Influence of Mechanical Property of Sea Ice on the Formation of the Dense Water
Jinro Ukita
IARC-FRONTIER
Shibaura, 1-2-1, N7
Minato, Tokyo, Japan 103 qpee@eorc.nasda.go.jp
1 Introduction
Brine rejection and subsequent formation of denser water with respect to the ambient water is the most direct way in which sea ice processes can influence the density field. In the context of the stability of the perennial Arctic sea ice cover, this process is of major importance to the formation and maintenance of the Arctic cold halocline, which is defined as a water with a constant temperature at near the freezing point with a salinity that increases with depth. It has been hypothesized that such a mechanism with an advection from the vast Arctic shelf region is responsible for the formation of this water mass [Aagaard et al., 1981]. There has been growing evidence to support this mechanism [Aagaard et al., 1985; Jones and Anderson, 1986; Wallace et al., 1987; Melling 1993; Cavalieri and Martin, 1994]. On the basis of chemical and stable isotope analyses, Jones and Anderson [1986] further pointed out that the upper halocline water has the signature of the North Ocean and, possibly, that of the Barents and Kara Seas for the lower halocline water. Nonetheless seems plausible that the coastal region around the Arctic Ocean is a major source region for the Arctic cold halocline layer, with a varying degree depending upon a seasonal cycle in salinity caused by river run-off .
There however arises the question as to how this ice formation process actually takes place. Depending on an exact processes involved, not only thermodynamic but also dynamic processes can influence an amount and of ice formed as well as its location. It is commonly assumed that over an open water region along the coast, often referred to as a coastal polynya, frazil ice is formed and rapidly removed under an offshore wind, resulting in a large amount of ice formation. In this scenario there is no strong influence of mechanical property of ice on the ice formation, except maybe in a process of frazil ice being carried. In addition, the area of ice production is relatively spread over the entire region except an accumulation zone. In terms of the dense water formation, it is associated with a near constant negative buoyancy flux over the region. The atmospheric pressure data however reveals that there is significant variability in the atmospheric forcing pattern. Over the entire fall and winter seasons there are many periods when offshore or along shore winds prevail, which lead to an interaction between sea ice dynamic processes and dense water formation. This can be realized at least in two different ways. For instance, a period with an onshore wind followed by that with an offshore wind results in ice locked in then moved off the coast. The way in which the ice field moves depends upon the mechanical property of ice. If ice is " rigid" , that is not easy to break up, then the entire ice field moves. As a result, a coastal polynya is formed with the maximum width and the least spacing. Or in an extreme case, ice can be locked to the coast even against offshore wind, becoming land fast ice. On the other hand, if ice is "breakable", then it is likely that many leads parallel to the coast would appear. In this case, the material property of ice affects both amount and location of new ice formation.
The second case is the situation wherein the wind direction is parallel to the coast, or there is a strong ocean current at the surface along the coast. This is typically referred to as a shear zone of ice, often observed off the northern coast of Alaska and Canada. It has been observed that this shear zone is associated with active lead formation, often results in a streak of leads with some angle with the coastline. It is clearly a dynamic feature. There is very little doubt that the mechanical property of a large scale ice field is related to characteristic width, frequency, and location of coastal leads, thus resulting in a link to the formation of the cold halocline water.
