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values, based on 5-day running means during January 199
to June 2007. The linear trends in the dates when SIC first
falls below 50% are consistently negative throughout the
Arctic, with amplitudes ~6-9 days per decade in many re-
gions and even greater values in the Barents Sea (Plate 2, top
left). Similarly, the linear trends in the dates when SIC first
exceeds 50% are positive throughout the marginal ice zone
except in the Bering Sea, with amplitudes ~6-12 days per
decade (12-18 days per decade in the Barents and Chuck-
chi seas; Plate 2, top middle). These patterns result in large
positive trends in the duration of SIC <50%, with values in
excess of 18 days per decade in the Labrador, Greenland,
Barents and Chuckchi seas (Plate 2, top right). The spatial
uniformity of the trend values in all three quantities (Plate
2, top panels) contrasts with the large meridional gradients
present in their background climatologies (Plate 2, bottom).
Qualitatively similar results are obtained with thresholds of
30% and 0% (not shown).
Figure 5 shows the time series of the area-averaged dates
when SIC first falls below or exceeds 50%. There is a trend
toward an earlier (later) date of occurrence of sea ice concen-
trations first falling below (exceeding) 50% of −3.0 days per
decade (3. days per decade). This results in an increasing
trend in the duration of SIC <50% (defined as the difference
between the dates when SIC first falls below 50% and first ex-
ceeds 50%) of 6.9 days per decade or 19 days over the period
1979-2006, statistically significant at the 99% level. The du-
ration of the sea ice melt season, determined from emissivity
changes associated with liquid and frozen water, has shown
an even larger increase (approximately 2 weeks per decade
[ Stroeve et al. , 2006]) than the duration of SIC <50%.
3.2. Arctic Sea Ice Concentration
3.2.1. SIC trends. Up to now, we have focused on trends
in sea ice extent for the Arctic as a whole. In this section, we
consider the spatial patterns of recent trends in Arctic sea ice
concentration (SIC), taking into account the seasonal depend-
ence of the trends. To reduce the amount of information, we
focus on 3-month seasons defined as follows: November-
January (NDJ); February-April (FMA); May-July (MJJ); and
August-October (ASO); we refer to these seasons as au-
tumn, winter, spring, and summer, respectively. This choice
of seasonal averaging retains the basic characteristics of the
annual cycle and seasonal dependence of recent trends. The
spatial patterns of the monthly SIC trends are highly coher-
ent within each season (not shown).
The climatological SIC distributions for each season based
on the period January 199 to April 200 are shown in the
far left-hand column of Plate 3. In winter (FMA), the season
of maximum sea ice extent, long-term mean SIC values be-
tween 10% and 90%, indicative of the location of the mar-
ginal ice zone, are found in the Labrador Sea, the Greenland
and Barents seas, the Bering Sea, and the Sea of Okhotsk. In
summer (ASO), the season of minimum sea ice extent, the
marginal ice zone retreats northward to coastal regions of
the Arctic Ocean and the Canadian Archipelago. The long-
term mean SIC distribution in autumn (NDJ) resembles that
in winter, albeit with reduced values in the peripheral seas,
particularly the Bering Sea and the Sea of Okhotsk. The cli-
matological SIC distribution in spring (MJJ) is not identical
to that in autumn: although similar values prevail over the
Atlantic sector, lower amounts occur within coastal regions
of the central Arctic Ocean, and the Pacific marginal seas are
nearly ice free.
These climatological SIC distributions provide a context
for the spatial patterns of recent SIC trends shown in the left,
middle, and right columns of Plate 3. In addition to showing
the trends over the full period of record (January 199 to
June 200), Plate 3 also shows the evolution of the trends
from the first half of the record (January 1979 to December
1993) to the second (January 1993 to June 200). Note that
the magnitudes of the trends in each period may be directly
compared as they are expressed in percent SIC per decade.
The regions of largest-amplitude SIC trends in each season
correspond to the marginal ice zone as depicted in the left-
hand columns. As a general rule of thumb, SIC trends ex-
ceeding 3% per decade in absolute value (corresponding to
Figure 5. Time series of the area-averaged dates when SIC first falls
below 50% (“melt,” dashed curve), first exceeds 50% (“freeze,”
dotted curve), and their difference (“duration,” solid curve).
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