Quasi-periodic variations of the polar vortex in the Southern Hemisphere stratosphere due to wave-wave interaction

The winter polar vortex in the Southern Hemisphere stratosphere is characterized by prominent quasi-stationary planetary waves: zonal wavenumber 1 (wave 1) and the eastward-traveling wave (wave 2). Quasi-periodic variations of the polar vortex are investigated in terms of the wave-wave interaction between wave 1 and wave 2 with both the NCEP-NCAR reanalysis dataset from 1979 to 2002 and a spherical barotropic model. A typical case shows that the transient wave 1 generated by the wave-wave interaction has comparable amplitude to those of the stationary wave 1 and the traveling wave 2, and has a node around 60degreesS, where these primary waves have large amplitude. The transient wave 1 travels eastward with the same angular frequency as that of the traveling wave 2. The polar night jet also vacillates with the same frequency such that it has its minimum when the stationary wave 1 and the transient wave 1 are in phase at the polar side of the node. The vacillation is basically due to quasi-periodic variations of the wave driven by the interference between the stationary and traveling wave 1s. Similar periodic variations of the polar vortex are obtained in the model experiment here, in the circumstance that stationary wave 1 generated by surface topography has comparable amplitude to the eastward-traveling wave 2 that is generated by the barotropic instability of a forced mean zonal wind. The winter polar vortex shows large interannual variability. Similar quasi-periodic variations due to wave wave interaction often occurred for the 24 yr in late winter when the transient wave 2 was vigorous.