Equatorial winds on Saturn and the stratospheric oscillation

The zonal jets on the giant planets have been thought to be stable in time(1-3). A decline in the velocity of Saturn's equatorial jet has been identified, on the basis of a comparison of cloud-tracking data across two decades(4), but the differences in cloud speeds have since been suggested to stem from changes in cloud altitude in combination with vertical wind shear, rather than from temporal changes in wind strength at a given height(5). Here, we combine observations of cloud tracks and of atmospheric temperatures taken by two instruments on the Cassini spacecraft to reveal a significant temporal variation in the strength of the high-altitude equatorial jet on Saturn. Specifically, we find that wind speeds at atmospheric pressure levels of 60 mbar, corresponding to Saturn's tropopause, increased by about 20m s(-1) between 2004 and 2008, whereas the wind speed has been essentially constant over time in the southern equatorial troposphere. The observations further reveal that the equatorial jet intensified by about 60m s(-1) between 2005 and 2008 in the stratosphere, that is, at pressure levels of 1-5 mbar. Because the wind acceleration is weaker near the tropopause than higher up, in the stratosphere, we conclude that the semi-annual equatorial oscillation of Saturn's middle atmosphere(6,7) is also damped as it propagates downwards.