21.02.2013: FB1-Seminar

14:00 h, Hörsaal (West), Düsternbrooker Weg 20


Abstract:

The QBO (quasi-biennial oscillation) has remote influences on the extratropical circulation by modifiying the propagation of the planetary waves. In the present study, we investigate seasonality of the QBO influence on the global circulation by making a time-lagged composite analysis, in which the QBO phase in a fixed key month is used to make composites for sequential months before or after the key month. The ERA40 data (1958-2002) are used in the analysis.

EOF analysis of the mean zonal wind in the equatorial lower stratosphere is made to define a QBO phase by the angle in the two-dimensional phase space spanned by PC1 and PC2. The probability density function of the phase angle for the analyzed 44 years shows bimodal structure, most clearly in June. We define two groups "Westerly" and "Easterly" according to the phase angle in June, to make composites for sequential months before and after the key month. In the present analysis, a composite obtained for six months before the key month is different from that obtained for six months after the key day, even though the calendar month is the same.

By extending the period of composite to several years before and after the key month, it is shown that descending of the QBO easterly or westerly winds starts in a specified phase (in September-October or in December-January, respectively) of the SAO (semiannual oscillation) in the upper stratosphere. It is also shown that the descending speed of the easterly or westerly winds has semiannual variation (fast in May and October). The composites of the EP (Eliassen-Palm) flux and the residual mean meridional circulation off the equator show annual and semiannual variations which are well corresponding to the variation of the descending speed. Statistical significance of the composite difference between "Westerly" and "Easterly" phase is assessed for all the analyzed quantities.