The association between the BWA index and winter surface temperature variability over eastern Canada and west Greenland

Since about 1970, winter surface temperature data from stations on coastal eastern Canada and western Greenland have shown detectable decadal cooling. In this study, we attempt to understand some aspect of this surface cooling trend by relating it to the variability of the Canadian Polar Trough (CPT). In order to facilitate the relationship, we introduce a new 50 kPa index called the Baffin Island–West Atlantic (BWA) index which, although reflecting the variability of the western structure of the North Atlantic Oscillation (NAO), is found to explain temperature variability better in north-eastern North America than the structure characterized by the NAO index. The decadal variability in the winter surface temperature is found to be associated with the BWA index at a statistically significant correlation of 0·85. Two distinctive winter climate regimes are found to exist in the climate record from 1947 to 1995, one before and one after about 1970. Although the magnitude of the variance does not change significantly from one regime to the next, the two regimes are characterized by statistically significantly different means and by two distinct spectral signatures. Variability before 1970 is dominated by interannual fluctuations, whereas afterwards much of the contribution to the variability comes from interdecadal fluctuations. Subtraction of the 1947–1969 winter 50 kPa mean height field from the 1970–1995 mean field shows that the change in the height field over the Northern Hemisphere is reflected in the enhancement of the negative phase of the NAO mode (which corresponds to a strong jet stream over the western Atlantic and a strong Icelandic low) and of the positive phase of the Pacific/North America (PNA) mode. © 1997 The Royal Meteorological Society.