Eddy–Zonal Flow Feedback in the Southern Hemisphere

The variability of the zonal-mean zonal wind in the Southern Hemisphere is studied using EOF analysis and momentum budget diagnostics of NCEP reanalysis data . The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north-south movement of the midlatitude jet. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of EOF1 and plays an important role in the selection of the leading EOF of midlatitude variability. Further analysis also shows a propagating feedback, common to both EOF1 and EOF2, which is responsible for the poleward drift of wind anomalies with time. The observations support the following feedback mechanism. Anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that, by thermal wind, coincides with the latitude of the anomalous zonal jet. The net propagation of baroclinic wave activity away from the jet gives momentum fluxes into the jet. This positive feedback is partially offset by low-frequency, equivalent barotropic eddies that propagate into the jet and remove momentum from it. The bias toward equatorward wave propagation on a sphere contributes to the poleward drift of the wind anomalies.