Analysis of the Characteristics of Zonal Wind Reverse Layer in the Lower and Middle Stratosphere of the Northern Hemisphere and Its Seasonal Variation

The zonal wind variation in the lower and middle stratosphere (from 300 hPa to 10 hPa) of the Northern 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 seasonal 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 direction, 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 levels, but the seasonal transition in lower stratosphere (100-200 hPa) is complex. In the extra-tropical stratosphere of the NH, the onset date and withdrawal date of stratospheric easterly have good correlation with the seasonal transition 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.