DEFINITION OF POLYNYAS

Pools or large, nonlinear openings in sea ice or

pack ice. Some polynyas are persistently open, others are

transient features that open and then disappear. They are

smaller in size than large ice-free regions called open-air

areas. Many form offshore from the mouths of large rivers.

In the Southern Hemisphere winter (June–September) large

polynyas may form in the Antarctic ice pack, and they are

thought to be of two main origins. Coastal polynyas are typically

30–60 miles (50–100 km) across and are formed by

winds blowing the ice away from the shore. Once these areas

open up, new sea ice forms in them, and it is thought that a

large percent of the sea ice in the Antarctic ice shelf may form

in this way. So much ice forms in Antarctic polynyas that the

amount of salt released (it is not incorporated into the ice) is

estimated to be 2.5 million cubic miles (10 million m3/s),

roughly equivalent to all of the salt released into the oceans

by river systems. This salt mixes with the cold Antarctic

waters, becomes very dense, and sinks to the bottom where

the salt is isolated from the atmosphere for long periods of

time. Open ocean polynyas form in the middle of sea ice, and

may recur in the same places year after year. They are probably

controlled by convective and other water currents that are

influenced by ocean bottom topography, influencing their

persistent location.

See also SEA ICE.

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