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 about200-490 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 - 156.4, 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 about1 Earth month (0.9 in L s). The strongest variability is at (near-)equatorial latitudes where the zonal winds are fastest (up to 240 plus or minus26 ms-1), with repeated about40 ms-1 fluctuations occurring between L s=146.0 - 156.4. 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 - 146.0, followed by a more moderate increase between L s=146.0 - 156.4. The wind speeds at mid-to-high latitudes (about-45 south) are also typically reasonably well reproduced by the GCMs (differing by 30 %). However, the equatorial wind speeds are up to a factor of about2 faster than model predictions (with the greatest discrepancy around solstice; L s about90). 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.