Friday, 1 August 2014

The Evolution and Fate of Saturn’s Stratospheric Vortex

As I was preparing for a presentation on Saturn's seasonal atmosphere in Wisconsin, I managed to track down a transcript of my 2012 presentation to the DPS.  These talks are so short that I tend to write down the key points in advance to make sure I don't miss anything.  As we're now seeing the demise of this large stratospheric anticyclone in 2013-14, I thought it'd be interesting to reproduce here.  It summarises the results of the following paper:

Leigh N. Fletcher, B.E. Hesman, R.K. Achterberg, P.G.J. Irwin, G. Bjoraker, N. Gorius, J. Hurley, J. Sinclair, G.S. Orton, J. Legarreta, E. García-Melendo, A. Sánchez-Lavega, P.L. Read, A.A. Simon-Miller, F.M. Flasar (2012), The Origin and Evolution of Saturn’s 2011-2012 Stratospheric Vortex, Icarus, Volume 221, Issue 2, Pages 560–586, (

DPS Reno, October 2012:

As we speak, the warm stratospheric vortex that developed in Saturn’s northern stratosphere in the aftermath of the 2010-2011 springtime storm is still persisting.  This presentation will summarise our understanding of conditions within the vortex and its long-term evolution. 

Reflected sunlight observations documented the evolution of the convective storm activity in Saturn’s northern mid-latitudes, which abated in July 2011.  Early in 2011 we first noted that the churning tropospheric storm had perturbed stratospheric winds, temperatures and composition, generating two regions of elevated temperatures to the east and west of the initial disturbance (phase 1).  These two beacons moved west at different velocities (one directly above the bulbous white storm head) until late April 2011 when they merged (phase 2), became disassociated with the storm head, and formed the single large beacon complex which is still present, and decaying (phase 3 and 4), at the present time.  The beacon continues to move west around the planet like clockwork at approximately 31 m/s, and appears disassociated from anything observed in the troposphere.

Conditions within the Vortex

So what do we know about the beacon?  Thermal imaging and spectroscopy from Cassini, VLT and IRTF have been used to monitor conditions within the vortex from pre-merger to the decay phase.  We know that the vortex exhibits peak temperatures at 2 mbar (deeper down than the original two beacons which were centred at 0.5 mbar), but has extended regions of hot emission stretching around the planet at even higher altitudes where the shape is less well-defined.  Using the thermal windshear relations to extrapolate zonal and meridional winds from the tropospheric cloud decks into the stratosphere, we know that the vortex is bounded by a peripheral anticyclonic jet (i.e., clockwise motion) which determines the width of the feature, which was some 80 degrees wide at its maximum extent but is now shrinking.  This jet entrains unique chemistry, with acetylene and other species enhanced within the vortex, suggestive of atmospheric subsidence.  From studying VIMS, ISS and amateur imaging, we know that the vortex has no tropospheric counterparts.

Evolution of the Vortex in 2011-2012

Full spatio-spectral maps of the northern hemisphere were acquired by Cassini during the decay phase, showing how the vortex has begun to cool and shrink in longitudinal extent over the observational period.   These plots show the 2-mbar temperatures and the magnitude of the peripheral winds.  The chart on the right shows that the shrinking is at a rate of approximately 0.16 deg/day, with the peak velocities of the peripheral jet moving inwards to its current longitudinal extent of around 30 degrees, hence the vortex has effectively circularised.  Acetylene remains entrained within the vortex, but the abundance is now decreasing.  At these rates, we’d expect the vortex to dissipate in the next 12-18 months.

Wave Origins

So far we’ve discussed the evolution and fate of this bizarre feature, but how did it develop in the first place?  Large scale weather disturbances on Earth are the sources of strong wave perturbations, from small-scale gravity waves to planetary-scale Rossby waves, thus it is reasonable to suggest that waves emitted by the tropospheric storm carried energy and momentum into the stratosphere.  How and where that energy was deposited, and why we saw the spin-up to produce anticyclonic vortices, requires more detailed models.  

The transmissivity of the atmosphere to waves depends on the details of the zonal wind structure.  However, simply assuming a quasigeostrophic atmosphere with no vertical shear of the zonal winds, we find that the conditions for the vertical propagation of Rossby waves to transport energy upwards (the difference between the ambient zonal velocity and the wave phase speed, u-c, must be positive and less than some critical prograde velocity, Uc) of this particular dimension (defined by the wavenumbers k, l and m in this expression) is only satisfied near the first two retrograde jets.  This simple idea would prevent the emergence of a stratospheric beacon if a storm erupted at any other latitude.  

Things aren’t that simple, however, as Saturn’s winds vary with both height and with season.  Terrestrial planetary wave propagation is seasonally-dependent.  By using the radiative-climate model of Saturn’s atmosphere from Tommy Greathouse to provide a first-order estimate of the seasonal temperature gradients, we see that Saturn’s stratosphere should experience summer easterlies and winter westerlies just as on Earth.  Furthermore, taking Cassini/CIRS temperature maps since 2004 and computing the zonal winds at 2 mbar, we start to see hints that the winds are becoming more retrograde in northern spring and more prograde in southern autumn.  Crucially, the winds currently measured at 2 mbar at 40N are sufficient to satisfy the condition for Rossby wave propagation.


In conclusion, the springtime tropospheric storm caused the formation and evolution of a large, hot stratospheric anticyclone, which is slowly shrinking and cooling but is still present nearly two years after the eruption.  Seasonal conditions (transitioning from winter prograde to summer retrograde) may have been ideal at mid latitudes for Rossby wave transport of energy into the stratosphere from the storm clouds below.  Dynamical and chemical modelling of the stratospheric response to this tropospheric forcing is now required, and as Saturn’s storm season may not yet be over, detailed observations are needed to see whether another Saturnian storm can have a similar stratospheric aftermath.

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