Sea ice cycle: chapter 1

Table of Contents

Formation

Water-fresh water, that is-freezes at a steady state of 0°C. However, the freezing point of sea water is not only lower than 0°C; it also varies depending on the degree of salinity. As salinity increases, the freezing point becomes lower.

In order to freeze and form ice, water must first be cooled to its freezing point. This cooling implies a loss of heat.

First Sign of Freezing

Diagram of ice growth from ice crystals to slush to pancake ice.

The first sign of freezing on the sea is an oily appearance of the water caused by the formation of needle-like crystals. These crystals are pure ice, free of salt. They increase in number until the sea is covered by a slush of a thick, soupy consistency.

 

Theoretically, the whole body of water must be cooled to the freezing point before any ice will form. In reality, the oceans are stratified with increasing salinity and hence become denser toward the bottom. The convection currents need only reach down to a layer of sufficient density to provide stable layers. Typically, convection currents will reach 50 m; therefore, ice begins to form on the surface long before the deep water has been cooled to the surface freezing temperature.

If a body of water originally of uniform density throughout is losing heat at the surface, the ice will form:

  • most readily in fresh water,
  • less readily in water of low salinity, and
  • least readily in water of high salinity.

The result is that all other conditions being equal, it takes less heat loss to produce ice in fresh water than it does in seawater. The greater heat removal required to freeze seawater is due to its lower freezing point, and the fact that the cooled surface water continues to sink, maintaining convection right up to the freezing point.

Where Does Sea Ice Form First?

Ice will form first in shallow water, near the coast or over shoals or banks, and particularly in bays, inlets, and straits in which there are no currents, and in areas of low salinity (near the mouths of rivers, for instance). Shallow water is conducive to ice formation because of the relatively small depth of water that has to be cooled. The greater the depth of high-salinity water, the later the time of freezing. In fact, deep waters may never freeze over entirely, as not enough heat can be removed from the water during the course of a winter to bring this about.

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