Connecting the dots II: Phase changes in the climate dynamics of tidally locked terrestrial exoplanets

TL;DR

This study uses detailed 3D atmospheric modeling to identify phase transitions in climate states of tidally locked terrestrial exoplanets, revealing how planetary rotation influences wave dominance, climate states, and observable atmospheric features.

Contribution

First to identify planetary wave types associated with different climate states and their dependence on rotation period and planet size in tidally locked exoplanets.

Findings

Three climate state transition regions linked to Rossby wave phase changes.

Multiple climate states possible for fast rotating planets, with distinct wave dominance.

Observable atmospheric features vary with climate state, affecting hot spot shifts.

Abstract

We investigate 3D atmosphere dynamics for tidally locked terrestrial planets with an Earth-like atmosphere and irradiation for different rotation periods ($P_{rot}=1-100$ days) and planet sizes ($R_P=1-2 R_{Earth}$) with unprecedented fine detail. We could precisely identify three climate state transition regions that are associated with phase transitions in standing tropical and extra tropical Rossby waves. We confirm that the climate on fast rotating planets may assume multiple states ($P_{rot}\leq 12$ days for $R_P=2 R_{Earth}$). Our study is, however, the first to identify the type of planetary wave associated with different climate states: The first state is dominated by standing tropical Rossby waves with fast equatorial superrotation. The second state is dominated by standing extra tropical Rossby waves with high latitude westerly jets with slower wind speeds. For very fast rotations ($P_{rot}\leq 5$~days for $R_P=2 R_{Earth}$), we find another climate state transition, where the standing tropical and extra tropical Rossby wave can both fit on the planet. Thus, a third state with a mixture of the two planetary waves becomes possible that exhibits three jets. Different climate states may be observable, because the upper atmosphere's hot spot is eastward shifted with respect to the substellar point in the first state, westward shifted in the second state and the third state shows a longitudinal 'smearing' of the spot across the substellar point. We show, furthermore, that the largest fast rotating planet in our study exhibits atmosphere features known from hot Jupiters like fast equatorial superrotation and a temperature chevron in the upper atmosphere.