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 experiments spanning a wide range of topographic and orographic realizations with different mean elevations and landmass distributions. Across a grid of $p_{\mathrm{N2}} = 0.5$-$8~\mathrm{bar}$ and $F_{\star} = 1200$-$1700~\mathrm{W\,m^{-2}}$, we find that surface relief breaks the flow symmetry, replacing the circumpolar vortices with mechanically forced stationary waves. Steep orography produces 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 a stronger cloud-greenhouse feedback and lower the critical fluxes 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 unconstrained for most exoplanets, can exert strong controls on the climatic bifurcations of tidally locked M-dwarf exoplanets.

Figures (11)

• Figure 1: Surface boundary configurations used in this study. Middle column: map view of continental distribution and topography with the substellar point centered at 0° longitude, or on the dayside. Left column: identical surface realizations centered on the nightside. Right column: corresponding three-dimensional renderings of the topography. Rows show (top) Baseline flat continent, (middle) Steep Uplift, and (bottom) Dayside Plateau. Although the mean elevation is comparable in the bottom two cases, the spatial distribution of relief differs substantially and sets up distinct stationary-waves and cross-terminator flow in the simulations.
• Figure 2: Open-water fraction across the $(F_\star, p_{\mathrm{N_2}})$ parameter grid. Color indicates the global fractional area that remains ice-free at equilibrium, with darker shades marking more extensive open-water regions. At low incident flux ($1200$--$1400~\mathrm{W\,m^{-2}}$), all simulations remain predominantly glaciated. At $1600~\mathrm{W\,m^{-2}}$, atmospheres exceeding a few bars develop measurable open-water areas, marking the onset of transitional climates. These maps summarize the broader parameter sweep referenced in Sections 3.1--3.2 and provide the context for the detailed single-forcing analyses presented later in the manuscript.
• Figure 3: Equilibrium sea-ice thickness for the three topographies. The panels shown are for: (Left) Baseline, (center) Steep Uplift, and (right) Plateau. Topographic control of the circulation strongly modulates volatile sequestration. The Steep Uplift case lead to open nightside ocean, while the Plateau tends reduced nightside ice and patchy thin ice emcompassing the antistellar point. Gray outlines mark the sea ice versus open water divide. Maximum sea ice is set at 9 m in the model.
• Figure 4: Time-mean total cloud cover. The panels shown are for: (Left) Baseline, (center) Steep Uplift, and (right) Plateau. Orography enhances ascent and cloudiness along the day–night boundary in the Steep Uplift run, producing a broader, darker band of clouds across the western terminator and nightside mid-latitudes.
• Figure 5: Net longwave flux at the planetary surface. Panels: (a) Baseline, (b) Steep Uplift, (c) Plateau. Topography enhances nightside cloud cover, increasing downwelling longwave flux and warming the surface. Unlike the clear‑sky Baseline, both the Steep Uplift and Plateau cases exhibit broader warming centered near the anti‑stellar point. The Steep Uplift case alone shows a distinct “red‑tongue” warming feature, driven by a stationary Rossby wave that channels moist air eastward.l.