We examine the linear instability of the observed two-dimensional (latitude/height) January 1979 zonal wind of the mid-latitude Southern Hemisphere. The model used is a 10-level, linear, quasi-geostrophic 45°S β-plane channel model, with 30 Fourier harmonics in the meridional direction and a single harmonic in the zonal direction. The fastest growing mode has a zonal wavelength corresponding to wavenumber 12 at 45°S. The mode corresponding to zonal wavenumber 5 at 45°S is also baroclinically unstable. Its latitude-height structure bears qualitative resemblance to the observed wavenumber 5 circulation, which frequently dominates the summer Southern Hemisphere mid-latitude circulation. The latitude of the maximum eddy amplitude at 50°s is simulated, but the maximum is located near the surface instead of aloft. Effects of surface dissipation and a normalization of the zonal wind by the cosine of latitude are also considered.