In this study 500 hPa geopotential heights from the NCAR-NCEP Reanalysis are used to calculate an index describing the spatial and temporal characteristics of zonal wave three (ZW3) in the high latitude Southern Hemisphere atmosphere. The index spans the period 1958--2001. Wavelet analysis of the index shows the strong interannual variation which is characteristic of ZW3 and also reveals significant, high amplitude decadal-scale variation. This interannual variation takes the form of marked fluctuation in sign and magnitude of the index. It appears to occur when there are rapid changes from strongly meridional to strongly zonal flow or vice versa. Initial indications are that some of this variability may be associated with ENSO events.