Inter annual and Interdecadal Variations of Soil Temperature over Tibetan Plateau from 1971 to 2005
西藏地温的年际和年代际变化

The variation trend,abrupt change and anomalous years of annual and seasonal mean soil temperatures at 0.8 m,1.6 m and 3.2 m over Tibet are investigated in the context of the observational data at 10 stations during the period 1971-2005 using modern statistical diagnostic methods such as the linear trend analysis. The results show that the annual mean soil temperature at 0.8 m decline in Chamdo and Nyingri of Tibet where the reduction rate is 0.15 oC/10a and 0.18 oC/10a respectively,but the increasing trends are observed at the other stations at a rate of (0.19-0.81) oC/10a (except for Tesdang,99% confidence level). Significant increasing trends of annual mean soil temperature at 1.6 m are found at five stations,with the increasing rate of (0.20-0.60) oC/10a (above 95% confidence level). Also,the increasing trends of annual mean soil temperature at 3.2 m are observed at six stations,with the rate of (0.13-0.52) oC/10a,especially in Lhasa. In the case of 0.8 m soil temperature,the following facts are found as: (1) The increasing trends of seasonal mean soil temperature are observed at most stations,with the most remarkable increasing rate in western and southern Tibet in summer,especially in Sengge Zangbu river basin where the rate is 1.61 oC/10a (99% confidence level); the maximum increasing rate is in northern Tibet in winter. But the decreasing trends of the soil temperature are found in most seasons of eastern Tibet. (2) In terms of interdecadal variations,the annual mean soil temperature increases apparently at most stations of Tibet,however it decreases in Chamdo. (3) The abrupt change of spring and summer mean soil temperature is observed in Sengge Zangbu in 1996 and 1983 respectively. The abrupt change of the annual and seasonal mean soil temperatures happens in Lhasa and Shigatse,the former in the 1980s and the latter in the early 1990s,a sudden change from the colder period to the warmer one. In addition,the abrupt change of summer and autumn mean soil temperature occurs in Nyingri in 1993,indicating a change from the warmer period to the colder one. (4) The annual and seasonal soil temperatures occur in the more anomalous higher temperature years at the end of the 1990s and the first five years of the 2100s in western Tibet,while in the mid-late 1990s in southern Tibet and the first five years of the 2100s in northern Tibet. The more anomalous lower years occur mainly in the 1980s while the anomalous lower years dominate in eastern Tibet.