Bistability of the Atmospheric Circulation on TRAPPIST-1e

Abstract

Using a 3D general circulation model, we demonstrate that a confirmed rocky exoplanet and a primary observational target, TRAPPIST-1e presents an interesting case of climate bistability. We find that the atmospheric circulation on TRAPPIST-1e can exist in two distinct regimes for a 1 bar nitrogen-dominated atmosphere. One is characterized by a single strong equatorial prograde jet and a large day-night temperature difference; the other is characterized by a pair of mid-latitude prograde jets and a relatively small day-night contrast. The circulation regime appears to be highly sensitive to the model setup, including initial and surface boundary conditions, as well as physical parameterizations of convection and cloud radiative effects. We focus on the emergence of the atmospheric circulation during the early stages of simulations and show that the regime bistability is associated with a delicate balance between the zonally asymmetric heating, mean overturning circulation, and mid-latitude baroclinic instability. The relative strength of these processes places the GCM simulations on different branches of the evolution of atmospheric dynamics. The resulting steady states of the two regimes have consistent differences in the amount of water content and clouds, affecting the water absorption bands as well as the continuum level in the transmission spectrum, although they are too small to be detected with current technology. Nevertheless, this regime bistability affects the surface temperature, especially on the night side of the planet, and presents an interesting case for understanding atmospheric dynamics and highlights uncertainty in 3D GCM results, motivating more multimodel studies.

The Effect of Interior Heat Flux on the Atmospheric Circulation of Hot and Ultra-hot Jupiters

“…As in Komacek et al (2022), all simulations contain a weak Rayleigh drag throughout the domain with a characteristic drag timescale τ drag,uniform = 10 7 s. Similarly to Liu & Showman (2013) and Komacek & Showman (2016), each simulation has a deep basal drag with a characteristic timescale that decreases linearly in pressure from τ drag,basal = ∞ at 10 bars to a characteristic drag of τ drag,basal = 10 5 s at the bottom of the domain (analogous to Equation (12) of Komacek & Showman 2016). This drag is applied in order to ensure that each simulation can reach an equilibrated state near the photosphere independent of initial conditions (Liu & Showman 2013), limiting any potential hysteresis (Thrastarson & Cho 2010;Sergeev et al 2022). The resulting specific drag force applied in the momentum equation at each individual pressure level is then t t = -v min , drag drag,uniform drag,basal…”

Large Interferometer For Exoplanets (LIFE)

“…Such studies are an example of potential future synergies between atmospheric retrievals and the theoretical modeling of atmospheres. However, recent intercomparison efforts show that both retrievals (e.g., ) and atmospheric modeling (e.g., Sergeev et al 2022) depend on the model choice. Community efforts, such as the CUISINES Working Group 9 , that benchmark, compare, and validate different models, are vital to the studies proposed above.…”

General Circulation Model Constraints on the Detectability of the CO2-CH4 Biosignature Pair on TRAPPIST-1e with JWST

“…It is also worth mentioning that, due to TRAPPIST-1e's position on the border between two different dominant rotation dynamics, there are fundamental differences between different GCM simulations in terms of atmospheric variability timescales (Fauchez et al 2022;Turbet et al 2022) and even basicstate climate (Sergeev et al 2022b). These can lead to disagreements of up to 50% on the number of transits needed to reach a 5σ spectral detection (Fauchez et al 2022).…”

“…For tidally locked planets such as TRAPPIST-1e, it is expected that the climate is characterized by day-night temperature contrasts, with a hot and cloudy substellar point and a cold and less cloudy antistellar point (Merlis & Schneider 2010;Showman et al 2013;Yang et al 2013;Koll & Abbot 2016). TRAPPIST-1e in particular is a special case that falls between the Rhines (or intermediate) rotator and the fast rotator regimes (Showman et al 2013;Haqq-Misra et al 2018;Sergeev et al 2022b). As such, it is expected to contain aspects of both regimes, depending on atmospheric composition; while the day-night temperature contrast should be large as in intermediate-rotating tidally locked planets, there may also be a superrotating eastward jet in the tropics, with large cyclonic eddies in the extratropics formed by the day-night contrast causing a global standing wave pattern, as expected on a fast rotator (Showman et al 2013).…”

“…In this case, the weak temperature gradient parameter, which encapsulates the effect of rotation on planetary-scale atmospheric dynamics, is Λ ≈ 2, as predicted in Pierrehumbert & Hammond (2019) for a Rhines rotator tidally locked planet 1 ( ) L > . However, the climate state of this model includes a superrotating equatorial jet, as predicted for a fast rotator, which arises from TRAPPIST-1e's unique bistability given its location at the edge of the fast and Rhines rotator regimes (Haqq-Misra et al 2018;Sergeev et al 2022b).…”