Relationship between Winter Surface Temperature Variation in Eastern Asia and Stratospheric Weak Polar Vortex

The variations of temperature in winter in eastern Asia during 17 stratospheric weak polar vortex events were investigated based on the standardized temperature departure and the Northern Annular Mode (NAM) index calculated by using NCEP data set. Results show that the circulation anomalies in the stratosphere occurred about 15 days before the temperature variation in the troposphere. In the latter period of the weak stratospheric polar vortex, the surface temperature departure reached its peak. At that time, the regions south of latitude 40°N were warmer than the climatic average and the regions north of 40°N were colder. The variation of potential vorticity and the Asian trough at 500 hPa are also taken into account when discussing the physical process and mechanism of the anomalous surface temperature. In addition, cold air with high potential vorticity remains near 60°N in the early and middle period of the weak vortex and begins to extend southwards in the late period. In the middle and upper troposphere, cold air from the high latitudes can migrate to 45°N. At the same time, the Asian trough at 500 hPa becomes stronger and the low pressure region moves eastward; the ridge near Lake Baikal becomes weaker, the activity of cold air behind the trough influences Northeast China, northern China and more northerly regions and causes the surface temperature to fall. Moreover, warmer air with low potential vorticity moves from the south into the regions south of 40°N causing the surface temperature there to rise.