Structure and energetics of medium-scale atmospheric waves in the Southern Hemisphere summer
Medium-scale waves (zonal wavenumbers 4-7) frequently dominate Southern Hemisphere summer midlatitude circulation patterns. This work is an observational study that focuses on characterizing their temporal and spatial characteristics, along with detailing the forcing mechanism(s) responsible for their formation;The medium-scale waves are observed to exhibit remarkably regular eastward phase progression, and wave maxima can often be traced continously around the globe. Frequent downstream development of existing wave patterns is observed. The medium-scale waves at times appear to be longitudinally localized features; at other times, they resemble truly global-scale modes. The time-mean wave structure is shown to be consistent with basic-state wave propagation characteristics and the conservation of wave activity;The energetics of the medium-scale waves are studied using the Transformed Eulerian Mean formalism of Plumb (1983). It is found that wave-zonal-mean exchange is a valid concept for describing the Southern Hemisphere summer atmospheric circulation, and that the flow vacillates between periods of highly perturbed and zonally symmetric states. The medium-scale waves result from nonlinear baroclinic instabilities, and exhibit a well-defined life cycle of baroclinic growth, maturity, and barotropic decay. These results are in excellent agreement with modeled global scale baroclinic waves;In addition to the transient waves, quasi-stationary medium-scale waves are also occasionally observed. The characteristics of these low-frequency features are discussed in terms of the dispersion of a localized stationary Rossby wave packet. Clear instances of stationary-transient medium-scale wave interference are observed and studied;It is argued that the zonal symmetry of the Southern Hemisphere summer atmosphere is responsible for many of the observed medium-scale wave characteristics.