Wave–Mean Flow Feedback and the Persistence of Simulated Zonal Flow Vacillation

Abstract

The structure of eddies forcing the vacillation of the southern midlatitude tropospheric zonal-mean zonal wind and the significance of wave?mean flow feedbacks on its persistence are assessed using a 100-yr 8-h dataset simulated by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 general circulation model. Using time-lagged regression and composite analyses relative to the vacillation index, it is shown that high-frequency (HF) eddy momentum flux anomalies near the mean jet latitude provide much of the forcing of the zonal-mean anomalies, as in observations. Low-frequency (LF) eddies also contribute, while the cross-frequency flux enhances short-term variation of the index. The HF band also provides a positive feedback, which is partly countered by an LF negative feedback. High- and low-index composites of representative midtropospheric waves (zonal wavenumbers 7 for HF and 3 for LF) are constructed, including those for waves phase shifted relative to the wave at the jet latitude at each of several lags. Such waves are coherent for only a week, but they provide most of the initial flux anomaly associated with the forcings and the feedbacks. Barotropic wave model simulations suggest that much of the feedback is due to the dependence of wave evolution on the zonal wind states of the composites, although energy variations also contribute. A stochastic model of the momentum equation terms is constructed. This matches the statistics of the index and the momentum terms well. The net feedback more than doubles the 30-day persistence of the index in the annual case. The vacillation index contains significant seasonal variation. Forcing and damping are both weaker in summer, while the negative and positive feedbacks almost negate each other in spring and autumn.