DEVELOPMENT OF SEASONAL PACK ICE IN THE BEAUFORT-SEA DURING THE WINTER OF 1991-1992 - A VIEW FROM BELOW

Subsea sonars moored in the Beaufort Sea acquired a spatial section of draft across 941 km of sea ice during the winter of 1991-1992. These observations document the development of seasonal sea ice from an open sea surface in October to an average accumulation exceeding 2.5 m dra ft by early April. Initially, level ice occupied 85% of the profile, b ut continued ridging reduced this fraction to 50% in late winter. Thre e modes of level ice were tracked: that of seasonal ice whose growth w as initiated at freeze-up, and two others created during openings of t he ice field in January and in March. The growth of these modes can be closely matched by calculations based on a slab model of ice formatio n forced using surface meteorological data. Initially, ridge keels wer e small and widely spaced. By midwinter an exponential dependence of k eel frequency on draft was obvious (e-folding scale, 2.16 m), and the frequency and mean draft of keels had stabilized. The maximum keel dra ft was a modest 17.4 m. The low incidence of deep keels at all times i s a statistically significant departure from the exponential dependenc e valid at lesser drafts. The truncation point of the exponential rela tion Is related to the draft of the thinnest level ice present. In lat e winter the formation of deeper keels from thick first-year ice was a pparently precluded by the presence of younger, thinner ice, which lim ited the force available for ridge building. Through calculation of th e seaward transport of ice over the sonars, the total production of ic e in the coastal flaw lead during 1991-1992 was determined to be about 60% of earlier indirect estimates. In general, the observations revea led winter ice conditions significantly less severe than those found o n the periphery of the polar pack only a few tens of kilometers to the north, but more so than conditions in a marginal first-year ice zone at the same latitude in Davis Strait.