NUMERICAL-SIMULATION OF BAROCLINIC JOVIAN VORTICES

We examine the evolution of baroclinic vortices in a time-dependent, n onlinear numerical model of a Jovian atmosphere. The model uses a norm al-mode expansion in the vertical, using the barotropic and first two baroclinic modes. Results for the stability of baroclinic vortices on an f plane in the absence of a mean zonal flow are similar to results of Earth vortex models, although the presence of a fluid interior on t he Jovian planets shifts the stability boundaries to smaller length sc ales. The presence of a barotropic mean zonal flow in the interior sta bilizes vortices against instability and significantly modifies the fi nite amplitude form of baroclinic instabilities. The effect of a zonal flow on a form of barotropic instability produces periodic oscillatio ns in the latitude and longitude of the vortex as observed at the leve l of the cloud tops. This instability may explain some, but not all, o bservations of longitudinal oscillations of vortices on the outer plan ets. Oscillations in aspect ratio and orientation of stable vortices i n a zonal shear flow are observed in this baroclinic model, as in simp ler two-dimensional models. Such oscillations are also observed in the atmospheres of Jupiter and Neptune. The meridional propagation and de cay of vortices on a beta plane is inhibited by the presence of a mean zonal flow. The direction of propagation of a vortex relative to the mean zonal flow depends upon the sip of the meridional potential vorti city gradient; combined with observations of vortex drift rates, this may provide a constraint on model assumption for the flow in the deep interior of the Jovian planets.