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. Copyright © 1999 Royal Meteorological Society

The duration of seasonal (September–May) snow cover over the central and northern Great Plains region of the USA has varied considerably during this past century. Annual and decadal fluctuations are embedded in a statistically significant trend toward greater seasonal snow cover, from 29 days in the early part of the century to 38 days in recent decades. The variability of snow cover duration also increased throughout the century, exhibiting the largest changes in a north‐west‐trending zone bisecting the region. Seasonal snowfall, temperature, and precipitation show fluctuations similar to those of snow cover; however, only snowfall has increased significantly since 1910. A post‐1970 shift toward longer autumn (September–November) snow cover duration and shorter spring (March–May) duration has been accompanied by decreases in autumn maximum and minimum temperatures and increases in spring minimum temperatures. The reliance on snow cover for hydrological and agricultural purposes in this part of the country, the sensitivity of the boundary layer climate to snow cover conditions in the Plains, and the projections by global climate models for this region to become warmer and dryer as a result of increasing atmospheric concentrations of greenhouse gases, necessitate increased study and hence a better understanding of the variability of snow cover over the Plains.

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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

ABSTRACT. Hi stori cal a nd reco nstru c ted sno w-co\'e r d a ta show e\'id e nce of a g radu a l in c rease in sno w cove r over th e co ntin enta l interi o r of K o rth Am eri ca (NA ) during mu ch of th e 20 t h century, primaril y in res po nse to in c reas ing snowfa ll, A ra pid d ec rease in C a nadi a n-pra iri e snow cove r a ft e r 1970 is no t observed O\'e r th e Grea t Pl a ins. An a lysis of snow-cove r-clim a te rela ti o nships re\'ea led sys tem a ti c in c reases in th e se nsitivity of snow cove r to North e rn H emisph ere (NH ) te mpera tures O\'e r th e 1940-65 pe ri od. Thi s ch a nge is mainl y du e to a n in c rease in snowfa ll-tempera ture se nsiti vity during this period , Season a l a na lysis re\'ealed th a t th e o bse rved in crease in snow-cove r a nd snowfa ll tempera ture se nsiti\'ity is prim a ril y a spring ph enom eno n. A m a rked in crease in th e importa nce of th e sprin g peri od is obse rved a ro und 1960, whi c h coin cid es with a well-d oc um e nted c ha nge in a tm os ph eri c circ ul a ti o n ove r NA . Th e pos t-1960 peri od is cha rac teri zed by a sig nifi ca n t in ve rse rela ti o nshi p be twee n sno \\' cover a nd hemi sph e ri c air te mpe ra ture ove r th e Ca nadi a n pra iri es a nd north ern G rea t Pl a in s regio ns,