] The SIC trends over the first (1979 – 1993) andsecond (1993 –2007) halves of the record, and over theperiod 1979 – 2007 as a whole, in (top) winter and (bottom)
summer (note that 2006 is the last year of data for summer)
are shown in Figure 1 (right). The magnitudes of the trends
in each period may be directly compared as they are
expressed in % per decade. The regions of largest SIC
trends in each season correspond to the marginal ice zone as
depicted in the left-hand columns. The pattern of winter SIC
trends in the first half of the record (1979– 1993) exhibits
positive values in the Labrador and Bering Seas andnegative values in the Greenland and Barents Seas andthe Sea of Okhotsk. In contrast, winter SIC trends in thesecond half of the record (1993 – 2007) are negative
throughout the marginal seas, with the largest declines in
the Atlantic sector. The winter SIC trends over the fullperiod of record (1979 – 2007) are also negative throughout
the marginal seas, except in the Bering Sea where the trends
are near zero. The change in pattern of winter SIC trends
between the first and second halves of the record is notable
and will be discussed further below in the context of
evolving atmospheric circulation trends.
[8] In summer, SIC trends in the first half of the record
are negative in the East Siberian Sea and positive in the
Barents, Kara, and eastern Beaufort Seas, with the area of
reduced SIC outweighing that of increased SIC. In thesecond half of the record (1993 – 2006), the area of negative
SIC trends has expanded to cover almost all longitudes. The
summer SIC trend over the full period of record (1979–
2006) is similar to that for the second half of the record,with the largest declines extending from the Laptev Sea
eastward to the Beaufort Sea.
[9] To investigate whether the SIC trend patterns shownin Figure 1 correspond to preferred structures of variability,
we applied empirical orthogonal function (EOF) analysis tothe covariance matrix of SIC anomalies over the full periodof record, using a separate EOF analysis for winter and forsummer. The results indicate that the two leading EOFs ineach season closely resemble the trend patterns for the firstand second halves of the record (see auxiliary materials1).
3.2. SIC Trends in the Context of SLP Trends[10] Figure 2 shows the SIC trends in the context of theoverlying trends in SLP (note the larger domain used in
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GL032023.

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Figure 2. (top) Winter and (bottom) summer sea ice concentration (color shading; % per decade) and sea level pressure(contours; hPa per decade) trends during (a) and (d) 1979 – 1993, (b) and (e) 1993 – 2007, and (c) and (f) 1979 – 2007. The
contour interval for sea level pressure is 1 hPa per decade, with negative values dashed, and the zero and positive values
solid. Note that 2006 is the last year of data in summer.
Figure 2 compared to Figure 1). The SLP trends are basedon the same seasonal definitions used for SIC, and are very
similar to those leading by one month (e.g., January – Marchand July – September: not shown). The simultaneous (or1-month lead) relationships between seasonal SLP and SIC
trends provide an indication of the role of atmosphericcirculation forcing of sea ice anomalies [Fang and Wallace,1994]. However, there may also be a longer response time(e.g., seasonal and multi-year) associated with atmosphericforcing of sea ice thickness changes which in turn feedback
upon SIC [Rigor et al., 2002; Rigor and Wallace, 2004].
This component of the SIC response to atmospheric circulation forcing will not be addressed with our approach.
[11] The pattern of winter SLP trends in the first half ofthe record resembles the positive phase of the NAM, withnegative values over the Arctic and northern North Atlantic(maximum amplitudes 4 – 6 hPa per decade) and positive
values farther south with regional maxima over the central
Atlantic and the Bering Sea. This pattern is physicallyconsistent with the notion that the atmospheric circulation
is driving the SIC anomaly trends, with anomalous northwesterly winds over the enhanced sea ice cover in the
Labrador and Bering Seas, and anomalous southerly winds
over the regions of reduced ice cover in the Barents Sea andthe Sea of Okhotsk. Such wind anomalies affect SIC byaltering the surface fluxes of latent and sensible heatthrough air temperature and wind speed changes, as well

as by inducing ice drift and changes in sea surface temperatures and ocean currents.
[12] The pattern of winter SLP trends in the second halfof the record is largely opposite to that in the first half over
the Arctic and north Atlantic; the monopole of positive
values over the north Pacific in the later period contrasts
with the dipole pattern in the early period. The change insign of the Arctic/North Atlantic SLP trends between thefirst and second halves of the record is consistent with the
behavior of the winter NAM index which reached a relative
maximum in 1993 (not shown). The inferred trends in thegeostrophic wind field during the second half of the record
are indicative of enhanced southeasterly (southwesterly)
flow over the reduced SIC in the Labrador (Bering) Sea,consistent with the notion that the atmospheric circulation isdriving the SIC trends as discussed above. Thus, the change
in sign of the SIC trends in the Labrador and Barents Seas
between the two halves of the record may be attributed tothe change in sign of the overlying wind trends.
[13] The relationship between the SLP and SIC trends in
the Greenland Sea and the Sea of Okhotsk in the second halfof the record appears to be inconsistent with the notion of
atmospheric forcing. In particular, negative SIC trends
persist in the Greenland Sea beneath weak geostrophic wind
trends; and a lack of SIC trends in the Sea of Okhotsk
occurs despite geostrophic southwesterly wind trends. The
continuation of negative SIC trends in the Greenland Sea

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DESER AND TENG: ARCTIC SEA ICE AND SLP TRENDS SINCE 1979

from the first half of the record to the second half despite thelack of wind forcing may indicate that other processes suchas oceanic warming are starting to play a dominant role inthis region [see also Polyakov et al., 2005; Francis andHunter, 2007; Stroeve and Maslowski, 2008]. In the Sea of
Okhotsk, the cooling trend of autumn sea surface temperatures (not shown) may offset the impact of wintertime
wind forcing and account for the lack of SIC trends in thisregion during the second half of the record [Ogi andTachibana, 2006].
[14] Given the opposition of winter SLP trends in the firstand second halves of the record, it is not surprising thatwinter SLP trends over the full period of record are weak(Figure 2c), with magnitudes generally less than 1 hPa per
decade. The lack of pronounced SLP trends is in striking
contrast to the negative SIC trends found throughout themarginal ice zone. This result strongly suggests that processes other than winter atmospheric circulation trends arecontrolling the long-term retreat of winter SIC, for example
increased ocean heat transport and residual absorption ofsolar energy during summer.
[15] Summer SLP and SIC trends are shown in Figures 2d–
2f. We note that SLP trends in spring (May – July) yield very
similar geostrophic wind trends over the marginal ice zone
to those in summer (not shown). The summer SLP trends inthe first half of the record are indicative of southerly wind
anomalies over the negative summer SIC trends in the East
Siberian, Chukchi and Beaufort Seas, and northerly wind
anomalies over the positive summer SIC trends in the
Barents and Kara Seas, consistent with the notion that
atmospheric circulation forcing contributes to the SIC trends
as discussed previously. It is unclear to what extent summer
atmospheric circulation trends play a role in the decline ofsummer SIC in the second half of the record. In particular,
easterly rather than southerly geostrophic wind anomalies
overlie the negative SIC trends in the East Siberian and
Beaufort Seas, and northerly wind trends are found over theregion of negative SIC trends between Greenland andSiberia: the latter is counter to the relationship expected if
the atmospheric circulation were forcing these SIC trends.
[16] Summer SLP trends over the record as a whole(1979 – 2006) are small, with negative values (magnitudes
1 hPa per decade) over coastal Siberia and near-zerovalues elsewhere [see also Ogi and Wallace, 2007]. The
weak amplitude of the SLP trends suggests that summer(and spring) atmospheric circulation anomalies are not thedominant factor driving summer SIC declines since 1979,
although Ogi and Wallace [2007] have speculated that the
implied increase in Ekman divergence may thin the summer
ice pack and increase it’s susceptibility to melt.
[17] It is worth noting from Figure 2 that there is norobust relationship between SLP trends over the Arctic
Ocean in summer and winter. Although Arctic SLP trends
are negative in both seasons in the first half of the record,
they are positive in winter but of mixed sign in summer inthe second half

4. Summary and Discussion[18] The purpose of this study was to document theevolution of Arctic SIC trends in summer and winter during
the period January 1979 – April 2007 in light of changing

