Sea ice stripes on CO2-rich aquaplanets with ROCKE-3D

The outer edge of the habitable zone (HZ) around M-dwarfs can host planets with atmospheres rich in CO2 (or other greenhouse gases), a prerequisite for liquid surface water to be present. This study investigates the climate state of such CO2-rich atmospheres on synchronously rotating Proxima Centauri b-like aquaplanets. We use the NASA GISS ROCKE-3D version 2 General Circulation Model (GCM). ROCKE-3D has been validated against other terrestrial exoplanetary GCMs in the THAI project, and is one of the few exoplanetary GCMs that includes a dynamic ocean component. This dynamic ocean allows for a physically-based calculation of the ocean heat transport, which is especially important in the outer HZ. We performed simulations for 3 configurations with 1 bar atmospheres ranging between 40% CO2 - 60% N2 to 99% CO2 - 1% N2. The importance of ocean heat transport in these configurations is demonstrated through surface energy budget considerations.Our results reveal two main features:At the permanent nightside, two persistent bands of sea ice stripes encompass the entire planetary nightside across all CO2 mixing ratios tested. These ice stripes modulate lower atmospheric climate and circulation which is separated from the upper atmosphere by a temperature inversion. An emphasis lies on their modulation of the hydrological cycle, both near the surface, through energy fluxes, and aloft, through cloud formation.At the substellar region (global dayside) a “trident” pattern, which may be described as an extension to the commonly observed surface “lobster” pattern, emerges. Its spatial distribution is modulated by the sea ice stripes through “drying” and “blocking” effects sensitive to the partial pressure of CO2. We provide explanations of connections and influences between the two patterns.These features are visible and different from N2-dominated aquaplanets in top-of-atmosphere radiative fluxes and may thereby be used to constrain surface features and planetary climate in future observations of CO2 -rich aquaplanets.