Seasonal variability of Saturn’s tropospheric temperatures, winds and para-H2 from Cassini far-IR spectroscopy

•A decade of seasonal temperatures, winds and para-H2 reconstructed from Cassini.•Summertime hemispheric temperature asymmetries have become symmetrical by 2014.•Para-hydrogen increasing in springtime hemisphere due to subsidence in the north.•Voyager/Cassini differences suggest tropical stratosphere oscillations not semi-annual.•Seasonal changes affect stability, winds, potential vorticity and wave activity. Far-IR 16–1000μm 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 (70–700mbar) 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 (100–250mbar) 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 (fp) distribution are subtle, with a 0.02–0.03 rise over the spring hemisphere (200–500mbar) perturbed by (i) low-fp air advected by both the springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of high-fp air at northern high latitudes, responsible for a developing north–south asymmetry in fp. Conversely, the shifting asymmetry in the para-H2 disequilibrium primarily reflects the changing temperature structure (and hence the equilibrium distribution of fp), rather than actual changes in fp 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 fp), 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 wit