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atmospheric circulation conditions, in particular an upward
trend in the NAM during the first half of the record (1979–1993) and a downward trend during the second half (1993–2007). The spatial patterns of the winter SIC trends in thetwo halves of the study period are distinctive, reflecting inpart opposing trends in the overlying atmospheric circulation. These distinctive trend patterns correspond to the twoleading EOFs of winter SIC anomalies during 1979 – 2007.
In the earlier period, the winter SIC trends are characterized
by positive values in the western Atlantic and eastern
Pacific, and negative values in the eastern Atlantic andwestern Pacific, a pattern consistent with that due toatmospheric circulation forcing associated with a positive
trend in the NAM. In the second half of the record, thewinter SIC trends are negative nearly everywhere within themarginal ice zone. The change from positive to negative
SIC trends in the western Atlantic (Labrador Sea) andeastern Pacific (Bering Sea) is physically consistent withthe change in sign of the overlying atmospheric circulation
forcing from the first to the second half of the study period.
However, the continuation of negative SIC trends in the
Barents and Kara Seas since 1993 is inconsistent with thenotion of atmospheric circulation forcing.
[19] We also find a residual trend of declining winter SIC
throughout the marginal ice zone over the period of study as
a whole (1979 – 2007). Since the trend in the winteratmospheric circulation over this period is negligible, this
implies that other factors are responsible for the residual
winter SIC decline, for example enhanced ocean heatstorage and transport [Polyakov et al., 2005; Shimada etal., 2006; Stroeve and Maslowski, 2008] as well as increased oceanic absorption of solar energy during summer
associated with the decline in summer ice extent [Perovich
et al., 2007].
[20] The spatial patterns of summer SIC trends in the two
halves of the record show modest differences compared totheir winter counterparts, and resemble the leading EOF ofsummer SIC anomalies during 1979 – 2006. The 1979 –1993 period is characterized by weak positive SIC trends
in the Greenland, Barents, and Kara Seas, and larger
amplitude negative trends in the East Siberian, Chukchi,
and eastern Beaufort Seas. Summer atmospheric circulation
trends contribute to forcing this dipole pattern of SIC trends.
The periods 1993 – 2006 and 1979 – 2006 exhibit negative
SIC trends throughout the marginal ice zone. The weak
amplitudes of the concurrent summer and previous spring
SLP trends suggest that the atmospheric circulation is not
the dominant factor driving overall summer SIC declines
since 1979 and 1993.
[21] Francis and Hunter [2007] examined some of thefactors responsible for winter sea ice extent variability in the
Barents and Bering Seas during 1979 – 2005, including
satellite-derived estimates of downwelling longwave radiation, low-level winds, and sea surface temperatures (SSTs).
Wind anomalies were found to be the dominant forcing
mechanism in the Bering Sea, while a combination of wind
and SST anomalies were the main factors in the Barents Sea.
In a related paper, Francis and Hunter [2006] showed that
downward longwave radiation was the primary cause ofsummer sea ice extent variations during 1979 – 2004
throughout the Arctic marginal ice zone. They also reported
that wind anomalies played a role in forcing summer sea ice

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extent anomalies in the Barents and Chukchi Seas before,
but not after, 1991. The results shown here based on SLP
trends are in general agreement with the findings of Francis
and Hunter [2006, 2007], although we note that a directcomparison is not possible due to the different time scales ofvariability examined in the two studies (interannual andlonger in the case of Francis and Hunter, and trends in ourcase). In future work, we plan to extend our analysis toinclude the forcing data sets of Francis and Hunter. A better
understanding of the role of atmospheric circulation forcingof Arctic sea ice trends remains an important task with
relevance to future predictions of Arctic climate change.
[22] Acknowledgments. We thank the anonymous reviewers for theirvaluable comments and the National Snow and Ice Data Center forproviding the sea ice concentration data. Clara Deser acknowledges support
from the NSF Office of Polar Programs, and Haiyan Teng acknowledges
support from the U.S. Department of Energy under Cooperative Agreement
DE-FC02-97ER62402. The National Center for Atmospheric Research issponsored by the National Science Foundation.

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