An EOF Analysis of the Vertical-Time Delay Structure of the Quasi-Biennial Oscillation

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Abstract
Empirical orthogonal function analyses of the time-height series of monthly mean zonal wind in the stratosphere have been performed. Conventional EOF analysis on the time series reveals that the quasi-biennial oscillation (QBO) of the zonal wind is a quasi-regular oscillation with a period near 28 months, but the noisy structure of the first two EOFs, 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 second 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 oscillation, the EOF analysis reveals that the QBO is essentially a linear feature, with successive wind regimes propagating smoothly downwards with a spectral peak near 28 months; this oscillation has a very smooth phase portrait and cross-correlation analysis of the first two EOFs reveals that it is a quasi-linear feature with good predictability. Delays in the downward propagation of the easterly phase of the QBO are shown to be nonlinear, unpredictable features, represented by higher-order EOFS.

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