Recent Variations of Snow Cover and Snowfall in North America and Their Relation to Precipitation and Temperature Variations

Contemporary large-scale changes in solid and total precipitation and satellite-derived snow cover were examined over the North American continent. Annual snow cover extent over the last 19 years decreased up to 6×105 km2 relative to a 0.93°C (0.33°C) increase in North American (Northern Hemisphere) temperature. A strong correlation exists between snow cover and temperature where up to 78% of the variance in regional snow cover and snowfall is explained by the anomalies of monthly mean maximum temperature. Over the last two decades the decrease in snow cover during winter (December-March) has largely occurred through reduced frequency of snow cover in areas that typically have a high probability of snow on the ground with little change in the frequency of snow cover in other areas. Similar characteristics were observed during spring (April-May) in areas with high snow cover probability except for an expansion of the snow-free regions. Anomalies in these two seasons dominate the interannual variability (nearly three-fourths of the variance) of snow cover. The apparent unprecedented global warmth of the 1980s was accompanied by a retreat of the mean annual North American snow cover, a 10% increase in annual Alaskan precipitation, a significant decrease (−7% ) in annual snowfall over southern Canada (while the total precipitation remained above normal), and a more than twofold increase in the variance of the ratio of frozen to total precipitation over the contiguous United States. An increase (4% –5% per decade) of both solid and total precipitation over northern Canada (zone 55°–70°N) occurred during the last four decades. A century-scale increase (1% per decade) of precipitation was found over southern Canada, but the proportion of the precipitation that falls in frozen form over this area decreased over the last four decades. Precipitation over the contiguous United States has significantly (2% –3% per decade) increased during the last four decades, but on a century time scale the increasing trend is not yet statistically significant.