中文摘要：

应用NCEP—NCAR再分析资料，分析了1958-2002年夏季北半球1000hPa和500hPa位势高度场的年际变化。发现整个北半球位势高度场都出现大范围的升高趋势，升高中心主要位于35°～55°N。90°～115°E范围内，即夏季东亚热低压区域；定义该范围内位势高度场的平均值为夏季东亚热低压指数（简写为SEAL）。在1958—1968年间SEAL值有非常明显的升高，数值达60gpm以上。应用欧洲中心的ERA-40再分析资料和中国气象局的500hPa位势高度场资料。计算同样范围的SEAL指数，并与NCEP—NCAR资料进行对比，结果表明，在1958-1968年间NCEP—NCAR资料的值远小于上述两份资料，相关计算结果是NCEP资料与两份资料无关。而在1968年以后，上述3份资料的时间序列却是十分相似，相关值都通过信度检验，表现为1968—1976年都有增大的趋势。1977年以后，3份资料的SEAL指数都趋向平缓，即在1970年代中后期北半球东亚夏季环流有一明显的突变。环流突变主要发生在东亚热低压区，在北半球中低纬度的大范围区域对流层中低层都有反映；但在中高纬度地区，相对而言，气候突变是不显著的。大范围的环流变化影响东亚夏季风的位置和强度，也影响我国的降水量和降水分布。

英文摘要：

The tendency variation for 500 hPa and 1000 hPa geopotential height fields of the Northern Hemisphere during the summer months （JJA） were calculated for the period of 1968-2002 based on the NCEP-NCAR dataset. The center was located on the typical East Asia low area over 35°-55°N, 90°- 115°E on 1000 hPa and 35°-55°N, 70°-110°E on 500 hPa, defined as Summer East Asia Low index （SEAL1 and SEAL5）. The SEAL1 and SEAL5 time series of 500 hPa and 1000 hPa were calculated during 1958-2002 based on the NCEP-NCAR dataset, both have an obvious dramatist increases as much as 60 hPa from 1958 to 1968, then they tend smoothly. Discussing the NCEP-NCAR data quarity, the NCEP-NCAR data/the ERA-40 of the ECMWF/ China NMA data were be used during 1958-2002, and the SEAL1 and SEAL5 indexes were calculated. Comparing with the SEAL1 and SEAL5 indexes for the three data during 1958-1968, there not exist relationship between the NCEP-NCAR data and another two data, only the NCEP-NCAR data have a dramatist increases, the another two data not have the variation. For 1968-2002 the three time series values of these data are very similar, and have increasing trend, and were passed the reliability test. Therefore the 1968-2002 data were analyzed in the studying. During 1968-2002, both SEAL indexes for 500 hPa and 1000 hPa geopotential heights have a fast increase 30 gpm on 1968-1977. Then it strays near mean geopotential meters during 1977-2002. It means that there exists an obvious decadal change for the Summer East Asia Low in later 1970＇s. Based on the difference fields on 500 hPa and 1000 hPa heights between the two periods, the main changes are characterized by a huge positive difference center over the north-middle of China and Mongolia around 35°-50°N, 90°-115°E. The Summer East Asia Low was weaked in later 1970＇s, it will effect on the moisture flux transfer, and influence on the intensity and position of precipitation in China. It means that the rainfall was obviously decreased near the lower reach of the