中文摘要：

在全球气候变暖背景下1980年代中期中国西北西部气候发生了年代际突变，降水明显增加且主要是极端降水的贡献．本文用欧洲中期预报中心再分析资料ERA-40计算了瞬变涡动、环流、水汽输送及其散度等的年代际变化并进行了分解分析．结果表明，1987年后大气环流年代际变化形成“东高西低”环流背景场，西风带向该区的水汽输送增加，水汽辐合增强，其中主要是定常波水汽输送的贡献，其结果是造成该区及周边空气水汽含量上升，有利于降水或极端降水的增加．夏季该区瞬变涡动增强且造成水汽辐合，增加了降水天气出现的频率：冬季瞬变涡动减弱，天气过程减少，说明降雪增加的原因应该是空气水汽含量增加，导致降雪强度增加．因此该区降水的增加即有中高纬度大气变暖变湿的贡献，也有瞬变涡动的动力作用．未来西北西部降水的趋势主要取决于瞬变涡动与大气水汽含量变化的竞争．

英文摘要：

There took place an inter-decadal climate change over the west part of Northwestern China at mid-1980s. One of the significant characteristics is the increase in precipitation with more extreme rainfall or snowfall event. For understanding its mechanism our calculation focuses on the difference of the geo-potential height on 500 hPa, transient eddy activity, column water vapor content or precipitable water, moisture transport and its divergence between 1987-2000 and 1973-1986. The results show that the change in height field is favorable for the increase in moisture transport to the region with moisture convergence, leading to more water vapor over the region. Meanwhile the eddy activity became weak in wintertime and strong in summertime, leading to more synoptic processes in summertime and less in wintertime. A further decomposition shows that the increase in the transport with convergence was contributed mainly by its stationary-wave part along the west-east direction, while the transient eddy played a positive role in summertime and a negative one in wintertime for precipitation. It turns out that the increase in precipitation in the region resulted from the moisture convergence and the increase in eddy activity in summertime while the moisture convergence played a dominant role in wintertime. Besides, the warming climate also played a positive role in increasing air water-vapor content in the region since the warm air can hold more water vapors than the cold one.