Atmospheric Heat Redistribution on Hot Jupiters

TL;DR

This paper presents a two-layer shallow water model explaining how heat redistribution efficiency on hot Jupiters depends on dynamical mechanisms, with wave propagation timescales controlling the transition from small to large day-night temperature contrasts.

Contribution

The study introduces a new dynamical model and scaling theory that identify the role of gravity wave timescales in atmospheric heat redistribution on tidally locked exoplanets.

Findings

Weak friction and irradiation lead to minimal temperature differences.

Strong irradiation or friction cause large day-night temperature contrasts.

Wave propagation timescale t_wave governs the transition in heat redistribution efficiency.

Abstract

Infrared lightcurves of transiting hot Jupiters present a trend in which the atmospheres of the hottest planets are less efficient at redistributing the stellar energy absorbed on their daysides---and thus have a larger day-night temperature contrast---than colder planets. No predictive atmospheric model has been published that identifies which dynamical mechanisms determine the atmospheric heat redistribution efficiency on tidally locked exoplanets. Here we present a two-layer shallow water model of the atmospheric dynamics on synchronously rotating planets that explains the observed trend. Our model shows that planets with weak friction and weak irradiation exhibit a banded zonal flow with minimal day-night temperature differences, while models with strong irradiation and/or strong friction exhibit a day-night flow pattern with order-unity fractional day-night temperature differences. To interpret the model, we develop a scaling theory that shows that the timescale for gravity waves to propagate horizontally over planetary scales, t_wave, plays a dominant role in controlling the transition from small to large temperature contrasts. This implies that heat redistribution is governed by a wave-like process, similar to the one responsible for the weak temperature gradients in the Earth's tropics. When atmospheric drag can be neglected, the transition from small to large day-night temperature contrasts occurs when t_wave ~ sqrt(t_rad/Omega), where t_rad is the radiative relaxation time and Omega is the planetary rotation frequency. Alternatively, this transition criterion can be expressed as t_rad ~ t_vert, where t_vert is the timescale for a fluid parcel to move vertically over the difference in day-night thickness. These results subsume the commonly used timescale comparison for estimating heat redistribution efficiency between t_rad and the global horizontal advection timescale, t_adv.