The Influence of Seasonal Ice Production within Marginal Seas on the Ventilation of the Canada Basin of the Arctic Ocean
Humfrey Melling
Institute of Sciences. P.O. Box 6000. Sidney B.C., Canada. V8L 4B2
Tel: 1-250-363-6552
Fax: 1-250-363-6746
MellingH@dfo-mpo.gc.ca
The low and uniform temperature of the Arctic halocline is evidence for its ventilation during winter. Seasonal freezing over the continental shelves has long been identified as the impetus for such ventilation. Freezing causes an increase in the salinity of surface water, because brine is not easily retained by growing ice. As the ice-free areas of late summer in Arctic seas cool and subsequently freeze, salt is delivered to the surface layer over a large area. Moreover, near the coast, where an episodic opening and closing of flaw leads occurs in response to varying winds, rapid ice growth and increase in shelf-water salinity continue locally through the winter. How much ice is actually produced, and how much salt added to surface waters, is, however, not well known. The salinity of surface water over Arctic continental shelves is relatively low at the end of summer, because of accumulated ice melt and river inflow. Therefore, in the absence of other factors, a very large volme of ice growth is needed to realize ventilation. Nonetheless, ventilation of the Arctic halocline in winter does occur. It has now been directly observed in the Beaufort, Chukchi and Barents Seas. But contrary to earlier belief, it is not a straightforward annual response to seasonal ice growth. Storms exert an important influence, through their effects on circulation and mixing over Arctic shelves, both prior to and during the ice-growth season. To improve our quantitative understanding of this process, the separate contributions of freezing and ocean dynamics to Arctic ventilation. Until recently, direct measurement of seasonal ice production was not possible. The rate of ice growth could only be estimated by calculation, typically using 1-D models forced by surface meteorology, and insensitive to ocean circulation and ice-thickness redistribution. With the development of moored sonar for long-term observation of ice draft and movement, it is possible to measure directly the draft and advection of the young sea ice in drifting pack over Arctic shelves. Ice-measuring sonata have been operating in the eastern Beaufort Sea near Bathurst Polynya since 1990. These observations provide insight into the kinematics of a system of recurrent flaw leads typical of those which encircle the Arctic Ocean. They provide evidence for the importance of ridging and deformation in the development of new ice fields. They yield direct quantitative estimates of the growth rates of new ice in offshore areas, and of the total seasonal ice production within recurrent flaw leads. The latter are consistent with the observed hydrochemical signature of ice production, and are somewhat smaller than earlier calculated values.
