中文摘要：

利用中国西北地区135个测站，1961年-2009年历年逐日地面最低气温和NCEP／NCAR资料，采用线性趋势分析、Mann—Kendall、子波分析、功率谱分析和合成分析等方法，分析了近2．49a中国西北区初霜冻的气候变化特征。结果表明：①中国西北地区初霜冻开始最迟的地方在陇南-陕南，平均出现在11月上旬，其次是南疆盆地出现在10月中下旬；初霜冻开始最早的地方在青海高原，一般开始在8月下旬，其余的地方初霜冻开始在9月中下旬-10月上旬。初霜冻开始期的稳定性较差，特别是北疆、高原北部和南部；②西北地区初霜冻平均以1．8d／10a的速率推迟，1986年有明显的突变现象，近49a来初霜冻有显著的2～3a周期；③西北地区有85％的站点以（2-4）d／10a的速率推迟，通过各年代际的变化来看，1980年代以来有50％以上的站偏迟，特别是近10a，有接近90％的站偏迟：④从环流特征量分析，影响西北地区初霜冻的环流特征量主要有副高正相关和极涡负相关；10月500hPa高度场合成分析表明，初霜冻偏迟年，东亚大槽浅，西太副高偏西偏强，亚洲中北部盛行纬向环流，中国西北区处于新疆脊控制之下，不利于出现降温天气；初霜冻偏早年，乌山脊强，东亚槽深，西太副高偏东偏弱，亚洲中北部盛行径向环流，西北地区上空不断有来自乌山脊前的冷空气，易形成强降温天气。

英文摘要：

Using daily surface minimum air temperature data from 135 observational stations from 1961 to 2009 in northwest China and NCEP/NCAR reanalysis data, we examined changes in the date of first frost over time. We found that first frost starts in southern Gansu and south Shanxi in the first 10 days in November and in southern Xinjiang in mid-October. On the Qinghai plateau first frost appears in late August. The first frost period has poor stability, especially in northern Xinjiang and the Qinghai plateau. The average first frost date occurred later at a rate of 1.8 d/10a, and 9 days for the whole area. There was an abrupt change in patterns in 1986. For 85% of stations, the date of first frost was postponed, and majority were postponed at a speed of （2-4） d/10a. Since 1980, 50% of stations have a late first frost date, especially in the last 10 years where a late first frost date was found at 90% of stations. The subtropical high and polar vortex affect first frost across northwestern China, the subtropical high has a positive correlation and the polar vortex is has negative correlation. The 500 hPa composite analysis indicated, in first frost late years the East Asian trough is week, the Western Pacific vice-high in the west strong, and the Mid-northern Asian is the prevalent zonal circulation. In first frost early years, the Ukraine mountain high ridge is strong, the East Asian trough is deep, the Western Pacific vice-high easterly is weak, and the Mid-northern Asian is the prevalent radial circulation. We suppose that a changing polar vortex and high-pressure results in the date of first frost being postponed; climate warming is also contributing. Frost is a by-product of cold air and climate warming causes increased vice-high and a decreasing vortex, and prevents cold air moving south.