Climatic Change of Sunshine Duration in Northwest China during the Last 47 Years

With EOF,REOF,Mann-Kendall,trend analysis and correlation analysis method,the spatial distribution features and time evolution rule of sunshine duration in the last 47 years in Northwest China are analyzed based on the monthly mean data for the period 1961 to 2007 collected from 135 observational stations.It is shown that sunshine duration of the region decreases from the southeast to the northwest,the value on both sides is low and the center is high.The border area of Gansu,Qinghai and Xinjiang is the high value region in sunshine duration of northwest.The winter sunshine duration is the least,summer is the most,and the spring's is more than the autumn's.Northwest sunshine duration in most part of northwest reduces remarkably,the average reduction rate of the entire region is 19.92 h/10 a;it has a sudden change in 1980.Southwest Xinjiang and southern Qinghai present an obvious increasing tendency,but in Gannan-Central Gansu-southern Ningxia-east Gansu-Central Shaanxi the increase in sunshine duration is not obvious.Using the REOF method we detailedly divide the region into seven subregions,i.e.,the eastern part of northwest,mid-northern Xinjiang,southwest Xinjiang,northwest Qinghai Plateau,Qilian mountainous area,southern Qinghai Plateau and Gansu Corridor.The sunshine duration is a single peak in eastern part and mid-northern Xinjiang,while it is a double peak pattern in Gansu Corridor,Qilian mountainous area and southwest Xinjiang,and a triple peak pattern in Qinghai plateau.Under background of the climate warming the relative humidity increases and the cloud cover increases in Northweat China,a primary cause for the reduction of sunshine duration in most part of the region.