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halves of the record, with the smallest (largest) trends oc-
curring during the cold season (October-March) during the
first (second) half of the record. In terms of relative magni-
tude (Figure 6, right), the seasonality of the trends in Arctic
sea ice extent remains similar between the two periods, with
the largest negative trends occurring during summer (mid
July to early October). The maximum relative trend ampli-
tudes have shifted from mid September in the first half of the
record to late September in the second half, and they have
also amplified (9% per decade in the first half to 15% per
decade in the second half).
our knowledge, the only other study documenting the spa-
tial pattern associated with EOF2 of winter SIC variability is
that of Deser and Teng [2008].
The leading EOF of summer SIC anomalies during 199-
2006 exhibits uniform polarity throughout most of the Arctic
marginal ice zone, with largest amplitudes from the Laptev
Sea eastward to the Beaufort Sea (Plate 4). This EOF re-
sembles closely the patterns of summer SIC trends during
199-2006 and 1993-2006 and projects substantially onto
the trend pattern for 199-1993 (recall Plate 3). This sim-
ilarity is consistent with the fact that the leading PC time
series exhibits an upward trend over the period of record.
The second EOF of summer SIC anomalies consists of out-
of-phase variations between the Barents/Kara seas and the
East Siberian/Beaufort seas, with no discernible trend in its
PC time series. This EOF does not correspond closely to any
of the summer SIC trend patterns shown in Plate 3, although
it captures some of the out-of-phase behavior evident in the
early period.
Although the SIC trend patterns in autumn resemble those
in winter (especially over the Atlantic sector), the ordering
of the EOFs is reversed, with the leading (second) EOF in
winter corresponding to the second (leading) EOF in autumn
(Plate 4). The leading EOF in spring consists of negative
values throughout the marginal ice zone, similar to the trend
patterns during 1993-2006 and 199-2006, while the sec-
ond EOF resembles the trend pattern during 199-1993.
Thus, the leading EOF of SIC anomalies in spring, summer,
and autumn (and the second EOF in winter) exhibit negative
values throughout the marginal ice zone, similar to the trend
patterns since 1993 (and 199). We expect that if the current
winter SIC declines continue, the leading EOF for that sea-
son will also eventually exhibit negative values throughout
the peripheral seas.
3.2.2. Empirical orthogonal function analysis of SIC
anomalies. Are the SIC trend patterns shown in Plate 3 pre-
ferred structures of variability, or are they simply a result of
dividing the record into halves? To address this question, we
have applied empirical orthogonal function (EOF) analysis
to seasonal SIC anomaly fields over the full period of record,
using a separate EOF analysis for each season. Note that the
SIC anomalies have not been normalized by their standard
deviation for this calculation. The two leading EOFs in each
season and their associated principal component (PC) time
series are shown in Plate 4. In both winter and summer, the
first and second EOFs account for 30% and 17% of the vari-
ance, respectively. In autumn (spring), EOF1 accounts for
29% (20%) of the variance and EOF2 for 21% (14%) of the
variance. In all seasons, EOFs 1 and 2 are well separated ac-
cording to the criterion of North et al. [1982].
In winter, the leading EOF exhibits out-of-phase varia-
tions between the eastern and western Atlantic and between
the eastern and western Pacific, strongly reminiscent of the
trend pattern during the first half of the record (recall Plate
3). This EOF is nearly identical to that given by Ukita et al.
[200] based on February-March averages over the period
199-2003 and consistent with results obtained using data
sets beginning in the early 1950s [ Walsh and Johnson , 199;
Fang and Wallace , 1994; Deser et al. , 2000]. The associated
PC time series exhibits an upward trend from 199 to 1995,
near zero values from 1996 through 2004, and positive val-
ues from 2005 through 200. EOF2 of winter SIC is charac-
terized by uniform polarity throughout the Arctic marginal
ice zones, with largest amplitudes in the Labrador Sea. This
EOF resembles the trend pattern during the second half of
the record (recall Plate 3). Its PC time series exhibits gener-
ally negative values before 1995 and positive values there-
after, indicative of a decreasing trend of winter SIC in the
peripheral seas. It is notable that the first and second EOFs
of winter SIC anomalies during 199-200 correspond to
the winter SIC trend patterns in the first and second halves
of the record, respectively, indicating that these two trend
patterns dominate the variability over the period of study. To
3.2.3. SIC trends in the context of atmospheric circulation
trends. As discussed in section 1, our motivation for examin-
ing the two halves of the record separately is not only to as-
sess in a simple fashion the evolution of the SIC trends over
time but also to examine the SIC trends in the context of a
rising and falling NAM index. Recall that the NAM is the
leading pattern of atmospheric circulation variability over
the extratropical Northern Hemisphere in all seasons [ Portis
et al. , 2001], and in winter it exhibits a positive trend in the
first half of the record and a negative trend in the second
half. To aid our interpretation of the role of SLP forcing of
SIC trends, we also consider trends in wind-induced atmos-
pheric thermal advection due to trends in the 1000 hPa zonal
and meridional wind components advecting the time-mean
zonal and meridional 1000 hPa air temperature gradients,
respectively, for each time period considered.
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