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

Area of Science:

• Planetary Science
• Fluid Dynamics
• Atmospheric Physics

Background:

• Saturn's polar regions exhibit dynamic activity, including zonal jets, polar cyclones, and the unique north polar hexagon (NPH).
• Understanding energy exchange between atmospheric components is crucial for explaining these phenomena.

Purpose of the Study:

• To analyze kinetic energy (KE) exchanges between Saturn's zonal jets and non-axisymmetric eddies in polar regions.
• To investigate the energy source of the north polar hexagon wave and its influence on the north polar jet.

Main Methods:

• Analysis of horizontal wind measurements from Cassini images (Antuñano et al., 2015).
• Spatial and spectral determination of kinetic energy transfer rates between eddies and zonal mean jets.

Main Results:

• Eddies generally supply KE to both eastward and westward zonal mean jets in Saturn's polar regions.
• The north polar jet (76°N) was energized by eddies, primarily the m=6 NPH wave, at approximately 10^-4 W kg^-1.
• The south polar jet's KE was sustained by eddies across various zonal wavenumbers (m).

Conclusions:

• The north polar hexagon wave likely derives energy from baroclinic instabilities or internal sources, rather than barotropic instability of the north polar jet.
• The north polar vortex may have been weakly barotropically unstable, with low-m eddies gaining KE from the axisymmetric cyclone.
• The south polar cyclone gained KE from non-axisymmetric components, indicating a decaying elliptical distortion.