= Solution
A <polynya> is a persistent or recurrent area of open water or relatively thin ice within an otherwise ice-covered region. Persistence of the opening does not mean absence of ice formation: a <latent-heat polynya> can continuously make ice and export it. In contrast, a <sensible-heat polynya> remains open because ocean heat inhibits freezing. Both processes may occur at the same site.
At an Antarctic coast, cold air descending from the elevated continent produces strong <katabatic winds>. Their offshore component drives <sea ice> away from a fixed coastline, fast-ice edge or grounded-iceberg barrier. Open water exposed to the cold atmosphere loses heat rapidly; <frazil ice> forms, aggregates into <pancake ice> or consolidated new ice, and is exported offshore. The opening continues as long as export balances production. Headlands, the shelter provided by <fast ice> and the direction of the prevailing winds determine the location and size. Terra Nova Bay is a characteristic coastal example. Such <latent-heat polynyas> are ice factories rather than permanently warm holes in the ice pack.
At an <ice shelf> front the fixed boundary is floating shelf ice instead of land. Offshore winds blowing across the shelf can similarly remove newly formed sea ice, so an ice-front <polynya> need not have a different thermodynamic mechanism from a coastal one; the Ross Ice Shelf front provides an important wind-driven example. Shelf-front geometry, ice tongues, grounded icebergs and changes in calving alter the shelter and the supply of drifting pack ice. Some ice-front openings also receive significant sensible heat from currents, tidal mixing or upwelling, and heat carried out of an ice-shelf cavity can affect freezing. A <sensible-heat polynya> requires a sufficiently warm ocean supply, whereas the latent-heat mechanism requires continuing removal of the ice that does form. Consequently a strict identification of all coastal openings with one mechanism and all ice-shelf openings with the other would be incorrect.
The oceanic roles are substantial. Rapid heat loss and freezing produce <brine rejection>, raising shelf-water <salinity> and <mass density>. The resulting convection and circulation can carry dense shelf water into ice-shelf cavities; melting beneath an <ice shelf> modifies it into colder, fresher <ice-shelf water>. Dense shelf and cavity-modified waters can subsequently descend the continental slope, mix with adjacent waters and contribute to <Antarctic bottom water>. The outcome depends on bathymetry and the density after mixing: not all rejected brine reaches the abyss.
These localized regions can therefore have a disproportionate effect on deep-ocean ventilation, oxygen supply, carbon transport and the large-scale <ocean circulation>. They are also intense sites of ocean-to-atmosphere heat and moisture transfer. Seasonal sunlight arriving in open water supports high biological production where nutrients are available, with consequences for marine food webs and carbon cycling. \b[Antarctic polynyas connect wind-driven ice export, concentrated ice production and dense-water formation; ice-shelf fronts add a floating boundary and possible cavity-related heat and freshwater exchanges.]
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