Temporal Variability of Titan's Middle-Atmospheric Zonal Winds from Southern Fall to Late Winter (2016-2023)

Previous ALMA observations have revealed unexpectedly strong and rapid variations in Titan’s high-altitude equatorial zonal winds, which present a challenge to our understanding of this moon’s atmospheric dynamics. Here we report further measurements of Titan’s zonal wind field at middle atmospheric altitudes $\approx 200$-490 km, based on ALMA observations of spatially and spectrally resolved CH${}_3$CN emission between 2016-2023 (corresponding to a solar longitude $L_s={81.6}^{\circ }$-${156.4}^{\circ }$, spanning Titan’s late southern autumn to late winter). These observations indicate substantial, ongoing, rapid temporal variability of Titan’s zonal winds, on timescales as short as $\approx 1$ Earth month (${0.9}^{\circ }$ in $L_s$). The strongest variability is at (near-)equatorial latitudes where the zonal winds are fastest (up to $240\pm 26$ ms${}^{-1}$), with repeated $\sim 40$ ms${}^{-1}$ fluctuations occurring between $L_s={146.0}^{\circ }$-${156.4}^{\circ }$. The observed longer-term trends are qualitatively well-reproduced by two independent, state-of-the-art general circulation models (GCMs), which match the observed decrease in zonal wind speeds over a broad range of latitudes between $L_s={81.6}^{\circ }$-${146.0}^{\circ }$, followed by a more moderate increase between $L_s={146.0}^{\circ }$-${156.4}^{\circ }$. The wind speeds at mid-to-high latitudes ($\sim -{45}^{\circ }$ south) are also typically reasonably well reproduced by the GCMs (differing by $\lesssim 30$%). However, the equatorial wind speeds are up to a factor of $\sim 2$ faster than model predictions (with the greatest discrepancy around solstice; $L_s\approx {90}^{\circ }$). Rapid temporal variability of the retrieved zonal winds indicates the presence of strong atmospheric instabilities that are not well reproduced by models, suggesting a need for future GCM improvements.