Temperature Structures Associated with Different Components of the Atmospheric Circulation on Tidally Locked Exoplanets

Abstract

Observations of time-resolved thermal emission from tidally locked exoplanets can tell us about their atmospheric temperature structure. Telescopes such as JWST and ARIEL will improve the quality and availability of these measurements. This motivates an improved understanding of the processes that determine atmospheric temperature structure, particularly atmospheric circulation. The circulation is important in determining atmospheric temperatures, not only through its ability to transport heat, but also because any circulation pattern needs to be balanced by horizontal pressure contrasts, therefore implying a particular temperature structure. In this work, we show how the global temperature field on a tidally locked planet can be decomposed into contributions that are balanced by different components of the atmospheric circulation. These are the superrotating jet, stationary Rossby waves, and the divergent circulation. To achieve this, we partition the geopotential field into components balanced by the divergent circulation and the rotational circulation, with the latter comprising the jet and Rossby waves. The partitioned geopotential then implies a corresponding partitioning of the temperature via the hydrostatic relation. We apply these diagnostics to idealized general circulation model simulations, to show how the separate rotational and divergent circulations together make up the total three-dimensional atmospheric temperature structure. We also show how each component contributes distinct signatures to the thermal phase curve of a tidally locked planet. We conclude that this decomposition is a physically meaningful separation of the temperature field that explains its global structure, and can be used to fit observations of thermal emission.

The Inhomogeneity Effect. III. Weather Impacts on the Heat Flow of Hot Jupiters

“…Mendonça (2020) analyzed the angular momentum and heat transport of hot Jupiters using a non-hydrostatic GCM and found that the heat can be transported downward by mean circulation and stationary waves. Hammond & Lewis (2021) and Lewis & Hammond (2022) analyzed the impact of the horizontal flow pattern on heat transport on a canonical hot Jupiter. Komacek et al (2022a) showed that a high-entropy interior and a large heat flux produce different horizontal temperature and flow structures than a low-entropy case.…”

“…The horizontal weather patterns in the stratified region can also be understood within the framework of Helmholtz decomposition (Hammond & Lewis 2021;Lewis & Hammond 2022). The horizontal circulation can be decomposed into divergent (vorticityfree) and rotational (divergence-free) components.…”

Two-dimensional Eclipse Mapping of the Hot-Jupiter WASP-43b with JWST MIRI/LRS

“…These maps contain small-scale structure that is not present in the observed eclipse maps in Section 3. A chevron-like structure is present in all the GCM maps, which is commonly attributed to the temperature structure associated with equatorial Kelvin and Rossby waves (Matsuno 1966;Showman & Polvani 2011;Lewis & Hammond 2022). THOR, expeRT/MITgcm, and the RM-GCM each show cold "lobes" on the nightside, which are likely associated with stationary equatorial Rossby waves (Matsuno 1966;Showman & Polvani 2011;Lewis & Hammond 2022).…”

“…Eclipse mapping is currently the only method by which 2D information can be measured for exoplanets, as outof-eclipse "phase curves" provide low-resolution information as a function of longitude only. 2D spatial information is crucial for understanding atmospheric circulation (Lewis & Hammond 2022), chemical composition (Taylor et al 2020;Yang et al 2023), and cloud structure (Parmentier et al 2016).…”