Climatic Change of Sunshine Duration and Its Influencing Factors over Tibet during the Last 35 Years

Using the data of monthly sunshine duration,mean cloudiness,surface vapor pressure and precipitation at 25 meteorological stations over Tibet from 1971 to 2005,the trend variation of the sunshine duration are analyzed by using linear trend analysis. Main results are as follows: (1) In terms of linear trend,the annual sunshine duration increases with an average rate of (8.8-52.1) h/10a in Ngari district,Tingri,Amdo,Mshung and Chali counties,especially in Mshung. But the tendency of annual sunshine duration decreases mostly in other stations with a decreasing rate of (10.9-138.4) h/10a,and the maximum is in Nagqu. In summer,the sunshine duration has a decreasing trend in most parts of Tibet. In winter,the increasing trend of sunshine duration occurred in Ngari district,northern Chamdo district and most parts of Nagqu district. In most parts of Tibet,the annual and seasonal total cloudiness has shown a decreasing tendency during recent 35 years,and the low cloudiness trend reduced. The trend of annual and seasonal mean surface vapor pressure increased over Tibet,especially in summer.(2) On an average in Tibet,the annual sunshine duration has shown a significant decreasing tendency during recent 35 years with a decreasing rate of 34.1 h/10a. Except for no obvious trend in winter,the decreasing trend occurred in other seasons. Specifically,in recent 25 years,the reducing range of sunshine duration increased in summer and autumn,as a result,the annual sunshine duration reduced. (3) In the 1970s,the sunshine durations are more sufficient in spring and summer,while lower in autumn and winter. The sunshine duration of positive anomaly occurred in all seasons in the 1980s,especially in autumn. But to the contrary in the 1990s,the sunshine durations are lower in all seasons,especially in summer. (4) Also,the annual and summer sunshine durations are more anomalous years in the 1980s,whereas less anomalous years in the 1990s. In winter,more anomalous years occurred in 1987 and less anomalous years in 1995 and 2005. (5) Results indicate that the principal causes for the increase of sunshine duration are the obvious decrease of cloudiness and decrease of precipitation in Ngari district,and that the decrease of sunshine duration is mainly caused by the significant increase of surface vapor pressure in other parts of Tibet.