Eastward moving, baroclinically forced, medium-scale waves are frequently observed to dominate the Southern Hemisphere summer circulation. In addition, strong quasi-stationary medium-scale waves were also observed during the summer of 1978/79. In this paper we present the results of an observational study for several weeks of this time period, during which the stationary and transient waves are found to exhibit clear linear interference characteristics. Energetic analyses indicate that the medium-scale waves grow barotropically and decay baroclinically during this period, although these interference induced contributions are secondary to the usual baroclinic growth-barotropic decay life cycle characteristics observed by Randel and Stanford. Data analyses and simple model calculations are presented which demonstrate that the observed baroclinic decay results from equatorward heat flux associated with the differing vertical structures of the stationary and transient waves. An interference induced feedback mechanism between the medium-scale waves and the zonal-mean flow is discussed.