Nonlinear aspects of atmospheric zonal-flow vacillation

Variability of the atmospheric zonal-mean flow in the Southern Hemisphere (SH) is dominated by irregular latitudinal shifts of the mid-latitude zonal jet about its climatological mean position. This phenomenon, known as atmospheric zonal-flow vacillation, is studied in SH observations and in a primitive equation (PE) model, emphasizing the vacillation's nonlinear aspects.; Advanced density estimation techniques are applied to zonal-mean flow profiles for SH observations as well as for PE model simulations. The estimated probability density functions indicate significant deviations from Gaussianity both in the observations and in the PE model. This strongly suggests the existence of multiple flow regimes in both cases, contrary to a widespread view that zonal-flow vacillation is a stochastic red-noise process.; Flow composites associated with two persistent regimes describe two extreme phases of the vacillation: the zonal jet is displaced equatorward of its climatological mean position in the one and poleward in the other. These two regimes are maintained by transient eddy forcing against surface friction, in the observations as well as in the PE model.; A low-order model is constructed in order to investigate the origin of the multiple regimes. Multiple equilibria are identified in this model's zonal-mean flow. These equilibria bear a strong resemblance to the two persistent flow regimes in the SH observations and the PE model. The two equilibrium states are also maintained by wave forcing against surface drag, as in the observations and the PE model. Successive bifurcations to periodic and irregular zonal-mean flow regimes occur as the model's dissipation parameter changes.; Finally, low-frequency oscillations in zonal-flow vacillation are examined using two advanced spectral analysis techniques, namely, the multi-taper method and singular spectrum analysis. These two independent methods are applied to the first principal component of the zonal-mean flow for the SH observations as well as the PE model. Both methods identify statistically significant oscillatory components on intraseasonal to seasonal time scales. Close examination of the relationship between the regimes and the oscillations suggests that the two persistent flow regimes are associated with the peak phases of the low-frequency oscillatory components.