A global primitive equation model of the stratosphere and mesosphere is integrated for specified planetary wave forcing at the 100 mb level with mean zonal flow conditions corresponding to the westerly and easterly phases of the equatorial QBO, respectively. The responses in the two QBO phases were compared for integrations with wavenumber 1 forcing amplitude maxima at 100 mb and 60°N varying from 100 m to 400 m. The phase of the QBO had little effect on the results in the weak wave (100 m) cases, which did not produce warmings, and the strong wave (400 m) cases, which produced major sudden warmings. At intermediate forcing amplitudes (particularly around 250 m), large wave amplitudes and stratospheric warmings occurred in both westerly and easterly QBO simulations. The warmings occurred earlier, however, and wave variability was stronger for the easterly phase, in qualitative agreement with observations.
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Holton et al. (1991) studied this question.