Structure of Medium-Scale Atmospheric Waves in the Southern Hemisphere Summer

Abstract

Recently reported medium-scale wave dominance of the Southern Hemisphere summer circulation is studied using NMC geopotential height fields for the 1978–79 summer. These features, corroborated by independent analyses of satellite microwave measurements, are apparent in meridional thermal winds derived from the NMC grids. In the time mean, we observe strong medium-scale waves which extend throughout the troposphere and lower stratosphere, in agreement with Kalnay et al. (1981). These zonally asymmetric features attain a maximum in low latitudes, exhibiting an equivalent barotropic vertical structure with maximum amplitude near the tropopause. A longitudinal phase versus time plot from daily analyses of zonal wavenumbers 4–7 (which contain the majority of the time variance) reveals periodic variations in both phase and amplitude: wave 5 frequently dominates, exhibiting eastward phase progression with period near 10 days. During other times, shorter scale waves (waves 6–7) exhibit enhanced amplitudes south and east of Africa, showing considerably faster eastward movement. Waves 6 and 7 both show remarkably regular eastward movement throughout the 90 day record, with periods near 5 and 4 days, respectively. The traveling waves exhibit maximum amplitude near the tropospheric jet core, often with an equivalent barotropic vertical structure. The amplitude of the medium-scale waves is observed to vary with approximately the same time scale as the period of their phase progression (10–15 days). The zonal wind exhibits fluctuations of amplitude and time scale which suggest that the medium-scale waves may grow at the expense of the zonal mean kinetic energy. Episodes of latitudinal phase structure indicative of barotropic energy exchange with the mean wind field are observed. An exceptionally clear case of stationary-transient wave interference is observed.