Energy Exchanges in Saturn's Polar Regions From Cassini Observations: Eddy‐Zonal Flow Interactions

• Published in: Journal of geophysical research. Planets 2022-05, Vol.127 (5), p.e2021JE006973-n/a
• Main Authors: Read, Peter L., Antuñano, Arrate, Cabanes, Simon, Colyer, Greg, Gaztelurrutia, Teresa del Río, Sanchez‐Lavega, Agustin
• Format: Article
• Language: English
• Subjects: Atmospheres; Atmospheric Composition and Structure; atmospheric dynamics; Baroclinic instability; Barotropic instability; Cassini mission; Circular vortex; Composition; Costs; Cyclones; Eddies; energetics; Energy; Energy sources; Energy transfer; Fluid flow; Geochemistry; Hurricanes; Image resolution; Internal energy; Jets; Jupiter; Kinetic energy; Latitude; Mineralogy and Petrology; Planetary Atmospheres; Planetary Atmospheres, Clouds, and Hazes; Planetary Geochemistry; Planetary Mineralogy and Petrology; Planetary Sciences: Astrobiology; Planetary Sciences: Comets and Small Bodies; Planetary Sciences: Fluid Planets; Planetary Sciences: Solid Surface Planets; Polar environments; Polar regions; Polar vortex; polar vortices; Saturn; Spacecraft; Vortices; Wavelengths; Zonal flow; Zonal flow (meteorology); zonal jets
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2021JE006973

Saturn's polar regions (polewards of ∼63° planetocentric latitude) are strongly dynamically active with zonal jets, polar cyclones and the intriguing north polar hexagon (NPH) wave. Here we analyze measurements of horizontal winds, previously obtained from Cassini images by Antuñano et al. (2015), https://doi.org/10.1002/2014je004709, to determine the spatial and spectral exchanges of kinetic energy (KE) between zonal mean zonal jets and nonaxisymmetric eddies in Saturn's polar regions. Eddies of most resolved scales generally feed KE into the eastward and westward zonal mean jets at rates between 4.3 × 10−5 and 1.4 × 10−4 W kg−1. In particular, the north polar jet (at 76°N) was being energized at a rate of ∼10−4 W kg−1, dominated by the contribution due to the zonal wavenumber m = 6 NPH wave itself. This implies that the hexagon was not being driven at this time through a barotropic instability of the north polar jet, but may suggest a significant role for baroclinic instabilities, convection or other internal energy sources for this feature. The south polar zonal mean jet KE was also being sustained by eddies in that latitude band across a wide range of m. In contrast, results indicate that the north polar vortex may have been weakly barotropically unstable at this time with eddies of low m gaining KE at the expense of the axisymmetric cyclone. However, the southern axisymmetric polar cyclone was gaining KE from non‐axisymmetric components at this time, including m = 2 and its harmonics, as the elliptical distortion of the vortex may have been decaying. Plain Language Summary Saturn's polar regions (polewards of ∼63° latitude) are strongly meteorologically active with high speed eastward zonal jets (at 76°N and 70°S), intense, hurricane‐like polar cyclones and the intriguing north polar hexagon wave (at 76°N). Here we analyze measurements of horizontal winds, previously obtained by tracking features in images from the Cassini Orbiter spacecraft, to determine how kinetic energy is exchanged between the longitudinally averaged zonal jets and various types of nonaxisymmetric eddy. As measured previously at low‐ and mid‐latitudes on Jupiter and Saturn, we found that Saturn's 76°N and 70°S jets were gaining energy at the expense of nonaxisymmetric waves and eddies, including the northern polar hexagonal meanders, suggesting an important energetic role for heat transporting processes in Saturn's circulation. Energy exchanges within the polar vortices themselve