The Earth’s invisible “Saturn ring”: The QBO

/ Paul Pukite (@whut)

This is puzzling:

“This temperature and zonal wind structure resembles those of Earth’s quasi-biennial oscillation (QBO) and Jupiter’s quasiquadrennial oscillation (QQO), in which temperature anomalies and eastward/westward winds alternate in altitude”
Fouchet, T., et al. “An equatorial oscillation in Saturn’s middle atmosphere.” Nature 453.7192 (2008): 200.

And recently the final results of the Cassini spacecraft mission were in the news:

“The density wave is generated by the gravitational pull of Saturn’s moon Janus.”
Wild! Cassini Probe Spots Weird Waves in Saturn’s Rings September 11, 2017 https://www.space.com/38114-weird-waves-saturn-rings-cassini-photo.html

But no one in the research literature has made the connection of the moon’s orbit to the dynamics of the QBO.

From Fouchet et al, again

On Earth, the alternating wind regimes repeat at intervals that vary from 22 to 34 months, with an average period of about 28 months. On Jupiter, the equatorial stratospheric temperature exhibits a 4.4-year period and the equatorial zonal winds in the upper troposphere oscillate with a 4.5-year period. Long-term ground-based monitoring reveals a period of 14.7±0.9 terrestrial years on Saturn. The observational similarities between Saturn’s oscillation and the QBO and QQO are the strong equatorial confinement of temperature minima and maxima and associated shear layers, a stronger eastward than westward shear layer, and the bounding of the equatorial oscillation at latitudes 15–20° north and south. Temperatures near these latitudes are relatively high when equatorial temperatures are relatively low, and vice versa.
On Earth and Jupiter, the quasi-periodic oscillations are triggered by the interaction between upwardly propagating waves and the mean zonal flow.

Both Jupiter and Saturn have 4 significant moons,making the collective lunar orbit difficult to describe. It’s possible that Saturn’s and Jupiter’s “QBO” are more like the Earth’s upper stratosphere oscillations, which align to the semiannual period (0.5 year period). This is suggestive as the values for Jupiter and Saturn’s “QBO” period are closer to 1/2 the planet’s full calendar year period, as show below

Planet “year” length “QBO” period “QBO upper” period
Earth 1 year 2.37 years 0.5 year
Saturn 29.46 years 14.7±0.9 years
Jupiter 11.86 years 4.5 years

 

One thought on “The Earth’s invisible “Saturn ring”: The QBO”

  1. Guerlet, S., T. Fouchet, B. Bézard, F. M. Flasar, and A. A. Simon‐Miller. “Evolution of the Equatorial Oscillation in Saturn’s Stratosphere between 2005 and 2010 from Cassini/CIRS Limb Data Analysis.” Geophysical Research Letters 38, no. 9 (May 16, 2011). https://doi.org/10.1029/2011GL047192.

    “5. Conclusion and Perspectives
    [19] This study reveals for the first time the temporal evolution of the thermal and zonal wind fields (with latitude and altitude) of Saturn’s equatorial oscillation. It shows that the main features of the oscillation, local extrema of temperature and jets, have moved downwards with time, consistently with a close to 15 years period (half a kronian year). The ‘phase’ speed of the oscillation pattern is ∼0.5 mm/s, whereas evidence for subsidence and upwelling are seen through temperature anomalies at a given epoch, with vertical speed motions estimated in the range 0.1–0.2 mm/s. This behavior is extremely reminiscent of the terrestrial QBO and strongly advocates for the similarity of the terrestrial, jovian, and kronian phenomena. However, in contrast with its terrestrial and jovian counterparts, the kronian equatorial oscillation has a period which is commensurate with the saturnian year. Numerical studies will have to state whether this proportionality is fortuitous or whether it arises from Saturn’s specificities, such as the ring’s shadow which enforces strong seasonal signal in the tropics.

    [20] On Earth, the QBO is forced from both eastward and westward waves, with zonal phase velocities of comparable magnitude. On Saturn, only westward waves have been observed in the equatorial stratospheric temperature field by Liming et al. [2008], and their vertical structure has not been measured. The potential importance of the influence of equatorial waves on the tropical dynamics of Saturn’s stratosphere requires that the nature, amplitude and spectrum of these waves should be much better characterized than they are currently. At a small vertical scale (typically a tenth of a scale height), radio occultation measurements seem the most powerful means to constrain the wave amplitude and spectrum. At a larger vertical scale (1–2 scale heights), CIRS limb observations spanning at least 120°–180° of longitude would be needed to simultaneously observe the zonal and vertical structure of equatorial waves. Such observations should be conducted in the course of the Cassini Solstice Mission.”

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