Wednesday, June 17, 2009 6:08:21 PM
But first a few words about polynyas: Polynia (in the US spelled polynya) is a loan-word from Russian полынья which means uncovered or a natural ice hole, and was adopted in the 19th century by polar explorers to describe navigable portions of the sea. It is a semipermanent area of open water in sea ice. Polynias are generally believed to be of two types. Coastal polynias characteristically lie just beyond landfast ice (also known as fast ice), i.e. ice that is anchored to the coast and stays in place throughout the winter. They are thought to be caused chiefly by persistent local offshore winds, such as the foehn, or katabatic (downward-driving), winds typically found off the coasts of Greenland and Antarctica. The zone of open sea may be 50-100 km wide. Open-ocean polynias, the larger and longer-lasting of the two types, form within the ice cover and are believed to be caused by the upwelling of deep warmer water. This type is best exemplified by the vast Weddell Polynia in the antarctic Weddell Sea. The Weddell polynia was an enormous area of open ocean that reappeared in the Weddell Sea during 3 consecutive winters (1974-1976). At its largest it measured about 350 by 1000 km. The Weddell polynia reappeared in approximately the same position each year above a sea-bed topographic high, known as the Maud Rise.
In open-ocean polynyas heat is lost from the sea-surface mainly by conduction/convection. Water cooled at the surface sinks and is replaced by warmer subsurface water which in turn is cooled and sinks, forming deep convection cells. After the Weddell polynya had formed, the temperature of deep water changed dramatically, decreasing by 0.8°C all the way down to a depth of 2500 m.
Coastal polynias have been referred to as ‘sea-ice factories’. They ‘manufacture’ ice on an enormous scale, perhaps producing much of the ice in the adjacent ocean. It has been calculated that the heat flux to the atmosphere from a coastal polynia is more than 300 Wm-2, enough to supply a ten centimetre thick layer of ice to the adjacent sea each day. The Storfjorden polynya at Svalbard is a coastal polynia, and in fact the heat flux here was indeed estimated to be about 300 Wm-2 - and the wind during the super-cooling event to be about 8 m/sec.
Polynyas are predictable in time and are of a high ecological significance. The most significant polynyas of the kanumas assessment areas are the North East Water (NEW) off Kronprins Christian Land, the waters off Wollaston Forland and the mouth of the Scoresby Sound. There are also some much smaller polynyas along the coast.
On the east coast the most important polynia is the North Water between the Qaanaaq area and Ellesmere Island in Canada. Polynyas become free of ice very early in spring (April) and also have ice-free parts throughout the winter, and particularly the North Water is an important winter habitat for marine
mammals. Smaller polynyas are found at several sites along the Greenland coast.
Map of the North Water Polynya from the Canadian Ministry of Environment:
Some polynyas, such as the North Water, occur at the same time and place each year. Because animals can adapt their life strategies to this regularity, such recurring polynyas are of special ecological significance. More information about polynyas is found here, in particular the North Water Polynia.