a semipermanent area of open water in sea ice. Polynyas are generally believed to be of two types. Coastal polynyas characteristically lie just beyond landfast 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. Open-ocean polynyas, 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 Polynya in the antarctic Weddell Sea.

Polynyas are as yet incompletely understood. Early explorers who ventured into open waters often mistakenly believed they had discovered a new ocean. Since the 1970s, satellites containing microwave sensors have enabled scientists to closely observe changes in polar ice cover. Many hypotheses about the formation of polynyas have been put forward. Among the other factors thought to influence their formation are cyclones (which pile up ice on some portions of the landfast ice while drawing ice away from other boundary areas), eddies and local gyres, and swift surface currents.

Polynyas vary in size, some being as large as inland seas. One of the larger arctic polynyas, known as North Water, is centred on Smith Sound, at the northern end of Baffin Bay on the Greenland coast; it has an area of approximately 85,000 square km (33,000 square miles). Some polynyas, like those found in the Weddell Sea in Antarctica, have been measured at 350 by 1,000 km (215 by 620 miles); some remain open for a number of years at a time. Smaller polynyas, often recurring at the same place and time each year, may slowly get smaller and close, or they may dramatically reopen at any point in the seasonal cycle.

Arctic polynyas support a significant ecosystem as the source of plankton, krill, and cod, and large colonies of arctic birds (including murres, kittiwakes, black guillemots, and Ross's gulls) breed nearby. Many marine mammals (walruses, seals, whales, and polar bears) also depend on the polynyas as feeding grounds and overwintering areas. Polynyas in the Antarctic support plankton, krill, squid, fish (such as the Antarctic cod), seals, and whales. Scientists also are studying the effects of polynyas on atmosphere and the processes involved in the circulation of the Earth's oceans.