Equatorial Winds on Saturn and the Stratospheric Oscillation

The zonal jets on the giant planets are generally thought to be stable with time. Recently, there are still some debates about the general thought. Here, we report a significant temporal variation of the equatorial jet at high-altitude on Saturn. Long-term (2004-2009) observations by Cassini reveal that wind speed at the 60-mbar level increased from 270 m/s in 2004 to 290 m/s in 2008, while the wind speed has been mostly constant over time at the 500-mbar level in the southern equatorial region. The Cassini observations further reveal that the equatorial jet intensified approximately 60 m/s in the stratosphere (1-5 mbar) from 2005 to 2008. The fact that the wind acceleration is weaker at the 60-mbar level (approximately 20 m/s) than at the 1-mbar level (approximately 60 m/s) demonstrates that the equatorial oscillation is damped when it propagates downwards to the tropopause around 60 mbar. The direct measurement of the varying equatorial jet around the tropopause also serves as a key boundary condition when deriving the thermal wind fields in the stratosphere.