GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 22, 2053, doi:10.1029/2001GL014293, 2002

NAO influence on sea ice extent in the Eurasian coastal region
Aixue Hu,1 Claes Rooth,2 Rainer Bleck,3 and Clara Deser1
Received 29 October 2001; revised 18 February 2002; accepted 22 April 2002; published 20 November 2002.

[1] Influence of winter pre-conditioning of Arctic sea ice
due to atmospheric forcing associated with the North
Atlantic Oscillation (NAO) on the reduction in summer sea
ice extent is studied. It is found that winter sea ice is about 50
cm thinner in high-NAO index years than in low-NAO index
years in the Eurasian coastal region mainly due to stronger
wind-driven ice export. The thinner wintertime ice combinedwith strengthened southerlies in spring promotes an earlier
break-up of the ice pack in the Eurasian coastal region,
resulting in significant sea ice export. The higher ice efflux,
in turn, further reduces the ice compactness, thus more solar
radiation is absorbed by the oceans which enhances thesummer melting process. Thus, winter and springatmospheric anomalies associated with the positive phaseof the NAO may underlie the reduction of summer sea ice
INDEX
extent observed during the 1980s and 1990s.
TERMS: 4207 Oceanography: General: Arctic and Antarctic
oceanography; 4540 Oceanography: Physical: Ice mechanics andair/sea/ice exchange processes; 4255 Oceanography: General:
Numerical modeling. Citation: Hu, A., C. Rooth, R. Bleck, and
C. Deser, NAO influence on sea ice extent in the Eurasian coastal
region, Geophys. Res. Lett., 29(22), 2053, doi:10.1029/
2001GL014293, 2002.

1. Introduction[2] Modulation of Arctic sea ice conditions has beenrecognized as a potentially significant climate feedback
mechanism for two primary reasons, namely, albedo modification and impact on the energy transfer across the air-sea
interface [Manabe et al., 1992; Randall et al., 1998].
Indirectly, variability of the mean oceanic buoyancy forcing
in the Arctic and adjacent seas induced by anomalous ice
formation (salt ejection) and anomalous ice efflux from the
Arctic affects the intensity of the Northern North Atlantic
deep convection, thus contributing to the forcing of the
global-scale oceanic meridional overturning circulation,
which plays a key role in the global meridional heat transport. Variability in the winter low-level atmospheric circulation over the Arctic and North Atlantic, as measured by
the North Atlantic Oscillation index [Hurrell, 1995], hasbeen empirically linked to fluctuations in annual mean sea
ice extent over the Arctic, with high NAO conditions
associated with reduced ice cover, especially in summer(Figure 1, left; see also Maslanik et al. [1996]).
[3] Using observed atmospheric conditions to define theforcing of a coupled sea ice-ocean general circulation
1

CGD/NCAR, Boulder, Colorado, USA.
MPO/RSMAS, University of Miami, Miami, Florida, USA.
EES-8, Los Alamos National Laboratory, Los Alamos, New Mexico,USA.

model, we investigate the Arctic-wide dynamic and thermodynamic response of sea ice to sustained NAO-related
atmospheric anomalies. The results show that wintertime
pre-conditioning of the sea ice in high NAO years comparedto low NAO years related to stronger wind-induced export
plays an important role in reducing summer sea ice extent.
Thus, we speculate that winter NAO trends may underlie
recent indications of a shrinking summer sea ice cover[Maslanik et al., 1996, and Deser et al., 2000].

