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

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.

Thermal structure and dynamics of Saturn's northern springtime disturbance.

Saturn's slow seasonal evolution was disrupted in 2010-2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn's stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced "beacons" of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N.

Sloping convection: A paradigm for large-scale waves and eddies in planetary atmospheres?

In laboratory studies and associated theoretical and numerical work covering a very wide range of conditions (as specified by the key dimensionless parameters of the systems used) the phenomenon of sloping convection in rotating fluids can manifest itself in one of several spatial forms (waves, closed eddies, and combinations thereof), but all with strong local gradients (fronts, jet streams) and exhibiting various types of temporal behavior [steady, periodic vacillation, aperiodic (geostrophic) turbulence]. These general properties were first discovered in cylindrical (annular) systems, but they do not depend critically on geometry; differences between spherical and cylindrical systems are largely to be found in quantitative details. In all cases, the raison d'e tre of sloping convection is horizontal advective transfer, a process accompanied by upward advective heat transfer, which affects and may control vertical potential density gradients. It has been argued that sloping convection is the basic dynamical process underlying a wide variety of large-scale flow phenomena seen in planetary atmospheres (e.g., irregular waves in the Earth's atmosphere, regular waves in the Martian atmosphere, the Jovian Great Red Spot and other long-lived eddies seen in the atmospheres of the giant planets). In this review the extent to which this paradigm is upheld in the atmospheres of the major planets by recent work is discussed.