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Sea ice is formed from ocean water that freezes. Because the oceans are salty, this occurs at about -1.8º C. Sea ice itself is largely fresh, since the ocean salt, by a process called brine rejection, is expelled from the forming and consolidating ice. The resulting highly saline (and hence dense) water is an important influence on the ocean overturning circulation.Pancake ice is sea ice that has been compressed by the action of waves on frazil ice. Plates are typically 1-3 metres across
The vast bulk of the world's sea ice forms in the Arctic ocean and the oceans around Antarctica. The Antarctic ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. Consequently, most Antarctic sea ice is first year ice, up to 1 m thick. The situation in the Arctic is very different (a polar sea surrounded by land, as opposed to a polar continent surrounded by sea) and the seasonal variation much less, consequently much Arctic sea ice is multi-year ice, and thicker: up to 3-4 m thick over large areas, with ridges up to 20 m thick.
View towards the underside of the ice. Image taken by an ROV. In the spring krill can scrape off the green lawn of ice algae from the underside of the pack ice. In this image most krill swim in an upside down position directly under the ice. Only one animal (in the middle) is hovering in the open 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, formed upturned edges. In time, the pancake ice plates may themselves be compressed into a solid ice cover, known as consolidated ice. Consolidated ice that floats freely on the ocean in pack ice; that which is attached to land is fast ice.
Sea ice has an important effect on the heat balance of the polar oceans, since it acts to insulate the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. Especially when covered with snow, sea ice has a high albedo - about 0.8 - and thus the ice also affects the absorption of sunlight at the surface. The sea ice cycle is also an important source of dense (saline) " bottom water ". While freezing, water rejects its salt content (leaving pure ice) and the remaining surface, made dense by the extra salinity sinks, leading to the productions of dense water masses, such as Antarctic Bottom Water . This production of dense water is a factor in maintaining the thermohaline circulation, and the accurate representation of these processes is an additional difficulty to climate modelling.
Reliable measurements of sea ice edge begin with the satellite era in the late 1970s using Scanning Multichannel Microwave Radiometer (SMMR) on SeaSat (1977) and Nimbus 7 (1978) satellites. The frequency and accuracy of passive microwave measurements improved with the launch of the DMSP F8 Special Sensor Microwave/Imager SSMI in 1987. Measurements since then indicate a downward trend in Arctic ice area and an insignificant (upwards?) trend in Antarctic ice.
Sea ice may be contrasted with icebergs, which are chunks of ice shelvesAn ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface, typically in Antarctica, Greenland, and elsewhere in the Arctic. The boundary between floating ice shelf and th or glacierA glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity. Equivalently, it is a multi-year ice accretion in mountainous terrain. The glacier fringe is the area where the glacier has recently melted. There ares that calve into the ocean.
The picture below shows the cycle of sea ice in both hemispheres (blue = northern, black = southern; units square meters).