Seasonal snow cover information over southern Canada was reconstructed from daily snowfall and maximum temperature data back to 1915 using a simple mass balance approach with snowmelt estimated via a calibrated temperature index method. The reconstruction method was able to account for 70%–80% of the variance in annual snow cover duration (SCD) over most of southern Canada for the 1955–1992 calibration period. The data were used to construct regional SCD anomaly series in four regions spanning the continent. The regional SCD series were characterized by high interannual variability, with most of the variance concentrated at periods less than 5 years. Spring (MAM) snow cover variability was characterized by a prominent spectral peak with a period of approximately 4 years, which appeared to be linked to tropical Pacific sea surface temperature variability.
There was no evidence of statistically significant long-term trends in snow cover in any of the regions, but the data suggested that winter (DJF) snow cover had increased and spring snow cover had decreased over much of southern Canada. One of the most prominent regional features was a systematic decrease in winter and spring snow cover over the prairies since approximately 1970. However, current low snow cover values in this region are still within the expected range of natural variability. Linear combinations of the regional SCD series, including data from the Great Plains, were able to explain 81% and 75% of the variance in North American winter and spring snow covered area (SCA) over the 1972–1992 period. Reconstructed values of SCA back to 1915 suggested that North American winter snow cover has exhibited a gradual increase of 11.0 × 103 km2 yr−1 during much of this century, while spring snow cover has decreased by an average −6.0 × 103 km2 yr−1. These represent rather small changes in SCA (<10% of current mean SCA over a 100-yr period).
Of the several teleconnection indices investigated, the Pacific-North American (PNA) pattern was observed to exert the strongest influence on snow cover variability; the positive phase of the PNA pattern was associated with reduced snow cover in all seasons over western Canada. The influence of ENSO on snow cover variability was found to be highly variable in both time and space, with lag 0 correlations indicating that El Niño was associated with less snow cover over western Canada. These correlations were much weaker than the PNA pattern. The influence of the North Atlantic oscillation pattern was observed to be mainly confined to winter snow cover variations across the eastern United States and southern Ontario.