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

Multiple regimes in the atmospheric zonal-mean flow of the Southern Hemisphere are examined using 53 years of daily observational data. Estimates of the probability density function in the reduced-phase space spanned by the two leading empirical orthogonal functions (EOFs) 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 high- and 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 high- and low-latitude regime.