Attribution of twentieth century winter warming over Canada

The detection and attribution of past trends, changes, and variability in climate parameters is essential toward the understanding of potential future changes resulting from anthropogenic activities. This is especially true for high latitude regions such as Canada where climate change signals are anticipated to be stronger. Recent studies have shown that since 1900, mean annual temperature over southern Canada has increased by an average of 0.9°C, with the largest warming occurring in winter and early spring. In addition, these increases were more pronounced on the left hand side of the daily temperature distribution (i.e. to extreme low daily temperatures) as opposed to the right hand side, particularly, over western regions. Winter and spring temperature trends during the second half of the 20th century were associated with distinct regional differences including significant increases over the south and west, and significant decreases over the northeast. The main purpose of this study is to obtain a better understanding of the causes of observed winter temperature trends and variability over Canada through examination of changes in the characteristics of various air masses influencing winter climate. Preliminary analyses of frequency distributions of daily surface temperature, 50-100 kPa thickness values, and tropospheric precipitable water strongly suggest that the cold air dome associated with the continental Arctic air mass has thinned in recent decades, but only over western regions of the country. This has resulted in more frequent incursions of warmer maritime air at upper levels (that occasionally reach the surface) and accounts for the significant increases in winter surface temperature over western Canada. The thinning of the cold dome is also consistent with the larger increases in extreme low daily temperatures observed over this region during winter. Conversely, results over eastern Canada indicate little change to the character of air masses affecting the area including the depth of the dome of Arctic air. These findings appear to be related to variations in large scale circulation patterns including the oft cited Pacific wide atmospheric and oceanic changes in 1976, and the recently identified Arctic Oscillation.