Impact of Titan's Polar Vortex on the Transport of Chemical Species

Abstract Distributions of trace hydrocarbons in the atmosphere of Titan possess a strong seasonality. In mid‐winter, hydrocarbons are thought to be able to escape through the polar vortex and develop “tongues” of enriched air which spread equatorward from the poles. However, recent studies have shown that the mixing barrier associated with a polar vortex of similar structure to the ones found on Titan should still be strong enough to isolate these hydrocarbons. Here, we examine the horizontal mixing associated with the polar vortices throughout their lifetime by using simulations of passive chemical tracers and contour advection for two Titan‐specific three‐dimensional (3D) general circulation models. Our analysis reveals that during the fall, the mixing barrier associated with the vortex extends above ∼0.1 hPa and isolates the stratospheric polar regions, allowing for strong enrichment by mesospheric descent. During winter, however, the vertical extent of the polar vortex is reduced due to the winter weakening, opening a region directly above the polar vortex (∼1–0.1 hPa) where tracer‐depleted air is transported poleward, locally reducing tracer concentrations in the upper stratosphere and creating an enriched tracer region below in the lower‐mid stratosphere. The presence of an extremely broad mixing barrier within and below the polar vortex (>1 hPa) would highly limit the movement of material equatorward and may indicate that previous observations of hydrocarbon tongues on Titan may be due to the influx of material above the polar vortex rather than escape across the vortex; further work is needed to verify this theory.