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]