Atmospheric Circulation of Brown Dwarfs and Jupiter- and Saturn-like Planets: Zonal Jets, Long-term Variability, and QBO-type Oscillations

Abstract

Brown dwarfs and directly imaged giant planets exhibit significant evidence for active atmospheric circulation, which induces a large-scale patchiness in the cloud structure that evolves significantly over time, as evidenced by infrared light curves and Doppler maps. These observations raise critical questions about the fundamental nature of the circulation, its time variability, and the overall relationship to the circulation on Jupiter and Saturn. Jupiter and Saturn themselves exhibit numerous robust zonal (east-west) jet streams at the cloud level; moreover, both planets exhibit long-term stratospheric oscillations involving perturbations of zonal wind and temperature that propagate downward over time on timescales of ∼4 years (Jupiter) and ∼15 years (Saturn). These oscillations, dubbed the Quasi Quadrennial Oscillation (QQO) for Jupiter and the Semi-Annual Oscillation (SAO) on Saturn, are thought to be analogous to the Quasi-Biennial Oscillation (QBO) on Earth, which is driven by upward propagation of equatorial waves from the troposphere. To investigate these issues, we here present global, three-dimensional, high-resolution numerical simulations of the flow in the stratified atmosphere-overlying the convective interior-of brown dwarfs and Jupiter-like planets. The effect of interior convection is parameterized by inducing small-scale, randomly varying perturbations in the radiative-convective boundary at the base of the model. Radiative damping is represented using an idealized Newtonian cooling scheme. In the simulations, the convective perturbations generate atmospheric waves and turbulence that interact with the rotation to produce numerous zonal jets. Moreover, the equatorial stratosphere exhibits stacked eastward and westward jets that migrate downward over time, exactly as occurs in the terrestrial QBO, Jovian QQO, and Saturnian SAO. This is the first demonstration of a QBO-like phenomenon in 3D numerical simulations of a giant planet.

Global climate modeling of Saturn's atmosphere. Part II: Multi-annual high-resolution dynamical simulations

“…This "stacked jets" equatorial signature is similar to the jet structure putatively associated with the equatorial oscillation of temperature (Fouchet et al, 2008;Guerlet et al, 2011). Nevertheless, our model does not reproduce the downward propagation of the observed equatorial oscillation in Saturn's stratosphere , also obtained by idealized simulations (Showman et al, 2018b). Furthermore, the modeled temperature contrasts between the equator and latitudes ±15 • associated with the stacked jets (∼ ±5−10 K, figure not shown) are much lower than the contrasts obtained by Cassini thermal infrared measurements (±15 − 20 K, Fouchet et al, 2008;Guerlet et al, 2018).…”

“…Most of those existing GCM studies for Saturn address the formation of tropospheric jets by angular momentum transfer through eddies and waves, often with either a theoretical approach aiming to address giant planets' atmospheric dynamics (Schneider and Liu, 2009;Lian and Showman, 2010;Schneider, 2010, 2015) rather than a focused approach aiming to address Saturn specifically, or with a limited-domain approach using a latitudinal channel enclosing one specific jet to explain structures such as the Ribbon wave or the String of Pearls (Sayanagi et al, 2010(Sayanagi et al, , 2014, to investigate the impact of convective outbursts (Sayanagi and Showman, 2007;García-Melendo et al, 2013), or to discuss the polar hexagonal jet (Morales-Juberías et al, 2011. The idealized GCM approach can also be employed to study equatorial oscillations in gas giants (Showman et al, 2018b). All those existing studies use simple radiative forcing rather than computing a realistic "physical package" that includes seasonally-varying radiative transfer.…”

Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

“…The actual rotational modulation profile may evolve with time, as seen in long-baseline observations of multiple L/T transition brown dwarfs (e.g., Apai et al 2017), most prominently detected in all three brown dwarfs (2M2139, 2M1324, and SIMP0136) monitored by the Spitzer Space Telescope in the Extrasolar Storms program (Yang et al 2016;Apai et al 2017). That study shows that the light curve evolution is the likely result of planetary-scale waves that modulated surface brightness (Apai et al 2017), possibly through the interplay of atmospheric circulations, condensations, and cloud formation/dispersal (Tan & Showman 2017Showman et al 2019). These mechanisms may also be present in GU Psc b and their presence could be revealed by continuous observations over 3-4 rotational periods.…”

Global climate modeling of Saturn’s atmosphere. Part IV: Stratospheric equatorial oscillation

“…A fully three-dimensional model to study in detail the global troposphere-to-stratosphere circulation in gas giants has only been recently developed, because of the huge computational resources required to resolve eddies arising from hydrodynamical instabilities that putatively force equatorial oscillations. Showman et al (2019) were the first to show the development of a QBO-like oscillation in an idealized 3D global primitive-equation model for gas giants and brown dwarfs. Their model uses a random wave forcing parameterization at the radiative-convective boundary to drive equatorial oscillations, and a Newtonian scheme to represent radiative heating/cooling in the thermodynamics energy equation.…”

“…Their model uses a random wave forcing parameterization at the radiative-convective boundary to drive equatorial oscillations, and a Newtonian scheme to represent radiative heating/cooling in the thermodynamics energy equation. A stack of eastward and westward jets that migrate downward over time is created in the stratosphere of Showman et al (2019)'s model, analogous to Earth's QBO, with a range of periods similar to Jupiter's and Saturn's equatorial oscillations. Nevertheless, the QBO-like oscillations depicted in their model occur at higher pressure (between 10 5 and 10 3 Pa) than the observations (Saturn's equatorial oscillation extends from 10 3 to 1 Pa).…”