JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. C5, PAGES 11,975-11,991, MAY 15, 1996 Development of seasonal pack ice in the Beaufort Sea during the winter of 1991-1992: A view from below Humfrey Melling and David A. Riedel Institute of Ocean Sciences, Sidney,British Columbia, Canada Abstract. Subsea sonars moored in the Beaufort Sea acquireda 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 draft by early April. Initially, level ice occupied 85% of the profile, but continuedridging reducedthis fraction to 50% in late winter. Three modesof level ice were tracked:that of seasonal ice whosegrowthwas initiated at freeze-up, and two others created during openings of the ice field in Januaryand in March. The growth of thesemodescan be closely matched by calculations basedon a slab model of ice formation forced usingsurface meteorological data. Initially, ridge keelswere small and widely spaced. By midwinter an exponential dependence of keel frequency on draft was obvious (e-folding scale, 2.16 m), and the frequency and mean draft of keelshad stabilized. The maximum keel draft was a modest 17.4 m. The low incidence of deep keels at all timesis a statistically significant departurefrom the exponential dependence valid at lesser drafts. The truncationpoint of the exponential relation is related to the draft of the thinnest level ice present. In late winter the formationof deeperkeelsfrom thick first-year ice was apparently precluded by the presence of younger, thinner ice, which limited the force available for ridge building.Through calculation of the seaward transportof ice over the sonars, the total production of ice in the coastal flaw lead during 1991-1992 was determined to be about 60% of earlier indirect estimates. In general,the observations revealed winter ice conditions significantly less severe than thosefound on the periphery of the polar pack only a few tensof kilometers to the north, but more so than conditions in a marginalfirst-year ice zone at the samelatitude in Davis Strait. Introduction In many of the marginal seas of the Arctic Ocean, waters within a few hundred kilometers of shore are commonly free of ice by early autumn. Such ice-free conditions occur in areas where the prevailing windsof summer blow off a warm conti- nentalland mass, forcing a seaward drift of the polar pack and accelerating the meltingof remaining seaice through a variety of interrelated mechanisms [Fissel andMelling,1990].The sub- sequent freezing of theseareas in autumn provides a situation resembling a controlled experiment in ice mechanics: a new ice field is grown"from scratch." Detailed ice observation during this time could provide a wealth of information on the ther- modynamic growthof seaice, and on the development of new populations of ridges and floesthrough the mechanical failure of the gradually thickening sheets of level ice. However,access to Arctic marginalseas by shipor aircraft is generally imprac- tical in autumn and winter. A broadregional view of the ice development in the seasonal sea ice zone has been available from satellite surveillance since the 1960s, initially in visible and thermal bands and more recently at microwave frequencies. Satellite-based remote sensing has providedvaluable data on the areal extent of de- veloping ice, and estimates of the thickness of new and young ice forms [e.g., Groves and Stringer, 1991]. However, the pre- cision of these thickness estimates is poor, and the spatial Copyright1996 by the American Geophysical Union. Paper number 96JC00284. 0148-0227/96/96J C-00284 $09.00 resolution of the sensors is too coarse to track the evolution of ridge and floe populations. For somedecades now, upward looking sonar mounted on nuclearsubmarines has been used to acquiredetailed "snap- shots" of seaice topography alongcruise tracks in the central Arctic Ocean [e.g.,Williams et al., 1975;Wadhams and Home, 1978; Wadhams,1981; McLaren, 1988]. More recently, time seriesobservations of sea ice topography at fixed locations have been acquiredby upward looking sonar moored to the seafloor [Hudson, 1990; Pilkington and Wright, 1991; Moritz, 1992]. Meanwhile, Belliveauet al. [1990] and Melling et al. [1995]have demonstrated that the drift velocity of seaice can be measured by mooredDoppler sonar under a wide variety of ice conditions. Now, if data are acquired simultaneously by ice-profiling and Doppler sonars, it is possible to map a time series of ice draft onto a spatialcoordinate derivedby integra- tion from icevelocity [Melling etal., 1995]. In this way,excellent topographic profilesof movingseaice can be obtainedfrom a fixed locationat any time of year [Melling and Riedel, 1995]. This paper discusses the seasonal evolution of the draft and underside topography of sea ice grown from an initially ice- free surface. In particular,it compares estimates of the growth rate of first-year ice in offshoreareas to model predictions, examines the net production of ice in a recurrent flaw lead, and establishes an empirical relationship between the depths of the largestkeels and the thickness of the surrounding matrix of level ice. The observations were obtainedby ice-profiling and Doppler sonars mooredin the shallow watersof southeastern Beaufort Sea during the winter of 1991-1992. 11,975