A dispersive wave pattern on Jupiter's fastest retrograde jet at 20S

A compact wave pattern has been identified on Jupiter’s fastest retrograding jet at 20°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 to 10.0° 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 °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 sufficient) condition for the growth of baroclinic instabilities, which is a potential source of the meandering wave pattern

Moist Convection and the 2010-2011 Revival of Jupiter's South Equatorial Belt

The transformation of Jupiter’s South Equatorial Belt (SEB) from its faded, whitened state in 2009-2010 (Fletcher et al., 2011b) to its normal brown appearance is documented via comparisons of thermal-infrared (5-20 µm) and visible-light imaging between November 2010 and November 2011. The SEB revival consisted of convective eruptions triggered over ∼ 100 days, potentially powered by the latent heat released by the condensation of water. The plumes rise from the water cloud base and ultimately diverge and cool in the stably-stratified upper troposphere. Thermal-IR images from the Very Large Telescope (VLT) were acquired 2 days after the SEB disturbance was first detected as a small white spot by amateur observers on November 9th 2010. Subsequent images over several months revealed the cold, putatively anticyclonic and cloudy plume tops (area 2.5 × 106 km2 ) surrounded by warm, cloud-free conditions at their peripheries due to subsidence. The latent heating was not directly detectable in the 5-20 µm range. The majority of the plumes erupted from a single source near 140−160◦W, coincident with the remnant cyclonic circulation of a brown barge that had formed during the fade. The warm remnant of the cyclone could still be observed in IRTF imaging 5 days before the November 9th eruption. Additional plumes erupted from the leading edge of the central disturbance immediately east of the source, which propagated slowly eastwards to encounter the Great Red Spot. The tropospheric plumes were sufficiently vigorous to excite stratospheric thermal waves over the SEB with a 20−30◦ longitudinal wavelength and 5-6 K temperature contrasts at 5 mbar, showing a direct connection between moist convection and stratospheric wave activity. The subsidence and compressional heating of dry, unsaturated air warmed the troposphere (particularly to the northwest of the central branch of the revival) and removed the aerosols that had been responsible for the fade. Dark, cloud-free lanes west of the plumes were the first to show the colour change, and elongated due to the zonal windshear to form the characteristic ‘S-shape’ of the revival complex. The aerosol-free air was redistributed and mixed throughout the SEB by the zonal flow, following a westward-moving southern branch and an eastwardmoving northern branch that revived the brown colouration over ∼ 200 days. The transition from the cool conditions of the SEBZ during the fade to the revived SEB caused a 2-4 K rise in 500-mbar temperatures (leaving a particularly warm southern SEB) and a reduction of aerosol opacity by factors of 2-3. Newly-cleared gaps in the upper tropospheric aerosol layer appeared different in filters sensing the ∼ 700-mbar cloud deck and the 2-3 bar cloud deck, suggesting complex vertical structure in the downdrafts. The last stage of the revival was the re-establishment of normal convective activity northwest of the GRS in September 2011, ∼ 840 days after the last occurrence in June 2009. Moist convection may therefore play an important role in controlling the timescale and atmospheric variability during the SEB life cycle