Waves on the SEBs jet: A paper for Icarus

‘A dispersive wave pattern on Jupiter’s fastest retrograde jet at 20ºS.’

by Rogers, JH, Fletcher, LN, Adamoli, G, Jacquesson M, Vedovato M & Orton, GS (2016).

Icarus 277, 354–369.  DOI: 10.1016/j.icarus.2016.05.028

Link for subscribers only:  http://dx.doi.org/10.1016/j.icarus.2016.05.028

Link to our final submitted version: see below.

A compact wave pattern has been identified on Jupiter’s fastest retrograding jet at 20 deg.S (the SEBs) on the southern edge of the South
Equatorial Belt. The wave has been identified in both reflected sunlight from amateur observations between 2010 and 2015, thermal
infrared imaging from the Very Large Telescope and near infrared imaging from the Infrared Telescope Facility. The wave pattern
is present when the SEB is relatively quiescent and lacking large-scale disturbances, and is particularly notable when the belt has
undergone a fade (whitening). It is generally not present when the SEB exhibits its usual large-scale convective activity (‘rifts’).
Tracking of the wave pattern and associated white ovals on its southern edge over several epochs have permitted a measure of the
dispersion relationship, showing a strong correlation between the phase speed (-43.2 to -21.2 m/s) and the longitudinal wavelength,
which varied from 4.4 – 10.0 deg longitude over the course of the observations. Infrared imaging sensing low pressures in the upper
troposphere suggest that the wave is confined to near the cloud tops. The wave is moving westward at a phase speed slower (i.e.,
less negative) than the peak retrograde wind speed (-62 m/s), and is therefore moving east with respect to the SEBs jet peak. Unlike
the retrograde NEBn jet near 17 deg.N, which is a location of strong vertical wind shear that sometimes hosts Rossby wave activity,
the SEBs jet remains retrograde throughout the upper troposphere, suggesting the SEBs pattern cannot be interpreted as a classical
Rossby wave. 2D windspeeds and thermal gradients measured by Cassini in 2000 are used to estimate the quasi-geostrophic
potential vorticity gradient as a means of understanding the origin of the a wave. We find that the vorticity gradient is dominated
by the baroclinic term and becomes negative (changes sign) in a region near the cloud-top level (400-700 mbar) associated with the
SEBs. Such a sign reversal is a necessary (but not sucient) condition for the growth of baroclinic instabilities, which is a potential
source of the meandering wave pattern.


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