Tuesday 24 October 2017

New Paper: Disruption of Saturn's Equatorial Oscillation

A decade ago, when Cassini was still in its prime mission at Saturn, thermal observations from the Composite Infrared Spectrometer revealed that Saturn’s equatorial atmosphere exhibited an alternating pattern of temperatures and winds that bore a striking resemblance to similar features on Earth and Jupiter.  Immediately this suggested some shared atmospheric traits between Earth and the giant planets, despite the considerable differences in the environments of the terrestrial and gas giant worlds.  Equatorial oscillations may be a fundamental feature of planetary atmospheres, a regular heartbeat that teaches us about the forces shaping the tropical stratosphere - namely atmospheric waves launched upwards by convective plumes at deeper levels.

When we started this particular project, the intention was to track the descending pattern over the entire length of the Cassini mission, through a full cycle.  We’d measure the descent rates and study the influence of the stratospheric pattern on the equatorial winds.  It was then a considerable surprise to see that the pattern was eradicated in 2011-2013, and the dates were a smoking gun for the cause - waves emanating from the Great Northern Storm, tens of thousands of kilometres away.

This connection between seemingly-unrelated patterns is well-known on Earth - for example, the influence of the El Nino Southern Oscillation on meteorological patterns across the globe.  Earth is a highly coupled system in delicate balance, and these new results suggest that the same is true of Saturn.  Indeed, in 2016 the Earth’s QBO exhibited a similar disruption, that was shown at the time to be unprecedented in the 60-year record of QBO observations.  The authors of that study suggested a source of waves in Earth’s northern hemisphere disrupting the regular pattern, and we were seeing exactly the same thing on Saturn.  Once again, the atmospheres of Earth and Saturn were shown to have similarities despite the vast differences between these two worlds.

This work helps us to understand the common forces driving the tropical atmospheres on multiple planets, and shows that these atmospheres are highly coupled and intricate systems that are susceptible to perturbations by grand meteorological events, like the Great Northern Storm of 2011.

Cassini carried an instrument called the Composite Infrared Spectrometer (CIRS), for which I’m a co-investigator.  This instrument measures thermal infrared spectra from 7 microns out to 1000 microns, and by modelling these spectra as a function of latitude and time, we can derive the oscillating pattern of temperatures and winds.  If you look at the four movies here (particularly the second one):
…you can see the shifting patterns.

Although Cassini has sadly come to an end, we will be continuing to track this oscillatory pattern and the eruptions of storm activity using Earth-based assets.  The University of Leicester is involved in a programme of observations from the VLT, Subaru and IRTF observatories to track Saturn’s seasonal evolution over long spans of time.  Furthermore, we will be employing the James Webb Space Telescope (JWST) when it launches in 2019 to catch another glimpse of Saturn’s tropical atmosphere, as part of a ERC-funded programme called GIANTCLIMES.

Full details of the article, published in Nature Astronomy, can be found here: https://t.co/wbZ5CGNbO5