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.