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

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 hotspot 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 hotspot shift and the positions of the cold standing waves on the nightside. We suggest that the eastward hotspot 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 that explained the hotspot shift as pure advection of heat from air flowing eastwards from the substellar point, or as equatorial waves traveling 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 hotspot 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.