Polynyas form where swift moving currents prevent the freezing of surface water
Polynyas occur in areas where swiftly moving sea currents prevent the freezing of surface water. Many polynyas are small, measuring only a few hundred square kilometres, and may freeze over at the height of winter. The larger polynyas, however, stay open all winter, although they may shrink in size in extremely cold temperatures. A distinct characteristic of all polynyas is that they are recurrent. The largest Arctic polynya, North Water, occurs at the head of Baffin Bay, and is like an inland sea surrounded by ice. This polynya was discovered in 1616, and is usually about as large as Lake Superior!
Compared to the surrounding air, the waters of a polynya are so warm that steam billows up. The average temperature of the waters within a polynya is about 0°C, and it never dips below -2°C, the freezing point of seawater. Polynyas allow heat from warm ocean currents to escape; this is important in the regulation of the Earth’s temperature.
In calm weather, a layer of new ice can form across a polynya in minutes. Inevitably, a strong wind or wave smashes this new ice layer, driving it out to sea and once again opening the surface of the polynya. This process occurs repeatedly, making polynyas an important source of new Arctic sea ice.