Titan's stratospheric temperatures: A case for dynamical inertia?

Abstract

Temperatures between the 1- and 0.4-mbar barometric pressure levels of Titan's atmospere have been retrieved from Voyager IRIS spectral radiances in the λ 4-band of CH 4. They exhibit a hemispheric asymmetry, being 4–8° K warmeraat 55° S than at 55° N. At the season of the Voyager observations (northern spring equinox), one would have expected a symmetric distribution if the opacity for solar and infrared radiation were uniformly distributed with latitude, because the radiative time constant is so short. Instead, the temperature field appears to lag the solar heating by nearly a quarter of a Titan year. The possibility of an asymmetry in the meridional distribution of opacity about the equator cannot be discounted, but an alternate explanation follows from the need for angular momentum transport concurrent with seasonally varying temperatures in Titan's stratosphere, so that the cyclostrophic thermal wind relation between zonal winds and temperatures can be maintained. A simple, zonally symmetric model predicts that the required transport can be affected by a circulation with upwelling at northern latitudes and subsidence at midsouthern latitudes at the time of the Voyager encounter. The adiabatic heating and cooling associated with these motions can produce the observed asymmetry in temperature. In effect, the circulation causes the temperature field to lag the radiative equilibrium field. The dynamical time scale for this lag is the radiative relaxation time multiplied by the Richardson number, and is comparable to a season in Titan's stratosphere.