18 August 2008

Types of Sea Ice

Earlier I talked some about types of ice in the climate system and types of ice ages. With the public discussion of what will happen to the Arctic ice pack this summer, it is time for some talk about the different types of sea ice and what their significance is. The main two involved are first year ice and multiyear ice. First year ice is in its first year of life. Multiyear ice has been around for more than 1 year. (We're really not very elaborate in our naming!)

Multiyear ice is generally thicker than first year. Part of this is because it has had an extra winter (or several) to freeze more ice on and grow this way. Part of it is because as the ice floes get shoved around by the winds and currents, they crash in to each other and can pile up. It also generally has a lower salt content than first year ice does. During the summer, the salt in ice makes the melting point lower there (same reason we put salt on roads in winter, at least if it's warm enough) and the saltier (brine) parts melt out of the ice floe, leaving behind nearly totally fresh water. Being fresh water or close to it also makes for mechanically stronger ice. It also makes the ice radiate differently than first year in the microwave, so it is possible to distinguish some between multiyear ice and first year ice from satellite sensors.

First year ice, then, is the (generally) thinner, mechanically weaker, saltier ice that formed some time during the most recent winter. All three of these properties make it easier to get rid of in the summer whether by atmospheric warming (straightforward melting), ocean warming (ditto), solar heating (easier to melt the saltier ice with the sun's rays), or by having a strong weather system hit the ice with high winds (breaking it up mechanically and helping it melt faster by exposing more surface area to the air and sea).

A different feature of thicker versus thinner ice is that thinner ice is harder to make weather-type predictions for/with. See, for example, The thermodynamic predictability of sea ice, Grumbine, Robert W., Journal of Glaciology, vol.40, Issue 135, pp.277-282, 1994.

There are many local names for various stages of growth in the first year ice. They include:
Grease ice -- small ice particles which give the ocean a 'greasy' appearance
Pancake ice -- ice floes maybe a meter or two across, more or less round like a pancake and with raised edges (collecting Grease ice)
Young ice -- let the pancakes grow and get thicker.

(The links will take you to places with good pictures and further explanation.)

A different sort is the 'fast ice'. This is ice which has frozen fast to the land. Otherwise it us much like the young and first year sea ice.

There are plenty more names and labels for sea ice types, and I'm not even starting in on the bestiary of names for iceberg types.

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