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A discontinuous ice coverage and lake temperature record of 36-yr duration has been compiled for Colour Lake (79°25'N, 90°45'W) on Axel Heiberg Island, Northwest Territories. About once every 6 yr, the ice cover remains through the entire summer, creating a residual ice cover the following winter. Residual ice covers are more frequent in the last 10 yr (1986-1995) than in the 20 yr from 1959 to 1978, indicating a reduction in climate factors controlling ice decay. These factors are identified as the number of thawing degree-days, suggesting a tendency toward cooler summers over the last decade. We report year-round meteorology observations that indicate a mean annual temperature of about -15.2°C, with ~ 500 thawing degree-days in summer. Following a residual ice year, spring lake-water temperatures are significantly greater than they are following nonresidual years. The increased temperatures can be attributed to the stabilizing effect of the residual ice pan the previous fall. Without an ice pan, convection cools the entire water column before the surface freezes. When the lake has an ice pan at the start of freezing, the surface freezes over quickly, effectively trapping water heated during summer. The warmer spring water temperatures act as negative feedback, decreasing the potential for a second consecutive residual ice year.

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Limnology and Oceanography

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