Arctic Winter Near Surface Freezing Intensity and its Impact on the Sea Ice Production
Sea ice growth during freezing season is a result of heat fluxes between relatively warm ocean and the cold air through snow covered sea ice. Sea ice thickness in NCEP/NCAR reanalysis is fixed at 3 meters, keeping the ocean well insulated from the atmosphere. Therefore the day-to-day and year-to-year changes in the surface air temperatures over sea ice covered ocean are entirely due to atmospheric processes in the given model, and not because of the upward heat flux from the ocean. Based on NCEP/NCAR reanalysis daily near surface air temperature fields (1979-2009) we explore winter freezing intensity and its eventual contribution to the total sea ice accumulation. Winter freezing intensity is an old idea advantageous for the analysis of long term high resolution air temperature data sets. These accumulated temperatures below freezing point are strongly (nonlinearly) related to sea ice growth. By means of the integration temporally (within one season, here between September and May) and spatially (within the entire region, here 60-90°N), surface thermal conditions between different years can be compared.
According to NCEP/NCAR lowest sigma level near surface air temperatures by the end of the Arctic freezing season vast territories accumulate about 4000-6000 freezing degree-days, 3000-5000 in the Eastern marginal seas, 5500-6500 in the Central Arctic and up to 8000 freezing degree-days in the Western Arctic northward from the Canadian Archipelago.
From year to year, the area [km2] dominated by very cold winter conditions was changing. In the 1980s, locations with values above 7000 freezing degree-days took place, disappearing entirely in the last decade. Significant winter long term warming, given as the decrease in the freezing intensity is found within the Central Arctic, East Siberian Sea, Chukchi Sea and the Eastern Beaufort Sea (up to 60 freezing degree-days loss per year). NCEP/NCAR air temperatures suggest that during the recent decade around the entire Arctic sea ice covered ocean new relatively mild conditions appear, compare to the earlier decades. High statistically significant correlation between the freezing intensity and the final fall freeze-up date is found over Siberian marginal seas, where the delay in freeze-up by several weeks reduces the resulting freezing degree-days accumulated during the whole freezing season.
