Atmospheric Controls On Eurasian Snow Cover Variability In Autumn And Winter

Snow cover is an important and active part of the climate system. Due to its impact on energy and mass exchange at the surface, snow cover affacts global and regional radiative and thermal energy budgets. Previous studies indicate that wide range of persistent anomalies of snow cover over Siberian and Tibet Plateau would induce large scale atmospheric circulation anomalies in the winter, through the propagation of planetary waves. Understanding how snow cover responses to changes of atmospheric circulation is especially important for improve weather and climate prediction skills. Previous studies have identified NAO, EU1, SIB these several atmospheric modes which control Eurasian snow cover effectively, as well as the controlling mechanisms. However, those several modes explain only a small fraction of snow cover variance. The importance of snow cover variability of transitional seasons is not less than that of winter. However, few studies concentrate on transitional seasons. This study seeks to find other atmospheric circulation modes which impact Eurasian snow cover significantly in autum and winter, and determine their physical mechanism on controlling snow cover. In China, scholars have analyzed some regional blizzard weather process, getting a lot of useful conclusions. However, the synoptic conditions that are responsible for large scale snow cover anomalies are still unclear. In light of this, we studied the synoptic conditions of large scale snow cover anomaly in China.EOF Analysis was applied to time series of Eurasia snow cover to determine spatial and temporal variation characteristics of Eurasian snow cover. The seven daily atmospheric circulation modes, namely the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific-North American (PNA), the West Atlantic (WA) type, Eurasian (EU), Eastern Atlantic (EA), the Western Pacific (WP), were regressed to the Eurasian daily snow cover data. By comparing the snow cover leading six EOF pattern and the regression pattern of these seven circulation indices, we discovered the relationship between these circulation modes and the snow cover variability as well as the importance of these modes to snow cover. The October, November, December, January, February’s cases were studied respectively.By comparison, AO and NAO are of great importance in controlling the snow cover variability for the five months respectively; PNA performs a strong sense of control effect on snow cover variability for these months except February; EA plays an important role in controlling October, November and January snow cover variability; EU’s influence effect on snow cover variability in October, November, December is obvious; WP of October, December, February has a strong control effect on snow cover variability; WA of December and January has a lot of control effect on snow cover variability. Composite analyses were used to determine how the circulation modes influence the air temperature, precipitation which in turn impacts the changes of snow cover. Classification was made according to the influence region and influence mechanism of the circulation modes.Principal component analysis (PCA) was applied to snow cover data of China area to distinguish separate regions of China. From the leading two EOF modes, we identified three different regions with separate snow cover signal. The synoptic conditions of snow cover anomaly in the each region were determined through composite analysis based on each PC extreme events. To sum up, increased snow cover in China is related with cold air activities:(1) Wide range of snow cover increase in the Yellow River basin, southern Xinjiang, Inner Mongolia, and the middle and lower reaches is related with the outbreak of cold wave. The circulation feature is that the Siberian high is enhanced, East Asian trough is deepened, west wind anomalies prevails in snow signal region, usually accompanied with water vapor transport from the Pacific. (2) Increased snow in Northeast China also relates with cold air activities, which cause temperature decrease. The difference is, Siberian abnormal high pressur center locates farther north, with weaker strength, leading to North-moving cold air path which in turn limits the cold air activities in the Northeast, China.