pack ice

Movements of the Sea ice between march and september

An icebreaker navigates through young (1 year old) sea ice

Nilas Sea Ice in arctic

Sea ice is formed from ocean water that freezes. Because the oceans consist of saltwater, this occurs at about -1.8 °C (28.8 °F).

Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Icebergs are compacted snow and hence fresh water.

Types of sea ice

Pancake sea ice in Ross Sea

Land-fast ice or simply fast ice is sea ice that has frozen along coasts ("fastened" to them) and extends out from land into sea.

Drift ice consists of ice that float on the surface of the water, as distinguished from the fast ice, attached to coasts. When packed together in large masses, drift ice is called pack ice. Pack ice may be either freely floating or blocked by fast ice while drifting past.

The most important areas of pack ice are the polar ice packs formed from seawater in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs significantly change their size during seasonal changes of the year. Because of vast amounts of water added to or removed from the oceans and atmosphere, the behavior of polar ice packs have a significant impact of the global changes in climate, see "Polar ice packs" for details.

An ice floe is a floating chunk of sea ice that is less than 10 kilometers (six miles) in its greatest dimension. Wider chunks of ice are called ice fields.

Formation of sea ice

Pancake ice is sea ice that has been compressed by the action of waves on frazil ice. Plates are typically 1–3 meters across

Satellite image of Scandinavia in winter. The Bothnian Bay and White Sea are covered with sea ice.

Only the top layer of water needs to cool to the freezing point. Convection of the surface layer involves the top 100–150 m, down to the pycnocline of increased density.

• In calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than 2-3 mm. Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent, it is called nilas. When only a few centimetres thick this is transparent (dark nilas) but as the ice grows thicker the nilas takes on a grey and finally a white appearance. Once nilas has formed, a quite different growth process occurs, in which water molecules freeze on to the bottom of the existing ice sheet, a process called congelation growth. This growth process yields first-year ice, which in a single season reaches a thickness of 1.5–2 m.

• In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere. The uppermost layer of the ocean is supercooled to slightly below the freezing point, at which time tiny ice platelets, known as frazil ice, form. As more frazil ice forms, the ice forms a mushy surface layer, known as grease ice. Frazil ice formation may also be started by snowfall, rather than supercooling.

Waves and wind then act to compress these ice particles into larger plates, of several metres in diameter, called pancake ice. These float on the ocean surface, and collide with one another, forming upturned edges. In time, the pancake ice plates may themselves be rafted over one another or frozen together into a more solid ice cover, known as consolidated ice pancake ice. Such ice has a very rough appearance on top and bottom.

The sea ice is largely fresh, since the ocean salt is expelled from the forming and consolidating ice by a process called brine rejection. The resulting highly saline (and hence dense) water is an important influence on the ocean overturning circulation.

Ecology

Main article: Sympagic ecology

ROV image of krill grazing under the ice.

Sea ice is part of the Earth's biosphere. Solid sea ice is permeated with channels filled with salty brine. These briny channels and the sea ice itself have its ecology, referred to as "sympagic ecology".

The decline of seasonal sea ice is putting the survival of Arctic species such as ringed seals and polar bears at risk.^[1][2][3]

References

1. ^ Barber, D.G.; Iacozza, J. Historical analysis of sea ice conditions in M'Clintock Channel and the Gulf of Boothia, Nunavut: implications for ringed seal and polar bear habitat. Arctic 57(1) Mar. 2004, pp. 1–14.
2. ^ Stirling, I.; Lunn, N.J.; Iacozza, J.; Elliott, C.; Obbard, M. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57(1) Mar. 2004, pp. 15–26.
3. ^ Stirling, I.; Parkinson, C.L. Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic 59(3) Sept. 2006, pp. 261–75

• Rothrock, D.A. and J. Zhang (2005). "Arctic Ocean Sea Ice Volume: What Explains Its Recent Depletion?". J. Geophys. Res. 110 (C1): C01002. 
• How Does Arctic Sea Ice Form and Decay?. NOAA Arctic theme page. Retrieved on April 25, 2005.
• NSIDC Sea ice. All About Sea Ice. Retrieved on October 6, 2005.
• Konstantin Y. Vinnikov, Donald J. Cavalieri, and Claire L. Parkinson, V33, L05704, doi:10.1029/2005GL025282, 2006 [1]

See also

• Ice pier
• Ice shelf
• Polynya
• Glacier
• Ice class

External links

• Ice in the Sea Chapter from Nathaniel Bowditch's American Practical Navigator
• Cryosphere Today: Current Arctic sea ice conditions
• Data source for sea ice picture
• Marine Modeling and Analysis Branch
• Everything you ever wanted to know about sea ice but were afraid to ask
• Animation of the movement of sea ice, September 2003 through May of 2004.
• NSIDC Sea Ice Index
• Arctic summer time: The short summer of 2004 (North Pole webcam view)
• Global Sea Ice Extent and Concentration: What sensors on satellites are telling us about sea ice
• BBC News, 28 September 2005, "Arctic ice 'disappearing quickly'"

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