season. This led to the accumulation of ice between 4 and 6 meter thickness in large areas south of Cape Evans.

At the same time, a combination of factors resulted in surface melting of the ice cover, likely increasing the strength of this ice due to the associated desalination of the underlying ice layers. The only relatively thin ice found in the inner sound (south of Cape Evans to Cape Armitage) is now confined to the icebreaker channel maintained over the past summer seasons (discernible in the synthetic aperture radar image shown in Figure 7.3). Because of the thickness of the surrounding level ice, it may become increasingly difficult to manage (i.e., displace) the mix of ice fragments and new ice forming each year as this channel is rebroken by icebreakers. As of summer 2006, the iceberg blockade had substantially reduced: B15A left the Ross Sea in October 2005; C16 is currently approaching Coleman Island; and B15J has historically remained east of Lewis Bay, close to Cape Crozier, where it has not had a substantial impact on sea ice. Also, collisions with B15A and C16 in 2005 and 2006, respectively, have shortened the Drygalski Ice Tongue by approximately 20 km. Nevertheless, there is a possibility that the level ice in McMurdo Sound that was fostered by the icebergs over the past several years has now reached such substantial thickness that only a major “centennial” storm is capable of breaking it up and clearing it from the sound. In summary, with several independent factors required to act upon the ice cover for decay and removal, it is at this point quite difficult to predict when the ice situation will improve again from the perspective of resupply operations. If the ice does not clear out on its own, then heavy ice conditions (level ice thicker than 4 meters and high concentrations of very thick ice fragments in a partially cleared channel) in subsequent years, starting with the 2006-2007 resupply season, may present a substantial challenge and could potentially thwart efforts to maintain an open channel to McMurdo pier.

At the same time, however, the factors that led to the development of the difficult sea-ice situation have now abated (with the departure of B15A and C16), and this means that once the sea ice does clear from McMurdo Sound, renewal of difficult sea-ice conditions would be unlikely until another period of unlucky iceberg circumstances (which may not develop again for 50 years, depending on the frequency of calving of the Ross Ice Shelf).

Ice Conditions in the Western Arctic

Ice-covered U.S. waters in the Arctic and sub-Arctic include the Bering Sea as well as parts of the Chukchi and Beaufort Seas, the latter typically referred to as the western Arctic (Figure 7.1). While the Bering Sea has not experienced substantial reductions in winter maximum ice extent (Comiso, 2003), the onset of spring ice retreat has occurred progressively earlier since the late 1970s (Stabeno and Overland, 2001). The Chukchi and Beaufort Seas have seen some of the most substantial changes in ice conditions anywhere in the Arctic. Thus, the greatest reduction in summer ice extent has been observed in the northern Chukchi Sea (Comiso, 2002; Overpeck et al., 2005). At the same time, changes in ice circulation, diminished ice growth, and enhanced summer melt have greatly reduced the amount of thick, multiyear ice in the region (Tucker et al., 2001; Perovich et al., 2003). Whereas the ice edge remained within <50 km of the coastline in waters off northern Alaska during most years in the 1970s and 1980s, it now typically retreats by more than 200 km to the north by the end of summer. However, due to the rapid response of a loose ice cover to shifting winds, and possibly aided by increases in summer storm intensity, the ice conditions overall have also become less predictable, with significant impacts on both wildlife and human activities. Thus, Native hunters and coastal residents report significant impacts on their traditional lifestyle by the changing ice regime (Huntington, 2000; Krupnik and Jolly, 2002). Among other factors, the amount of local rescue operations (in northern Alaska typically supported by the North Slope Borough’s Search and Rescue Operations Center) due to hazardous ice or open water conditions has increased substantially in the past decade or two (George et al., 2004).

Regardless of the recent changes, the western Arctic remains one of the areas with the most diverse, complex ice conditions in the northern hemisphere. This is due to the fact that the clockwise Beaufort Gyre is still advecting thick multiyear ice into coastal regions (and even through the Bering Strait in the winter of 2005-2006) while the coastal wind regime still fosters growth and export of ice in coastal polynyas and leads in the Chukchi Sea. Interactions between drifting ice and the coastline result in some of the largest ridges produced anywhere in the coastal Arctic. The changes observed so far in the ice regime have not resulted in the complete loss of any of these ice types, but rather have increased spatial and temporal variability, arguably rendering prediction and hazard mitigation more difficult. This situation is exemplified by difficulties encountered by a number of vessels in the Chukchi and Beaufort Seas during the 2006 summer. Thus, a Russian icebreaker carrying tourists into the Chukchi Sea encountered thick multiyear ice that impeded the ship progress and significantly altered cruise plans. More important, offshore oil and gas exploration activities that are resurgent as a result of past and planned federal lease sales in the Chukchi and Beaufort Seas were significantly affected, with a larger number of vessels confined to coastal stretches of the Beaufort Sea for most of the summer.



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