The Ice/Ocean Interface

At first glance it seems that describing heat exchange at the ice/ocean interface would be a reasonably straightforward exercise in applying the first law of thermodynamics. There are essentially only three important factors in the enthalpy balance: (i) upward (or downward) heat conduction within the ice column; (ii) heat flux from or to the underlying ocean; and (iii) latent heat associated with the phase change as ice grows or melts. However, salt greatly complicates the process. In the same way that spreading salt may help remove ice from a cold roadway, diffusion of salt from the ocean lowers the freezing point at the interface so that whenever the IOBL temperature is above freezing, upward heat transfer occurs. Melting occurs when upward heat flux from the ocean exceeds upward conduction in the ice column; freezing happens when the ice conduction exceeds ocean heat flux (which is why ice can and does form even when the upper ocean is warmer than its salinitydetermined freezing temperature).

In the initial efforts at modeling sea ice, the prescribed ocean heat flux was important in maintaining the modeled equilibrium mass balance. Maykut and Untersteiner (1971), for example, found that a constant 2W m⁻² heat flux from the ocean was required for a realistic equilibrium thickness of Arctic pack ice. For Southern Ocean sea ice, Parkinson and Washington (1979) also utilized a constant ocean heat flux in their model, but found that it needed to be an order of magnitude greater—about 25W m⁻². If the early models considered the ocean mixed layer at all, it was assumed that if sea ice was present, the mixed layer would remain at its freezing temperature, which in essence meant that any heat entering the upper ocean, either by absorption of solar radiation or upward conduction from below, would be instantaneously transferred to the ice.

When summer measurements of IOBL characteristics with modern instrumentation became available, it was obvious that the polar mixed layers could remain above freezing for extended periods. A smoothed time series of mixed-layer temperature at AIDJEX station Blue Fox, on the eastern side of the Beaufort Gyre over the Canadian Basin illustrates (Fig. 6.1) that even in the central Arctic with perennially high ice concentrations, a large amount of heat is stored in the upper ocean for at least a third of the year. The time series is more or less typical of the other AIDJEX stations, the SHEBA data in 1998, and several unmanned buoy drifts covering most of the Arctic Ocean. In this chapter we will explore how this storage comes about and what it implies about survivability of sea ice.