An assessment of changes in winter cold and warm spells over Canada

The recent Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) indicated that observed 20th century changes in several climatic extremes are qualitatively consistent with those expected due to increased greenhouse gases. However, a lack of adequate data and analyses make conclusive evidence of changing extremes somewhat difficult, particularly, in a global sense. In Canada, extreme temperature events, especially those during winter, can have many adverse environmental and economic impacts. In light of the aforementioned IPCC report, the main focus of this analysis is to examine observed trends and variability in the frequency, duration, and intensity of winter (Jan-Feb-Mar) cold and warm spells over Canada during the second half of the 20th century. Cold spell trends display substantial spatial variability across the country. From 1950-1998, western Canada has experienced decreases in the frequency, duration, and intensity of cold spells, while in the east, distinct increases in the frequency and duration have occurred. These increases are likely associated with more frequent occurrences of the positive phase of the North Atlantic Oscillation (NAO) during the last several decades. With regard to winter warm spells, significant increases in both the frequency and duration of these episodes were observed across most of Canada. One exception was found in the extreme northeastern regions, where warm spells are becoming shorter and less frequent. The results of this study are discussed within the context of climate warming expectations.