An analysis was carried out to determine the duration of the summer shipping season for Deepwater vessels transiting through the Northwest Passage Route. The most likely route segment to obstruct shipping is in Viscount Melville Sound, which is typically characterized by the presence of high concentration mixtures of the deformed, thick first-year and multiyear ice. The period for ship transits through the Passage is determined from the computer-based analysis of digital Canadian Ice Service weekly ice charts which are available from the late 1960s to the present. Automated computer-based algorithms were developed to estimate the number of if any, weeks with ice conditions that would successfully allow transit. The results show a very large year-to-year variability in the duration of the summer shipping season with the trend towards slightly improving ice conditions. The possibility of future increases in old ice concentrations in western and central portions of Parry Channel due to an apparent trend towards more rapid passage of this old ice through the Queen Elizabeth Islands to the north may impede ship passages in the next decade by comparison with the last decade or two.

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