Abrupt changes in the seasonal cycle of North American snow cover

Continental-scale snow cover extent has now been monitored from space for more than 20 yr in visible wavelengths. Here, the authors utilize weekly snow cover extent charts derived from such analyses to identify unusually rapid (1 week) spatially extensive snow cover accumulation and ablation events across the North American continent. Ancillary data are employed to describe the atmospheric patterns associated with the events. These episodes, which occur irregularly from year to year, bring about important changes in the total albedo of the continent. Rapid extensive accumulation events occur during two preferred portions of the accumulation season. The early season accumulation events average 1-week snow cover increases of 3.9 X 10(6) km2 and begin near the end of October. Late season accumulation events occur 1 month later and lead to average increases of 3.5 X 10(6) km2. These rapid advances in the North American snowpack are associated with distinct and consistent atmospheric anomalies that are conducive to spatially extensive snowfalls. Rapid ablation events also fall into two groupings based upon their timing within the annual cycle. Early season ablation episodes occur near the middle of March and account for snow cover losses averaging 2.1 X 10(6) km2. Early ablation events are associated with fluxes of sensible and latent heat induced by atmospheric disturbances moving along the Canadian-U.S. border. Late season events occur near the middle of May and are generally associated with anomalous high pressure at the surface and aloft over eastern Canada. This category of ablation events is not associated with large sensible heat flux to the snowpack. The loss of snow cover is more likely associated with downwelling longwave radiation fluxes from cloudy skies or shortwave radiation fluxes under clear-sky conditions.