Arctic Sea Ice Retreat in 2007 Follows Thinning Trend
R. W. LINDSAY, J. ZHANG, A. SCHWEIGER, M. STEELE, AND H. STERN
Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington
(Manuscript received 11 March 2008, in final form 9 June 2008)
The minimum of Arctic sea ice extent in the summer of 2007 was unprecedented in the historical record.
A coupled ice–ocean model is used to determine the state of the ice and ocean over the past 29 yr to
investigate the causes of this ice extent minimum within a historical perspective. It is found that even though
the 2007 ice extent was strongly anomalous, the loss in total ice mass was not. Rather, the 2007 ice mass loss
is largely consistent with a steady decrease in ice thickness that began in 1987. Since then, the simulated
mean September ice thickness within the Arctic Ocean has declined from 3.7 to 2.6 m at a rate of ?0.57 m
decade?1. Both the area coverage of thin ice at the beginning of the melt season and the total volume of
ice lost in the summer have been steadily increasing. The combined impact of these two trends caused a
large reduction in the September mean ice concentration in the Arctic Ocean. This created conditions
during the summer of 2007 that allowed persistent winds to push the remaining ice from the Pacific side to
the Atlantic side of the basin and more than usual into the Greenland Sea. This exposed large areas of open
water, resulting in the record ice extent anomaly.
Arctic sea ice retreated dramatically in the summer
of 2007, shattering the previous record low ice extent
set in 2005 by 23% (Stroeve et al. 2008; Comiso et al.
2008). Figure 1 shows the extent of the Arctic sea ice
each September (the month of minimum extent) since
the beginning of the satellite data record in 1979. The
extent in 2007 falls 4 standard deviations of the residu-
als (4?) below the downward linear trend for 1979–
2006. What caused this precipitous drop?
The monthly average extent of Arctic sea ice has
been declining in every season and every region since
the beginning of the satellite record (Meier et al. 2007;
Parkinson and Cavalieri 2008). The decline of perennial
sea ice has been faster than the decline of the ice cover
as a whole (Comiso 2002), implying a shift toward less
multiyear ice and more first-year ice and thus a thinner
ice pack (Maslanik et al. 2007).
The anomalous sea ice retreat in the summer of 2007
occurred mainly on the Pacific side of the Arctic basin.
Kwok (2008) used satellite data to calculate the areal
advection of sea ice from the Pacific to the Atlantic
sector of the basin during the summers of 2003–07 using
ice motion derived from passive microwave measure-
ments. The largest flux (0.48 ? 106km2) occurred in the
summer of 2007, accounting for 15% of the total ice
retreat in the Pacific sector, the balance being lost to
melt. Persistently high atmospheric surface pressure
over the Beaufort Sea and low pressure over the Laptev
Sea drove southerly winds from eastern Siberia that
brought warm air to the Pacific sector and pushed the
ice northward (Maslanik et al. 2007).
The motion of sea ice is mainly wind driven. The
anticyclonic Beaufort Gyre recirculates ice in the west-
ern Arctic, allowing it to become older and hence
thicker, while the Transpolar Drift Stream advects ice
across the basin and out through Fram Strait. Rigor et
al. (2002) describe how high Arctic Oscillation (AO)
conditions from 1989 until about 1995 flushed large
amounts of the older, thicker ice out of the basin. Lind-
say and Zhang (2005) call this episode of high AO years
the trigger that propelled sea ice into the current regime
of increasing amounts of summer open water, solar heat
absorption, and reduced winter ice growth. This results
in thinner first-year ice that is more likely to melt com-
pletely away during the subsequent summer.
What special factors in 2007 led to the precipitous
decrease in summer ice extent? Was a critical threshold
in the ice–ocean system passed or was it simply unusual
weather? Was the state of the ice–ocean system at the
beginning of the year the most important factor or were
Corresponding author address: Ron Lindsay, Polar Science
Center, 1013 NE 40th St., Seattle, WA 98105.
1 JANUARY 2009LINDSAY ET AL.
© 2009 American Meteorological Society
highly anomalous winds or temperatures in 2007 solely
responsible for the decline? Our objective in this paper
is to quantify the importance of each factor and to use
the events of 2007 to further our understanding of how
the ice–ocean system is evolving in the modern epoch
of a warming Arctic.
