Sea Ice
Sea ice usually covers the northern Bering and Chukchi Seas from late autumn
through early spring (Figure 43). Wind-induced ice movement causes ice ridge
and hummock formation—convergence of ice floes—much as it does in the Arctic
Ocean, although ridges do not attain as great heights because of thinner one-year
ice in the Northwest Region.
Winter ice is fairly solid in the Chukchi Sea, although many leads are present
along the Alaskan coast, particularly between Cape Lisburne and Icy Cape where
strong longshore currents exist. In the northern Bering Sea, a discontinuous
changing mass of irregular fields, floes, and cakes of ice are intersected by
numerous breaks and leads. One-year, winter ice in the Bering Sea generally
averages only two to four feet (0.6 to 1.2 m) in thickness.
Ice formation begins in October and the permanent polar ice pack edge reverses
direction and begins to move counterclockwise, generally toward Siberia, as
evidenced by impingement of ice against the USSR coast. However, the area adjacent
to Alaska and the central Bering Sea and Bering Strait have well-developed shore
leads. The ice edge reaches its maximum southern position during late March
because of freezing and wind drift. Commonly, a southwesterly or westerly drift
in the southern Bering Sea between November and April causes a seaward flow
of ice from the more shallow area where it forms near the Alaskan coast (Potocsky
1975). In April the ice begins to break up and melt, and the ice edge begins
to retreat northward.
By early summer the Bering Sea is essentially free of ice. Bering Strait generally
clears by late June. Ice concentration north of Bering Strait decreases as summer
progresses, and the ice edge retreats further into the northern Chukchi Sea,
eventually merging into one continuous edge that reaches a maximum northward
position during the latter half of September.
During
years of moderate climatic conditions, the ice edge may not retreat as far northward
from the Chukchi Sea coast at the end of summer as during years with warmer,
windy summers. Throughout summer, in general, belts or patches of ice may break
from the main pack and pose a threat to vessels with unreinforced hulls navigating
the Chukchi Sea. Winter ice thicknesses on the northern Bering Sea coast range
from 28 to 48 inches (71 to 122 cm) compared to 12 to 28 inches (30 to 71 cm)
in more southern areas. Bering Strait is covered throughout the ice growth cycle
with ice thicknesses varying from 12 to 48 inches (30 to 122 cm) that may cause
extensive ice pressure buildup due to its geographic constriction. North of
Bering Strait ice cover is medium to thick with first-year growth greater than
48 inches (122 cm) during most of the growth cycle. Figure 43 summarizes seasonal
ice concentrations in marine waters of the Northwest Region. Other means of
determining ice cover are also used, however, such as percent coverage of sea
surface or ice coverage measured in Oktas (scale from one to eight) (Potocsky
1975; Alaska University, 1975).
Freezeup and breakup conditions in the region are important since ice may restrict
or immobilize man's marine activities. Figure 44 illustrates mean freezeup and
breakup information for selected locations in the Northwest Region.
[Alaska Regional Profiles, Northwest Region, p. 38]
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