Variations Trend of Soil Temperature at Deep Layers in Lhasa from 1961 to 2005

Using the data of monthly mean soil temperatures at 0.8 m, 1.6 m and 3.2 m in Lhasa from 1961 to 2005, the linear trend, climatic anomalous and climate abrupt of deep soil temperature during recent 45 years are analyzed by using of linear trend analysis, accumulated variance and signal noise ratio methods.In terms of linear trend, the annual mean soil temperatures at 0.8 m and 1.6 m display a significant increasing trend in Lhasa in the past 45 years, and an increase of (0.54—0.62 ℃)/10a, especially at 0.8 m is indicated. During recent 35 years (1971—2005), the maximum rising range of mean soil temperature at 0.8 m and 1.6 m is in spring, the rate of rising is (0.84—0.69 ℃)/10a, its minimum is in autumn. But the maximum rising range of mean soil temperature at 3.2 m is in summer, the rate of rising is 0.71 ℃/10a, and its minimum is in winter. Comparing to the air temperature, the rising range of soil temperature is bigger in the same period. In addition, the rising range of the mean soil temperature at deep layer increases, specifically at 0.8 m and 1.6 m.There is an obvious increase tendency of inter-decadal variations of the annual mean soil temperatures at 0.8 m and 1.6 m from the 1960s to the 1990s. The seasonal mean soil temperatures at 0.8 m and 1.6 m are low from the 1960s to the 1970s, obviously in spring, and they are still low slightly in the most seasons of the 1980s. But there is positive anomaly of the mean soil temperature in all the seasons of the 1990s, specifically in spring and summer.The inter-decadal variations of the seasonal mean soil temperature at 3.2 m show an obvious increase trend from the 1970s to the 1990s. There is negative anomaly of the mean soil temperature at 3.2 m in all the seasons of the 1970s, especially in summer. To the contrary in the 1990s, there is positive anomaly of mean soil temperature in all seasons. It is also much evident in summer.The annual mean soil temperatures at 0.8 m, 1.6 m and 3.2 m are anomalous warm in 1999, whereas anomalous cold year occurs only in 1963 at 1.6 m.In summer, the anomalous warm of the mean soil temperature at deep layer takes place in 1999. In winter, the soil temperatures at 0.8 m and 1.6 m are anomalous cold in years in 1960s, and the mean soil temperatures at 3.2 m are anomalous warm in 1999, 2002—2005.Except for no abrupt change at 3.2 m in winter, the seasonal and annual mean soil temperatures at deep layer have abrupt changes, and it occurs in the 1980s. In summer and autumn there is a sudden change in 1986, in spring and winter there is a climate abrupt in 1983.