Your source has the Danish definition (right above your quoted excerpt) as "hvor havet holdes åbent hele vinteren igennem" -- see the first link. Second link is a Wikipedia entry to acquaint you with the English term, borrowed in turn from Russian.

("Polynya" is a very familiar term to those of who have spent a lot of time dreaming of the Arctic. :-) )

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Note added at 57 mins (2011-12-18 20:17:22 GMT)
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Some corroboration at http://www.protectplanetocean.org/explore/arctic --

" An important feature of the Arctic marine environment are areas of open water or 'polynyas' and 'leads"' in the pack ice. Some open sporadically and some are open year round. They act as lifelines for over-wintering wildlife or as stopovers on long migration routes."


(You'll see "lead" used in explorers' accounts and the like, but "polynya" is what's typically used in geographical literature.)

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Note added at 4 days (2011-12-23 11:24:39 GMT) Post-grading
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Thanks for your choice of my answer, Diarmuid. As for polynya vs. polynia, you might want to consider that the IPCC, in a glossary using UK English spelling, uses polynya: http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-app... Some Google searching indicates a preference for polynya as well, even among some UK English writers in technical fields. Compare http://tinyurl.com/2z7wz with http://tinyurl.com/2z7wz.