Wave-Mean Flow Interactions in the Atmospheric Circulation of Tidally Locked Planets

TL;DR

This paper uses a linear shallow-water model to analyze how wave-mean flow interactions shape the atmospheric circulation and hot-spot shifts on tidally locked planets, providing a new explanation for observed phenomena.

Contribution

It introduces a linear model highlighting wave-mean flow interactions as key to understanding atmospheric circulation on tidally locked planets, differing from previous advection-based explanations.

Findings

Eastward hot-spot shift explained by zonal flow Doppler-shifting stationary waves.

Model solutions match climate simulations in wind and height patterns.

Wave-mean flow interaction is crucial for equilibrium circulation understanding.

Abstract

We use a linear shallow-water model to investigate the global circulation of the atmospheres of tidally locked planets. Simulations, observations, and simple models show that if these planets are sufficiently rapidly rotating, their atmospheres have an eastward equatorial jet and a hot-spot east of the substellar point. We linearize the shallow-water model about this eastward flow and its associated height perturbation. The forced solutions of this system show that the shear flow explains the form of the global circulation, particularly the hot-spot shift and the positions of the cold standing waves on the night-side. We suggest that the eastward hot-spot shift seen in observations and 3D simulations of these atmospheres is caused by the zonal flow Doppler-shifting the stationary wave response eastwards, summed with the height perturbation from the flow itself. This differs from other studies which explained the hot-spot shift as pure advection of heat from air flowing eastward from the substellar point, or as equatorial waves travelling eastwards. We compare our solutions to simulations in our climate model Exo-FMS, and show that the height fields and wind patterns match. We discuss how planetary properties affect the global circulation, and how they change observables such as the hot-spot shift or day-night contrast. We conclude that the wave-mean flow interaction between the stationary planetary waves and the equatorial jet is a vital part of the equilibrium circulation on tidally locked planets.