The vacillation of the southern midlatitude zonal wind in the CSIRO R21 9-level general circulation model has been recently studied by Kidson and Watterson (1999), who showed that eddy fluxes associated with increased storm activity produced the fluctuations in zonal momentum. They found that a simple model with a stochastic representation of the fluxes could mimic the short term variation of the index of the vacillation, but not its persistence. The zonal momentum budget of the 30-year 8-hourly dataset has been reexamined to provide insights into this discrepancy. At each model level, composites have been constructed to show the evolution of the terms in the budget equation around the times of large values of the index. The eddy flux term clearly drives the index variation, but remains non-zero for several weeks beyond the peak index times, consistent with a weak positive feedback of the wind anomalies on the eddies. At midlevels, however, the flux term is briefly negative soon after the peak, showing the need for an analysis with a high time-resolution. The role of meridional circulation anomalies in balancing the surface drag and in maintaining the vertical coherence of the mode is clear. An enhanced stochastic model for the vertically integrated budget is developed. The flux term is modeled by a superposition of `storms' with finite lifecycle, but fluctuating amplitude. The model is used to explore the importance of both the short term negative feedback, and the longer term positive feedback.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics