Quasi-normal eastward-traveling modes in the Southern Hemisphere.

The repeated appearance of eastward-traveling medium-scale waves in the Southern Hemisphere having quasi-discrete spectra suggests that there must be some mechanism of their maintenance, but basic states with realistic zonal flows do not have eastward-traveling Rossby normal modes in the discrete spectrum. Nevertheless, we show within the linearized shallow-water model by applying forcing to each hemisphere separately, that eastward-traveling modes can be excited in the same way as normal-mode Rossby waves. The obtained spectra and structures of these modes well agree with observations. We have made also calculations of the atmospheric response to each hemisphere separately within the linearized primitive equations global model. Confirming the above results for medium-scale modes, we received also that the zonal wavenumber 1 hemispheric mode is quasi-stationary and zonal wavenumber 2 and 3 modes exhibit eastward propagation with periods 2-3 weeks in accordance with observations.