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

Abstract

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 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 (<10m/s per scale height near the tropopause) and mostly affects the broad retrograde jets, with the notable exception of large variability on the northern flank of the equatorial jet. The meridional potential vorticity (PV) gradient, and hence the ‘staircase of PV’ associated with spatial variations in the vigour of vertical mixing, has varied over the course of the mission but maintained its overall shape. PV gradients in latitude and altitude are used to estimate the atmospheric refractive index for the propagation of stationary planetary (Rossby) waves, predicting that such wave activity would be confined to regions of real refractivity (tropical regions plus bands at 35–45° in both hemispheres). The penetration depth of these regions into the upper troposphere is temporally variable (potentially associated with stratification changes), whereas the latitudinal structure is largely unchanged over time (associated with the zonal jet system).