The analysis of Zhang et al. (2008) gives a month-by-
month interpretation of the events of 2007. They show
that there was increased ice mass advection from the
Pacific sector to the Atlantic sector of the Arctic Ocean
and into the Greenland Sea, caused primarily by
anomalous winds in July, August, and September, and
that this increase in advective loss increased the open-
water area and set the stage for increased absorption of
solar flux and consequently increased melt of ice vol-
ume. The present analysis provides a longer-term per-
spective and puts the events of 2007 into the context of
a changing ice cover over the period 1987–2006. We
also expand our analysis and include ice mass and ice
area budgets to help further illuminate the mechanisms
involved in the sea ice anomaly of 2007.
The paper is organized as follows: section 2 intro-
duces the model, the forcing fields, and the validation
studies. Section 3 presents an analysis of the changes in
the ice mass, area, and extent; the ice thickness distri-
bution; and the open-water formation efficiency. Sec-
tion 4 presents budget estimates for the mass, area, and
extent in terms of the net melt and advection. Section 5
presents the results of a numerical experiment that
changes the wind forcing for the model, and the final
section presents a short discussion of the many factors
leading to the thinning of the ice and the sharp reduc-
tion in the ice extent.
2. The coupled ice–ocean model
a. Model description
In this study, we use the Pan-Arctic Ice–Ocean Mod-
eling and Assimilation System (PIOMAS) based on the
Parallel Ocean and Ice Model (POIM) of Zhang and
Rothrock (2003). PIOMAS consists of the Parallel
Ocean Program (POP) ocean model (e.g., Smith et al.
1992; Dukowicz and Smith 1994) coupled to a multicat-
egory thickness and enthalpy distribution (TED) sea
ice model (Zhang and Rothrock 2001; Hibler 1980).
The POP model is a Bryan–Cox–Semtner-type ocean
model (Bryan 1969; Cox 1984; Semtner 1986) with nu-
merous improvements, including an implicit free-
surface formulation of the barotropic mode and model
adaptation to parallel computing. The TED sea ice
model consists of five main components: 1) a momen-
tum equation that determines ice motion, 2) a viscous–
plastic ice rheology that determines the internal ice
stress, 3) a heat equation that determines the ice tem-
perature profile and ice growth or decay, 4) an ice
thickness distribution equation that conserves ice mass,
and 5) an enthalpy distribution equation that conserves
ice thermal energy. The TED sea ice model has 12
categories each for ice thickness, ice enthalpy, and snow
depth; it uses a line successive relaxation (LSR) dynam-
ics model (Zhang and Hibler 1997) to solve the ice
momentum equation with a teardrop plastic ice rheol-
ogy that allows biaxial tensile stress (Zhang and Roth-
The PIOMAS configuration is based on a general-
ized orthogonal curvilinear coordinate system, covering
the Arctic Ocean, North Pacific, and North Atlantic.
The northern grid pole is displaced into Greenland and
the mean horizontal resolution is about 22 km for the
Arctic Ocean. The ocean model’s vertical dimension
has 30 levels of varying thicknesses, with six 5-m-thick
levels for the surface waters. The POP ocean model is
modified so that open boundary conditions can be
specified along the model’s southern boundaries along
43°N. The open boundary conditions of sea surface
height and ocean velocity, temperature, and salinity are
obtained from a run of a global version of POIM
The model is forced with daily National Centers for
Environmental Prediction–National Center for Atmo-
spheric Research (NCEP–NCAR) reanalysis (Kalnay
et al. 1996) fields of 10-m surface winds, 2-m surface air
temperature (SAT), specific humidity, precipitation,
evaporation, downwelling longwave radiation (DLR),
sea level pressure (SLP), and cloud fraction. SAT and
cloud fraction are used to calculate downwelling short-
wave radiation (DSR) following Parkinson and Wash-
ington (1979). No data assimilation is used in this study.
Figure 1 shows a time series of modeled and satellite-
observed September sea ice extents from 1979 through
FIG. 1. Modeled and satellite-observed ice extent for the entire
Arctic in September. The observed extent is from the Special
Sensor Microwave Imager (SSM/I) NASA team ice concentration
dataset (Meier et al. 2006).
