Alternating Jets in a Coupled Two-Layer Atmosphere-Interior Model of the Extratropical Latitudes of a Gas-Giant Planet
T. E. Dowling (U. of Louisville), G. R. Flierl (MIT)
We study the simplest possible coupled atmosphere-interior model for a gas-giant planet. The model is quasigeostrophic and consists of two active layers. Layer 1 models the thin spherical-shell extratropical atmosphere and is characterized by planetary-vorticity gradient . Nondimensionalizing by the horizontal wind speed, U, and the horizontal length scale, L, the value for Jupiter is , which expresses the violation of the sufficient barotropic shear-stability criterion ( ) found on Jupiter. To capture the deep spherical geometry and Taylor-Proudman effect, we follow Ingersoll and Pollard and examine . We study the linear stability problem, calculate growth rates for unstable initial configurations, identify stable and marginally stable configurations, and run fully nonlinear simulations of unstable configurations. Two key parameters are the ratio of upper-layer to lower-layer depth, , which is typically kept small, and the inverse square of the upper-layer deformation radius, or Froude number, . When initialized with random turbulence, the model spontaneously forms alternating jets in the deep layer that evolve to become barotropically stable ( ), basically following the Rhines scaling, and forms similar alternating jets in the upper layer that violate the barotropic stability condition but are nevertheless stable with respect to Arnol'd's 2nd stability criterion. Isolated eddies tend to persist in the simulations after the jets have emerged, which is characteristic of the gas-giant planets. These results are consistent with the Voyager data analysis and modeling of Dowling (1993, J. Atmos. Sci. 50: 14-22), and motivate full-scale atmosphere-interior modeling efforts.