Cold- and warm-deep-snow winters in Mongolia

摘要

Eurasian snow cover in spring has followed a decreasing trend since the mid-1960s, but winter conditions remain unknown because of a lack of data. To address this issue with a regional focus on the eastern part of Eurasia, we conducted an observational study of winter temperature, precipitation, and snow depth in Mongolia and the associated atmospheric circulation. We used the meteorological data at 21 representative Mongolian weather stations for four winter months (November to February) from 1960 through 2007. Time series analysis was applied to three indices: standardized deviations from the mean for this 4-month period averaged over the 21 stations in Mongolia for snow depth, precipitation, and temperature. This analysis revealed a significant multi-decadal trend in temperature, but not in snow depth.During the 1960s and 1970s deep-snow winters coincided with extreme cold. However, beginning in the winter of 1992-1993, a new type of deep-snow winter with warmer conditions has occurred in some years. Moreover, a synoptic analysis demonstrated that a trough at the 500-hPa level that is usually climatologically located east of Mongolia extended westward to Mongolia during the cold-deep-snow winters. This indicates that enhanced cold surges from the north to Mongolia led to the historically typical deep snow conditions. On the other hand, the warm-deep-snow winters were characterized by a weakened trough, weakened cold surges, and concurrently intensified moisture transport from the west into Mongolia. The new circulation pattern observed here shows that warm winters, which may become more frequent in the future, still have the potential to cause deep snow in Mongolia.