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Sea Ice

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Sea Ice

• Definition: Sea ice is a thin layer of ice floating on the sea surface that forms when the temperature of the water falls below its freezing point.

• The salinity of sea water depresses the freezing point to about -1.9oC.

• Sea ice is a predominant feature of the polar oceans that has dramatic effects on the physical characteristics of the ocean surface.

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• The normal exchange of heat and mass between the atmosphere and ocean is strongly modulated by sea ice, which insulates the sea surface from the usual atmospheric forcing.

• Sea ice also affects albedo, exchange of heat and moisture with the atmosphere, the thermohaline circulation of the ocean, and the habitats of marine life.

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Formation of an ice cover

• The simplest case is that of a shallow pond of calm water on which a thin and often transparent layer of ice cover forms.

• It is a continuous sheet of ice called skim and can be seen on pools, reservoirs, lakes and sluggish streams (velocities <0.5 m s-1).

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• Deeply supercooled aqueous solutions generate downward growing dendrites, commonly known as candle ice.

• More dramatic changes occur when the water is not calm; turbulent exchange interrupts the undisturbed crystal growth, and multiplies the effective nucleation rate, thus creating a greater number of ice crystals known as frazil ice.

• Over time, floes of frazil generate slush patches.

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• The increasingly congealed slush then forms into pancake ice and, in turn, pancakes may collect and freeze together to produce ice floes.

• In the ocean, the consolidation of pancakes into floes eventually leads to a continuous sheet of primary sea ice.

• Further growth may take two forms: secondary ice that is produced on the bottom of the ice cover either by direct freezing or by the accretion of frazil or superimposed ice that forms on top of the ice cover when it is inundated with water.

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Pancake Ice

8Source: Slaymaker and Kelly (2007)

Nilas

9Source: Wikipedia

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• The other major source of superimposed ice is snow.

• The additional weight of the snow causes the ice to submerge and water is added to the ice by capillarity.

• Bottom ice growth by addition is strongly influenced by the temperature and velocity fields below.

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• Once a continuous sheet of ice covers a huge area of sea, lake, or river, the process of secondary growth will depend on the removal of latent heat upwards through the cover; this is thermal growth or congelation.

• Open fracture lines or “leads” commonly occur in sea ice and are often sites of high biological activity.

• A “polynya” is an ice clearing or area of open water, possibly containing some thin ice, in the ice pack.

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• A polynya is distinguished from a lead by being a broad opening rather than a long, narrow fracture.

• 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 floats on the surface of the water, as distinguished from the fast ice, attached to the coast.

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• When packed together in large masses, drift ice is called pack ice that may either be freely floating or blocked by fast ice while drifting past.

• Sea-ice draft is the thickness of the ice that is submerged under water, whereas the sea ice freeboard is the thickness of the ice that is above water.

• The marginal ice zone (MIZ) is an interfacial region that forms at the boundary between open and frozen oceans.

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Source: Wikipedia

Source: Wikipedia

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Distribution

• Sea ice is a key element of the global energy and mass transfer system since it has a strong insulation capacity that restricts energy, mass, and momentum transfer between the atmosphere and ocean.

• Through brine rejection during formation, it also influences the salinity content, and, therefore, the ocean density of the upper ocean layers which, in turn, can influence ocean circulation and bottom water formation.

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• Sea ice extent changes seasonally in the polar regions; at its minimum extent in the northern and the southern hemisphere, sea ice covers 6-8 × 106 km2 and 2-3 × 106 km2 respectively and at its maximum, it covers 15-16 × 106 km2 and 18-19 × 106 km2 respectively.

• The key variables of interest for sea ice characterization are extent, concentration, ice type, and thickness.

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• In the Arctic, some sea ice persists year after year, termed multiyear ice (MYI) and some as first year ice (FYI) which is “seasonal ice” meaning it melts away and reforms annually.

• Almost all Southern Ocean or Antarctic sea ice is FYI in character.

• Characterization of sea ice variations is challenging because not only does the extent of sea ice change seasonally, but also sea ice does not remain in one place; it drifts and moves as the total extent expands and shrinks seasonally.

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• Atmospheric synoptic-scale and larger atmospheric weather systems tend to drive the dynamics of sea ice extent and concentration at temporal scales of a few days to weeks.

• Hence, remote sensing observations of sea ice need to be frequent and consistent to effectively characterize sea ice variables.

• Attempts at measuring sea ice variables using in situ techniques are limited at best because the hostile polar climate tends to constrain human activity.

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The “egg” code

Source: Canadian Ice Service

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Annual cycle of sea ice extent in Antarctica

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Source: Wikipedia

Source: Wikipedia

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Energetics of sea ice

• The energy balances of snowpacks and glaciers are comparatively simple when compared with those of sea ice because of its high spatial variability, including its ability to be transported by ocean currents.

• There are many varieties of sea ice and each of them have different spectral albedos.

• Large changes can occur rapidly over the course of a summer melt season and also during the fall freeze back.

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• Perhaps the most important aspect of the temporal behavior of the spectral albedos of sea ice is the general decrease that takes place with the onset of the melt season.

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Mass balance of sea ice

• Sea ice's relatively straightforward (compared to land snow cover) and rapid (compared to land ice sheets and glaciers) response to atmospheric forcing suggests that observations of sea ice cover may provide early strong evidence of warming in the Arctic.

• Moreover, the sea ice cover is a spatially integrated indicator of environmental change; by contrast with the spotty temperature records available for the Arctic.

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• On the other hand, the dynamic response of sea ice to environmental change depends on a complex interplay of mechanical and thermodynamic processes. Because of ice deformation, a typical 100 km2 patch of sea ice will contain a variety of ice thicknesses.

• These thicknesses range from open water to very thick ice, including pressure ridges extending possibly 30 m or more below the surface.

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• On top of this matrix there is often a relatively thin snow cover that can cause substantial insulation of the ice and reduce its growth rate.

• This spatial heterogeneity, especially around the edges of the sea ice, makes the mass balance of sea ice the most complex in the cryosphere.

• Local growth and melt and horizontal transport and deformation alter the local mean thickness (ice volume per unit area) and involves exchanges of mass (fresh water) with the atmosphere and ocean.

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Source: Dery and Tremblay (2004)

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