Self-Organization of Zonal Jets in Outer Planet Atmospheres: Uranus and Neptune
A.J. Friedson (JPL, Caltech)
We present a theoretical calculation of mean zonal wind profiles for Uranus and Neptune, based on the hypothesis that they are determined primarily by the spontaneous self-organization of turbulence under anisotropic flow conditions in a shallow spherical shell. According to this view, weak thermodynamic forcing enters only to maintain the flow against weak dissipation, while the character of the flow is controlled by a statistical equilibrium of advective processes. By combining recent advances in the equilibrium statistical mechanical theory of 2D turbulent flows with a barotropic model, we have found analytical expressions for the latitude profiles of mean zonal wind for Uranus and Neptune in terms of the global-average kinetic energy and angular momentum of their atmospheres. The solutions are found in a linear limit of the theory that corresponds to the case where the constraint to conserve energy does not significantly restrict the efficient mixing of absolute vorticity throughout the domain. The mean zonal wind profile derived for Uranus is in excellent agreement with available observations, differing from the schematic interpolation/extrapolation of Voyager-2 cloud-tracked winds given by Allison et al. (1991) by no more than 7 meters per second at any latitude. For Neptune, the theory underestimates the strength of the prograde jet at 70 S; a solution forced to recover this jet (having a slightly different value for global-average angular momentum) forms too narrow an equatorial jet. We are in the process of applying the full nonlinear theory to Jupiter and Saturn. We will be particularly interested to determine whether the statistical mechanical theory predicts equatorial superrotation and multiple alternating jets for these planets.
Allison, M. et al. 1991. Uranus atmospheric dynamics and circulation.
In Uranus, J.T. Bergstralh, E.D. Miner and M.S. Matthews, Eds.
(Tucson: Univ. of Arizona Press). pp. 253-295.