Planetary-Scale Thermal Waves in Saturn's Upper Troposphere

Abstract

We have used temperatures retrieved from the Voyager IRIS (Infrared Interferometer Spectrometer) data at pressures of 130 and 270 mbar to search for waves in the upper troposphere of Saturn. The data are from three global mapping sequences taken by Voyagers 1 and 2, which together provide coverage between 60°S and 80°N latitudes. To analyze the temperatures, we grouped them within 10° wide latitude bins and applied a periodogram analysis of their zonal variance. Since the data for each map were taken over periods of 12 to 18 hours, the phase velocity of any spectral component identified is ambiguous. We estimated the phase velocity of these components by varying the assumed phase velocity to maximize the amplitude of the periodogram response at integer zonal wavenumbers. The results are dominated by zonal wavenumber 2 at 130 mbar in northern midlatitudes. In the Voyager 1 data this component is statistically significant at the 99% confidence level over latitudes 20°N to 40°N; the zonal phase of the wave is roughly constant over this latitude range. Estimates of the phase velocity using the combined Voyager 1 inbound and outbound maps allow a solution for the midlatitude zonal wavenumber 2 wave that is quasi-stationary in the reference frame defined by the rotation period of Saturn's magnetic field. Raytracing models suggest that the midlatitude waves are quasi-stationary Rossby waves. The waves are confined meridionally by meridional variations of the zonal mean winds, and confined vertically by variations in the static stability of Saturn's atmosphere. The meridional structure of these features suggests that they may be excited by overreflection of waves incident on a critical surface at midlatitudes, and that the low phase velocities are a consequence of the gradient in potential vorticity vanishing where the zonal winds are small. The waves can leak into the upper stratosphere, saturating or “breaking” at pressure levels ∼10−2–10−1mbar. The viscous damping times are ∼10 years, comparable to the radiative damping time in the tropopause region and in the stratosphere.