North American snow extent: 1900-1994

Historical fluctuations of North American snow extent from November through March are reconstructed back to 1900 using a combination of satellite and station observations. Using results of principal components analyses (PCA) from a companion study (Frei, A. and Robinson, D.A. Int. J. Climatol., this volume), simple and multiple linear regression models are used to take advantage of the spatial coverage of satellite observations and the temporal extent of station observations. This analysis more than triples the remotely-sensed record length, which begins in 1972. Model results indicate that North American winter snow extent tended to increase between the 1930s and around 1980, followed by a subsequent decrease during the 1980s. Long-term trends during November are less dramatic, with small increases since the 1960s. During March a different signal is observed, with snow extent decreasing since the 1950s. These results suggest a possible shift in the snow season. Historical signals from smaller regions within North America are identified during December and January. During December, the continental-scale signal is driven mainly by fluctuations over the western US, while January fluctuations are more strongly driven by an eastern signal. Models are sufficiently accurate to estimate changes in interannual variability over North America only during February, as well as over the eastern portion of the continent during December and January. Continental-scale interannual variability during February has been high since the mid-1970s compared to any previous time this decade. Regional-scale interannual variability over eastern North America in January has also been higher in recent years, but in December the highest interannual variability occurred during the 1940s.