Arctic ice

The Arctic ice pack is the ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter. Summer ice cover in the Arctic is about 50% of winter cover. Some of the ice survives from one year to the next. Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 meters (9.8–13.1 ft) thick over large areas, with ridges up to 20 meters (65.6 ft) thick. As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades.

Sea ice has an important effect on the heat balance of the polar oceans, since it insulates the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. Sea ice is highly reflective of solar radiation, reflecting about 60% of incoming solar radiation when bare and about 80% when covered with snow. This is much greater than the reflectivity of the sea (about 10%) 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. The remaining surface water, made dense by the extra salinity, sinks, leading to the production of dense water masses such as North Atlantic Deep Water. This production of dense water is a factor in maintaining the thermohaline circulation, and the accurate representation of these processes is important in climate modelling.

In the Arctic, a key area where pancake ice forms the dominant ice type over an entire region is the so-called Odden ice tongue in the Greenland Sea. The Odden (the word is Norwegian for the headland) grows eastward from the main East Greenland ice edge in the vicinity of 72–74°N during the winter because of the presence of very cold polar surface water in the Jan Mayen Current, which diverts some water eastward from the East Greenland Current at that latitude. Most of the old ice continues south, driven by the wind, so a cold open water surface is exposed on which new ice forms as frazil and pancake in the rough seas.

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