Longitudinally Asymmetric Stratospheric Oscillation on a Tidally Locked Exoplanet

Abstract

Using a three-dimensional general circulation model, we show that the atmospheric dynamics on a tidally locked Earth-like exoplanet, simulated with the planetary and orbital parameters of Proxima Centauri b, support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth’s quasi-biennial oscillation (QBO). In our simulations, the LASO has a vertical extent of 35–55 km, a period of 5–6.5 months, and a peak-to-peak wind speed amplitude of −70 to +130 m s−1 with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localized in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapor and trace gases such as ozone, which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundances of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets.

Sporadic Spin-orbit Variations in Compact Multiplanet Systems and Their Influence on Exoplanet Climate

“…For instance, the impact of stellar UV flare events (Paudel et al 2021;Howard 2022;Louca et al 2022) and stellar plasma (e.g., protons and α-particles; Tilley et al 2019;Chen et al 2021a) on varying sides of the hemisphere may lead to different global chemical and particle precipitation rates. Moreover, distinct atmospheric dynamics due to oscillating planetary spin (Figure 5) could drive the planet into other transport regimes (Carone et al 2018;Chen et al 2019) and change how chemical species and aerosols are advected from the dayside to the nightside (Boutle et al 2020;Cohen et al 2022). Modulations in the vertical mixing region and strength would lead to altered photolysis rates and shallower flux-abundance curves of biosignature gases.…”

“…Because of the small orbital separation (semimajor axes) of worlds around M-dwarfs, theory suggests that they will be locked in a synchronized state due to strong tidal forces from the host star. As such, the majority of previous work using three-dimensional (3D) general circulation model (GCM) to simulate M-dwarf planets assumed 1:1 spin-orbit rotation ratios for planets with short (50 days) orbital periods (e.g., Way et al 2016;Kopparapu et al 2017;Chen et al 2019;Yang et al 2019;Guzewich et al 2020;Joshi et al 2020;Cohen et al 2022;Hammond & Lewis 2021;Lefèvre et al 2021;Braam et al 2022;Sergeev et al 2022;Wolf et al 2022). This assumption is acceptable due to the computational expense and time-consuming nature of many state-of-the-science GCMs.…”

Lightning-induced chemistry on tidally-locked Earth-like exoplanets

“…From Figure 2 we can see that deep convection occurs around the substellar point and falls off radially. Furthermore, deep convective mixing tends to occur more intensely westward of the substellar point, coincident with a source of gravity waves (Cohen et al 2022). Due to a decreasing depth of convection as a function of radial distance from the substellar point, lower-altitude clouds become more frequent (Boutle et al 2017;Sergeev et al 2020).…”

“…The circulation regime also varies with orbital period (e.g. Merlis & Schneider 2010;Edson et al 2011;Carone et al 2015Carone et al , 2018 and time-dependent wave phenomena can further impact our ability to interpret observations (Cohen et al 2022). Yang et al (2013) show that the dayside is covered by a thick cloud deck, resulting from vigorous convection centred at the substellar point.…”

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

“…Significant temporal variability is expected in the climates of tidally locked planets with rotation periods similar to TRAPPIST-1e, although its mechanism may be difficult to identify and is likely related to planetary-scale wave propagation (Pierrehumbert & Hammond 2019;Song & Yang 2021;Cohen et al 2022a). Additionally, a longitudinally asymmetric stratospheric oscillation, analogous to Earth's quasi-biennial oscillation and caused by vertical propagation of gravity waves, emerges in general circulation models (GCMs) of Proxima Centauri b, an intermediate-rotating tidally locked rocky planet (Cohen et al 2022b). As such, there may be a temporal variation in the stratospheric temperature and humidity, and thus in the resulting cloud cover, which could amplify the expected variability in transmission signal strength at the limb.…”