Saturn’s Northern Hemisphere Ribbon: Simulations and Comparison with the Meandering Gulf Stream

Abstract

Voyager observations of Saturn in 1980-81 discovered a wavy feature engirdling the planet at 478N planetographic latitude. Its latitude coincides with that of an eastward jet stream, which is the second fastest on Saturn after the equatorial jet. The 478N jet's wavy morphology is unique among the known atmospheric jets on the gas giant planets. Since the Voyagers, it has been seen in every high-resolution image of this latitude for over 25 years and has been termed the Ribbon. The Ribbon has been interpreted as a dynamic instability in the jet stream. This study tests this interpretation and uses forward modeling to explore the observed zonal wind profile's stability properties. Unforced, initial-value numerical experiments are performed to examine the nonlinear evolution of the jet stream. Parameter variations show that an instability occurs when the 478N jet causes reversals in the potential vorticity (PV) gradient, which constitutes a violation of the Charney-Stern stability criterion. After the initial instability development, the simulations demonstrate that the instability's amplitude nonlinearly saturates to a constant when the eddy generation by the instability is balanced by the destruction of the eddies. When the instability saturates, the zonal wind profile approaches neutral stability according to Arnol'd's second criterion, and the jet's path meanders in a Ribbon-like manner. It is demonstrated that the meandering of the 478N jet occurs over a range of tropospheric static stability and background wind speed. The results here show that a nonlinearly saturated shear instability in the 478N jet is a viable mechanism to produce the Ribbon morphology. Observations do not yet have the temporal coverage to confirm the creation and destruction of eddies, but these simulations predict that this is actively occurring in the Ribbon region. Similarities exist between the behaviors found in this model and the dynamics of PV fronts studied in the context of meandering western boundary currents in Earth's oceans. In addition, the simulations capture the nonlinear aspects of a new feature discovered by the Cassini Visual and Infrared Mapping Spectrometer (VIMS), the String of Pearls, which resides in the equatorward tip of the 478N jet. The Explicit Planetary Isentropic Coordinate (EPIC) model is used herein.

Emergence of polar-jet polygons from jet instabilities in a Saturn model

“…Fig. 13 shows the time evolution of four kinetic energy components (zonal barotropic, zonal baroclinic, eddy barotropic, and eddy baroclinic) for the case which develops a quasi-stationary Hexagon (H2 in Table 2), calculated following the scheme used by Sayanagi et al (2010). As the eddy barotropic kinetic energy is much greater than both the eddy baroclinic and zonal baroclinic components, we identify barotropic instabilities as the dominant instability mode.…”

The universal aspect ratio of vortices in rotating stratified flows: theory and simulation

“…Examples in planetary atmospheres include the rows of cyclones and anticyclones near Saturn's Ribbon (Sayanagi, Morales-Juberias & Ingersoll 2010) and near 41 • S on Jupiter (Humphreys & Marcus 2007), and Jupiter's anticyclonic great red spot (Marcus 1993). In the Atlantic ocean, meddies persist for years (McWilliams 1985;Armi et al 1988), and numerical simulations of the discs around protostars produce compact anticyclones (Barranco & Marcus 2005).…”

Observations and Numerical Modeling of the Jovian Ribbon

“…Previous modeling of the Saturnian Ribbon investigated instabilities and saturation processes of the 47 N  jet where this feature is located with U z = 0 (Sayanagi et al 2010). Those results showed noticeable patches of PV at 45°N.…”

“…We looked for a strong PV gradient in our simulations of a Ribbon-like feature which would be consistent with our observational analysis. We then performed spectral analysis on the relative vorticity contour equal to zero (RV0), which approximated the jet's path as done in Sayanagi et al (2010). We identified Ribbon-like features that met the following criteria:…”

“…We aim to understand what underlying atmospheric conditions and processes favor the genesis of the Jovian Ribbon's irregular cloud morphology near 30°N. Previous studies of Jupiter and Saturn have shown how the properties of some observed waves are dependent on the conditions of the background flow and static stability of the atmosphere (Gierasch 2004;Sayanagi et al 2010). We explored the effects of vertical wind shear, static stability, and location of perturbations to determine their impact on reproducing a Ribbon-like feature.…”