Polynya





A polynya is an area of open water surrounded by sea ice. It is now used as geographical term for an area of unfrozen sea within the ice pack. It is a loanword from полынья (polynya), which refers to a natural ice hole, and was adopted in the 19th century by polar explorers to describe navigable portions of the sea.

There are two main types of polynyas; coastal polynyas that are found year-round near the coast of Antarctic and Arctic and are mainly created by strong winds pushing the ice away from the coast. The other type is mid-sea or open-ocean polynyas that are found in the middle of the ice pack in certain locations especially around Antarctica. These locations are generally preconditioned by certain oceanic dynamics.

For example, one of the famous mid-sea polynya is the Weddell Polynya or the Maud Rise Polynya which occurs in the Lazarev Sea over the seamount Maud Rise. It was spotted first in September 1973 and have lasted over multiple winters (1974–1976) and recently reoccurred in September 2017. [12]

Formation
Coastal Polynyas are formed through two main processes:


 * Sensible heat polynya: this is thermodynamically driven, and typically occurs when warm water upwelling keeps the surface water temperature at or above the freezing point. This reduces ice production and may stop it altogether.
 * Latent heat polynya: is formed through the action of katabatic wind which act to drive ice away from a fixed boundary, such as a coastline, fast ice, or an ice bridge. The polynya forms initially by the first year pack ice being driven away from the coast, which leaves an area of open water within which new ice is formed. This new ice is then also herded downwind toward the first year pack ice. When it reaches the pack ice the new ice is consolidated onto the pack ice. This process continues over time, thus naming the latent heat polynyas as a major source of sea ice production in the Antarctic. The latent heat polynya is the open water region between the coast and the ice pack.

Latent heat polynyas are regions of high ice production and therefore are possible sites of dense water production in both polar regions. The high ice production rates within these polynyas leads to a large amount of brine rejection into the surface waters. This salty water then sinks. It is an open question as to whether the polynyas of the Arctic can produce enough dense water to form a major portion of the dense water required to drive the thermohaline circulation.

Mid-sea Polynyas are formed when specific atmospheric conditions occur over the preconditioned oceanographic areas. Such atmospheric conditions should favor ice drift in opposite directions to open the ice pack. Polar cyclones are a typical atmospheric trigger for the occurrence of mid-sea polynya as the cyclonic winds push the ice in opposite directions away from the cyclone center. Also, cold fronts, where two opposite flows in direction are found, are ideal for creating a mid-sea polynya.

Ecology
Some polynyas, such as the North Water Polynya between Canada and Greenland occur seasonally at the same time and place each year. Because animals can adapt their life strategies to this regularity, these types of polynyas are of special ecological research significance. In winter, marine mammals such as walruses, narwhals and belugas that do not migrate south, remain there. In spring, the thin or absent ice cover allows light in, through the surface layer as soon as the winter night ends, which triggers the early blooming of microalgae that are at the basis of the marine food chain. So, polynyas are suspected to be places where intense and early production of the planktonic herbivores ensure the transfer of solar energy (food chain) fixed by planktonic microalgae to Arctic cod, seals, whales, and polar bears. Polar bears are known to be able to swim as far as 65 kilometres across open waters of a polynya.

The presence of polynyas in McMurdo Sound in the Antarctic provides an ice-free area where penguins can feed, thus they are important for the survival of the Cape Royds penguin colony.

Antarctic Bottom Water (AABW)
Antarctic Bottom Water is the dense water with high salinity that exists in the abyssal layer of the Southern Ocean and it plays a major role in the global overturning circulation. Coastal Polynyas (Latent heat polynyas) are a source of AABW as brine rejection during the formation of sea ice at these polynyas increase the salinity of the sea water which then sinks down to the ocean bottom as AABW. Antarctic polynyas form when ice masses diverge from the coast and move away in the direction of the wind; creating an exposed area of sea water which subsequently freezes over, with brine rejection, to form another mass of ice.

Arctic navigation
When submarines of the U.S. Navy made expeditions to the North Pole in the 1950s and 60s, there was a significant concern about surfacing through the thick pack ice of the Arctic Ocean. In 1962, both the USS Skate and USS Seadragon surfaced within the same, large polynya near the North Pole, for the first polar rendezvous of the U.S. Atlantic Fleet and the U.S. Pacific Fleet.