JOURNAL OF CLIMATEVOLUME 22
2007. The modeled September ice extent is highly cor-
related with the observations, with a linear correlation
coefficient of R ? 0.92 (R ? 0.83 if the trends are
removed). The model overestimates ice area and extent
in 2007 relative to the observations. It underestimates
the large drop in 2007 extent, so it also possibly under-
estimates ice thickness decreases as well. Figure 2
shows maps of the mean September simulated ice thick-
ness and the simulated and satellite-observed ice ex-
tents. The modeled ice extent (the region with ice con-
centrations greater than 0.15) exceeds that of the ob-
servations but there is a good match in the overall
pattern. The simulated ice motion is validated through
comparison with buoy tracks from the International
Arctic Buoy Program. The annual mean vector corre-
lation for daily velocities is R ? 0.88, with simulated ice
speed exceeding observations by 8% on an annual av-
erage. The bias in recent years, since 1991, is less, av-
eraging 5%, and in 2007 it was near zero. The overes-
timate of the ice speed in some years may mean that the
advective processes in the analysis below are overesti-
mated as well.
The model ice draft is compared to recent observa-
tions of the ice draft made with upward-looking sonar
(ULS) instruments moored to the ocean floor in five
locations (Fig. 3) between 2001 and 2005. The ice draft
exhibits a bias between the North Pole, where the model
drafts are too thin, and the Beaufort Sea, where the
model drafts are too thick. This bias has been observed
in a similar model before (Rothrock et al. 2003; Lindsay
and Zhang 2005). Within each region the correlation of
the model draft with the measured draft is high: R ?
0.81 at the North Pole and R ? 0.86 in the Beaufort Sea.
The correlation between the model-simulated ice thick-
ness and submarine observations available over 1987–
97 is 0.71, with a mean bias of 0.06 m.
FIG. 3. Comparison of the model draft to ULS draft measurements from five mooring
locations: triangle, Institute of Ocean Sciences, 2003–05; diamonds, Beaufort Gyre Explora-
tion Project, 2003–05; and square, North Pole Environmental Observatory, 2001–03.
FIG. 2. Simulated (a) ice thickness and (b) ice concentration for September 2007. The
observed ice extent line (0.15 contour) is drawn in black and the simulated ice extent is in
white (dashed). The observed extent is from the SSM/I NASA team ice concentration dataset
(Meier et al. 2006).
1 JANUARY 2009 LINDSAY ET AL.
3. Basinwide properties of the ice cover
We first consider area-averaged quantities to inves-
tigate temporal changes in the ice. Our analysis focuses
on the Arctic Ocean basin defined to include the mar-
ginal seas but not including the Bering, Barents, and
Greenland Seas nor the Canadian Archipelago. Clear
definitions for different diagnostics of sea ice are
needed. Mean ice thickness in our analysis includes the
open-water category. Ice volume is the total volume of
sea ice and is thus the product of the mean ice thickness
and the total area of a region. Here, we report the
volume in terms of the mean ice thickness over the
Arctic Ocean basin. Ice area is the total area of a region
covered by sea ice. Ice concentration is the fractional
area of a region that is covered with sea ice. Ice extent
refers to the area that is covered by ice with a concen-
tration of greater than 0.15. Thus, the ice area is equal
to or smaller than the ice extent because little of the ice
is in regions with concentrations of less than 0.15.
a. Ice thickness, area, and extent
The simulated mean ice thicknesses in May (annual
maximum) and September (annual minimum) for the
Arctic Ocean are shown in Fig. 4. The thickness began
its modern decline in about 1987 with a period of very
strong positive Arctic Oscillation index in 1989–93
(Rigor and Wallace 2004; Lindsay and Zhang 2005).
The 1987–2006 trend lines in Fig. 4 show a rate of re-
duction in the thickness of ?0.47 m decade?1in May
and ?0.57 m decade?1in September (see Table 1). If
extrapolated forward in time (always a chancy exercise
and not meant as a forecast here), the September ice
will have zero thickness between 2025 and 2030. Sig-
nificance tests for the trend are not appropriate because
the fit interval was selected based on the data.
In a changing climate, a meaningful benchmark for
the magnitude of the anomalies is with respect to the
anomalies about the trend. We use the quantity ?, de-
fined as the standard deviation of the residuals of the fit
for the linear trend, to measure how far from the trend
line a quantity is relative to the variability about the
trend, that is, how unusual it is relative to the trend. The
2007 mean thickness values are record minima, but are
not far from the trend lines for May and September,
?0.19 ? and ?0.75 ?, respectively. The trend line ac-
counts for 83% of the variance (R2) in the September
mean ice thickness.
The September ice area and extent also show consis-
tent declines since 1987 but the interannual variability
is much higher, as reflected in the percent of the vari-
ance explained by the trend lines, 39% and 17%, re-
spectively. However, the deviations of the 2007 values
from the trend lines are much higher than for the ice
thickness, ?2.24 ? and ?2.86 ? (Table 1).
