Effects of salinity on tidally-locked aqua planets’ climate with a coupled atmosphere and ocean GCM

 Habitable terrestrial exoplanets around M dwarfs are remarkable targets due to the ease to detect and characterize. These planets are thought to have a different climate from solar system planets because these planets are expected to be in a tidally-locked state due to closer orbits and strong tidal forces. According to the studies of planet formation, terrestrial exoplanets around M dwarfs could sustain a large amount of surface water (tens or hundreds of Earth ocean mass). Such planets with large amount water (deep ocean) would be globally ocean covered, resulting in no continents.

 Earth’s ocean circulation largely affects the climate (e.g. meridional heat transport, water cycle). Ocean circulation is roughly classified into wind-driven circulation and density-driven circulation. In particular, density-driven circulation, which passes deep ocean, is affected by sea water temperature and salinity, resulting from heat exchange to atmosphere and salinity change by precipitation or sea ice formation. Although salinity has large influences on ocean behavior such as strength of ocean circulation and condition of sea ice formation, this effect on tidally-locked ocean planet has been still unknown.

 To investigate the influence on climate by salinity, first, we developed an atmospheric and oceanic global climate model (AOGCM) for exoplanets based on MIROC4m. Both atmospheric part and oceanic part solve 3-dimensional hydrodynamics, thermodynamics, and tracer transports (such as water vapor in the atmosphere and salinity in the ocean), and a coupler exchanges the information about the sea surface between both parts for example radiative heating and evaporation. By using the AOGCM, we simulated the TRAPPIST-1e’s climate assuming  an ocean covered planet. We set 1 Earth ocean mass (3200 m depth because of its planetary radius) with Earth-like salinity (35.4 psu) or pure water (0 psu). In addition to the AOGCM simulations, we also run AGCM simulation which includes slab ocean instead of ocean GCM part.  We integrated over 2000 years for AOGCM simulations and 50 years for AGCM simulation.

 An eyeball-shape open sea appears on the dayside according to the AGCM simulation, however, AOGCM simulations result in slushball-shape open sea due to the equatorial ocean currents. Comparing the results of AOGCM results, we found that salinity expands open sea distribution due to freezing-point depression and affects intensity of surface ocean current and vertical circulation. In this presentation, we will show the physical mechanisms of these differences by salinity.