Energy exchanges in Saturn’s polar regions from Cassini observations:
Eddy-zonal flow interactions
Simon Cabanes
DICEA, Sapienza Università di Roma, Via Eudossiana 18, 00184 Rome, Italy., DICEA, Sapienza Università di Roma, Via Eudossiana 18, 00184 Rome, Italy.
Author ProfileAbstract
Saturn’s polar regions (polewards of ∼ 63◦ planetocentric latitude) are
strongly dynamically active with zonal jets, polar cyclones and the
intriguing north polar hexagon wave. Here we analyse measurements of
horizontal winds, previously obtained from Cassini images by Antun ̵̃ano
et al. (2015), 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 energised at a rate of ∼ 10−4 W kg−1,
dominated by the contribution due to the zonal wavenumber m = 6 north
polar hexagon 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.