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A mechanistic approach is used to determine the nature and frequency dependence of transient eddy feedbacks on zonally symmetric low-frequency variability. The zonal momentum of a simple though fully nonlinear atmospheric model is driven by an imposed periodic forcing. In the absence of the imposed forcing this is a model that spontaneously generates its own low-frequency zonally symmetric variability. The response to the imposed forcing is determined by the amplitude and phase of the transient eddy driving of the zonal flow, which depends strongly on the imposed forcing frequency. At short periods, less than 30 days, the driving of the zonal flow by transient eddies lags the zonal angular momentum by more than a quarter-cycle and is, therefore, a negative feedback. As the period is increased, the eddy driving becomes more in phase with the angular momentum, and for steady imposed forcing the eddy feedback is strongly positive. These results are consistent with the suppression of high frequencies in the spontaneous zonally symmetric fluctuations in this model. The present results may be understood as an example of eddy–zonal flow interaction induced by zonal flow transience. This occurs in flows for which high densities of eddy enstrophy or wave activity tend to coincide with maxima in the zonal wind. A displacement of a zonal jet then involves a displacement of eddy activity. The increase of eddy activity at the new latitude of the jet and the decrease of eddy activity at its former latitude imply convergent and divergent Eliassen–Palm fluxes at the new and old jet latitudes, and therefore a negative eddy feedback on rapid changes in the zonal flow.
Walter A. Robinson (Thu,) studied this question.