4. OTHER FACTORS
Continuing research is advancing the understanding of other factors which have influence on the occurrence of ventilation over shelves of the western Arctic in winter.
In the estuaries of large Arctic rivers, the ice itself provides a solution to the dilution of shelf waters in winter by continuing river inflow, it builds dams. Systems of huge grounded pressure ridges are formed at the edge of the land-fast ice as a result of reciprocating offshore/onshore movements of the pack ice. Extending in places to the seafloor in 25 m of water, they block the spread into the flaw lead of the thin (< 5m) layer of freshwater at the surface (Macdonald and Carmack, 1991)
Storms exert an important influence, through their effects on circulation and mixing over Arctic shelves, both prior to and during the ice-growth season. Melling (1993) and Melling and Moore (1995) have demonstrated that during autumn prior to ventilation events in the Beaufort Sea, a particular sequence of strong wind events occurs: first northwest gales which force fresh surface waters off to the east in a fast baroclinic coastal current; then sustained southeast winds which drive more saline water to the freezing interface by upwelling from intermediate depths. Winds apparently play a similar role in ventilation events in the Chuckchi Sea (Weingartner et. al, 1998).
How does the atmospheric circulation differ prior to winters when no ventilation is observed? Figure 3 illustrates progressive vectors for geostrophic wind in the Beaufort Sea during autumn.
5. FREEZING RATES AND ICE PRODUCTION
To improve our quantitative understanding of the ventilation process, the separate contributions of freezing and ocean dynamics must be identified. 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 the redistribution of ice thickness via deformation. With the development of moored sonar for long-term observation of ice draft and movement, it is possible to measure the draft and advection of the young sea ice in drifting pack over Arctic shelves.
Figure 4 compares observations of the draft of level ice drifting out of the Bathurst polynya with predictions using a simple 1-D ice-growth model. Despite the dynamic conditions of growth in a polynya in winter, the simple model does very well. Although very high rates of growth occur at polynya opening they are not sustained. The export of ice from the polynya may also be measured using sonar (Melling and Riedel, 1996). Such export measurements reveal that while the average rata of freezing is higher in polynyas than in stable ice fields, it is not exceptionally so.
International Workshop on Exchange Processes between the Arctic Shelves and Basins
Yokohama, Japan, 17-19 February 1998
