Belt-Zone Contrasts in Vertical Motion and Cloud Structure in Jupiter's Troposphere

Abstract:

It is known that the eddy fluxes of angular momentum in Jupiter's upper troposphere converge in prograde jets and diverge in retrograde jets. Away from the equator, this implies convergence of the Eulerian mean meridional flow in zones (anticyclonic shear) and divergence in belts (cyclonic shear). It suggests lower-tropospheric downwelling in zones and upwelling in belts because the mean meridional circulation almost certainly closes at depth. Yet the banded structure of Jupiter's clouds and hazes suggests that there is upwelling in the brighter zones and downwelling in the darker belts. Here we show that this apparent contradiction can be resolved by considering not the Eulerian but the transformed Eulerian mean circulation, which includes a Stokes drift owing to eddies and is a better approximation of the Lagrangian mean transport of tracers such as ammonia. The structure of the potential vorticity inferred from observations paired with mixing length arguments suggests that there is transformed Eulerian mean upwelling in zones and downwelling in belts, and that this transformed Eulerian mean flow is about an order of magnitude stronger than the Eulerian mean flow. Simulations with a general circulation model of Jupiter's upper atmosphere demonstrate the plausibility of these inferences.