Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars

Abstract

The distinctive feature of tidally locked exoplanets is the very uneven heating by stellar radiation between the dayside and nightside. Previous work has focused on the role of atmospheric heat transport in preventing atmospheric collapse on the nightside for terrestrial exoplanets in the habitable zone around M dwarfs. In the present paper, we carry out simulations with a fully coupled atmosphere-ocean general circulation model to investigate the role of ocean heat transport in climate states of tidally locked habitable exoplanets around M dwarfs. Our simulation results demonstrate that ocean heat transport substantially extends the area of open water along the equator, showing a lobster-like spatial pattern of open water, instead of an "eyeball." For sufficiently high-level greenhouse gases or strong stellar radiation, ocean heat transport can even lead to complete deglaciation of the nightside. Our simulations also suggest that ocean heat transport likely narrows the width of M dwarfs' habitable zone. This study provides a demonstration of the importance of exooceanography in determining climate states and habitability of exoplanets.planetary climate | exoplanet habitability | superEarth M dwarf stars are the most common type of star in the Universe (1). The habitable zone (HZ) around M stars is close to such stars because of their weak luminosity (2). In consequence, habitable exoplanets orbiting M stars are likely to be tidally locked to their primary stars, so that one side of tidallocking exoplanets permanently faces stars, and the other side remains dark. Previous studies have demonstrated the role of atmospheric heat transport in preventing atmospheric collapse on the nightside of terrestrial exoplanets located in the HZ of M stars (3-10). For a planet with an extensive ocean, its climate and habitability also involves ocean heat transport, which is known to be important in Earth's climate (11). None of the existing studies has considered the role of ocean heat transport. Moreover, the climate also involves the spatial distribution of open water versus ice and the question of whether the planet becomes locked in a globally glaciated Snowball state. Simulation with a comprehensive Earth atmospheric general circulation model (AGCM) coupled to a slab ocean, without dynamic ocean heat transport, revealed an "eyeball" climate state, with a round area of open ocean centered at the substellar point and complete ice coverage on the nightside, even for very high CO 2 concentrations (4). In the presence of sea ice, ocean heat transport is likely to be especially important, because it is known from studies of the Snowball Earth phenomenon in Earth-like conditions that ocean heat transport is very effective in holding back the advance of the sea-ice margin (12-14). The distribution of sea ice on tidally locked exoplanets is only an issue for M stars, because planets with sufficiently dense atmospheres orbiting hotter stars in orbits close enough to yield tidal locking are likely to be too hot to permit ice and may ...

Demarcating Circulation Regimes of Synchronously Rotating Terrestrial Planets within the Habitable Zone

“…But subsequent investigations with simplified climate models (Haberle et al 1996) and general circulation models (GCMs) (Joshi et al 1997;Joshi 2003;Merlis & Scheider 2010;Edson et al 2011;Showman et al 2010Showman et al , 2013Leconte et al 2013;Yang et al 2013Cullum et al 2014;Hu & Yang 2014;Carone et al 2014Carone et al , 2015Carone et al , 2016Wordsworth 2015;Way et al 2016;Kopparapu et al 2016Kopparapu et al , 2017Noda et al 2017;Fujii et al 2017) have demonstrated that energy transport from the day to night hemisphere is generally sufficient to avoid atmospheric collapse across a wide range of atmospheric compositions and rotation periods.…”

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…The zonal oceanic heat transport caused by the dynamical ocean circulation was found to reduce the seaice thickness on the nightside to 5 m (Hu and Yang 2014) compared to a sea-ice thickness of 100 m in the uncoupled simulations (Pierrehumbert 2010). However, their synchronous rotation period is 36.7 Earth-days, while ours is 365 Earth-days.…”

“…However, their synchronous rotation period is 36.7 Earth-days, while ours is 365 Earth-days. We therefore expect a smaller day-tonight oceanic heat transport in our case because the geostrophic balance breaks down at longer rotation period, and thus the ocean currents won't be necessarily zonal as in Hu and Yang (2014). Nonetheless, we expect that ocean circulation would reduce the temperature gradients and the sea-ice cover for non-synchronous rotations.…”

“…In order to overcome the limitations of a fixed depth mixed-layer ocean, Hu and Yang (2014) have performed a full atmosphere-ocean coupled aquaplanet simulation with the orbital parameters of the tidally-locked planet Gliese 581g. The zonal oceanic heat transport caused by the dynamical ocean circulation was found to reduce the seaice thickness on the nightside to 5 m (Hu and Yang 2014) compared to a sea-ice thickness of 100 m in the uncoupled simulations (Pierrehumbert 2010).…”

Importance of ocean salinity for climate and habitability

“…There are many studies describing simulations of exoplanetary atmospheric circulations and climate (4), but most incorporate either no ocean (5-7) or a heavily simplified slab ocean (8)(9)(10)(11)(12). Only recently have studies considered a dynamical ocean (13)(14)(15)(16), and even in these studies, the ocean is consistently assumed to have similar properties to the oceans on Earth. Such an assumption may prove to be an oversimplification.…”