On the Large-Scale Modeling of Sea Ice and Sea Ice—Ocean Interactions

Abstract

Sea ice is ice that forms in the ocean as a result of the freezing of seawater. Unlike ice in calm freshwater, such as that found in quiescent lakes and slow current rivers, sea ice forms not only at the surface but also underwater. This is due to the fact that, for typical ocean salinities, the seawater density increases monotonically with decreasing temperature, and therefore surface cooling generates continued convective overturning until the entire oceanic mixed layer reaches the freezing temperature. At this stage, additional heat losses produce supercooled water and ice growth commences. Upon generation of the ice crystals, most of the salt content of seawater is expelled out of the ice lattice. A consequence of this salt rejection process is that, during sea ice formation, the salinity and hence the density of the surrounding waters increase. Convective mixing induced by the expulsion of salt carries relatively warm pycnocline water up into the mixed layer and may retard further freezing or even bring it to a halt. As freezing progresses, minute ice particles collect at the top few millimeters of the sea. This is known as frazil ice. If there is wind, swell, or wave action, frazil ice agglomerates into pan-shaped pieces, termed ice pancakes. Through welding and rafting (sliding of one piece of ice up over another), pancake ice amalgamates into larger blocks of ice, called ice floes, which are the basic constituents of the sea ice cover, or ice pack. Once the ice cover is formed, it will go on thickening mainly via accretion of congelation ice at the ice-water interface.