2. Model and Experiments[4] A coupled sea ice-ocean general circulation model the Miami Isopycnic Coordinate Ocean Model coupled with
the Elastic-viscous-plastic dynamic sea ice model of Hunkeand Dukowicz [1997] and a simple thermodynamic sea ice
model of Semtner [1976] - is used in this study. The modelbasin consists of the Arctic Ocean and the North Atlantic.
The southern basin boundary is located at 6°N. A bufferzone is used there. As illustrated by Paiva [1999], the basinscale thermohaline circulation is not sensitive to the locationof the southern buffer zone. The oceanic initial state isderived from Levitus data modified by Steele et al. [2001].
(For details of the model, see Hu [2001].)[5] Two experiments have been carried out to establish
the expected differences in sea ice extent in the Eurasiancoastal region under sustained NAO-related forcing anomalies, referred to as the high NAO case and the low NAO
case. In each case, the model is integrated for 100 years,
starting from a 30-year spin-up run forced by a climatological annual cycle based on 51 years of observations. The
results reported here are mainly based on the mean state of
the two 100-year integrations.
[6] Composite monthly mean atmospheric forcing components - wind stress and speed, air temperature andhumidity, radiation fluxes, and precipitation - for high andlow NAO conditions and the 51-year climatology were
derived from NCEP/NCAR reanalysis data (1948 to1998). The high NAO index climatology is the mean annual
cycle of years 1973, 1981, 1983, 1989, 1990, 1992 to 1995
whose winter NAO index was higher than +2 standard
deviations based on the definition of Hurrell [1995]. The
low NAO index climatology is the mean annual cycle ofyears 1955, 1962 to 1965, 1969, 1977, 1979 and 1996
(NAO index lower than 2 standard deviations). Note that,while the years are chosen on the basis of wintertime(December to March) NAO conditions, all 12 months of
the respective years are used to derive the composite forcing
functions.

2
3

Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL014293$05.00

3. Model Results[7] The mean annual change of sea ice extent in the
Arctic between high and low NAO cases in the model

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10 - 2

HU ET AL.: HIGH NAO RELATES TO LOW SEA-ICE COVER

Figure 1. Variations in annual mean sea ice extent in the
Northern Hemisphere (left panel), an average over 5
observed time series [solid line, see Vinnikov et al., 1999],
and the North Atlantic Oscillation index [dashed line,
Hurrell, 1995] since 1970. Note the significant negative
correlation between the NAO index and the annual mean ice
extent, especially from 1989 to 1995. The right panel is the
Arctic portion of the model domain. The line AB marks thenorthern edge of the Eurasian coast basin.
solution is 4.4%, coincidentally almost the same as theestimated loss in sea ice extent from 1978 to 1996 based onsatellite observations (4.5% loss in ice extent, Maslanik et
al. [1996]). In general, the basinwide sea ice extent reaches
its winter maximum in April and its summer minimum inearly September (Figure 2, left panel). The reduction of thewinter maximum sea ice extent is about 1.6% from the highto the low NAO case (Figure 2, right panel); however, thereduction of the summer minimum ice extent is about 17%,much greater than the reduction of the winter maximum.
[8] The focus in this study is on the variation of sea ice
extent in the Eurasian coastal basin (see right panel of
Figure 1). This represents about 22% of the Arctic basin in
the model domain, including the coastal region from west of
the Laptev Sea to east of the Chukchi Sea. Compared tovariations in sea ice extent in the basin as a whole, about
92% of the reduction in summer sea ice extent in high NAO
years in the model occurs in the Eurasian coastal basin, anumber which qualitatively agrees well with satellite observations [Maslanik et al., 1996]. The maximum difference
between high and low NAO years in sea ice extent in thisbasin is about 0.9  106 km2 in September (Figure 3).
[9] The changes in sea ice extent in this region can be
related to the winter sea ice thickness and compactness. The
Seasonal variation of Arctic ice extent

Ice extent difference between high and low NAO cases

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case (dotted line), and the low NAO case (dashed line) andthe percentage difference of ice extent between low NAO
and high NAO cases (solid line). The winter maximum sea
ice extent appears in late April and the summer minimum
sea ice extent appears in early September. The solid (dotted)
line in the right panel is the daily (annual) mean percentage
difference between the low and high NAO cases.

