Atmospheric dynamics of Earth-like tidally locked aquaplanets (1001.5117v1)

Abstract: We present simulations of atmospheres of Earth-like aquaplanets that are tidally locked to their star, that is, planets whose orbital period is equal to the rotation period about their spin axis, so that one side always faces the star and the other side is always dark. As extreme cases illustrating the effects of slow and rapid rotation, we consider planets with rotation periods equal to one current Earth year and one current Earth day. The dynamics responsible for the surface climate (e.g., winds, temperature, precipitation) and the general circulation of the atmosphere are discussed in light of existing theories of atmospheric circulations. For example, as expected from the increasing importance of Coriolis accelerations relative to inertial accelerations as the rotation rate increases, the winds are approximately isotropic and divergent at leading order in the slowly rotating atmosphere but are predominantly zonal and rotational in the rapidly rotating atmosphere. Free-atmospheric horizontal temperature variations in the slowly rotating atmosphere are generally weaker than in the rapidly rotating atmosphere. Interestingly, the surface temperature on the night side of the planets does not fall below ~240 K in either the rapidly or slowly rotating atmosphere; that is, heat transport from the day side to the night side of the planets efficiently reduces temperature contrasts in either case. Rotational waves shape the distribution of winds, temperature, and precipitation in the rapidly rotating atmosphere; in the slowly rotating atmosphere, these distributions are controlled by simpler divergent circulations. The results are of interest in the study of tidally locked terrestrial exoplanets and as illustrations of how planetary rotation and the insolation distribution shape climate.

• The paper simulates atmospheric dynamics of tidally locked Earth-like aquaplanets using a general circulation model, comparing slow and fast rotation scenarios.
• Slowly rotating aquaplanets exhibit nearly isotropic winds, minimal horizontal temperature variations, and efficient heat transport keeping the night side above 240 K.
• Rapidly rotating aquaplanets display predominantly zonal winds, significant temperature variations, and dynamics driven by Rossby waves and eddies, similarly maintaining night-side temperatures above 240 K.

Atmospheric Dynamics of Earth-Like Tidally Locked Aquaplanets

The paper by Merlis and Schneider presents a detailed analysis of atmospheric dynamics on hypothetical Earth-like aquaplanets that are tidally locked to their star. This state of being tidally locked means that one hemisphere perpetually faces the star while the opposite remains in darkness. The research primarily utilizes simulations with a three-dimensional general circulation model (GCM) to investigate the climatic and atmospheric characteristics of these planets under varying rotational conditions. Two extreme rotation scenarios are considered: a slowly rotating case with a single-planet-year rotation and a rapidly rotating one that completes a rotation within a single Earth day.

Key Simulation Outcomes

The primary focus of this paper is to contrast the atmospheric dynamics under slow and rapid planetary rotation, correlating them with existing theories of atmospheric circulation. The simulations reveal nuanced insights into wind patterns, temperature distributions, and precipitation mechanics. Notably, rotation rate significantly influences atmospheric behavior:

• Slow Rotation: The atmospheric dynamics under slower rotation exhibit nearly isotropic and divergent wind patterns. Horizontal temperature variations are minimal, reflecting the inertial and Coriolis acceleration balance or dominance of the latter as rotation is reduced. Despite constant insolation on one side, efficient heat transport mechanisms maintain a night-side temperature above 240 K.
• Fast Rotation: In contrast, rapidly rotating planets exhibit predominantly zonal and rotational winds, with significant horizontal temperature variations. Rossby waves and eddies become fundamental in shaping wind, temperature, and precipitation distributions. Additionally, temperature remains above 240 K on the night side due to the heat redistribution across hemispheres.

Implications for Exoplanets

These findings are particularly relevant in the context of tidally locked terrestrial exoplanets. As planets around their stars experience heightened detection probability when orbiting closely, understanding their potential climatic states could aid in assessing habitability. The paper indicates that heat transport mechanisms can mitigate temperature extremes, potentially influencing surface habitability conditions.

Theoretical and Practical Significance

Theoretically, this paper bolsters insights into how planetary rotation and insolation distribution mold atmospheric behavior. The slow rotation results underscore a dynamic regime where isotropic circulations arise, aligning with phenomena observed in Earth's tropics. Fast rotation scenarios illustrate vorticity and wave-dominant dynamics familiar in extratropical Earth settings, supporting theories posited by Charney and others on geostrophic balance in atmospheric circulation.

Practically, this research may guide the development of observational strategies for exoplanets, highlighting regions where certain climatic parameters can be optimized for detection or habitation potential. The emphasis on atmospheric heat exchange processes further assists in modeling other extreme planetary environments.

Prospects for Future Research

Continued exploration might explore the interactions between atmospheric chemistry, surface characteristics, and rotation-induced dynamics. Additionally, refining GCM models to include cloud radiative effects could elucidate their impact under varied insolation disparities. Expanding the range of planetary parameters, such as atmospheric composition or ocean presence, might also enhance our understanding of the climates of tidally locked Earth-like exoplanets.

In summary, the paper by Merlis and Schneider contributes significant insights into the behaviors and characteristics of atmospheric circulations on Earth-like tidally locked planets, emphasizing the potential for dynamically stable climates even under extreme rotational conditions. This work forms a foundation for both observational and theoretical pursuits within the exoplanetary science community.