In a, series of idealized numerical experiments, Butchart et al (1982) have recently established that the configuration of the mean zonal wind occurring immediately before the wavenumber-2 major stratospheric warming of February 1979 was crucial in subsequently focusing upward propagating planetary wave-activity into the high latitude stratosphere. In this sense, it was concluded that the stratospheric circumpolar flow should evolve to some preconditioned state before a wavenumber-2 major warming could occur.

In the present paper, the mechanisms responsible for the transition of the circumpolar flow from its normal midwinter state to this preconditioned state are investigated through a combination of observational numerical and theoretical studies. For the 1978–79 winter, this transition occurred during the substantial wavenumber-1 minor warming of January 1979, and the characteristic structure associated with the preconditioned mean zonal flow was established four days after the peak of this warming, during a period of intense high latitude acceleration. This latter phenomenon is referred to as a stratospheric sudden cooling. Observations of Eliassen-Palm flux cross-sections indicate that while wave, zonal mean-flow interaction theory could account for the qualitative evolution of the circumpolar flow during the warming, substantial nonlinear wave interactions were active during the cooling period, and these interactions significantly influenced the evolution of the circumpolar flow.

In a series of numerical experiments using a truncated semi-spectral model, we show that this sudden cooling phenomenon can be realistically reproduced in an idealized integration in which wave-wave interactions are present. By contrast, we were unable to simulate this phenomenon with these interactions removed.

Two different mechanisms are put forward to account for these nonlinearities. One mechanism is that of wave-breaking and associated potential vorticity mixing, as suggested by McIntyre (1982). The second mechanism is based on the notion of wave-activity, forced in the troposphere, propagating relative to isopleths of potential vorticity of some zonally asymmetric basic state.

Results of the observational and numerical study suggest that the first mechanism was dominant, and that potential vorticity mixing in the outer regions of the polar vortex was central to the process of preconditioning. Nevertheless, we believe that the second mechanism plays an important role in the dynamics of the stratosphere.

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