Seasonal and interannual variability in Saturn's stratosphere

Thesis

The stratosphere of Saturn is highly variable. With an axial tilt of 26.7°, Saturn experiences seasons like Earth and is currently approaching northern summer solstice in 2017. In addition to general seasonal change, previous studies have highlighted that Saturn's stratosphere is host to a range of dynamical phenomena. These processes have an observable effect on the vertical temperature profile and stratospheric concentrations of acetylene (C₂H₂) and ethane (C₂H₆), which may be determined or retrieved from thermal infrared observations of Saturn. This thesis presents an analysis of observations of Saturn acquired by Voyager's IRIS (Infrared Interferometer Spectrometer, 180 - 2500 ^cm-1, Hanel et al.,[1980]) instrument in 1980, Cassini's CIRS (Composite Infrared Spectrometer, 10 - 1400 ^cm-1, Flasar et al.,[2004]) instrument from 2005 to 2012 and the Celeste spectrometer (400 - 2000 ^cm-1, Moran et al.,[2007]) on NASA's IRTF (Infrared Telescope Facility) in 2012 in order to track seasonal and interannual changes in Saturn's stratosphere.

The concentrations of C₂H₂ and C₂H₆ were seen to decrease at 15°S and increase at 25°N from 2005 to 2009/2010. These changes at 15°S and 25°N respectively indicate upward and downward branches associated with cross-equatorial seasonally-reversing Hadley circulation that has been predicted by a general circulation model [Friedson and Moses, 2012]. Strong cooling of up to 17 K at high-southern latitudes from 2005 to 2010 suggests an autumnal weakening of a vortex that appears to form at the pole of the summer hemisphere [Fletcher et al., 2008]. The emergence of a similar northern polar vortex as northern summer solstice approaches was yet to be observed in 2012. Interannual differences in the equatorial temperature structure between 1980 and 2009/2010 suggest Saturn's semiannual oscillation (or SSAO, Fouchet et al. [2008]; Orton et al. [2008]) has been captured in a different phase from one year to the next. This is puzzling since the oscillation would be expected to have undergone two cycles assuming its period is half a Saturn year (14.7 years). This contrast is suggestive that the period of the SSAO is more quasisemiannual.

Authors: Sinclair, JA

• Publication date: 2014
• DOI: 10.5287/ora-dqedbvzrr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Saturn; stratosphere; seasons; climate
• Subjects: Atmospheric,Oceanic,and Planetary physics