Elevated levels of greenhouse gas concentrations have prompted the scientific community to assess the impact of such increases on the Earth's climate. With the doubling of carbon dioxide, many Global Climate Models are projecting a significant rise in global mean temperatures. Moreover, these models also indicate changes to the frequency of both extreme high and extreme low temperature events. Recent studies have shown a clear trend toward fewer low minimum temperature extremes in many parts of the globe. The objective of this study is to document the trends and variability in the number, duration and intensity of winter cold spells over Canada during the 20th century.

Individual cold spells are identified at 210 stations across Canada for the periods 1900-98 (southern Canada) and 1950-98 (entire country). The 20th percentile of the daily winter (JFM) minimum temperature distribution (relative to the 1961-90 climatology) is firstly determined for each station. Cold spells are then defined as those events in which the minimum temperatures remained below this threshold for at least three consecutive days. Another measure of 'abnormally cold' events includes the number of days in which the minimum daily temperature remains below -25°C from October to March.

Results show a significant decrease in both the number and duration of winter cold spells over most of western Canada. In particular, during the period 1950-98, there has been a decrease of approximately two to three spells, and the length of these spells is about 2 days shorter. Even though the number and duration of cold spells are decreasing, preliminary analyses suggest that the intensity of the spells has been increasing over time. In contrast, several eastern Canadian stations show significant increases in the number of cold spells for the shorter 1950-98 period. The association of the cold extremes with atmospheric circulation anomalies is also presented including relationships with the Arctic and the Pacific Decadal Oscillations.