SEASONAL ICE PRODUCTION WITHIN ARCTIC MARGINAL SEAS AND ITS ROLE IN SHALLOW VENTILATION OF THE CANADA BASIN
Humfrey Melling
Institute of Ocean Sciences, Fisheries and Oceans, Canada
1. INTRODUCTION
The low temperature of the halocline within the deep Arctic basins is evidence for its ventilation during winter. Seasonal freezing over the peripheral continental shelves has long been proposed as one 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 oceanic 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 may continue locally throughout the winter.
This paper focuses on the ventilation of the upper Arctic halocline, centred at 33.0-33.5 salinity, which is thought to occur primarily on the Pacific Ocean side of the Arctic Ocean. It makes a contribution to a quantitative understanding of the relationship between seasonal ice production and ventilation in the Arctic.
2. EARLY OBSERVATIONS
Only since the mid 1970's have observations been made to search for such waters over Arctic continental shelves in winter. Water with T-S characteristics appropriate to halocline ventilation have now been observed in winter in the Chukchi Sea in 1977, (Agagaard, Coachman and Carmack, 1981), 1982 (Aagaard, Swift and Carmack, 1985) and 1992 (Weingartner et al., 1998); and in the Beaufort Sea in 1979 (Melling and Lewis, 1982), 1981 (Melling, 1993), and 1988 (Melling and Moore, 1995). Moreover, ventilation of the halocline has actually been observed adjacent to these seas: in Barrow Canyon in 1973 (Mountain and Aagaard, 1976), 1975 (Garrison and Becker, 1976) and 1992 (Weingartner et al., 1998), and over the continental slope in the eastern Beaufort Sea in 1979, 1981, 1982 and 1988 (Melling and Lewis, 1982; Melling, unpublished 1982 data; Melling and Moore, 1995). These observations corroborate the original hypothesis for the source of waters which maintain the cold stratified character of the upper Arctic halocline.
Figure 1, based on observations made in the Beaufort Sea in November 1979, contains, in somewhat enigmatic guise, four clues to the nature of halocline ventilation in the western Arctic. These are: intrusions of cold water within the halocline over the continental slope at 130 m depth; water at 40 m depth at two stations on the shelf with salinity equal to that at 140 m over the slope, despite a date only 6 weeks into the freezing season; at one shelf station high-salinity water is at freezing temperature; at the other it is well above freezing.
3. C0MPLICATIONS
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. Rivers inundate seas to several metres depth in summer and substantial (1-2m) ice melt can occur. Aagaard, Coachman and Carmack (1981) estimate that more than 4 m of ice must be grown over the shelf of the Beaufort Sea to raise the salinity to 33.5, where typical seasonal growth is of order 2 m! Moreover, rivers continue to flow at reduced rates during winter. The winter flow of the Mackezie River can flood an area of 1000 k? to a depth of 35 cm each day.
Observations have also shown that halocline ventilation is intermittent; it does not occur every year. Mountain & Aagaard (1976) report cold drainage flows in Barrow Canyon in 1973, but such were not observed during a similar programme of observation in 1987 (Aagaard & Roach, 1990). Observations in the southern Beaufort Sea have revealed a wide inter-annual range of variation in surface salinity, despite rather less changeable ice conditions in late winter. Only for the most extreme values of surface salinity does ventilation occur (Figure 2).
International Workshop on Exchange Processes between the Arctic Shelves and Basins Yokohama, Japan, 17-19 February 1998
