Constraining the Vertical Structure of Zonal Jets on the Gas Giant Planets

The zonal mean zonal velocities in the upper tropospheres of Jupiter
and Saturn have been well-constrained by several decades of ground and
space-based observations combined with tracking algorithms of the
visible features of cloud tops. The observations reveal jets that are
remarkable steady in time and highly aligned in the zonal direction.
The vertical structure of the jets has been inferred above the cloud
level from infrared spectrometers and thermal wind balance, but the
opacity of the clouds to both visible and infrared wavelengths means
that the vertical structure of the jets deeper in the troposphere
remains poorly constrained: some studies have argued that wind
magnitude must increase going downward through the cloud layer, while
others have argued the just opposite.

In this study, we combine the zonal mean zonal velocity profiles
available from cloud-tracking observations with a simple argument
based on the potential vorticity distribution throughout the
troposphere. Specifically, we assume that below the cloud tops the
potential vorticity is organized into a monotonic distribution in
accordance with the potential vorticity staircase. Such a
distribution may be expected on two counts: one, from considerations
of shear stability and, two, from material conservation, that mixing
by waves and turbulence is unlikely to overmix potential vorticity in
the zonal mean into a nonmonotonic distribution. Taken together, the
relative vorticity obtained from the observed cloud top zonal mean jet
profile, with the assumption of monotonic potential vorticity implies
a latitudinal variation of potential temperature in the region below
the cloud level. The sense of the temperature anomalies is such that,
immediately below the cloud tops, density stratification is weaker on
the poleward flanks of midlatitude jets and stronger on the
equatorward flanks, while at great depth the opposite relation holds.
Thermal wind balance then yields the associated vertical shears of
midlatitude jets in an altitude range bounded above by the cloud-tops
and bounded below by the level where the latitudinal gradient of
static stability changes sign. The inferred vertical shear below the
cloud tops is consistent with the existing thermal profiling of the
upper troposphere, with jet magnitude increasing with depth below the
cloud level. Finally, the sense and approximate magnitude of the
associated mean meridional circulation in the upper troposphere may be
inferred based on existing estimates of the radiative timescales on
each planet.