Stratosphere circulation is important to interpret abundances of photochemically produced compounds like ozone which 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: Porb = 1–100 d. The circulation of exoplanets with Porb ≤ 25 d 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 an ‘Anti-Brewer-Dobson’-circulation that confines airmasses to the stratospheric equatorial region. Thus, the distribution of photochemically produced species and aerosols may be limited to an ‘equatorial transport belt’. In contrast, planets with Porb > 25 d, like GJ 667 C f, exhibit efficient thermally driven circulation in the stratosphere which allows for a day side-wide distribution of airmasses.
The influence of the standing tropical Rossby waves on tidally locked ExoEarths with Porb ≤ 25 d can, however, be circumvented with deep stratospheric wind breaking alone – allowing for equator-to-pole transport like on Earth. For planets with 3 ≤ Porb ≤ 6 d, the extratropical Rossby wave acts as an additional safeguard 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.
