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Spectrum analysis of the mean monthly zonal winds at 50 mb indicates energy peaks near 24-, 12-, 8-, and 6-month periods. However, an analysis of the mean biennial and annual waves shows that the observed annual spectral peak is made up of harmonic energy from the 26-month wave plus energy of the annual variation itself. The 8-month wave peak can be totally explained as the third harmonic contribution of the biennial wave. The 6-month peak appears to contain harmonic energy from both the biennial and annual waves.
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Dartt et al. (1964) studied this question.
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