Polynya

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A persistent area of open water surrounded by sea ice - the hole rather than the ice. Two mechanisms are distinguished: a latent-heat polynya, where wind or current pushes newly formed ice away from a coast or ice shelf as fast as it forms, and a sensible-heat polynya, where warmer water rising from below keeps the surface ice-free. They matter out of all proportion to their area. Biologically they are where light reaches water in spring before the surrounding pack breaks up, and where air-breathing animals can surface in winter, so they concentrate life and the hunting that depends on it. Physically the latent-heat kind are ice factories: continuously freezing new ice rejects salt into the water beneath, and around Antarctica that dense, cold, salty water sinks off the shelf and becomes the bottom water of the world ocean.