Why is the relative variance about the trend line (1 ?
R2) for ice area and extent greater than for the thick-
ness? The relative variances for the ice area and extent
are larger because a small change in ice thickness can
result in a large change in the area covered by ice. For
thin ice a small amount of melt can rapidly decrease the
ice area or, conversely, ice growth can rapidly cover
large areas of open water with ice. The ice extent is
further subject to variable amounts of movement of the
ice pack, which depends on wind forcing but also on ice
compactness, which may limit how much the ice can
move. The compactness of the ice pack (the ratio of ice
area to ice extent) in August, the month of minimum
compactness, has been decreasing in the last 20 yr be-
cause the area is declining faster than the extent. A less
compact ice pack is more easily moved to one side of
the basin and thus allows for greater variability in the
b. Thickness distribution
The relationship between changes in ice thickness
and area is represented in Fig. 5, which shows the cu-
mulative distribution of the ice thickness averaged over
the Arctic Ocean for the months of May and Septem-
ber, GMay(h) and GSept(h). For both months, more area
is covered with thin ice in recent years. In May 1987,
FIG. 4. (top) Annual mean ice thickness in the Arctic Ocean in
May and September and (bottom) the September fractional ice
area and ice extent. The trend lines for the period 1987–2006 are
shown as dotted lines.
JOURNAL OF CLIMATEVOLUME 22
about 50% of the area (the 0.5 line) is covered by ice
thinner than 2.0 m and in May 2007 about 70% of the
area (the 0.7 line) has ice less than 2 m thick. The area
covered by ice less than 1 m thick in September in-
creases from 50% in 1987 to 70% in 2007. The most
notable aspect of the changes in the simulated ice thick-
ness distribution is that 2007 is not unusual when con-
sidered as part of a 20-yr trend that started in about
1987. In September 2007 about 60% of the area of the
basin was ice free. The 0.6 line has been dropping to-
ward the zero thickness level since 1987, albeit with
some considerable interannual variability.
The reduction in the September ice area arises from
both a decline in the area covered by thick ice and a
steady increase in the amount of summer ice melt and
export. The dashed line in the May distribution plot
shows the mean loss of ice over the Arctic basin from
May to September (the difference in the two ice thick-
ness lines in Fig. 4 and also the “change” column in
Table 1). A rough approximation of the amount of
open water in September may be found in the May
thickness distribution as the amount of area in May
with ice thickness less than the amount lost through
melt and export from May through September, ?h:
GSept(0) ? GMay(?h). In 1987 the equivalent of slightly
more than 1.0 m of ice was either melted or exported,
enough to produce 20%–30% open water as seen in the
September distribution at zero thickness. In 2007 the
summer loss of ice is near 1.5 m. This amount of ice loss
would have produced 30%–40% open water in 1987,
but with the changes in the thickness distribution, it
produces 50%–60% open water in 2007. This relation-
ship between the ice thickness distribution and the sum-
mer melt is summarized as the open-water formation
This concept was introduced by Holland et al. (2006)
to analyze the nature of abrupt ice declines seen in
general circulation model simulations. The open-water
formation efficiency (OWFE) is the open-water frac-
tion formed between May and September for each
meter of ice melt. We expect that the OWFE would
increase if more of the spring ice area is thin. As with
the thickness distribution, the year 2007 can also be
viewed as being consistent with recent trends in terms
of the OWFE. Figure 6 shows the OWFE over the
Arctic Ocean and the OWFE plotted against the mean
ice thickness in March. The years are color coded to
make it apparent that the most recent year has the
greatest OWFE. In 2007 more open water was formed
per meter of melt than in any other year. This rate,
however, is not unexpected given the gradual increase
in OWFE seen over the past decades. The OWFE is
expected to increase sharply as the ice thickness in
March decreases even more (Holland et al. 2006).
TABLE 1. Ice thickness, area, and extent for the Arctic Ocean, May–September.*
May meanSeptember meanTotal meltTotal exportTotal change Fram Strait export
1987–2006 mean (m)
Trend (m decade?1)
2007 trend anomaly (m)
1987–2006 mean (fraction)
Trend (fraction decade?1)
2007 trend anomaly (fraction)
1987–2006 mean (fraction)
Trend (fraction decade?1)
2007 trend anomaly (fraction)
* The May and September columns are the mean for the month. The melt and export columns are the total contribution of each process
for 1 May–30 Sept averaged over the basin. The Fram Strait export is the total for May–September; the volume export is expressed
as meters of ice over the entire basin and the area and extent export as a fraction of the basin area. Here, ? is the trend anomaly
divided by the std dev of the residuals of the linear trend line. Values with magnitude greater than 2 are set in boldface.
