Variability of Arctic sea ice: The view from space, an 18-year record

A recently compiled 18-year record ( 1979 to 1996) of sea ice concentration s derived from four passive-microwave satellite instruments has allowed the quantification of a variety of measures of Arctic sea ice variability. Ear lier maps generated using data through August 1987 have been updated to 18- year summaries of the annual range of sea ice distributions, the interannua l variability of average monthly sea ice distributions, the frequency of se a ice coverage over the 18-years, the length of the sea ice season, and tre nds in the length of the sea ice season. Linear least squares trends over t he 18-year record show the sea ice season to have lengthened over some size able regions, especially in the Bering Sea, Baffin Bay, Davis Strait, the L abrador Sea, and the Gulf of St. Lawrence, but to have shortened over a muc h larger area, including the Sea of Okhotsk, the Greenland Sea, the Barents Sea, and all the seas along the north coast of Russia. The area with trend s showing sea ice seasons shortening by over 0.5 days/year is 7.5 x 10(6) k m(2), over 2.5 times the area experiencing a lengthening of the sea ice sea son by over 0.5 days/year. Neither the shortening nor the lengthening, howe ver, is uniform or monotonic over the 18-year record. Instead, the ice cove r exhibits widespread interannual variability, not just in the length of th e sea ice season but for each month-a fact well illustrated by the monthly average September ice coverage, which was at its lowest extent in 1995 but at its second highest one year later, in the final year of the record. The maps of ice frequency and ice variability can help identify how anomalous i ndividual years are. In some cases, they can help forestall unnecessary con cern over seemingly unusual conditions which, upon examination of the maps, are found to fall well within the observed variability.