AN EOF ANALYSIS OF THE VERTICAL TIME-DELAY STRUCTURE OF THE QUASI-BIENNIAL OSCILLATION

Empirical orthogonal function analyses of the time-height series of mo nthly mean zonal wind in the stratosphere have been performed. Convent ional EOF analysis on the time series reveals that the quasi-biennial oscillation (QBO) of the zonal wind is a quasi-regular oscillation wit h a period near 28 months, but the noisy structure of the first two EO Fs, representing 57.18% and 36.24% of the variance of the time series, leads to little new insight concerning the dynamics of the QBO. A sec ond type of analysis is performed by applying windows to the data and calculating the EOFs of the time development of the spatial structure. By using a window of around one-half of the period of the dominant os cillation, the EOF analysis reveals that the QBO is essentially a line ar feature, with successive wind regimes propagating smoothly downward s with a spectral peak near 28 months; this oscillation has a very smo oth phase portrait and cross-correlation analysis of the first two EOF s reveals that it is a quasi-linear feature with good predictability. Delays in the downward propagation of the easterly phase of the QBO ar e shown to be nonlinear, unpredictable features, represented by higher -order EOFs.