Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

Abstract

We analyse the potential vorticity of Saturn's polar regions, as it is a fundamental dynamical tracer that enables us to improve our understanding of the dynamics of these regions and their seasonal variability. In particular, we present zonally averaged quasi-geostrophic potential vorticity maps between 68{\deg} planetographic latitude and the poles at altitudes between 500 mbar and 1mbar for three different epochs: (i) June 2013 (early northern summer) for the north polar region, (ii) December 2008 (late northern winter) for both polar regions and (iii) October 2006 (southern summer) for the south, computed using temperature profiles retrieved from Cassini Composite Infrared Spectrometer (CIRS) data and wind profiles obtained from Cassini's Imaging Science Subsystem (ISS). The results show that quasi-geostrophic potential vorticity maps are very similar at all the studied epochs, showing positive vorticities at the north and negative at the south, indicative of the dominance of the Coriolis parameter 2${\Omega}sin{\phi}$ at all latitudes, except near the pole. The meridional gradients of the quasi-geostrophic potential vorticity show that dynamical instabilities, mainly due to the barotropic term, could develop at the flanks of the Hexagon at 78{\deg}N, the jet at 73.9{\deg}S and on the equatorward flank of both polar jets. There are no differences in potential vorticity gradients between the two hemispheres that could explain why a hexagon forms in the north and not in the south. No seasonal variability of the potential vorticity and its meridional gradient has been found, despite significant changes in the atmospheric temperatures over time.