The detection and attribution of changes in climatic extremes is one of the main themes of current climatological research. Analyses of recent trends in Canadian climatic extremes have shown substantial decreases in the frequency, duration and intensity of winter cold spells over western regions of the country. Conversely, eastern Canada was associated with opposite trends in these variables. This study analyzes the influence of low frequency variability modes (namely, ENSO, the AO and the QBO) on the frequency, duration, and intensity of winter cold and warm periods over Canada during the second half of the 20th century.

Results show that for western Canada, El Niņo events are associated with a significant reduction in the frequency of cold spells, and a significant increase in the frequency of warm spells. During these events, the duration of warm spells increase throughout most of southern Canada, while the number of cold days (warm days) decrease (increase) from British Columbia to the Great Lakes. The high index phase of the AO is related to increases in both the frequency and the duration of winter cold spells in eastern Canada. The coupling between the stratospheric circulation and the climatic extremes are also examined by compositing the number of winter cold and warm days according to the two phases of the QBO. When compared to the westerly phase, the easterly phase of the QBO favors more cold days in western Canada and fewer cold days along the east coast of Canada.