Growth and Vacillation Cycles of Disturbances in Southern Hemisphere Flows

Abstract

The nonlinear interaction of initial single zonal wavenumber small-amplitude waves with Southern Hemisphere zonal flows characteristic of January and May is studied in a multilevel primitive equation spectral model with spherical geometry and viscous dissipation. For January, zonal wavenumber 7 and 10 waves are considered which occlude after a period of 8-10 days of almost exponential growth, with growth rates comparable with linear theory. Thereafter the eddy kinetic energy and wave structures vacillate with large-amplitude variations. Similar vacillations of the maximum mean zonal wind and temperature occur with the vertical shear considerably reduced as the flow becomes increasingly barotropic with the lower layers spinning up to a maximum speed comparable with that at the original jet center. As the waves amplify, their structures and poleward heat fluxes penetrate increasingly into the troposphere so that at the first peak in the vacillation cycle the streamfunction with dominant zonal wavenumber 7 is largest in the upper troposphere. The penetration of the shorter wavenumber 10 wave is less effective. At this stage and during the first barotropic decay period following the occlusion, the momentum fluxes also compare closely, in most respects, with observations with largest convergence in the upper troposphere. For May, viscous and inviscid integrations with wavenumber 10 waves are studied to examine the role of viscosity in determining the structure of waves in the nonlinear regime. During the viscous simulation, an initial wave growing on the polar jet rearranges its structure during the first 11 days and thereafter grows exponentially on the subtropical jet, occludes and shows indications of a vacillation cycle. In contrast, the wave grows exponentially on the polar jet in the inviscid integration. The changes in structure that occur during the viscous integration produce a dramatic improvement in the comparison with observations compared with linear results.