Dynamics of Saturn's south polar vortex.

The camera onboard the Cassini spacecraft has allowed us to observe many of Saturn's cloud features. We present observations of Saturn's south polar vortex (SPV) showing that it shares some properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPV's combination of properties is unique among vortices in the solar system

Equatorial winds on Saturn and the stratospheric oscillation

The zonal jets on the giant planets have been thought to be stable in time1-3. 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 decades4, 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 height5. 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 20 m 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 60 m 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 atmosphere6,7 is also damped as it propagates downwards. © 2011 Macmillan Publishers Limited. All rights reserved