Analysis Of Strong Cold Waves In China During Warm Winter Periods

Using 740 stations daily surface temperature data in China and NCEP/NCARdaily data from 1986 to 2004, the effect of mid-high latitude circulation and southtrough intensity on the cold waves in China during the warm winters are studied, andthe main reasons for the strong cold waves, together with the propagation andvertical structure of the strong cold air, during the winter of 2004/2005 are furtheranalyzed. The main conclusions are summarized as follow:1) From the winter of 1986/1987, countrywide and regional cold waves, andstrong cold air are prone to onset during the negative periods of the East Asiaregional index of Arctic Oscillation (EastAOI), when the main circulation of theEurasian continent is dipole-type with blocking ridge to the north and cut-off low tothe south. Moreover, there is obvious negative correlation between the dailyEastAOI and blocking ridge or the Siberian high intensity index, and positive onebetween the daily EastAOI and Aleutian low intensity index. Through its effects onthe blocking ridge, Siberian high and Aleutian Low; EastAO influences the strongcold wave outbreak in China.2) During the warm winters, strong positive correlation exists between southtrough intensity winter anomalies and the north or south winter temperatureanomalies. 84.6% of the countrywide cold waves, 42.9% regional ones areinfluenced by the south trough, while only 15.4% of the countrywide ones and 40%of the regional ones are not affected by it. Further analysis shows that from thewinter of 1986/1987, almost all the countywide cold waves during the positiveperiods of EastAOI are correlated with the south trough activities.3) Strong cold waves outbreak frequently in the east of China during the winterof 2004/2005. Results show that blocking ridge to the east the Ural Mountain andover the Caspian Sea are the main preconditions of the strong cold waves, while twointense cold waves are under the circulation of dipole type and the revising oftransverse trough. Moreover, there is a strong 10-20 oscillation in the temperatureand Siberian high during the winter of 2004/2005, which is the main intraseasonallow frequency model of cold waves onset. Further analysis also indicates that twowave trains exist and propagate respectively on the north and south west wind beltover the Eurasian continent. When propagating to the East of China, they couplewith each other, thus strong cold waves outbreak.4) According to the isentropic potential analysis of the strong cold waves from Dec 22nd, 2004 to Jan 1st, 2005, strong cold air of this cold wave event is from thelower stratosphere and upper troposphere over the north of Eurasian continent andArctic area. High potential vorticity cold air propagates to the south of the BaikalLake and is cut off by the low potential vorticity air from the low latitude, thus formthe dipole type circulation on the Eurasian continent before the cold wave onset,with low potential vorticity to the north and high ones to the south. Then after theweakening of low potential vorticity center, the cold air moves towards the southeastdirection along the north of the Tibet Plateau. When it arrives at the East Asia, thestrong cold air strongly extends in the vertical direction, leading to the deepening ofthe East Asia trough and thus the outbreak of the countrywide cold wave.