Satellite Analysis of Interannual Variab Ility in the Snow-free Season and Its Relation to Vegetation Activity in Northern Land Areas

This study investigated variability and trends in the annual snow cover cycle in regions covering high latitude and high elevation land areas in the Northern Hemisphere. The annual snow cover cycle was examined with respect to the week of the last observed snow cover in spring (WLS), the week of the first observed snow cover autumn (WFS), and the duration of the snow-free period (DSF). The analysis employed a corrected version of a 29-year time-series (1972-2000) of weekly, visible-band satellite observations of Northern Hemisphere snow cover from NOAA. Substantial interannual variability was observed in WLS, WFS, and DSF (standard deviations of 0.8-1.1, 0.7-0.9, and 1.0-1.4 weeks, respectively), which is directly related to interannual variability in snow cover area in the regions and periods of snow cover transition. Over the nearly 3-decade study period, WLS in all study regions shifted earlier by 3-5 days/decade as determined by linear regression analysis. DSF increased by about 5-6 days/decade, primarily as a result of earlier snow cover disappearance in spring. No strong evidence of any systematic trend in WFS was observed. In addition to altering the surface energy balance through the snow-albedo feedback effect, the observed variability and trends in the annual snow cover cycle are potentially significant factors for terrestrial ecosystem functioning, including annual primary production and net ecosystem CO2 fluxes.