Jupiter's Overturning Circulation: Breaking Waves Take the Place of Solid Boundaries

Abstract

Cloud-tracked wind observations document the role of eddies in putting momentum into the zonal jets. Chemical tracers, lightning, clouds, and temperature anomalies document the rising and sinking in the belts and zones, but questions remain about what drives the flow between the belts and zones. We suggest an additional role for the eddies, which is to generate waves that propagate both up and down from the cloud layer. When the waves break they deposit momentum and thereby replace the friction forces at solid boundaries that enable overturning circulations on terrestrial planets. By depositing momentum of one sign within the cloud layer and momentum of the opposite sign above and below the clouds, the eddies maintain all components of the circulation, including the stacked, oppositely rotating cells between each belt-zone pair, and the zonal jets themselves.

Figure 1

Ammonia vapor concentration (upper panel) in parts per million derived from Juno Microwave Radiometer (MWR) observations compared with dynamical features of Jupiter's atmosphere. Belts (gray bands) and zones (white bands) are defined by the cyclonic or anticyclonic vorticity $\partial \overline{\mathbf{u}}/\partial \mathbf{y}$ of the zonal winds (middle panel). The eddy momentum flux (EMF, northward flux of eastward momentum) divided by the density (lower panel) is poleward in the zones and equatorward in the belts (Salyk et al., 2006). The points are $\overline{{\mathbf{u}}^{\prime }{\mathbf{v}}^{\prime }}$ and the smooth curve is $\partial \overline{\mathbf{u}}/\partial \mathbf{y}$. The MWR map differs from earlier maps (Bolton et al., ; Ingersoll et al., ; C. Li et al., 2017) because it is an average of seven north‐south scans of the planet and is an inversion that uses not only the nadir brightness data but also the center‐to‐limb darkening data (Oyafuso et al., 2020). Notable features of the MWR map are the extreme dryness (depleted ammonia vapor) from 1 to 6 bars in the belts on either side of the equator, the ammonia increase with altitude from 1 to 6 bar both at the equator and at the zones in midlatitudes, and wavy contours implying rising and sinking motion in the belts and zones, respectively, at 40–60 bars.

Figure 2

Inertia‐gravity waves propagating in the x‐z plane. The x coordinate is to the east (velocity u) and the z coordinate is vertical (velocity w). The left two panels show a latitude where the zonal wind $\overline{\mathrm{u}}\text{is}\text{to}\text{the}\text{west}$ relative to the phase velocity c of the waves ($\overline{\mathbf{u}}-\mathbf{c}<0)$, and the right two panels show the opposite ($\overline{\mathbf{u}}-\mathbf{c}>0).$ The top two panels show waves that are carrying momentum upward and exerting a drag force on the flow above the source region. The bottom two panels show the opposite—a drag force below the source region. The figure shows a snapshot of each of the four wave types. The thick black arrows are in the direction of phase propagation and are perpendicular to the crests and troughs of the wave. Arrows along the crests and troughs are the fluid velocities. Phase velocity and group velocity are denoted by k and ${\mathit{c}}_{\mathit{g}}$, respectively. The words high and low refer to the gravitational potential at the crests and troughs. The words warm and cold refer to temperature. Circles with crosses and dots refer to poleward and equatorward flow, respectively. The figure in the upper left corner is a copy of Figure 4.19 on p. 200 of AHL. The figures in the other three corners were created by flipping and relabeling the original figure.