Abstract:Using the daily data of NCEP/DOE Reanalysis II and the temperature data established by conventional observation stations of the National Meteorological Information Center,cold waves during the extreme event of regional averaged temperature in China in the winter of 2011/2012 and the characteristics of the low-frequency oscillation(LFO) of the mean temperature of 828 stations were analyzed.Then,the evolution of the low-frequency temperature field with phases was also analyzed,as well as the impact of the spatial configuration of the low-frequency circulation system in the mid-troposphere in the extratropical Northern Hemisphere on temperature changes by using the SVD method.The results showed that:
(1)The increased cumulated frequency of the cold waves and the declining-temperature periods from November 2011 to February 2012,accompanied by the wide temperature reduction,led to the extreme event of regional averaged temperature in China and an abnormally low temperature anomaly in North and Southeast China in the winter of 2011/2012.
(2)Cold-temperature anomalies were the result of overlapping of the strong 10-30-day oscillation of temperature,based on the seasonal temperature change trend.Low-frequency temperature in the western Tibetan Plateau and Yellow River basin and middle and lower reaches of the Yangtze River and the region to its south changed coherently,and two centers of strong oscillation were separately located in the Hetao area in the north and in southern China in the south.Whereas,the eastern Tibetan Plateau showed the opposite,with a decrease(increase) of the low-frequency temperature in Northeast Inner Mongolia and Northeast China lagging behind the large area.As for northwest north,the LFO of temperature was mostly consistent with both of the two former.
(3)The results of coupling spatial patterns and their temporal variations of the low-frequency height field at 500 hPa and the low-frequency temperature field of 2 days later by using the SVD diagnosis method indicated that,when the low-frequency high between the Urals and Lake Baikal was enhanced,and the low-frequency wave train in mid-low latitudes was coupled with that in the mid-high latitudes,these circulation systems led to strong cold air spells south along the easterly path.Furthermore,they caused the eastern area from the west of Inner Mongolia to southern China to undergo cold waves and declining temperature in winter's eve and in mid to late winter,with the two centers of severe declining temperature separately located in the east of the Hetao area in the north and in southern China in the south.However,the eastern Tibetan Plateau showed the opposite,with the low-frequency wave train along Kara-east of the Urals-Northwest China being the low-frequency circulation system that led to strong cold air spells in the south,which then dispersed along the westerly and northerly path before sequentially impacting Northwest and Northeast China during the cold waves in the latter half of winter.