中文摘要：

2009年12月北半球中纬度出现大范围持续低温、暴风雪等天气。采用NCEP／NCAR再分析资料研究了平流层AO（Arctic Oscillation，北极涛动）异常信号下传的特征及其对本次极端气候事件的影响，并讨论了与平流层异常信号下传相关的行星波活动。结果表明：1）与此次极端气候事件相联系的负位相A0异常11月首先发生在平流层，维持将近1个月后于12月初开始下传，并且迅速传至地面。12月整个对流层的位势高度及温度在极区附近出现强的正异常，而中纬度地区则为负异常。2）平流层AO异常信号下传后，地面出现有利于低温降雪过程的环流异常。12月上旬，亚洲大陆东部及北美大陆西部出现异常偏北风，造成了俄罗斯、北美西部大面积负的温度异常；12月中下旬，欧洲大陆盛行偏西北气流，同时蒙古高压增强，欧亚大陆北部包括中国北方出现大片负的温度异常。3）在此次极端气候事件之前，北半球高纬度地区有异常强的行星波上传至平流层，导致平流层出现负位相的AO异常，并维持了一个月；随后，上传到平流层的行星波减弱，同时平流层负位相的AO异常迅速传至地面，导致了有利于低温降雪的环流异常。

英文摘要：

The extreme cold and snow weather occurred at the mid-latitudes of Northern Hemisphere in December 2009. Based on the NCEP/NCAR reanalysis data, this paper studies the characteristics of stratospheric AO（ Arctic Oscillation） abnormal signal downward propagation and its influence on the extreme weather ,and the planetary wave propagation mechanism related to the abnormal signal down-ward. It shows that： 1 ） The Arctic Oscillation （AO） plays an important role in this cold event, and its a-nomalous negative signal occurs and maintains in the stratosphere during the preceding November. In the early December, the associated anomalies spread into the troposphere and then propagates fast onto the surface, which leads to remarkably positive （negative） geopotential height and temperature anoma-lies in the northern polar（mid-latitude） region in the whole troposphere in December. 2 ）Induced by the negative AO signal, the anomalous northerlies dominate the eastern Asia and the west part of the North- ern America during the early December, facilitating the extreme cold condition in these areas. The asso-ciated northwesterly anomalies in the Europe and the strengthened cold Mongolian high also result in the prevalence of low temperature in the northern Eurasian continent during the mid-to-late December.3 ）The stationary planetary wave propagation into the stratosphere is extremely strong at the high-mid latitudes of Northern Hemisphere in November, which induces negative AO anomaly in the strato-sphere. Accompanied by weakening of the planetary wave upward propagation during the following De-cember, the negative AO anomaly exhibits from the upper troposphere to surface,leading to cold-condi-tion circulation anomalies.