Atmospheric Overturning Circulation on Dry, Tidally Locked Rocky Planets Is Mainly Driven by Radiative Cooling

Abstract

In this study, we examine the driving mechanism for the atmospheric overturning circulation on dry, tidally locked rocky planets without the condensation of water vapor or other species. We find that the main driving process is the radiative cooling of CO2 (or other noncondensable greenhouse gases) rather than CO2 greenhouse warming or stellar radiation. Stellar radiation is the ultimate mechanism but not the direct mechanism. Due to the combination of the uneven distribution in the stellar radiation and effective horizontal energy transports in the free troposphere, there is strong temperature inversion in the area away from the substellar region. This inversion makes CO2 have a radiative cooling effect rather than a radiative warming effect for the atmosphere, the same as that in the stratosphere of Earth’s atmosphere. This cooling effect produces negative buoyancy and drives large-scale downwelling, supporting the formation of a global-scale overturning circulation. If CO2 is excluded from the atmosphere, the overturning circulation becomes very weak, regardless of the level of stellar radiation. This mechanism is completely different from that for the atmospheric overturning circulation on Earth or on moist, tidally locked rocky planets, where latent heat release and/or baroclinic instability are the dominated mechanisms. Our study improves the understanding of the atmospheric circulation on tidally locked exoplanets and also on other dry planets, such as Venus and Mars in the solar system.

3D Global climate model of an exo-Venus: a modern Venus-like atmosphere for the nearby super-Earth LP 890-9 c

“…The dynamics obtained in our simulation with a modern Venuslike atmosphere substantially differ from previous simulations of tidally locked terrestrial planets with Earth-like atmospheres (surface pressure of 1 bar) but with similar stellar irradiation and rotation rate values to this work. These studies consistently found a single equatorial superrotation jet located at ∼500 mbar with speeds on the order of dozens of m s −1 (Penn & Vallis 2018 ;Hammond, Tsai & Pierrehumbert 2020 ;Hammond & Lewis 2021 ;Sergeev et al 2022 ;Wang & Yang 2022 ). Ho we ver, at least two distinct dynamical regimes can occur in these worlds (e.g.…”

Lorenz Energy Cycle: Another Way to Understand the Atmospheric Circulation on Tidally Locked Terrestrial Planets

“…Atmospheric compositions should also have impacts on the LEC. Ding & Wordsworth (2019) showed that excluding greenhouse gases from the atmosphere would decrease the day-night temperature contrast and wind speeds; Wang & Yang (2022) showed that the atmospheric circulation in a pure N 2 atmosphere without any greenhouse gas is very weak. They suggested that the potential and kinetic energy would be smaller in an atmosphere without any greenhouse gas.…”

“…For tidally locked terrestrial planets, especially slowly rotating ones, the Eulerian mean should match the monotonically decreasing stellar radiation and temperature along an arbitrary direction from dayside to nightside (Figure 2(d)). So, for slowly rotating, tidally locked terrestrial planets, the tidally locked coordinates are employed in this study, similar to that used in previous studies (e.g., Koll & Abbot 2015Ding & Pierrehumbert 2020;Ding & Wordsworth 2020;Hammond & Lewis 2021;Sergeev et al 2022;Turbet et al 2022;Wang & Yang 2022). In the tidally locked coordinates, the nominal "north/south pole" is the substellar/antistellar point, the tidally locked latitude lines are a series of concentric circles around the substellar point, and the tidally locked longitude lines are the great circles linking the substellar and antistellar points (Figure 2(b)).…”

“…The substellar point of 1:1 tidally locked (or synchronously rotating) terrestrial planets is fixed with time. Such a state can drive different atmospheric circulation compared to Earth, which has been simulated by general circulation models (GCMs) in previous studies (e.g., Joshi et al 1997;Merlis & Schneider 2010;Edson et al 2011;Leconte et al 2013;Wordsworth 2015;Koll & Abbot 2016;Noda et al 2017;Haqq-Misra et al 2018;Pierrehumbert & Hammond 2019;Hammond & Lewis 2021;Sergeev et al 2022;Turbet et al 2022;Wang & Yang 2022). These simulations showed that the atmospheric circulation on tidally locked terrestrial planets is dominated by a global-scale overturning circulation consisting of winds with upwelling in the substellar region, horizontally flowing from the dayside to the nightside in the upper troposphere, downwelling in the region away from the substellar point, and flowing back from the nightside to the dayside near the surface.…”