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 coa stal eastern Canada and western Greenland have shown detectable decada l cooling. In this study, we attempt to understand some aspect of this surface cooling trend by relating it to the variability of the Canadi an Polar Trough (CPT). In order to facilitate the relationship, we int roduce a new 50 kPa index called the Baffin Island-West Atlantic (BWA) index which, although reflecting the variability of the western struc ture of the North Atlantic Oscillation (NAO), is found to explain temp erature variability better in north-eastern North America than the str ucture characterized by the NAO index. The decadal variability in the winter surface temperature is found to be associated with the BWA inde x at a statistically significant correlation of 0.85. Two distinctive winter climate regimes are found to exist in the climate record from 1 947 to 1995, one before and one after about 1970. Although the magnitu de of the variance does not change significantly from one regime to th e next, the two regimes are characterized by statistically significant ly different means and by two distinct spectral signatures. Variabilit y before 1970 is dominated by interannual fluctuations, whereas afterw ards much of the contribution to the variability comes from interdecad al fluctuations. Subtraction of the 1947-1969 winter 50 kPa mean heigh t field from the 1970-1995 mean field shows that the change in the hei ght 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 o f the positive phase of the Pacific/North America (PNA) mode. (C) 1997 by the Royal Meteorological Society.