A similarity model for the windy jovian thermocline

Abstract

A new conceptual model for the observed banded wind structure of the giant outer planets is proposed, assuming a smoothly distributed potential vorticity (PV) atop a layer of deeply seated, latitudinally variable stratification. With a vanishing horizontal entropy contrast presumably enforced at the bottom of the flow layer by the underlying convection, the thermal-wind balance of observed cloud-top motions implies a mapping of constant potential temperature surfaces, nearly vertical at upper tropospheric levels, down to a deeper, flatter, but variably sloped “thermocline”. For a generally colder-poleward distribution of isentropes consistent with the strong equatorial jets of both Jupiter and Saturn, this kind of mapping would also imply a poleward decreasing static stability. The proposed temperature-stability distribution is just the reverse of the Earth’s troposphere, where the strongest latitudinal potential temperature gradients are at the bottom instead of the top, with static stability generally increasing toward the pole. The warmer–stabler correlation for Jupiter would be consistent, however, with a nearly monotonic distribution of PV from low to high latitudes, over planetary scales for which the planetary vorticity dominates the relative vorticity of the jets. In this way efficient PV mixing for the isentropically bounded thermocline can account for the dynamical maintenance of the cyclonic flanks of the equatorial jet. Over latitudinal intervals comparable to the internal deformation scale, however, the gradient of the absolute vorticity is dominated by the flow curvature, correlated for a well-mixed PV distribution with the local latitudinal gradient of the static stability, itself proportional for the assumed thermocline structure to the local potential temperature gradient and therefore the local departures in geostrophic velocity. The resulting correlation of velocities and vorticity gradients would imply an alternation of the flow over the internal deformation radius set by the vertical entropy contrast. The requisite size of the deep stability is itself roughly consistent with the diagnostic interpretation of the apparent dispersive character and vertical wavelength of observed temperature oscillations in Jupiter’s upper troposphere and stratosphere, as well as a plausible extrapolation of the deep sub-adiabatic lapse rate detected by the Galileo probe.