A dispersive wave pattern on Jupiter's fastest retrograde jet at $20^\circ$S

TL;DR

A wave pattern on Jupiter's SEBs jet at 20°S, observed in multiple wavelengths, exhibits dispersive properties and likely has a baroclinic origin, especially prominent during quiescent belt conditions.

Contribution

This study identifies and characterizes a dispersive wave pattern on Jupiter's SEBs jet, providing insights into its dynamics and potential baroclinic origin, which differs from classical Rossby waves.

Findings

Wave pattern observed in reflected sunlight, infrared imaging, and thermal data.

Wave's phase speed correlates with wavelength, varying from -43.2 to -21.2 m/s.

Wave confined near cloud tops and prominent during quiescent belt states.

Abstract

A compact wave pattern has been identified on Jupiter's fastest retrograding jet at 20S (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 17N, 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. Cassini-derived windspeeds and temperatures reveal 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, suggesting a baroclinic origin for this meandering wave pattern. [Abr]