Multiple regimes and low-frequency oscillations in the Southern Hemisphere’s zonal-mean flow

[1] Multiple regimes in the atmospheric zonal-mean flow of the Southern Hemisphere are examined using 53 years of daily observational analysis data. Estimates of the probability density function in the reduced-phase space spanned by the two leading empirical orthogonal functions (EOFs) are found to exhibit significant deviations from bivariate Gaussianity. Two major regimes are identified, which describe the extreme phases of atmospheric zonal-flow vacillation: in the high-latitude regime the jet is displaced poleward from its climatological mean position, while in the low-latitude regime it is displaced equatorward. The two zonal-flow regimes are maintained by the forcing due to transient eddies against surface friction. Significant low-frequency oscillatory components, with peaks near 135 and 70 days, are found in the EOF mode associated with zonal-flow vacillation. These oscillations are described using both the multitaper method and singular spectrum analysis. Close examination of the relationship between the regimes and the oscillations suggests that the highand low-latitude regimes tend to be associated with the extremes of either low-frequency oscillation. We illustrate how the phase information of either low-frequency oscillation could be used as a predictor for the highand low-latitude regime.