[Paper Review] Angular Momentum and Heat Transport on Tidally Locked Hot Jupiter Planets

João M. Mendonça|arXiv (Cornell University)|Oct 23, 2019

Astro and Planetary Science65 references43 citations

TL;DR

This study uses a 3D atmospheric general circulation model to show that the semi-diurnal thermal tide—driven by intense stellar irradiation—dominates angular momentum transport in tidally locked hot Jupiters, coupling with the mean circulation to drive a strong prograde equatorial jet. The key finding is that this mechanism, not the diurnal tide, is primarily responsible for the jet, and the model reveals multiple steady-state solutions, including indirect atmospheric cells that transport heat and momentum vertically from the upper to lower atmosphere, acting as a global heat pump.

ABSTRACT

The atmospheric circulation in the upper atmosphere of hot Jupiter planets is strongly influenced by the incoming stellar radiation. In this work we explore the results from a 3D atmospheric model and revisit the main processes driving the circulation in hot Jupiter planets. We use the angular momentum transport as a diagnostic and carry out a Fourier analysis to identify the atmospheric waves involved. We find that the coupling between the angular momentum transported horizontally by the semi-diurnal tide and the mean circulation is the mechanism responsible for producing the strong jet at low latitudes. Our simulations indicate the possible formation of atmospheric indirect cells at low latitudes. The formation of these cells is induced by the presence of the semi-diurnal tide that is driven by the stellar irradiation. The tropical circulation has an important impact transporting heat and momentum from the upper towards the lower atmosphere. One of the consequences of this heat and momentum transport is a global increase of the temperature. We show that the initial conditions do not affect the output of the reference simulation. However, when the period of rotation of the planet was increased ($P_{rot} > 5$ Earth days), vertical transport by stationary waves became stronger, transient waves became non-negligible, and Coriolis influence less dominant, which allowed a steady state with a strong retrograde jet to be stable. We found that at least two statically steady state solutions exist for the same planet parameters.

Motivation & Objective

• To identify the dominant physical mechanisms driving atmospheric circulation in tidally locked hot Jupiters.
• To investigate the role of angular momentum transport in forming strong equatorial jets.
• To determine the influence of planetary rotation rate on circulation stability and multiple steady-state solutions.
• To explore the formation and impact of indirect atmospheric cells induced by the semi-diurnal tide.
• To assess the sensitivity of the circulation to initial conditions and model parameters such as rotation period and dissipation.

Proposed method

• A 3D quasi-hydrostatic general circulation model (GCM) with a gray radiative transfer scheme and convective adjustment was used.
• The model employed a simplified dynamical core based on the HadAM3 framework, enabling efficient numerical integration.
• Fourier analysis was applied to decompose wave components and identify contributions from stationary, transient, and mean flows to momentum and heat transport.
• Angular momentum transport was diagnosed via vertical and horizontal fluxes: [w][M] for mean circulation, [w*][M*] for stationary waves, and [w'][M'] for transient waves.
• Simulations were run from rest and from initial retrograde wind states to test solution multiplicity and stability.
• The model was run for up to 26,500 Earth days to reach equilibrium, with output analyzed over the final 100 days.

Experimental results

Research questions

• RQ1 What is the dominant mechanism responsible for angular momentum transport that drives the strong prograde equatorial jet in tidally locked hot Jupiters?
• RQ2 How does the semi-diurnal thermal tide influence the formation of indirect atmospheric cells and their role in vertical momentum and heat transport?
• RQ3 Can multiple steady-state solutions exist for the same planetary parameters, and what conditions stabilize them?
• RQ4 How does the rotation period affect the relative importance of stationary, transient, and mean circulation in momentum transport?
• RQ5 What is the role of wave-induced vertical angular momentum flux in maintaining the jet and coupling upper and lower atmospheric layers?

Key findings

• The semi-diurnal thermal tide, not the diurnal tide, is the dominant driver of angular momentum transport in the equatorial jet, with its horizontal structure identified as a coupled equatorial Rossby-Kelvin wave system.
• The coupling between the semi-diurnal tide and the mean circulation generates a pressure torque that transports angular momentum up-gradient toward low latitudes, accelerating the prograde jet.
• Indirect atmospheric cells form at low latitudes due to large eddy momentum convergence from the semi-diurnal tide, enabling vertical transport of heat and momentum from the upper to lower atmosphere.
• These indirect cells act as a global heat pump, depositing energy in deeper atmospheric layers and increasing the global temperature.
• When the rotation period exceeds 5 Earth days, vertical transport by stationary and transient waves strengthens, allowing multiple steady-state solutions to coexist, including a stable retrograde jet.
• Initial conditions do not affect the final state in the reference simulation, but solution multiplicity emerges at longer rotation periods, implying that observational light curves could reflect multiple dynamical states.