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these warming trends provide favorable environmental con-
ditions for Arctic-wide sea ice losses since 199, coupled
with positive ice-albedo feedback and sea ice thinning.
noted that the first half coincides with an upward trend in the
NAM, the leading pattern of winter atmospheric circulation
variability over the extratropical Northern Hemisphere that
is known to exert a strong influence on Arctic sea ice cover,
while the second half coincides with a downward trend in
the NAM. We used 5-day running mean sea ice concentra-
tion data on a 25 km × 25 km grid derived from passive
microwave measurements from 1 January 199 through 31
October 200.
Our main findings are as follows. Arctic sea ice extent has
been retreating throughout the year, with the largest declines
occurring from mid July to mid October. Overall, the pace
of retreat as estimated from linear least squares regression
analysis is −0.52 × 10 6 km 2 per decade (~ −5% of the mean
per decade) or −1.76 × 10 6 km 2 in total during 199-200.
The rate of retreat has accelerated from −0.35 × 10 6 km 2 per
decade in the first half of the record (1979-1993) to −0.9 ×
10 6 km 2 per decade in the second half of the record (1993-
200). The date of maximum (minimum) sea ice extent has
increased by approximately 4 (1) days per decade, equivalent
to a delay of approximately 10 (3) days in 200 compared to
199. The number of days with sea ice concentrations less
than 50% over the Arctic as a whole has increased by 19
days from 199 to 200.
In each season, the spatial patterns of the SIC trends in
the two halves of the record are distinctive. The first half is
characterized by regional trends of opposing sign, and the
second half is characterized by uniformly negative trends
that resemble those over the full period. These distinctive
trend patterns correspond in each season to the first two lead-
ing EOFs of SIC anomalies during 199-200. In spring,
summer, and autumn, the leading (second) EOF corresponds
to the trend pattern over the full record (first half), while in
winter the order of the EOFs is reversed.
SIC trends in the first half of the record are character-
ized by positive values in the sub-Arctic seas of the west-
ern Atlantic and eastern Pacific and negative values in the
peripheral seas of the eastern Atlantic and western Pacific
in autumn, winter, and spring. In summer, the first half of
the record exhibits positive SIC trends in the eastern Atlan-
tic (Greenland and Barents seas) and negative trends in the
Arctic (East Siberian, Chukchi, and eastern Beaufort seas).
Atmospheric circulation trends, in particular a positive trend
in the NAM, contributed to forcing the SIC trends in the first
half of the record in all seasons via wind-induced low-level
atmospheric thermal advection. In the second half of the
record, the SIC declines in the Labrador, Barents, Kara, and
Bering seas in fall and winter are associated with increased
warm air advection in part because of a negative trend in
the NAM. However, the pronounced SIC declines within the
Arctic Ocean in spring and summer in the second half of the
3.2.4. SIC and SLP anomaly maps for spring/summer
2007. The drastic reduction of Arctic sea ice extent during
the summer of 200 deserves additional mention. Plate 8
shows the monthly SLP and SIC anomaly maps from May
2007 through October 2007, where anomalies are defined
relative to the 199-2006 long-term monthly means. Simi-
lar maps were presented by Comiso et al. [2008]. Large
SIC losses within the central Arctic developed in June and
reached peak amplitudes from late August to early Septem-
ber, leaving much of the eastern Arctic Ocean ice free [see
also Stroeve et al. , 2008]. The SLP field was highly anom-
alous in all months of reduced SIC, featuring persistent
high-pressure anomalies over the central Arctic Ocean and
low-pressure anomalies over Eurasia and adjacent seas: this
pattern resembles the distribution summer SLP trends during
1993-2007 (recall Plate 5). This configuration of SLP anom-
alies resulted in large geostrophic easterly wind anomalies
over the marginal ice zone in June and July and strong south-
erly wind anomalies over the Beaufort, Chuckchi, and East
Siberian seas in August through October where the largest
SIC losses were observed. It is likely that the low-level wind
anomalies contributed to the massive sea ice reductions dur-
ing summer 200, a point also made by Slingo and Sutton
[200]. Reduced cloudiness and associated enhancement of
downwelling shortwave radiation also played an important
role [ Kay et al. , 2008].
We note that the role of atmospheric circulation forcing
of the 200 summer SIC anomalies does not negate our con-
clusion that the overall retreat of Arctic sea ice since 199
is not directly controlled by long-term atmospheric circu-
lation changes. Indeed, we expect that atmospheric circu-
lation anomalies will continue to play an important role in
individual years, especially as the ice pack continues to thin,
but that over the long term they become less important com-
pared to other factors such as the positive ice-albedo feed-
back mechanism and greenhouse gas-induced warming of
the atmosphere and ocean.
4. SUMMARY AND DISCUSSION
The purpose of this study was to document aspects of the
evolving trends in Arctic sea ice extent and concentration
during 199-200 and to place them within the context of
overlying changes in the atmospheric circulation. In addition
to examining trends over the period as a whole, we inves-
tigated trends over the two halves of the record separately
as a simple way of characterizing their evolution. It was
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