Figure 3. Seasonal variations of sea ice extent (upper left),thickness (upper right), compactness (lower left), anddifferences of the surface heat flux between high and low
NAO index cases (lower right) in the Eurasian coast basin.
Solid/dashed ice property curves represent high/low NAO
index cases respectively. Heat flux differences are broken
down into net surface heat flux (thick solid curve), net
radiation flux (thin solid curve), sensible heat flux (dashed),
and latent heat flux (dot-dashed). Positive values represent
oceanic heat lost.
mean ice thickness in winter is about 50 cm less under highthan under low NAO conditions (Figure 3). Ice compactness
in winter is slightly lower in the high NAO case. When
spring melting starts, the thinner, less compact ice would
melt faster than the thicker, more compact ice even underidentical atmospheric conditions. In fact, the air is 1 to 2°C
warmer in high NAO years than in low NAO years. Thisfurther enhances the summer melting process in high NAO
years. The faster melting process reduces the ice compactness, resulting in a higher rate of oceanic absorption of solar
radiation, which further enhances summer melting. The net
radiation flux difference between high and low NAO conditions, the thin solid line in the bottom right panel of Figure
3 shows that the net radiative heat input is higher in high
NAO years from late April to September. The maximum
difference is 18 W/m2 in August (a 31% increase from the
low NAO case) and the mean difference in this period is 10
W/m2 (a 5% increase). With the additional radiation
absorbed by the ocean, the rate of ice melting in springand summer increases by 18% in high NAO years.
[10] The formation of thinner, less compact ice in high
NAO winters is related to the atmospheric forcing, previous
summer sea ice condition, and the oceanic response. In high
NAO years, winds blowing from the Eurasian side of the
Arctic to the Canadian side are stronger. The stronger winddrives more ice away from the Eurasian coastal region.
Except in January, the advection of sea ice from theEurasian coastal region to the interior Arctic is always
higher in high NAO winters than in low NAO winters.
The total winter sea ice production in high NAO years isabout 3000 km3, 26% more than the ice volume change
between the summer minimum and the winter maximum. In
low NAO years, the total ice formation is about 2000 km3,

HU ET AL.: HIGH NAO RELATES TO LOW SEA-ICE COVER

slightly lower than the ice volume variation between summer and winter. In other words, the ice influx from the
Arctic interior to the Eurasian coastal region in early winteralso contributes to the ice volume change in this basin, as
shown in Figure 4. From December to March, 20% of the
new ice (600 km3) formed in high NAO years is advected
into the interior Arctic from the Eurasian coastal region, afigure that drops to 6.1% (122 km3) in low NAO years. As aresult, the mean sea ice flux difference in this period isabout 0.023 Sv (Sv = 106 m3/s) between high and low NAO
years.
[11] Further analysis of the model results indicates that byneglecting all of the possible feedbacks, if the export of sea
ice were the same in high and low NAO winters, the ice
thickness difference between high and low NAO cases would
be reduced more than 80% in this basin. Thus, the more
efficient sea ice advection results in thinner, less compact ice
in high NAO years. This facilitates enhanced winter sea ice
formation, as indicated in the previous paragraph.
[12] In high NAO summers, most of the ice in theEurasian coastal region either melts or is exported into the
Arctic interior. The minimum sea ice volume there is only
5% of the winter maximum in high NAO years, but about
38% in low NAO years. As a result, only 17% of the area inthis basin is covered by ice in early winter during high NAO
years, in contrast to 73% in low NAO years. This may
indicate the importance of the effect of summer direct
forcing on the reduction of sea ice extent.
[13] Once the open ocean region in this basin is cooled to
the freezing point of sea water, the rate of ice formation ishigher in high NAO years than in low NAO years. Becauseof this, the rate of new ice formation in November andDecember in the high NAO case is more than twice as large
as in the low NAO case. The higher rate of heat loss due tolower ice compactness in the high NAO case plays animportant role in this higher rate of ice formation. A
consequence of these differences is expressed in the net
surface heat balance (thick solid line in Figure 3). The graphshows that from October to December, the ocean loses moreheat to the atmosphere in high NAO years than in low NAO
years. The mean difference in this period is 39 W/m2 with amaximum of 46 W/m2 in November. This difference in net
surface heat flux represents a 59% increase from low NAO
years. The 1 to 2°C warmer air temperature over this region
in high NAO climatology might be the atmospheric
response to such changes in the net surface heat balance
which needs to be studied further.
[14] The most significant difference in heat loss occurs in
October. The enhanced oceanic heat loss in the high NAO
case relative to the low NAO case is related to the thinner,less compact ice and smaller ice extent in this region. The
lower ice compactness and smaller ice extent exposes more
ocean directly to the atmosphere which results in an
increase in sensible heat flux from ocean to atmosphere(dashed curve in the lower bottom panel of Figure 3) and anincrease in outgoing longwave radiation (thin solid curve inthe same panel) in winter, especially in early winter (October to December). In early winter, the release of latent heat(dot-dashed curve) due to evaporation is also very importantto the higher heat loss in high NAO years.
[15] While the higher rate of heat loss in high NAO winters
results in a higher rate of sea ice production, the strong ice

