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Atmospheric controls on Eurasian snow extent

Composite analyses, based on weekly snow-cover charts, temperature, sea level pressure, cyclone tracks and a rotated PCA of daily filtered 700 hPa geopotential height are used to examine relationships between the dominant modes of low-frequency atmospheric variability and mid-winter snow extent over the Eurasian continent. Two of the circulation modes examined have been identified previously and represent the North Atlantic Oscillation (NAO) and the Eurasian Type 1 (EU1) pattern. A third, termed the Siberian pattern (SIB), has not been identified previously, and describes variability in 700 hPa height over central Asia and southern Siberia. The most coherent snow-cover signals occur in the transient snow regions over Europe and south-western Asia, where variations in snow extent are largely controlled by temperature. Snow signals in east Asia are difficult to explain, but appear to be primarily determined by the availability of precipitation. For the NAO, snow-cover signals are largely restricted to central Europe. This result is initially surprising, as the NAO is associated with large temperature anomalies over a large part of the Eurasian continent. However, east of the Ural Mountains temperature anomalies in NAO extremes are confined to northern regions where mean temperatures are well below freezing, and air temperatures have little influence on snow extent. In extremes of the EU1 and SIB patterns, significant snow-cover signals are found in south-western Asia, where variability in the amplitude of the Eurasian wave train results in large differences in air temperature and cyclone activity over the transient snow regions. No coherent snow-cover signals are associated with extremes of the Siberian High. Copyright © 1999 Royal Meteorological Society