Winter Weakening of Titan's Polar Vortices

Abstract:

Polar vortices on Titan have been observed for many years, and our work analyzes the structure and time evolution of the polar vortices, using a combination of Cassini observations and results from the TitanWRF atmospheric model. The jet at the equatorward edge of the polar vortex is observed to lie between 60-30 degrees latitude in each hemisphere during its respective winter and exhibits a maximum velocity of ~200 m/s within the jet core at a pressure level of approximately 0.1 mb. Although Cassini observations have been a treasure trove of information about Titan’s atmosphere, they are unfortunately limited to less than a full Titan year. Using TitanWRF model simulations of an entire Titan year to help fill in the spatial and temporal gaps of the observations, the polar vortex is found to have a non-monotonic variation in wind strength, temperature, and potential vorticity, and in particular we observe a noticeable minimum in the strength of the vortex at a time between winter solstice and spring equinox (around L_S=120° in the Southern hemisphere and L_S=310° in the northern hemisphere). An annular vortex is also observed to develop during this period of weakening. Analysis of the model results indicates that the formation of an annulus is linked to enhanced subsidence over the winter pole causing elevated heating and decreased potential vorticity within the vortex at those most poleward latitudes.