中文摘要：

使用1979-2005年NCEP／NCAR再分析数据，分析了北半球平流层中低层（300hPa至10hPa）纬向风的季节转换规律，并采用二维空间场相似性方法确定了平流层的季节过渡日期。分析表明，平流层大气环流基本为冬夏二元状态，冬夏转换具有突变性；其季节过渡在纬向是接近同步进行的，而在经向则有时间差异，无论是冬夏转换还是夏冬转换高纬都要早于低纬。在平流层中部（10-70hPa）季节过渡是自上而向下进行的；而在平流层下部（100-200hPa）季节过渡的上下传递关系则比较复杂，在不同的纬度带有不同的表现。在北半球热带外地区，平流层中部东风期的起止日期与相似性方法计算得到的平流层季节过渡日期之间具有较好的对应关系，在东风期之前和之后往往各存在持续10天左右的零风一弱风期。

英文摘要：

The zonal wind variation in the lower and middle stratosphere （from 300 hPa to 10 hPa） of the North- ern Hemisphere （NH） is analyzed using the NCEP/NCAR reanalysis data during 1979 - 2005. Because the reversal of zonal wind direction mainly occurs during the stratospheric seasonal transition period, the classification of the sea- sonal change in stratosphere is necessary. To determine the seasonal transition in stratosphere, a method named similarity coefficient is used. The atmospheric general circulation in the stratosphere is of bimodal, i.e. winter and summer modes, with abrupt change during the winter- summer transition. Analyses show that in the zonal direc- tion, the seasonal transition is synchronous; in the meridional direction, however, it is non synchronous, and the winter- summer and summer- winter transitions are both earlier in higher latitudes than that in lower latitudes. In the middle stratosphere （10 - 70 hPa） the seasonal change begins from the higher levels downward to the lower lev- els, but the seasonal transition in lower stratosphere （100 - 200 hPa） is complex. In the extra-tropieal stratosphere of the NH, the onset date and withdrawal date of stratospheric easterly have good correlation with the seasonal tran- sition date in stratosphere determined by the measure of similarity coefficient. In general, there is a period of ten days with zero zonal wind （weak wind） before and after stable easterly period in the middle stratosphere.