A New Quasi-biennial Index And The QBO Signal In Meridional Wind

A new quasi-biennial oscillation (QBO) index, the QBOI, is defined as the difference of normalized zonal-averaged zonal wind at 10 hPa and 70 hPa. The new index can include the variation of zonal wind in most levels in the lower stratosphere. At the same time, the new index can reflects the downward propogation in the lower stratosphere. Compareing with the zonal wind at 30 hPa and 50 hPa, the new index shows its excellent performance with the closer association of variation in the new index with the equational temperature in stratosphere. The influence of QBO on the zonal mean stratospheric zonal and meridional wind, temperature and geopotential height is analyzed. We also use this new index to study the influence of QBO on areas in the tropical troposphere. In the positive (negative) phase, which means westerly (easterly) shear in the lower stratosphere, the temperature is comparatively higher (lower) and correspondingly the altitude of the tropopause is lower (higher). The vertical motion in the tropical troposphere will be restrained (stimulated). By comparison with the performance of zonal wind at 30 hPa and 50 hPa, we can find the new index can better describe the influence of QBO in tropical troposphere. A spectral analysis of the stratospheric zonal mean meridional wind is made by using the monthly NCEP/NCAR reanalysis data for the 40-year period 1961-2000. The analysis provides distinct evidence for the quasi-biennial oscillation (QBO) of meridional wind in the tropic areas outside of the equatorial band (5°S?5°N). The QBO appears a dipole pattern in the Northern and Southern Hemispheres. Both the northerly (south to north) and southerly (north to south) transitions in wind direction show a strong seasonal preference to occur from May to September. The QBO in meridional wind has certain relationship with the QBO in zonal wind in the tropical stratosphere. Some statistic characters of the QBO signal such as periodicity...