中国西北夏季降水特征及其异常研究

A Study of Summer Precipitation Features and Anomaly in Northwest China

本文利用中国西北五省（区）９０个测站，１９６０～１９９０历年６～８月降水量资料，采用ＥＯＦ、ＲＥＯＦ、波谱分析及大气环流模式，对夏季降水量的空间异常特征、时间变化规律以及降水异常的主要成因进行了诊断研究和数值试验。结果表明：未旋转的前３个载荷向量场可以较好地反映西北夏季降水整体异常结构，即全区一致的多雨或少雨型；东西相反变化的东多（少）西少（多）型；南北相反变化的南多（少）北少（多）型。旋转后的前６个载荷向量可较好地代表西北夏季降水的６个主要异常敏感区：青藏高原东侧区、渭水流域区、青海高原区、北疆区、沙漠盆地区和河西走廊区。旋转主分量和代表站资料反映出近３０～６０年中，西北地区最干旱的时段在６０至７０年代。８０年代至９０年代初，除沙漠盆地外，各区程度不同地呈现出降水增加趋势。当初夏（６月）青藏高原下垫面感热异常偏强时，有利于同期西北大部地区降水偏多，而使７～８月西北西部、北部降水偏少；东部、南部降水偏多。利用ＩＡＰ２－ＬＡＧＣＭ数值试验表明，在夏季青藏高原下垫面感热大面积异常增强时，由于西太平洋副热带高压脊明显西伸，江淮处于副高西伸脊控制下，冷暖气流在长江和黄河上游交绥，有利于中国西北东南部降水偏多，多雨区?

With EOF, REOF, spectral analysis and atmospheric circulation model, the spatial anomaly features and time evolution rule and the mechanism of summer precipitation anomaly in Northwest China are studied by using the precipitation data in June, July and August for the period 1960～1990 from 90 observational stations. The results show that the first three loading vector fields could reflect the whole anomaly structures of summer rainfall in Northwest China, for example, wet or dry in all area pattern, wet (or dry) in east and dry (or wet) in west pattern, wet (or dry) in north and dry (or wet) in south pattern. The first six rotated loading vector fields represent six principal precipitation anomaly areas: east of the Qinghai-Xizang Plateau, the Weishui River basin, the Qinghai plateau, northern Xinjiang, the desert basin and Hexi corridor. The rotated principal component and the data in the representative stations show that in the recent 30～60 years, the driest period was in the 1960′s and 1970′s in Northwest China. From the 1980′s to early 1990′s, an increasing rainfall trend, more and less, appeared in every area except desert basin. When the sensible heat anomaly in the underlying surface is strong in the Qinghai-Xizang Plateau in early summer (June), the northern Northwest China is wet in the same time, but the western, northern Northwest China is dry and the eastern and southern is wet in July and August. The numerical experiment of IAP 2-L AGCM shows that when the sensible heat anomaly in the underlying surface increases in the Qinghai-Xizang Plateau in summer, the subtropical ridge in the western Pacific Ocean elongates westerly and the warm and cold air meets in the upstream of the Yangtze River and Yellow River. As a result, the rainfall increases in the eastern Northwest China. The wet areas are in the southeastern Qinghai, southern Gansu and south-eastern Shaanxi, and the dry center is in the west of the northwestern area.