Guest essay by Eric Stephens
Many people think that once a sea freezes, it is one complete sheet. In reality, there are several open areas that never freeze. One such area is a polynya (common US spelling) or polynia (common UK spelling). It is a loanword from Russian: 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.
Polynyas are large, persistent regions of open water and thin ice that occur within much thicker pack ice, at locations where climatologically, thick pack ice would be expected. Polynyas have a rectangular or oval aspect ratio with length scales of order 100 km; they persist with intermittent openings and closings at the same location for up to several months, and recur over many years. In contrast to polynyas, leads*another open water feature*are long, linear transient features associated with the pack ice deformation, are not restricted to a particular location, and generally have a much smaller area than polynyas.
Polynyas can be classified into coastal and open ocean polynyas. Coastal polynyas form adjacent to a lee shore, where the winter winds advect the adjacent pack ice away from the coast, so that sea water at temperatures close to the freezing point is directly exposed to a large negative heat flux, with the resultant rapid formation of new ice.
This new ice is advected away from the coast as fast as it forms. For these polynyas, a typical along shore length is 100 – 500km; a typical offshore length is 10 – 100 km.
In contrast, the less common open ocean polynyas have characteristic diameters of 100km and are driven by the upwelling of warm ocean water, which maintains a large opening in the pack ice. Because the atmospheric heat loss from the open-ocean polynyas goes into cooling of the water column, they are sometimes called ‘sensible heat’ polynyas; because the heat loss from coastal polynyas goes into ice growth, they are called ‘latent heat’ polynyas. Finally, some polynyas, notably the North Water polynya in Baffin Bay, are maintained by both upwelling and ice advection.
Open ocean polynyas are self-maintaining, in that as heat is lost to the atmosphere at the surface, the surface water becomes denser and sinks, driving future convection. The convection ceases when the atmosphere warms in spring, or if sufficient fresh water, either produced locally by melting, or advected into the region, places a low-salinity cap on the convection
In the winter Canadian Arctic, because the marine mammals living under the ice need breathing holes, these mammals tend to concentrate in the polynya regions. For example, the North Water contains large concentrations of white whales, narwhals, walruses, and seals, with polar bears foraging along the coast. Also, the major winter bird colonies in the Canadian islands are located adjacent to polynyas.
Polynyas are persistent openings in the ice cover that in winter ventilate the warm ocean directly to the cold atmosphere. The major physical importance of the coastal polynyas is due to their large production of ice and brine, where the resultant dense water contributes to various Arctic, Antarctic, and North Pacific water masses.
During the cold war the location of polynyas were classified as secret, because the Subs from both sides would use them to surface.
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
In January 2013 the media was reporting a pod of orcas were trapped in the ice in Hudson Bay. With headlines such as: “Little hope for killer whales trapped by ice in Hudson’s Bay”. Some people were calling for the government to send an ice breaker to rescue them. Just when the media had given up hope – they disappeared.
The truth was they were at a polynya. There are several known and studied polynya fields in the eastern side Hudson Bay, with the Belcher Islands being one. From there they can swim under the ice to other polynyas in the Hudson Strait.
Polynyas give off a lot of heat to the atmosphere. For example the area surrounding the polynya can be cloudy, but over the polynya it will be clear.
In Antarctica many mainland penguin colonies are located near places where annual polynyas are known to recur, as there is a guaranteed source of food for the nesting penguins. There has been some concern that with increased sea ice around Antarctica that the penguins would be in danger, because they have to walk further. This is not a concern because they use polynyas.
This blog found some open water where they were to drill. Instead of blaming AGW/ CAGW etc. he correctly said it was a polynya.
This is just an introduction to polynyas as the title says. There is much more available on the net such as thermodynamics of polynyas – how they cool the water. Also there are many pictures of them.