A Model of Large-Scale Instabilities in the Jovian Troposphere

The cloud-top circulation of Jupiter is characterized by latitudinal cloud bands and near-steady jets which decay with height. We developed a quasi-geostrophic mid-latitude beta-plane model to study whether the jet decay and meridional circulations are driven by large-scale instabilities of a deep zonal flow extending upward into the troposphere of Jupiter. Linear barotropic and baroclinic instabilities of simple analytic zonal flows thought to be representative of the Jovian tropospheric circulation are investigated. When the vertical shear zones are not too thin, the shear confines disturbances which are essentially barotropic, and reduces their growth rates; the westward jets are the most unstable and the eastward jets are readily stabilized by a weak shear. The instabilities are associated with residual mean meridional circulations: they consist of a bottom cell with rising motions on the anticyclonic side of the jet, and an opposite cell stacked above it. The least unstable flows are shown to have a Burger number one at the most. A regime of baroclinic instability arises for narrower shear zones. A quasi-linear model was also used to investigate how an initially barotropically unstable flow develops a steady shear zone in the lower scale heights as a result of the action of the eddy fluxes. The instabilities are then acting as a vertical drag term and the jets decay to a non-zero barotropic component. In the equilibrated state, the eddy to mean flow kinetic energy ratio is about 20% for westward jets and less than 1% for eastward jets. For adjacent jets, unstable modes can propagate meridionally into the core of eastward jet and contribute to the jet decay even if it is stable. We also introduce a Rayleigh drag, forcing the mean flow back to its unstable state, while the eddies are damped radiatively. When the forcing time scale is the same as the radiative damping time, we found an asymmetric life-cycle type of behavior, with the eddy energy growing quickly to saturation and decaying slowly. For faster forcings, the eddy energy can be steady, and there are time-averaged eddy fluxes which drive meridional circulations.