Is spontaneous generation of baroclinic flows possible?

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Abstract
Large-scale flows in the gaseous planets lacking a strong mean meridional thermal gradient are believed to be sustained by thermally driven small scale turbulence. In this work self-organization of turbulence in a two-layer fluid with no imposed mean north-south thermal gradient and turbulence supported by an external random stirring is studied. The tool employed is a second order closure of the statistical state dynamics (S3T) with a Reynolds averaging ansatz that allows the identification of statistical turbulent equilibria and the investigation of their stability. The bifurcation of the statistically homogeneous equilibrium state to inhomogeneous equilibrium states comprising of zonal jets and/or large scale waves is analytically studied. The theory predicts that when the flow transitions to a statistical state with large-scale structures, these states are barotropic if the scale of excitation is larger than the deformation radius. Mixed barotropic-baroclinic states with jets and/or waves arise when the excitation is at scales shorter than the deformation radius with the baroclinic component of the flow being subdominant for low energy input rates and non-significant for higher energy input rates. The results of the S3T theory are compared to nonlinear simulations and are found to predict accurately both the critical transition parameters and the finite equilibrium states.

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