Diving into the new dynamical slab ocean of the Generic-PCM: Implications for the climate of TRAPPIST-1e

Abstract

Ocean modelling is often sidelined by exoclimate modellers, mostly due to the associated computational expense of spinning up dynamic oceans. However, oceanic heat transport can critically impact the climate and observables for M-planets in the middle of their habitable zones (e.g., [1]) like TRAPPIST-1e. The oceanic description can also affect the number of final stable climatic states of the planet ([2]). Short of using a fully dynamic ocean model, a compromise used in most exoplanet General Circulation Models (GCMs) is a slab ocean model without oceanic heat transport. Here, we will first present our improved compromise - the new dynamical slab ocean model integrated into the Generic-PCM ([3]), previously known as the LMD Generic GCM (e.g., [4]). Our parallelisable ocean model not only accounts for sea-ice/snow evolution, but also features wind-driven ocean transport (Ekman transport), horizontal eddy diffusion and convective adjustment between oceanic layers. When coupled with the atmosphere, it effectively reproduces critical attributes observed on modern Earth, including the major oceanic heat flows, an annually averaged surface temperature of 13 C, planetary albedo of 0.32 and sea ice coverage spanning 18 million sq. km. Further, we will delve into the implications of a dynamical slab ocean model for TRAPPIST-1e. Despite recent JWST observations indicating the lack of a (thick) atmosphere for TRAPPIST-1b ([5]) and 1c ([6]), the planets farther away from the star, like 1e, may have retained moderately thick atmospheres. Assuming this, and if 1e formed with a substantial water reservoir ([7]), it could have sustained liquid water oceans ([8]). In general, the presence of oceanic heat transport can give rise to distinct oceanic patterns, as illustrated by the “lobster” pattern observed for Proxima Centauri b by [9], in contrast to the “eyeball” pattern in [4], observed in its absence. Moreover, studies suggest that the climates of Proxima Centauri b and TRAPPIST-1e may share similarities ([4], [8]). In this context, we will present findings from our new dynamical slab ocean within the Generic-PCM for TRAPPIST-1e. These results will then be systematically compared with those of [9], which used ROCKE-3D ([10]) with a dynamic ocean model. We believe that this will help in strengthening our understanding of the climate of TRAPPIST-1e and also offer insights into comparative exoplanetary climate research. Finally, we will discuss our findings in the context of habitability, particularly emphasising the role of a dynamical ocean model in informing our understanding of habitable conditions.