Topography-induced Stationary Waves and the Onset of Nightside Warming on Rocky Planets around M Dwarf Stars

Abstract Among potentially habitable worlds, rocky planets orbiting M dwarfs offer the most favorable prospects for atmospheric characterization, yet their climates may differ substantially from those of Earth analogs. In the tidally locked limit, the nightside’s tendency to radiatively cool and potentially trap volatiles as permanent ice introduces a strong dependence of habitability on the planet’s surface and atmospheric boundary conditions. We perform a suite of synchronously rotating exoplanet climate experiments spanning a series of topographic and orographic realizations with different mean elevation and landmass distributions. Across a grid of p N2 = 0.5–8 bar and F ⋆ = 1000–1700 W m −2 , we find that surface relief breaks the flow symmetry, replacing circumpolar vortices with mechanically forced stationary waves. For example, Steep orography produces coherent standing Rossby gyres that strengthen the cross-terminator jet and align vertical uplift with the day/night boundary. These new circulation regimes enhance moisture transport, increasing the infrared optical depth and promoting additional nightside cloud formation, which produces stronger cloud–greenhouse feedback, and ultimately lower the critical stellar flux required for global planetary deglaciation. Broad elevated plateaus drive a similarly fragmented but slightly weaker circulation, yielding less effective moisture transport. These results show that the relief and spatial distribution of landmasses, parameters that are unconstrained for most exoplanets, can exert strong controls on the climatic bifurcations of tidally locked M dwarf exoplanets.