ENSO对中国西北地区秋季异常降水的影响

以奇异值分解（SVD）方法为基础，用1960～1994年35年的资料，分析了ENSO对中国西北地区秋季降水异常的影响，然后用奇异向量的海温场时间系数与500hPa高度场和海平面气压场进行交叉相关，讨论了在ENSO年引起西北地区降水异常的环流特征。结果表明，赤道中东太平洋海表温度异常与西北秋季大范围的区域性降水异常有较好的对应关系，在ElNino年，西北地区秋季大部分地区降水异常偏少，其中青藏高原东北侧的青海东部、甘肃中东部、宁夏南部和陕西北部等西北主要的雨养农业区降水明显偏少。引起这种异常的环流特征表现为500hPa高度场出现PNA异常流型，强异常中心多分布在低纬度，印缅槽偏弱，西太平洋副高加强西伸，脊线南移，南海副高和北美副高加强；东亚中纬度冬季盛行的1波流型在秋季异常发展，新疆脊偏强，东亚大槽加深西移。海平面气压场上，乌拉尔山北部高压、欧洲南部低压及阿留申低压等较高纬度系统异常加强（加深）。LaNino年情况相反。

Effect of ENSO on the Autumn Rainfall Anomaly in Northwest China

Based on the SVD method, the effect of ENSO on the autumn rainfall anomaly in northwest China were analyzed by using the data from the preiod 1960-1994. Crossed-correlation between the time coefficient of sea surface temperature in SVD and the geopotential height at 500 hPa and the sea level pressure were used to study the circulation features that cause the rainfall anomaly in an El Nio/La Nia year. The results show that good correlation really exists between the SSTA in east equatorial Pacific and the autumn rainfall anomaly in northwest China. In the year of El Ni o, northwest China is dry in autumn, where the rainfed area, the middle east Gansu, the southern Ningxia and the northern Shaanxi is the significant dry region. The circulation features of geopotential height at 500 hPa are that the PNA pattern appears, the strong anomaly centers are located in low latitudes, the west Pacific subtropical high is intensified and extends westward and its ridge line moves southward; the winter prevailing 1-wave flow pattern is abnormaly developed in autumn, Xinjiang ridge is intensified, East Asia trough is also intensified and moves westward. At the sea-surface pressure level, the stronger anomaly centers are located in high latitudes, the Ural high is strengthened, the Aleutian low is enhanced. The case of the La Nia year is opposite.