PLANETARY-SCALE FLOW REGIMES IN MIDLATITUDES OF THE SOUTHERN-HEMISPHERE

The transient eddy height variance in midlatitudes during Southern Hemisphere (SH) winter achieves its largest value at zonal wavenumber 3. The presence of two highly statistically significant modes in the probability density distribution of the zonal wavenumber 3 amplitude allows one to conclude that this variance is the result of the circulation switching between two statistical flow regimes. One regime is characterized by a predominantly wavenumber 1 pattern and the other by an amplified wavenumber 3 pattern. The probability density distributions of the duration of these two regimes can be roughly approximated as exponential functions with e-folding times of 6 to 12 days. Thus, on the intraseasonal time scale during SH winter, the time mean flow is not the most probable state of the circulation. In contrast, during SH summer no bimodality occurs and the wavenumber 3 probability density distribution is strongly skewed toward the winter low amplitude mode. Simple energetics considerations suggest that in SH winter wave-wave interaction between intermediate-scale eddies (wavenumbers 5 to 7) and wavenumber 3 is a significant energy source to maintain the amplified wave pattern, while wave-mean flow interactions are a sink for wavenumber 3 kinetic energy. In contrast, during SH summer wave-wave coupling between intermediate-scale transients and wavenumber 3 is sharply reduced compared to winter. This suggests that wave-wave interaction is an important component in the mechanism of the SH bimodality.