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

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