Canadian pressure observations and circulation variability: links to air temperature

A set of 71 station series of surface pressure from Canada and Greenland have been examined for quality control and homogeneity. These records range in length from 50 to 130 years. The object of this exercise was to investigate station-based surface pressure series and atmospheric circula
tion on a decadal time scale, and to examine the effects of the
atmospheric circulation on climate. The data considered here are monthly means.
Several major inhomogeneities were disc overed during the course of this exercise, the most serious of which relates to a Canadian-wide change in reporting practice which took place in 1977. This type of inhomogeneity is almost impossible to uncover using conventional homogeniza
tion techniques based upon reference series. The final homogenized series show appreciable differences in regional trends of atmospheric p
ressure compared with the unhomogenized series, particularly
in southern Canada.
Empirical orthogonal function (EOF) analyses on the station series revealed three main modes of circulation over Canada and Greenland; these patterns were compared with results from the UK Hadley Centre’s gridded pressure dataset.
There are appreciable differences between the leading EOF modes of the two datasets, which may be due to an artificially enhanced number of degrees of freedom in the gridded datas et. Trends in atmospheric pressure were also calculated; these suggest an intensification of zonal flow during winter o
ver the period 1950 – 98, but these variations appear to be
much less pronounced and not statistically significant when considered over the whole of the 20th century.
The new station database was also compared with a gridded surface temperature dataset. There are strong correlations between the various circulation indices and temperature anomalies. Some of the trends of temperature in Canada during the period 1950 – 98 can be attributed to these changes of atmospheric circulation. The regional atmospheric circulation
indices described here are shown to have considerable influence on the surface temperature variability and trends for all seasons of the year