Interannual Variability of Eurasian Snow Cover in Spring and its Possible Link With Atmospheric Circulation

We investigate the interannual variability of Eurasian spring snow cover extent (SCE) during 1973-1998, especially focused on the relationship between the recent decreasing trend and the year-to-year variation. Two major patterns are detected by applying EOF analysis to the interannual component of the snow cover duration at each grid point derived from the NOAA satellite measurement data set. Recent Eurasian SCE change, mainly characterized by an abrupt decrease after the end of 1980's, is identified as the first component of EOF analysis (representing 18.3% of the total variance), which is located over the eastern part of the continent and has an apparent persistency from February to August. This pattern is the representative of spring-to-summer snow cover variability over the whole Eurasia. On the other hand, another dominant mode is found over the western Russia (50-65N,40-80E) as the second component (8.6%), which has a fluctuation of about 4-6-year cycle without a significant linear trend. The regression analysis shows that in April there is a close relationship between the variation of this mode and air temperature and geopotential height fields over western Russia from surface up to stratosphere, which indicates that this mode is strongly controlled by the atmospheric circulation in spring. Furthermore, the results of the atmospheric heat budget analysis over the western Russia shows that adiabatic compression in relation to its vertical motion plays an important role for heating the atmosphere and the decision of the timing of snow cover disappearance over western Russia, while horizontal advection is always working as a heating sense independently of the interannual variation of snow cover disappearance. Diabatic processes merely affect the fluctuation of the atmospheric heating. Many numerical studies have shown the influence of the continental scale variability of land surface processes induced by snow cover on the following climate variation, and some diagnostic studies pointed out that western Russia (including central Asia) is a key region. But the results of this work suggest that another scenario to explain the mechanism of these relationships is needed as for that over western Russia.