1 JANUARY 2009 LINDSAY ET AL.
4. Budgets for the ice mass, area, and extent
a. Ice mass budget
To gain a better understanding of the physical pro-
cesses that link changes in ice properties over the past
29 yr with the sea ice anomaly of 2007, we compute
budgets that allow the separation of advective and ther-
modynamic processes. To understand how ice area and
mass are affected by these processes during summer,
separate mass and area budgets are computed for the
melt season. Changes in the ice mass within the Arctic
Ocean are due to ice export or ice production (either
melt or freezing). The local change in ice thickness ?h
over a time interval ?t can be described by a simple
imbalance between the local net thermodynamic ice
production ?hpand the local net ice advection (mass
flux convergence), ?hadv? ??(uh) ?t, due to ice mo-
tion u, such that ?h ? ?hp? ?hadv. Integrating the
imbalance between local ice advection and ice produc-
tion over the whole Arctic Ocean yields ?H ? Ph? Eh,
where the change in the basin-wide mean ice thickness
(?H) in a given time is due to an imbalance between
the total ice production P inside the ocean domain and
ice export E at its open boundaries, mainly at Fram
Figure 7 shows the May–September total ice export
and production for the Arctic Ocean. Ice production is
substantially negative, reflecting the strong summer
melt. [Note that the total annual production is in fact
positive, reflecting net growth, which is approximately
balanced by the net export; e.g., Steele and Flato
(2000)]. May–September ice production is more vari-
able than the ice export and shows a general downward
trend since 1987. The total melt in 2007, a near-record
maximum, is not far below the 1987–2006 trend line
(?0.62 ?; Table 1). The net summer ice export in 2007
is farther below the trend line relative to the variability
(?0.97 ?). The trend slopes slightly upward, indicating
decreasing ice export due to thinner ice transiting Fram
Strait (Lindsay and Zhang 2005).
The annual cycle of the mean ice thickness for the
FIG. 5. May and September cumulative ice thickness distribu-
tions for the Arctic Ocean. The contours show the fraction of the
total area with ice less than the indicated thickness. The median
ice thickness line, marked as 0.5, is set in boldface. The May–
September mean ice thickness loss, a thick line, and the linear
trend line are also shown in the May plot.
FIG. 6. The open water formation efficiency versus the year and
versus the mean March ice thicknesses. The colors in the bottom
panel correspond to the year.
FIG. 7. May–September net ice mass export and ice production
for the Arctic Ocean. The values for 2007 are marked with dia-
monds and the trend lines for the period 1987–2006 are shown as
JOURNAL OF CLIMATEVOLUME 22
most recent 8 yr is shown in Fig. 8. The mean thickness
for 2007 is below that of all the recent years and is
particularly low in the early fall. The cumulative ice
export since the first of the year is not anomalous in
2007 compared to recent years, although there is
greatly accelerated export in the late summer (as in
Zhang et al. 2008). However, the cumulative ice pro-
duction, while it began the year at a greater than nor-
mal rate, was below normal by the end of the summer.
July shows a steep drop in production (increased melt-
ing) compared to recent years, while the late fall shows
a rapid increase in ice production as would be expected
for the large areas of thin ice and open water when the
The anomalous ice production and advection terms
for 2007, integrated from May through September, are
shown in the maps in Fig. 9. A large area of anomalous
advective ice mass loss in the Pacific sector extends
toward the North Pole. This strong transpolar drift is
also observed by the drift of buoys and in the Advanced
Microwave Sounding Radiometer for the Earth Ob-
serving System (AMSR-E) passive microwave mea-
surements (Kwok 2008). The anomalous drift is on the
order of 1000 km over the summer (half the length of
the box containing the color bar). An area of anoma-
lous ice mass convergence is seen near the Laptev Sea
and northwest near Severnaya Zemlya. A broad band
of anomalous melt, over 1 m in some locations, extends
from the Beaufort Sea to the Laptev Sea, mostly south
of the final ice edge. In most of the Eurasian sector, the
melt rate was near normal. Small areas of positive
anomaly in the East Siberian Sea and southern Beau-
fort Sea are caused by a lack of ice, so the ice loss there
is less than normal. Averaged over the basin, there was
1.48 m of ice loss from the first of May to the end of
FIG. 8. Mean ice thickness, cumulative net ice export, and cu-
mulative ice production, starting in January, for the most recent
FIG. 9. (a) May–September anomalous net ice mass advection
and (b) ice production for 2007. The anomalous mean ice velocity
over the 5-month period is shown as vectors and a green line
shows the simulated ice extent in September. The anomalies are
relative to the 1979–2006 period.
