Exo-oceanography, climate, and habitability of tidal-locking exoplanets in the habitable zone of M dwarfs

The distinctive nature of tidal-locking exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Thus, the permanent nightside can be extremely cold. It had been worried about that atmosphere and water could be condensed on the nightside of habitable exoplanets around M dwarfs. Previous studies have demonstrated that atmospheric circulations are able to transport sufficient heat to warm the nightside and prevent atmosphere collapse there. However, it remains a question of how ocean heat transports and sea-ice feedbacks play important roles in determining climates and habitability of such kind of exoplanets and whether water could be completely frozen on the nightside. Here, we apply a coupled atmospheric and oceanic general circulation model and a three-dimensional ice-sheet model to this problem. It is found that oceanic zonal heat transport plays important roles in determining climate states of habitable aqua-exoplanets orbiting M-type stars. For sufficiently high greenhouse gas levels, the nightside can be completely ice free due to ocean heat transport. Futhermore, we show that for an ocean planet surface winds drive sea ice toward the dayside and the ocean carries heat toward the nightside, both of which keep the nightside sea ice thin. Our results show that the thickness of nightside sea ice is only about 10 m or less. Thus, nightside water trapping on a water-world should not be significant. We also test whether a large ice sheet could grow on a nightside super-continent using an ice sheet model driven by the climate model output. We find that for weak precipitation generated by the climate model the ice-sheet thickness is strongly dependent on the geothermal heat flux, and could reach 1-2 km if the geothermal heat is similar to Earth's or lower. These suggest that complete nightside water trapping would not happen if exoplanets have fairly deep and extensive oceans.