Medium-scale waves (zonal wavenumbers 4-7) frequently dominate Southern Hemisphere (SH) summer midlatitude circulation patterns. This work is an observational study that focuses on their temporal and spatial characteristics, along with detailing the forcing mechanisms responsible for their formation.
Medium-scale wave characteristics for three SH summers (1978/79 through 1980/81) are discussed. It is shown that the time-mean medium-scale wave structure is consistent with the basic state linear wave propagation wave characteristics. The energetics of the medium-scale waves are studied using the transformed Eulerian-mean formalism of Plumb. It is found that wave-zonal mean exchange is a valid concept for describing the SH summer atmospheric circulation, and that the flow vacillates between periods of highly perturbed and zonally symmetric states, with a time scale on the order of 10-20 days. These vacillations result from nonlinear baroclinic instabilities, and the medium-scale waves exhibit well-defined life cycles of baroclinic growth, maturity, and barotropic decay.
The observational characteristics of the medium-scale waves are discussed in terms of Northern Hemisphere observational studies, modeled baroclinic waves, and laboratory annulus experiments. It is argued that the zonal symmetry of the SH summer atmosphere is responsible for many of the observed wave characteristics.