Seasonal variability of Saturn's tropospheric temperatures, winds and para-H-2 from Cassini far-IR spectroscopy

Far-IR 16–1000 μμm spectra of Saturn’s hydrogen–helium continuum measured by Cassini’s Composite Infrared Spectrometer (CIRS) are inverted to construct a near-continuous record of upper tropospheric (70–700 mbar) temperatures and para-H2 fraction as a function of latitude, pressure and time for a third of a saturnian year (2004–2014, from northern winter to northern spring). The thermal field reveals evidence of reversing summertime asymmetries superimposed onto the belt/zone structure. The temperature structure is almost symmetric about the equator by 2014, with seasonal lag times that increase with depth and are qualitatively consistent with radiative climate models. Localised heating of the tropospheric hazes (100–250 mbar) create a distinct perturbation to the temperature profile that shifts in magnitude and location, declining in the autumn hemisphere and growing in the spring. Changes in the para-H2 (fpfp) distribution are subtle, with a 0.02–0.03 rise over the spring hemisphere (200–500 mbar) perturbed by (i) low-fpfp air advected by both the springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of high-fpfp air at northern high latitudes, responsible for a developing north–south asymmetry in fpfp. Conversely, the shifting asymmetry in the para-H2 disequilibrium primarily reflects the changing temperature structure (and hence the equilibrium distribution of fpfp), rather than actual changes in fpfp induced by chemical conversion or transport. CIRS results interpolated to the same point in the seasonal cycle as re-analysed Voyager-1 observations (early northern spring) show qualitative consistency from year to year (i.e., the same tropospheric asymmetries in temperature and fpfp), with the exception of the tropical tropopause near the equatorial zones and belts, where downward propagation of a cool temperature anomaly associated with Saturn’s stratospheric oscillation could potentially perturb tropopause temperatures, para-H2 and winds. Quantitative differences between the Cassini and Voyager epochs suggest that the oscillation is not in phase with the seasonal cycle at these tropospheric depths (i.e., it should be described as quasi-periodic rather than ‘semi annual’). Variability in the zonal wind field derived from latitudinal thermal gradients is small (