Mechanisms of Superrotation in Slowly-Rotating and Tidally-Locked Planets

Abstract

Superrotation is a common feature of quickly rotating gas giants, slowly rotating planetary bodies, and tidally-locked planets. In this paper we compare and contrast the mechanisms of superrotation in slow rotators and tidally-locked planets. We cover a wide range of planetary properties, varying in particular the thermal Rossby number Ro_T (controlled by planetary size, rotation rate, and instellation) and a radiative relaxation timescale T_rad (which parameterizes atmospheric optical thickness). We use a two-level model that contains the principal mechanisms for superrotation in both regimes yet remains analytically tractable. Linearizations of the model elucidate the behavior of superrotation-inducing eddies. In tidally-locked planets a Matsuno-Gill-like structure organizes the eddy effects but of itself is insufficient to produce superrotation; baroclinicity and low-level drag are additional essential ingredients. Nonlinear integrations further explore the superrotating regimes and exhibit significant time variability even in statistical equilibrium. Not all tidally-locked regimes superrotate: subrotation arises at high T_rad (optically thick atmospheres) and weak low-level drag. On axisymmetrically-forced slow rotators, superrotation is always linked to a previously identified Rossby-Kelvin instability. Perhaps surprisingly, the instability itself is also linked to the spinup of superrotation in some tidally-locked regimes. Finally, we explore the continuous transition in the mechanisms of superrotation from axisymmetrically-forced to tidally-locked planets by applying a progressively stronger asymmetric equatorial forcing. The Matsuno-Gill pattern quickly dominates over traveling planetary Rossby-Kelvin waves in forcing superrotation, although both mechanisms can coexist. These results provide a unified view of superrotation mechanisms across a wide range of planetary bodies.

Submitted to the Planetary Science Journal

References in corpus (15)

• Accurate, Empirical Radii and Masses of Planets and their Host Stars with Gaia Parallaxes
• Atmospheric Circulation of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing
• Tidal Locking of Habitable Exoplanets
• Atmospheric dynamics of Earth-like tidally locked aquaplanets
• 3D Structures of equatorial waves and the resulting superrotation in the atmosphere of a tidally locked hot Jupiter
• Linking the Climate and Thermal Phase Curve of 55 Cancri e
• Comparison of the deep atmospheric dynamics of Jupiter and Saturn in light of the Juno and Cassini gravity measurements
• Temporal Variability in Hot Jupiter Atmospheres
• A Low-order Model of Water Vapor, Clouds, and Thermal Emission for Tidally Locked Terrestrial Planets
• JWST Thermal Emission of the Terrestrial Exoplanet GJ 1132b
• Atmospheres on Nonsynchronized Eccentric-tilted Exoplanets I: Dynamical Regimes
• The Equatorial Jet Speed on Tidally Locked Planets: I -- Terrestrial Planets
• Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics
• Traveling planetary-scale waves cause cloud variability on tidally locked aquaplanets
• In-depth characterization of the Kepler-10 three-planet system with HARPS-N radial velocities and Kepler transit timing variations