The Characteristics Of Wintertime Persisting Cold Anomaly In China And Its Relationship With Low Frequency Oscillation

Based on the daily mean temperature datasets at496stations in China from51winters (1959/1960～2009/2010), two objective methods are adopted to depict the variations and characteristics of the persistent cold anomaly in the last51winter seasons over China. One is the definition of the LPCA (local persistent cold anomaly), and the other is the definition of the RPCA(regional persistent cold anomaly). The LPCA means that sequential cold day occur at a certain station at least5days. According to the definition, the result shows91RPCAs in all, and the distribution and trend of the LPCA frequency are analyzed. Further, the impacts of low frequency activities in the atmosphere are explained taking the lasting snowstorm in2008as an example. Then the work analyzes general linkage between the low frequency oscillation and the PCA (persistent cold anomaly) in wintertime China in the climatology perspective. The main conclusions are as follows:(1) The frequency, duration and influenced stations of PCA presents downtrend.The1986/1987winter seems to be a dividing line, before which PCA tends to be high frequency, long duration and more influenced stations, and shows a reverse trend after1986/1987.(2) The occurance of PCA in winter mainly locate in north of Xinjiang and south of Yangtze River, and larger duration of PCA concentrates in north of Xinjiang. there is great decrease in both frequency and duration in Tibet and Qinghai. (3) The time-spatial distribution analyzed from EOF of frequency and duration have similar pattern. The first mode shows the same variability over China; second mode indicates the opposite pattern between southwestern China and other regions; the third mode notes the opposite pattern between northeastern China, southwestern China and other regions; and the forth mode demotes the inverse pattern of northeastern China, south of the Yangtze River and other areas. Corresponding time serials contains both inter-annual and decadal variability.(4) The evolution of PCA in China is revealed by combined EOF, which exhibits four typical modes. The first one shows that PCA starts from northwestern of China, invades mainland of China in northwestern path, and retreats when it weakened, finally ends in north of China. The second mode notes that PCA origins from northwestern of China, invades mainland of China in western path, then turn to northern path. The third mode indicates similar invading route with the first one, but gradually die out at the southwest of China. The forth mode demotes a different original region with other modes, it is originated from the northeastern China. It mainly develops in the southern China.(5) The durative snowstorm processes in07/08winter are resulted from the cooperation of intraseasonal activities in the tropics and in the mid-high latitudes. The central of negative temperature anomalies in the western China is found in MJO Phase3, which corresponds to an enhanced MJO activity over the Indian Ocean and a reduced convective activity near the tropical central Pacific. In the meantime, the positive center temperature anomaly occurs in the northeast of China. On the other hand, when the MJO is detected in Phase7, the central of negative temperature anomaly appears over the northeastern China, and the western China undergo a warming anomaly. An analysis of the evolution of the500hPa geopotential height, sea level pressure and1000hPa wind anomalies indicate that low frequency activities in the mid-high latitudes provide climate background for favoring the PCA. Hence, the PCA in2008winter is resulted from the cooperation of intraseasonal activities in the tropics and in the mid-high latitudes.(6) Accompanied by the convective activity change from reduced to enhanced over eastern Indian Ocean, the wintertime abnormal temperature fields in China exhibit the variation on the intraseasonal time scale. When the strong reduced convections locate over the eastern Indian Ocean, significant warm temperature anomaly occurs over the whole China. There are two warm centers, one is located over inner Mango, and the other is at mid reaches of the Yangtze River. With the deep convective propagating eastward, the areas with cold anomaly stretched to the south of the Yangtze River, while warm temperature anomaly over the southwestern and southern China. While the convective activity enhanced intensively over the eastern Indian Ocean, significant cold temperature anomaly over the whole China, and the maximum anomaly appears in mid reaches of the Yangtze River.(7) In the mid-high latitudes, during the PCA events, The SH index is correlated significantly with major winter temperature anomalies at the intraseasonal time scale. When the SH intensity decreases abnormally, there occurs abnormal temperature in China, increasing in Southwest China and decreasing in the other parts of China, especially in the inner Mango and a part of northern China. When the SH begin to enhance, the PCA break out, cold anomaly occur in the north of China. While the SH enhanced intensively, significant cold temperature anomaly over the whole China, and the maximum anomaly appears in the western inner Mango, northern Xinjiang and the south of the Yangtze River.The PCA are resulted from cooperation of intraseasonal activities in the tropics and in the mid-high latitudes, possibly through EAP wavetrain propagation.