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of frequent episodes of cyclonic ice motion, forced by the frequent migration of
atmospheric lows into the region, the calculated divergence in the interior ice pack
can be large. For example, during the thirty-day period of August 13 to September
11, 1980, when the ice motion was strongly cyclonic, aggregate divergence rates of
up to 0.75 percent per day were calculated. Over the thirty-day period, this amounts
to a total opening of more than 20 percent. Estimates based in the IPAB data were
somewhat lower (0.50 percent per day). This is counter to the situation for most of
the year, when ice motion in this area tends to be anticyclonic (the Beaufort Gyre)
and hence slightly convergent.
As part of the SHEBA field study, H. Stern and Moritz (
2002
) evaluated time
series of ice divergence at several different scales centered over the main SHEBA
camp (
Figure 7.15
). They based these estimates on the Radarsat (synthetic aperture
radar, or SAR) Geophysical Processor System, using same basic feature-tracking
approach just described with respect to SSM/I. However, the ice motion fields from
Radarsat are at a much finer (5 km) spatial resolution. Although the basic time series
structure as well as the magnitudes of divergence are similar at the different scales,
(all fairly large, ranging from 50x50 km to 200x200 km), there are differences.
Divergences/convergences exceeding 1 percent per day are not uncommon. The
extremes are around 4-5 percent per day.
Stern and Moritz (
2002
) discuss the annual cycle with reference to
Figure 7.11
.
In autumn and early winter, as the pack ice is growing and becoming stronger,
the divergence is mostly positive (new leads form and new ice grows). There is a
cumulative divergence of about 25 percent between November 1 and December
25. January contains large divergence and convergence events. Winds are moder-
ate through the month, varying from northerly to easterly. The ice drifts generally
westward and leads form with a generally northwest-southeast orientation. At the
end of January, wind speeds pick up quickly, and the direction changes to slightly
south of easterly. This causes large convergences. Leads that were formed earlier
in the month close and ridging occurs. From February through July, divergent and
convergent events are small, but the ice undergoes a gradual convergence of about
15 percent as the SHEBA station drifts within the Beaufort Gyre. The large diver-
gence at the end of July is associated with a storm. After this time, the nature of
the deformation becomes more random, consistent with low summer ice strength
associated with “free-drift.” After about September 11, the ice enters a period when,
in response to seasonal cooling, it begins to gain strength and eventually redevelops
the more winter-like property of “plates and cracks.” The ice pack became well
consolidated by October 4.
7.4
Drivers of Large-Scale and Regional Sea Ice Variability
7.4.1
Overview
As already pointed out, there are downward linear trends in Arctic sea extent over
the period of satellite observations for all months, smallest in the winter months and