10 - 3Figure 4. Seasonal variations of sea ice export from theEurasian coastal region to the interior Arctic for the high(solid curve) and low (dashed curve) NAO cases.
export prevents a buildup of sea ice in this basin. The higher
net sea ice production causes more salt to be ejected into theocean. Thus the mixed layer becomes saltier in high NAO
years. Our model solutions show that the mixed layer salinity
is about 0.5 ppt higher in high NAO winters, consistent withobserved surface salinity variation in this basin [Carmack etal., 1997]. Facilitated by salt extrusion in the freezing
process, the mixed layer is deepened about 10 m in high
NAO winters relative to low NAO winters.
[16] From Figure 4, we also notice that in the spring
season (March to May), the sea ice flux from the Eurasiancoastal region to the interior Arctic in the high NAO case isabout 150% higher than in the low NAO case. Thisconfirms that the increase in spring cyclonic activity inhigh NAO years relative to low NAO years indeed helps toadvect sea ice away from the Eurasian coastal region asargued by Maslanik et al. [1996] and Deser et al. [2000].
However, as suggested by this study, winter pre-conditioning of the sea ice in high NAO years also plays an importantrole in the reduction of summer sea ice extent in theEurasian coastal region.

4. Conclusions[17] Our model results indicate that variations in Arctic
sea ice conditions are directly connected to changes inatmospheric circulation. Specifically, the model solutions
suggest that atmospheric circulation and temperature
anomalies associated with the NAO may be the major factor
influencing the recent observed shrinking of sea ice in theEurasian coastal region. The importance of wind-driven ice
motion to the ice mass redistribution and thickness variations addressed here is consistent with other model studies[Zhang et al., 2000 and Holloway and Sou, 2002]. However, these studies did not focus on how NAO-related
atmospheric anomalies influence summer sea ice extent
along the Eurasian coastal region.
[18] Some specific implications of our work are:
1. The thinner, less compact ice in high NAO winters
plays an important role in the reduction of summer sea ice
extent in high NAO years. The thinner wintertime ice
accompanied by strengthened southerlies in spring promotes an earlier break-up of the ice pack in the Eurasiancoastal region, resulting in significant sea ice export.
2. The higher ice efflux, in turn, further reduces the icecompactness, thus more solar radiation is absorbed by theocean which enhances the summer melting process.

10 - 4

HU ET AL.: HIGH NAO RELATES TO LOW SEA-ICE COVER

3. In winter, the stronger wind in high NAO years drives
more ice away from the Eurasian coastal region, suppressing sea ice buildup there. As a result, the ice is thinner andless compact, which reduces its insulating effect and allows
more heat loss from the ocean to the atmosphere, ultimately
resulting in enhanced net sea ice production.
4. The higher net sea ice production in the Eurasiancoastal basin in high NAO years causes more salt to be
injected into the ocean. The resulting buoyancy flux leads tomixed layer deepening.
5. The estimated surface heat flux variability in ourmodel solutions may be potentially important to the regional
air-ice-sea interaction. The changes in oceanic stratificationinduced by the variability of net sea ice production in theEurasian coastal region may also affect the shelf-basininteraction in the Arctic, and may further modify the watermass exchange between the Arctic and the North Atlantic
marginal seas.
[19] Acknowledgments. This work was supported by DOE grant DEFG02-98ER62608.

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A. Hu and C. Deser, CGD, National Center for Atmospheric Research,
Boulder, CO, USA. (ahu@ucar.edu)
C. Rooth, MPO/RSMAS, University of Miami, Miami, FL, USA.
R. Bleck, EES-8, Los Alamos National Laboratory, Los Alamos, NM,
USA.