Planetary waves propagating against the background of the zonal flow

In the context of a one-level model, it is shown that the atmosphere can sustain the propagation of not only global Rossby waves but also hemispheric modes, which can propagate to both the west and the east. Calculated periods of eastward-moving middle-scale modes of the summer Southern Hemisphere are found to be in good agreement with observational data. It is shown that, at certain frequencies, either hemisphere significantly strengthens disturbances moving westward from the other hemisphere.