The thickness and ridging of pack‐ice causing difficult shipping conditions in the Beaufort Sea, summer 1991

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Abstract
Once or twice in a decade, reversals of the Beaufort Gyre in the Arctic Ocean close the shore lead along the North American coast in summer. This occurred in 1991, causing heavy ice conditions which severely hindered the annual re‐supply by sea of communities in the western Canadian Arctic. At this time, sonar instruments moored on the continental shelf of the Beaufort Sea measured the draft and topography of sea‐ice along a 1135 km transect. Perennial pack‐ice normally found further to the north was observed in the shipping lanes at high concentration prior to mid‐May and subsequent to mid‐July. This pack was of high average draft (3–6 m) and heavily deformed (43–61%). Level ice occurred at drafts typical of both first‐year (1.5–2 m) and multi‐year ice (2.5–4.5 m). The deepest keels exceeded 30 m in draft. The margin of the perennial pack was more heavily ridged and contained less level ice than its interior. The old‐ice mode, which appeared at 4.5 m draft in the margin, decreased to 2.5 m in the interior. In the margin, both first‐year and old‐ice modes were observed, whereas first‐year ice was not an obvious component in the interior. Two of the three deepest keels (>32 m) were also observed in the marginal zone. The polar pack of the present study was, on average, similar to ice observed by submarine north of Barter Island in 1976. It was, on average, thicker and contained thicker level floes than ice in the interior of the Beaufort Gyre. The deepest features of 1991 resembled keels, but were much larger than those observed to date in the western Arctic. They were also anomalously common. It is suggested that they originated within the multi‐year landfast ice of the Canadian Arctic Archipelago.