Wave-Driven Zonal Flow Vacillation in the Southern Hemisphere

The variability of the zonal mean flow in the Southern Hemisphere during the period 1985–94 is studied using European Centre for Medium-Range Forecasts analyses. The dominant mode of variability has approximately equivalent barotropic variations of opposite signs centered at 408 and 608S. This structure is dominant in all seasons and has similar variance in all seasons. The temporal variance of the amplitude of this mode is well modeled as Gaussian red noise with a correlation e-folding time of about 10 days. Zonal wind anomalies are maintained against frictional drag by variations in the zonal flow accelerations driven by transient eddies and associated mean meridional circulations. The eddy structures suggest that equatorward propagation is favored when the jet is displaced poleward and zonal propagation is favored when the jet is displaced equatorward.