[Paper Review] Three-dimensional Atmospheric Circulation of Warm and Hot Jupiters: Effects of Orbital Distance, Rotation Period, and Non-Synchronous Rotation

Adam P. Showman, Nikole K. Lewis+1 more|arXiv (Cornell University)|Nov 18, 2014

Stellar, planetary, and galactic studies1 references4 citations

TL;DR

This study presents 3D atmospheric circulation models of warm and hot Jupiters across a wide range of orbital distances, rotation rates, and stellar fluxes, revealing a dynamical transition from superrotating equatorial jets (driven by day-night heating) to midlatitude eastward jets (driven by baroclinic instabilities) as rotation increases and irradiation decreases. The key contribution is identifying how infrared light curves and spectra can distinguish these circulation regimes, enabling rotation rate and heating regime inference from future observations.

ABSTRACT

Efforts to characterize extrasolar giant planet (EGP) atmospheres have so far emphasized planets within 0.05 AU of their stars. Despite this focus, known EGPs populate a continuum of orbital separations from canonical hot Jupiter values (0.03-0.05 AU) out to 1 AU and beyond. Unlike typical hot Jupiters, these more distant EGPs will not in general be synchronously rotating. In anticipation of observations of this population, we here present three-dimensional atmospheric circulation models exploring the dynamics that emerge over a broad range of rotation rates and incident stellar fluxes appropriate for warm and hot Jupiters. We find that the circulation resides in one of two basic regimes. On typical hot Jupiters, the strong day-night heating contrast leads to a broad, fast superrotating (eastward) equatorial jet and large day-night temperature differences. At faster rotation rates and lower incident fluxes, however, the day-night heating gradient becomes less important, and baroclinic instabilities emerge as a dominant player, leading to eastward jets in the midlatitudes, minimal temperature variations in longitude, and, in many cases, weak winds at the equator. Our most rapidly rotating and least irradiated models exhibit multiple eastward jets in each hemisphere--similar to the jets on Jupiter and Saturn--and illuminate the dynamical continuum between highly irradiated EGPs and the weakly irradiated giant planets of our own Solar System. We present infrared (IR) light curves and spectra of these models, which show that the amplitude and offset of the IR phase variation, as well as the shape of the spectra, depend significantly on incident flux and rotation rate. This provides a way to identify the regime transition in future observations and suggests that, in some cases, IR light curves can provide constraints on the rotation rate of non-synchronously rotating planets.

Motivation & Objective

• To investigate atmospheric circulation dynamics in warm and hot Jupiters beyond the canonical synchronous hot Jupiter regime.
• To determine how orbital distance, rotation rate, and non-synchronous rotation affect atmospheric circulation patterns.
• To explore the transition between circulation regimes dominated by day-night heating contrast versus baroclinic instabilities.
• To predict observable signatures—specifically infrared light curves and spectra—linked to different circulation regimes for future observational comparison.
• To assess the potential of phase curve variations to constrain planetary rotation rates in non-synchronously rotating exoplanets.

Proposed method

• Numerical simulations using a 3D general circulation model (GCM) to solve the primitive equations of atmospheric dynamics on a spherical planet.
• Systematic variation of stellar flux (from 0.01 to 10 times Earth's insolation) and rotation period (from 1.5 to 15 days) to span warm and hot Jupiter conditions.
• Incorporation of non-synchronous rotation by adjusting the rotation rate relative to the orbital period, simulating planets not locked in tidal lock.
• Modeling includes radiative transfer, thermal tides, and non-hydrostatic dynamics to capture wave-mean flow interactions and baroclinic instability.
• Synthesis of synthetic infrared light curves and spectra across multiple wavelengths to simulate observable emission features.
• Analysis of zonal wind profiles, temperature structures, and wave activity to classify circulation regimes based on Rossby number and heating mechanisms.

Experimental results

Research questions

• RQ1 How does the atmospheric circulation of warm and hot Jupiters change with increasing rotation rate and decreasing stellar flux?
• RQ2 What dynamical mechanisms—day-night heating contrast or baroclinic instabilities—dominate in different regions of parameter space?
• RQ3 Can infrared light curves and spectra distinguish between circulation regimes driven by thermal contrast versus baroclinic instability?
• RQ4 To what extent can non-synchronous rotation be inferred from phase curve variations in secondary eclipse observations?
• RQ5 How do the circulation patterns of weakly irradiated, rapidly rotating exoplanets compare to those of Jupiter and Saturn?

Key findings

• The circulation transitions from a superrotating equatorial jet regime (driven by strong day-night heating) to a midlatitude jet regime (driven by baroclinic instabilities) as rotation rate increases and stellar flux decreases.
• At fast rotation rates and low flux, models exhibit multiple eastward jets in each hemisphere, resembling Jupiter and Saturn, indicating a dynamical continuum between hot Jupiters and ice/gas giants.
• Equatorial winds weaken or reverse in direction in the fast-rotating, weakly irradiated regime, while midlatitude jets become dominant and eastward.
• Infrared light curves show significant phase variations only at high stellar flux and slow rotation, with amplitude decreasing by a factor of ~2 for a factor-of-two change in rotation rate.
• Spectral variations in phase curves are largest in absorption bands (low-pressure regions with strong day-night temperature contrasts), while windows (higher-pressure regions) show smaller variations.
• Non-synchronous rotation with deviations greater than a factor of two from synchronous rotation can be detectable in light curves, enabling observational inference of rotation rate.