Atmospheric controls on Eurasian snow extent

Composite analyses, based on weekly snow-cover charts, temperature, sea lev el pressure, cyclone tracks and a rotated PCA of daily filtered 700 hPa geo potential height are used to examine relationships between the dominant mod es of low-frequency atmospheric variability and mid-winter snow extent over the Eurasian continent. Two of the circulation modes examined have been id entified previously and represent the North Atlantic Oscillation (NAO) and the Eurasian Type 1 (EU1) pattern. A third, termed the Siberian pattern (SI B), has not been identified previously, and describes variability in 700 hP a height over central Asia and southern Siberia. The most coherent snow-cov er signals occur in the transient snow regions over Europe and south-wester n Asia, where variations in snow extent are largely controlled by temperatu re. Snow signals in east Asia are difficult to explain, but appear to be pr imarily determined by the availability of precipitation. For the NAG, snow- cover signals are largely restricted to central Europe. This result is init ially surprising, as the NAO is associated with large temperature anomalies over a large part of the Eurasian continent. However, east of the Ural Mou ntains temperature anomalies in NAO extremes are confined to northern regio ns where mean temperatures are well below freezing, and air temperatures ha ve little influence on snow extent. In extremes of the EU1 and SIB patterns , significant snow-cover signals are found in south-western Asia, where var iability in the amplitude of the Eurasian wave train results in large diffe rences in air temperature and cyclone activity over the transient snow regi ons. No coherent snow-cover signals are associated with extremes of the Sib erian High. Copyright (C) 1999 Royal Meteorological Society.