HST-STIS image cubes over 22 years provide a unique probe of atmospheric variations.
Two switches of Titan's two-component north-south asymmetry were observed.
Both components at each latitude follow remarkably well a harmonic oscillation.
The low-altitude component lags 2.3 years behind the high-altitude component.
Our seasonal model fits all data at latitudes to +/− 70° at all eight observed dates.
We present an analysis of three new image cubes of Titan by the Space Telescope Imaging Spectrograph taken in 2017, 2018, and 2019, half a Titan year after previously analyzed image cubes. Both data sets probe periods when Titan's seasonal north-south-asymmetry switched. The new observations show that the new reversal came exactly half a Titan year after the previous opposite reversal. On the other hand, the phase lag of the reversals with respect to Titan's equinoxes was different indicating that the seasonal variation is close to harmonic and does not follow variations due to Saturn's orbital eccentricity. The reversal had two components, a major one at altitudes below 80 km reversing two years after a minor one above 150 km. The observations further revealed small temporary deviations of <10% of the seasonal amplitude. The new observations provide an improved seasonal model of Titan that gives accurate constraints for future global circulation models.