1 JANUARY 2009 LINDSAY ET AL.
September in 2007, of which 91% was from melt and
9% from export. This loss was ?0.11 m less than the
1987–2006 trend line (?0.84 ?).
b. Ice area budget
As with the ice thickness, a budget may be formu-
lated for the area covered by ice. The changes in the ice
area during the summer melt can be quite different
from changes in the ice thickness. If the ice is thick,
melt may not significantly change the ice-covered area,
while if it is thin, it will. The ice area budget shows
where and when changes in the ice area, as opposed to
changes in the ice thickness, are caused by melt or ice
area convergence. The change in the fractional area
covered by ice (the ice concentration) can be computed
in terms of the production of the area due to melt or
freezing and the change due to advection. Let the frac-
tion of the area covered by sea ice be a, the area pro-
duction due to net ice area advection (net area conver-
gence) be ?aadv? ?? (ua) ?t, and that due to produc-
tion (thermodynamic melt or freezing) be ?ap(some ice
area can be lost to local ridging of ice and it is also
included in this term). The change in the ice area over
a time period is then ?a ? ?ap? ?aadv.
The monthly change in the ice-covered area and the
monthly advection of ice area are computed from the
monthly model output (mean ice velocity and ice area)
for each month and each location, and the thermody-
namic production is determined as a residual. Integrat-
ing the imbalance between local ice advection and ice
production over the whole Arctic Ocean yields ?A ?
Pa? Ea, where the change in the basin-wide mean ice
area ?A in a given time is due to an imbalance between
the total ice area production Painside the ocean do-
main and the ice area export Eaat its open boundaries,
mainly at Fram Strait.
The spatial patterns of melt and advection are similar
to those for the ice thickness, but the budget is able to
quantify just what the proportion of the ice area loss is
from each of these two processes (see Table 1). The
annual May–September ice area losses from advection
and melt are represented in Fig. 10. Both the melt and
export terms for 2007 are below their trend lines (?2.31
? and ?1.75 ?, respectively; Table 1). The spatial pat-
terns of the net area advection anomalies and area pro-
duction anomalies are shown in Fig. 11. These maps
clearly show that the melt in 2007 created most of the
open water in the Pacific sector, similar to the mass
melt pattern, but more widespread. Advection is also
important in some locations and a broad region of net
advective loss is formed in the Transpolar Drift Stream
as low ice concentrations are advected farther north.
Ice area divergence is important primarily in the Beau-
fort Sea. Some area convergence is seen in the region
north of Canada and northwest of the Laptev Sea. The
area budget shows that, in 2007, 38% of the basin was
cleared of ice by melt and 10% by export (Table 1).
c. Ice extent
The persistent southerly winds in some parts of the
Pacific sector had an important influence on the ice
extent in that the edge was moved 1000 km north. Fig-
ure 12 shows the end-of-August ice edge if it were ad-
vected backward 4 months to the likely end-of-April
position using the monthly mean ice velocities. A large
region of the Pacific sector was cleared of ice by the
persistent southerly winds. The area between the end-
of-May and end-of-August positions (the shaded area
in Fig. 12) amounts to 0.85 ? 106km2, or 13% of the
basin. According to these simulations, of the 40% of the
basin that was ice free in September, about one-third
(13%/40%) was created by ice moving out of the Pacific
The area cleared of ice by the persistent southerly
winds in the simulations is about 75% more than that
obtained in the observational study by Kwok (2008).
He used AMSR-E 18-GHz brightness temperature im-
ages to track the summer ice (June–September) for the
years 2003–07 and determined that in 2007 the net ice
area exported from the Pacific sector to the Atlantic
sector over a straight line across the basin amounted to
0.48 ? 106km2, 15% of the summer ice retreat. Much of
the discrepancy between our estimate and Kwok’s may
be due to the different methods used to determine the
ice transport, but the difference is also due to errors in
the model’s representation of ice velocity.
