Stratosphere circulation on tidally locked ExoEarths

Abstract

Stratosphere circulation is important to interpret abundances of photo-chemically produced compounds like ozone that we aim to observe to assess habitability of exoplanets. We thus investigate a tidally locked ExoEarth scenario for TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri~b and GJ 667 C~f with a simplified 3D atmosphere model and for different stratospheric wind breaking assumptions. These planets are representatives for different circulation regimes for orbital periods: $P_{orb}=1-100$~days. The circulation of exoplanets with $P_{orb} \leq $ 25~days can be dominated by the standing tropical Rossby wave in the troposphere and also in the stratosphere: It leads to a strong equatorial eastward wind jet and to 'Anti-Brewer-Dobson'-circulation that confines air masses to the stratospheric equatorial region. Thus, the distribution of photo-chemically produced species and aerosols may be limited to an 'equatorial transport belt'. In contrast, planets with $P_{orb}>25$~days, like GJ~667~C~f, exhibit efficient thermally driven circulation in the stratosphere that allows for a day side-wide distribution of air masses. The influence of the standing tropical Rossby waves on tidally locked ExoEarths with $P_{orb} \leq 25$~days can, however, be circumvented with deep stratospheric wind breaking alone - allowing for equator-to-pole transport like on Earth. For planets with $3 \leq P_{orb} \leq 6$~days, the extratropical Rossby wave acts as an additional safe-guard against the tropical Rossby wave in case of shallow wind breaking. Therefore, TRAPPIST-1d is less prone to have an equatorial transport belt in the stratosphere than Proxima~Centauri~b. Even our Earth model shows an equatorial wind jet, if stratosphere wind breaking is inefficient.