GISS Lunch Seminar Speaker: Denis Sergeev (Univ. Exeter) Title: Dichotomy of the atmospheric circulation on TRAPPIST-1e Abstract: The era of atmospheric characterization of terrestrial exoplanets is just around the corner with the advent of the next generation of telescopes, marked by the successful deployment of the James Webb Space Telescope (JWST). In order to make the best use of telescope time and future observational data, it is imperative to improve our theoretical understanding of exoplanet atmospheres. This includes using numerical models to reproduce various patterns of circulation in the atmospheres of exoplanets. A confirmed rocky exoplanet and a primary candidate for future atmospheric characterization, TRAPPIST-1e has planetary parameters that place it right at the boundary of different atmospheric circulation regimes. Consequently, the atmospheric circulation on TRAPPIST-1e may reside in two distinct states, as was recently demonstrated by 3D general circulation model (GCM) simulations within the TRAPPIST-1 Habitable Intercomparison (THAI) project. In this study, we use one of the THAI GCMs, the Met Office Unified Model, to explore the mechanism behind this regime dichotomy in detail. One of the regimes is dominated by a strong superrotating equatorial jet and a large day-night temperature difference; the other regime is dominated instead by strong mid-latitude jets and has a relatively small day-night contrast. The circulation regime appears to be highly sensitive to the choice of parameterizations of unresolved processes such as convection. Moreover, we find that the circulation regime depends on the model's initial conditions, revealing an interesting case of climate bifurcation. Across a series of sensitivity experiments, our model is more susceptible to the bifurcation when a mass-flux parameterization of convection is used, while using a convection adjustment parameterization strongly favors only one of the regimes. The two circulation regimes have a different imprint on the transmission spectrum due to the differences in cloud extent and water vapor concentration at the terminators. However, the inter-regime differences in the transmission spectrum in our simulations are smaller than the instrumental noise of JWST and are thus unlikely to be detected soon.