If we divide the basin into an Atlantic sector and a
Pacific sector based on the ice edge in September, the
Pacific sector occupies 38% of the basin. From May
through September, ice representing 13% of the basin
FIG. 10. May–September changes in the fractional ice area within
the Arctic Ocean due to the area export and due to melt.
JOURNAL OF CLIMATEVOLUME 22
was advected out of the Pacific sector and 25% was
removed by melt. In the Atlantic sector, ice represent-
ing 13% of the basin area was advected in from the
Pacific side, 4% was exported through Fram Strait
(Table 1), 2% through other passages, and 7% was lost
by melt. The total net convergence of the ice pack in the
Atlantic sector amounted to only 1% of the basin area.
5. Sensitivity experiment with alternate winds
Was the atmospheric forcing of 2007 particularly un-
usual or would winds or air temperatures from other
years have produced ice extents just as low? This ques-
tion is addressed with model simulations in which the
wind forcing is taken from each of the last 28 yr but the
daily thermal forcings (SAT, DLW, and DSW) are
taken to be always the same, the 28-yr mean from 1979
to 2006. The initial ice and ocean conditions each year
are always from the recent thin-ice conditions of 1
January 2007. This experiment is designed to test the
importance of the 2007 winds in establishing the large
ice retreat and to determine if the 2007 winds were
highly unusual in this regard. The results are compared
to a control run (Fig. 13, dashed line), which is a ret-
rospective analysis in which all forcings change nor-
mally and January conditions evolve normally.
The 2007 result for the experiment shows that the ice
is both thicker and more extensive than in the control
run because of the much cooler thermal forcing derived
from the mean of the previous years. However, the
winds from 2007 did not produce an ice thickness or
extent that was particularly unusual compared to the
other years in the experiment. Winds from 10 of the
years produced ice thinner than that from 2007 and
winds from five years produced ice extents lower than
those from 2007, all given the same thermal forcing.
Even though constant thermal forcings dampen the re-
sponse of the system to the different wind forcings be-
cause the air temperature cannot respond to the chang-
ing surface conditions, we note that winds alone can
produce significant variability in the ice extent from
year to year. We therefore conclude that 2007 was not
an unusual year in this respect. Winds that pushed the
ice from the Pacific side of the basin to the Atlantic side
FIG. 11. (a) May–September anomalous net ice area advection
and (b) ice area melt for 2007. The anomalous ice velocity over
the 5-month period is shown as vectors and a green line shows the
simulated ice extent in September.
FIG. 12. The end-of-August 2007 ice edge is traced back 4
months using the monthly mean ice velocity fields.
1 JANUARY 2009 LINDSAY ET AL.
were not highly unusual in their impact on the ice ex-
tent and would not have produced the ice extent
anomaly without prior thinning or anomalous melt.
Analogous experiments (not shown) were also per-
formed with January initial conditions taken from 2007
and both the winds and the thermal forcings taken from
each of the past years. The ice thickness is thinner than
the control, but consistently thicker than in 2007. How-
ever, the ice extent is very strongly forced by the ther-
mal fluxes, so the ice extent more closely follows the
control run. These experiments demonstrate the impor-
tance of ice–ocean–atmosphere interactions and illus-
trate that only limited conclusions can be drawn from
uncoupled models about the relative importance of
6. Discussion and conclusions
The total volume of summer sea ice has been on a
downward trend since 1987, and the volume in the sum-
mer of 2007 was not far below the trend line. While the
amount of ice volume melt and ice export in summer
2007 were higher than normal, they were not greatly
different from what might be expected based on the
trends over the last 20 yr. Thus, the unusual retreat of
the sea ice was preconditioned by decades of gradually
warming temperatures and the replacement of older ice
by younger ice, resulting in a thinner ice pack. Though
winds were anomalous during 2007, it appears that they
would have had little impact on the sea ice extent with-
out this preconditioning.
The ice has now reached a stage in the thinning pro-
cess where large reductions in ice extent can be ex-
pected for modest increases in the amount of melt
(Maslanik et al. 2007). The thinner ice is more suscep-
tible to large reductions in ice extent because (a) more
open water is created per meter of vertical melt (the
open-water formation efficiency) and (b) thinner ice is
weaker and hence responds more readily to wind forc-
ing, allowing the ice to be more easily driven away from
the coast or out of the basin.
