Saturn's Global Zonal Winds Explored by Cassini/VIMS 5‐μm Images

Abstract

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) 5‐μm images are used to derive Saturn's global zonal winds around the 2,000‐hPa level. The comparison of zonal winds between 2,000 and 300–500 hPa shows a general consistency of wind structure between the two pressure levels on a global scale. However at some latitudes, the magnitude of the zonal winds differs between these levels. The equatorial zonal winds are stronger downward, while the zonal winds in the middle and high latitudes are generally weaker downward. These new wind measurements also imply that barotropic and baroclinic instabilities probably exist through the relatively deep atmosphere at some latitudes. Finally, our analysis reveals that the VIMS winds in the two polar regions are basically constant with time except for a westerly jet centered at ~88°N, which decreased from 135 ± 7 m/s in 2008 to 91 ± 12 m/s in 2017.

Global climate modeling of Saturn's atmosphere. Part II: Multi-annual high-resolution dynamical simulations

“…Melendo et al, 2010;Del Genio and Barbara, 2012); the jet intensity actually tends to slightly increase upwards from the troposphere to the stratosphere in our simulations. Using Cassini VIMS images, Studwell et al (2018) found that the intensities of mid-latitude jets were generally increasing from the 2-bar level to the 300-500 hPa level, which tends to confirm our Saturn DYNAMICO GCM results at and below the cloud level. Accounting for the preferential zonal wavenumber n = 6 (hexagonal) mode in the circumpolar jet structure on Saturn is still an open question (Morales-Juberías et al, 2011, given the narrow parameter space which allows for this mode to predominate over other modes (Barbosa Aguiar et al, 2010;Rostami et al, 2017).…”

“…We conclude that both barotropic and baroclinic instabilities could account for the maintenance, migration and (in polar regions) disappearance of eastward zonal jets in our Saturn DYNAMICO GCM simulations. The putative role of baroclinic instability in driving the mid-latitude jets was also highlighted by Liu and Schneider (2010); the possibility that barotropic and baroclinic instabilities exist in Saturn's troposphere is also argued in Studwell et al (2018). In that respect, the pole-to-equator meridional gradient of temperature in the deep troposphere (e.g.…”

Saturn atmospheric dynamics one year after Cassini: Long-lived features and time variations in the drift of the Hexagon

“…These values are closer to the equatorial zonal winds at cloud level at the time of the Voyagers than during the Cassini mission. Zonal winds obtained in Cassini VIMS images sensitive to the ∼ 2 bar level (Choi et al, 2009;Studwell et al, 2018) • A similar bright and fast feature has been found in the South Equatorial Zone at 6.7 ± 2.2 • S during most of 2018. Except for its elongated shape, this South equatorial bright spot is very similar in size, brightness, drift rate and closeness to the deep winds obtained by VIMS.…”

“…(ii) The coincidence in the zonal speed of the WS with the VIMS zonal winds from 2007 data. However, the VIMS zonal winds in 2015 published later (Studwell et al, 2018) suggest that the WS moves sligthly faster than the deep winds, complicating this interpretation. The new WS groundbased data in 2017-2018 are more precise and show a sustained fast feature with a continuous acceleration of its zonal speed from 444.3 ± 3.1 ms −1 in 2014 to 452.4 ± 1.7 ms −1 in 2018 with an overall acceleration of 1.9 ms −1 per year.…”

How Well Do We Understand the Belt/Zone Circulation of Giant Planet Atmospheres?

“…Jupiter's winds were measured by the Galileo probe in a single location (Atkinson et al 1998), and were found to increase with depth in Jupiter's tropics down to ∼ 4 bar, and then to remain approximately constant (or show a very weak decrease) to 21 bar, where the probe signal was lost. On Saturn, observations at 5-µm by Cassini/VIMS could probe down to the 2-3 bar level, and revealed that tropical winds were generally stronger at depth than at the 500-mbar level, whereas extra-tropical winds were slightly weaker at depth Choi et al 2009;Li et al 2011;Studwell et al 2018), although Sanchez-Lavega et al (2019) cautions that this extra-tropical trend is relatively weak and may not be universal across all of Saturn's jets. Nevertheless, this contrast between the tropics and extra-tropics is important, as we shall see later, and suggests that the winds should decay away with increasing depth.…”