Abstract
The instability characteristics of Southern Hemisphere zonally averaged flows are studied for January, May and August basic states in a spherical, inviscid, adiabatic, quasi-geostrophic multilevel model. The growing disturbances may have considerably more complex growth rate curves, structures and momentum and heat fluxes than found for idealized single jet and Northern Hemisphere flows. For August, the largest growth rate occurs at the largest zonal wavenumber studied (m = 16), while for January and May it occurs at a more conventional intermediate value (m = 10). The modes for January have many properties in common with modes found with idealized single jet basic states, but in addition to eastward-propagating disturbances long-wave westward propagating disturbances occur. For May and August, the presence of both the subtropical and polar jets is felt by the eastward-propagating disturbances. Some of these grow primarily on one jet or the other, while other modes have two maxima of the disturbance streamfunctions at latitudes near the two jet streams. The presence of the two jets may also result in more complex momentum fluxes than found previously.
For each month, appropriate modes do have maximum streamfunction amplitude and eddy fluxes at the correct latitudes, but the usual vertical structure problem of linear theory occurs, viz., the amplitudes of streamfunctions, momentum and heat fluxes are too large at the surface compared with at the tropopause in relation to observations. However, for May, the instability results appear to contradict the usual hypothesis of baroclinic instability theory that one of the members, lying on the growth rate curve, as a function of zonal wavenumber, with largest growth rates, should be of the most meteorological significance. In fact, the second fastest growing modes of intermediate zonal wavenumbers appear to correspond most closely with observations, emphasizing the importance of finding all the growing modes by using, for example, an eigenvalue approach.