What we do not directly address in this study are the
basic causes of the thinning ice pack. The fundamental
cause is likely the warming of the global atmosphere
and ocean due to increasing greenhouse gases, though
the physical processes linking this warming to the de-
cline in sea ice are not clear at this point. The Arctic
atmosphere appears to have warmed not only at the
surface, which could be a response to the thinning ice as
much as a cause, but at all levels up to at least 300 mb.
The Arctic warming to this level is linked to global
warming in a study by Graversen et al. (2007), who
conclude that much of the present warming above the
surface is not due to surface changes but to atmospheric
energy transport from southern latitudes.
A number of feedbacks exaggerate the warming
trends. The warming of the atmosphere, particularly in
the spring, has led to earlier onset of melt (Belchansky
et al. 2004). This earlier melt onset changes the albedo
of the snow surface in a way that has repercussions
throughout the melt season. Perovich et al. (2007) con-
clude that the total amount of solar energy absorbed by
the ice and ocean is strongly related to the date of melt
onset, but only weakly related to the total duration of
the melt season or the onset of freeze-up. The timing of
melt onset is significant because the solar elevation is
high in the late spring and a change in the surface al-
bedo and the surface energy balance at this time propa-
gates through the entire melt season, affecting the ab-
sorbed solar flux until the sun is again low. The increas-
ing amounts of ice loss during the summer are largely a
result of the changing albedo of the surface caused by
earlier onset of melt, more thin ice, and more open
water (Lindsay and Zhang 2005).
The ice–albedo feedback was particularly strong in
2007. Perovich et al. (2008) found a sixfold increase
(relative to the 1990s) in bottom melt at the location of
a mass balance buoy in the Beaufort Sea but only nor-
mal amounts of surface melt. This was caused by a
500% increase (relative to 1979–2005 average) in the
absorbed solar flux due chiefly to more open water and
a small anomaly (6%) in downwelling solar radiation.
FIG. 13. September ice thickness and extent over the Arctic
Ocean for the control run and the annual winds experiment.
JOURNAL OF CLIMATEVOLUME 22
The anomaly in downwelling solar radiation and poten-
tially increased melt rates were due to persistent high
pressure in the Beaufort Sea region that brought un-
usually clear skies (Kay et al. 2008). However, the
anomalous downwelling solar flux was not a key com-
ponent of the large retreat of ice in 2007 according to a
modeling study by Schweiger et al. (2008). They con-
clude that the anomalous radiative flux was not in the
region where the ice retreated most dramatically and
numerical experiments without the anomaly produced
ice extents similar to those with the anomaly.
The anomalous winds of 2007 contributed to the re-
duction in ice extent by pushing the ice to one side of
the basin, but if the sea ice had been of near-normal
thickness at the start of the year, the unprecedented
reduction in extent would likely not have occurred.
This increase in the advection of ice from the Pacific
sector to the Atlantic sector may be amplified by two
dynamic feedbacks, one in which thinner (and hence
weaker) ice is more easily compacted (Maslanik et al.
2007) and one in which thinner ice responds more
readily to wind forcing, which is manifested in higher
ice drift speeds (Rampal et al. 2007). The thinner ice is
more easily compacted and is flushed out of the basin
more quickly. In addition, winds favorable for seques-
tering multiyear ice within the basin have been rare
since the 1980s.
The most notable aspect of the changes in the simu-
lated mean ice thickness and the basin-wide ice thick-
ness distribution is that 2007 is not unusual when seen
as part of a 20-yr trend that started in about 1987. Both
the area coverage of thin ice at the beginning of the
melt season and the total volume of ice lost during the
summer have steadily increased. The combined impact
of these two trends and the persistent southerly summer
winds caused the 2007 ice extent reduction. The simu-
lated ice thickness for December 2007 is at yet another
minimum, so we expect the high variability and gradual
mean decline in the September ice extent to continue.
Acknowledgments. This study was supported by the
National Aeronautics and Space Administration (Grants
NNG06GA84G, NNG04GH52G, and NNG06GA76G)
and the National Science Foundation (Grants ARC-
0629312 and ARC-0629326). We thank Humfrey Mel-
ling, Institute of Ocean Sciences; Andrey Proshutinsky,
Woods Whole Oceanographic Institute; and Dick
Moritz, Polar Science Center, for ice draft data; the
NOAA/Earth System Research Laboratory for the
NCEP–NCAR Reanalysis data; the International Arc-
tic Buoy Program for buoy drift track data; and the
National Snow and Ice Data Center for passive micro-
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