To build on concepts introduced in Chapter 2, it is useful to draw from the review of Maykut (1985) and compare the processes of ice formation in a freshwater body with those that occur in the Arctic Ocean. Figure 7.1 gives the temperature versus density relationship for freshwater. For most substances, decreased temperature results in higher density. But freshwater is a very unusual substance. Down to a fixed threshold temperature, cooling results in increased density. Below this temperature of density maximum (Tm) of 3.98 °C, further cooling results in lower density. The solid form of water is in turn roughly 10% less dense than liquid water at the freezing point, which is another way of saying that ice floats.
Imagine that winter is approaching, and that the temperature in a freshwater column (e.g., in a lake) is initially higher than the maximum density temperature Tm. The water column is cooling from the top. This sets up a vertical temperature gradient in the water. However, as density increases as temperature decreases, the cooling destabilizes the column. This generates vertical convection, mixing the cooler surface water downward and the warmer water at depth upward. This process continues until the entire water column reaches the temperature of maximum density. From this point on, any further surface cooling is associated with a decrease in density. This allows the water column to become stably stratified, meaning that the lower density water is at the top. Once the column is stably stratified, conduction, rather than convection, dominates the heat transport. As compared to convection, conduction is a comparatively inefficient mechanism for heat transport. Because it is difficult to bring up warm water from lower depths, surface cooling can proceed quickly until the freezing point (Tf) is reached. Further cooling results in a slight supercooling and ice formation proceeds. Because of the stable stratification, ice can readily form at the surface even though the bulk of the water column is significantly above the freezing point.
Figure 7.1 Temperature versus density plot for freshwater and ice at standard atmospheric pressure (from Maykut, 1985, by permission of Applied Physics Laboratory, University of Washington, Seattle, WA).
Add salt and the situation changes radically. With salinities below 24.7 psu, the temperature of maximum density is higher than the water's freezing point. At salinities above this value, the temperature of maximum density equals the freezing point (Figure 7.2). Furthermore, the presence of salt in solution depresses the freezing point of water. For a typical ocean salinity of about 35 psu, the value is -1.8 °C. Imagine that it is autumn in the Arctic Ocean. As in the case of a freshwater body, cooling of the water surface initially results in a density increase and vertical mixing. The plot in Figure 7.2 could lead one to believe that the entire water column would have to be cooled to the salinity-adjusted freezing point before ice could form. Since much of the Arctic Ocean is 2-3 km deep, ice formation would seem to be rather difficult. The reason it can readily form is that there is strong, pre-existing stability in the upper ocean, which limits the depth of water that needs to be cooled.
The basis of this salinity structure was examined in Chapter 2. To reiterate, the low-salinity surface layer (Figure 2.7) is maintained by discharge from the rivers draining into the Arctic Ocean, the inflow of comparatively low-salinity waters through Bering Strait, and net precipitation over the Arctic Ocean itself. Below the surface layer is the Atlantic layer. The Atlantic layer is comparatively warm, with temperatures above 0 °C. It represents a potential source of ocean heat to the surface, which would inhibit ice formation. But at the low temperatures of the Arctic Ocean, the vertical density structure is determined by salinity, rather than temperature. The halocline between the surface and warm Atlantic water hence acts as a strong density gradient (pycnocline) that suppresses vertical mixing.
In the Arctic Ocean, the depth to which the water must be cooled before freezing can commence (Zc) is typically 10-40 m, although values in excess of 70 m have been observed (Doronin and Kheisin, 1975). Where Zc is 50 m, there can be a delay of up to two months in the date of initial ice formation compared to where Zc is 10 m. The surface mixed layer also varies seasonally. As new ice forms in autumn, brine is rejected from the ice. This brine mixes downward, increasing the density of the surface layer and weakening the pycnocline. In summer, ice melt freshens the surface layer.
This enhances the density stratification in the uppermost 30-50 m, strengthening the pycnocline, further decoupling heat exchange with Atlantic water (Carmack, 1990).
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