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Joint evolution of equatorial oscillation and interhemispheric circulation in Saturn’s stratosphere

Nature Astronomy volume 6, pages 804–811 (2022)Cite this article

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Abstract

Planetary stratospheres are characterized by a subtle interplay between dynamics, radiation and chemistry. Observations of Saturn’s stratosphere have revealed a semi-annual equatorial oscillation of temperature and hinted at an interhemispheric circulation of hydrocarbon species. Both the forcing mechanisms of the former and the existence of the latter have remained debated. Here we use a new troposphere-to-stratosphere Saturn global climate model to argue that those two open questions are intimately connected. Our Saturn climate model reproduces a stratospheric oscillation exhibiting the observed semi-annual period, amplitude and downward propagation. In the same Saturn simulation, a prominent stratospheric summer-to-winter hemispheric circulation develops at the solstices, controlled by both the seasonal radiative gradients and Rossby-wave pumping in the winter-subsiding branch, analogous to Earth’s Brewer–Dobson circulation. Furthermore, we show that Saturn’s equatorial oscillation is driven by the seasonal variability of both the resolved planetary-scale wave activity and the interhemispheric circulation, akin to Earth’s Semi-Annual oscillation.

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Fig. 1: Eddy-to-mean flow interactions at the origin of the time evolution of Saturn’s equatorial oscillation.
Fig. 2: Circulation during different seasons and the main acceleration terms driving it.
Fig. 3: Schematic of the inter-connection between Saturn’s equatorial oscillation and its seasonal interhemispheric circulation.

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Data availability

Simulation outputs are too data-heavy to be shared in a public repository. A minimal dataset derived from simulation outputs (zonally averaged eddy-to-mean flow interaction diagnostics) is available via IPSL-ESPRI at https://doi.org/10.14768/6494348c-3304-4838-a379-a7ccfefac872.

Code availability

The Python codes developed to produce the transformed Eulerian-mean formalism diagnostics are available via GitHub at https://github.com/aymeric-spiga/dynanalysis.

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Acknowledgements

The authors acknowledge the exceptional computing support from the Grand Equipement National de Calcul Intensif and the Centre Informatique National de l’Enseignement Supérieur (CINES). All the simulations presented here were carried out on the Occigen cluster hosted at CINES. This work was granted access to the high-performance computing resources of CINES under the allocation A0080110391. D.B., A.S. and S.G. acknowledge funding from the Agence Nationale de la Recherche, project EMERGIANT ANR-17-CE31-0007. The authors acknowledge T. Dubos, Y. Meurdesoif and E. Millour for key contributions in developing the DYNAMICO-Saturn model, F. Lott for insightful comments on terrestrial stratospheric dynamics and T. Fouchet for long-standing support of modelling studies of Saturn’s stratosphere.

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Authors and Affiliations

  1. Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Université, Centre National de la Recherche Scientifique (CNRS), École Polytechnique, Institut Polytechnique de Paris, École Normale Supérieure (ENS), PSL Research University, Paris, France

    Deborah Bardet, Aymeric Spiga & Sandrine Guerlet

  2. Institut Universitaire de France (IUF), Paris, France

    Aymeric Spiga

  3. LESIA, Observatoire de Paris, Université PSL, Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, Université de Paris, Meudon, France

    Sandrine Guerlet

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  1. Deborah Bardet
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Contributions

D.B. performed the simulations, carried out the analysis (including figures), interpreted the results and led the manuscript writing. A.S. and D.B. developed the post-processing calculations. A.S. and S.G. contributed to discussion and interpretation of the results. All authors designed the methodology and contributed to the manuscript.

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Correspondence to Deborah Bardet.

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Nature Astronomy thanks Ricardo Hueso, Glenn Orton and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Temperature phase of modelled Saturn’s equatorial oscillation.

Zonal-mean temperature profiles for two different times to highlight temperature oscillation phases change. These profiles are averaged between ± 2° of latitude.

Extended Data Fig. 2 Descent over time of equatorial temperature anomalies.

Altitude/latitude sections of zonal-mean temperature departures from mean temperature, which varies with seasons in Saturn’s stratosphere at six dates for the ninth simulated Saturn year. To see the variations of the descent of the temperature anomalies over 10 Saturn years, we refer the reader to the video in DOI data center repository here: https://doi.org/10.14768/6494348c-3304-4838-a379-a7ccfefac872.

Extended Data Fig. 3 Vertical structure of temperature anomalies at and surrounding the equator.

Altitude-time cross section of the zonal-mean temperature anomalies at (a) 11°N, at (b) the equator and at (c) 11°S for the tenth simulated Saturn years.

Extended Data Fig. 4 Differences of zonal-mean temperature at two different planetocentric latitudes over time to highlight the equatorial oscillation periodicity.

These figures were produced so as to be comparable to ground-based observations and can be compared to ref. 2. the temperature was averaged between 100 and 200 Pa to account for the coarse vertical resolution of nadir observations, and were also averaged from 0 to ± 3° and from ± 10 and ± 12° of latitude to emulate the 3° spatial resolution of these observations. Then, we compute temperature differences between the average 0 − 3°N and the average 10 − 12°N (denoted by blue line) and between the average 0 − 3°S and the average 10 − 12°S (denoted by red line).

Extended Data Fig. 5 Seasonal activity of planetary-scale waves around ± 30° of latitude to highlight Rossby waves surf zones driving the main downward branch of the seasonal inter-hemispheric circulation.

Two-dimensional Fourier transforms32of the symmetric component of u - (a) and (b) northern winter from Ls=250° to Ls=340°, (c) and (d) northern spring from Ls=340° to Ls=70°, (e) and (f) northern summer from Ls=70° to Ls=160° and (g) and (h) northern autumn from Ls=160° to Ls=250°- at 100 Pa. Gray shaded areas depict the waves identified by the spectral analysis. Colour curves represent relation dispersion from the linear theory. Seven equivalent depths are used for theoretical curves: 5 km (cyan), 10 km (blue),20 km (purple), 50 km (magenta), 100 km (red), 200 km (light green), 500 km (dark green).

Extended Data Fig. 6 Equatorial eddy- and RMC-forcing during the northern winter season.

Northern winter seasonal mean (Ls=250°to Ls=340°) of RMC (a) and of the eddy-kinetic energy (b), defined by EKE =1/2(u’2+v’2), superimposed to the residual mean circulation (v*, ω* in streamlines) for a typical simulated year from DYNAMICO-Saturn. The associated spectra are shown in A.6c to identify waves involved in the interplay between Saturn’s equatorial oscillation and Saturn’s inter-hemispheric circulation.

Extended Data Fig. 7 Equatorial eddy- and RMC- forcing during the northern spring season.

Same as Extended Data Fig. 6 for northern spring seasonal mean (Ls=340° to Ls=70°).

Extended Data Fig. 8 Equatorial eddy- and RMC-forcing during the northern summer season.

Same as Extended Data Fig. 6 for northern summer seasonal mean (Ls=70° to Ls=160°).

Extended Data Fig. 9 Equatorial eddy- and RMC-forcing during the northern autumn season.

Same as Extended Data Fig. 6 for northern autumn seasonal mean (Ls=190° to Ls=280°). The seasonal mean for the autumn equinox is shift by 30° of Ls to clearly brought out the fall overturn, that occurs later is the season compared to the spring overturn.

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Bardet, D., Spiga, A. & Guerlet, S. Joint evolution of equatorial oscillation and interhemispheric circulation in Saturn’s stratosphere. Nat Astron 6, 804–811 (2022). https://doi.org/10.1038/s41550-022-01670-7

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