Multiple moist climate equilibrium states on arid rocky M-dwarf planets: A last-saturation tracer analysis

TL;DR

This study uses a last-saturation tracer technique in GCM simulations to reveal multiple stable moist climate states on arid M-dwarf planets, influenced by rotation and cold-trapping dynamics.

Contribution

It introduces the application of last-saturation statistics to understand the stability and multiplicity of moist climate states on tidally locked exoplanets.

Findings

Water vapor distribution weakly depends on rotation on the nightside upper troposphere.

Fast rotation leads to more water vapor buildup in the nightside lower troposphere.

Multiple moist climate states result from cold-trapping competition between atmosphere and surface.

Abstract

Terrestrial-type exoplanets orbiting nearby red dwarf stars (M dwarfs) are the first potentially habitable exoplanets suitable for atmospheric characterization in the near future. Understanding the stability of water in cold-trap regions on such planets is critical because it directly impacts transmission spectroscopy observations, the global energy budget, and long-term surface water evolution. Here we diagnose the humidity distribution in idealized general circulation model (GCM) simulations of terrestrial-type exoplanets. We use the `tracer of last saturation' technique to study the saturation statistics of air parcels. We find that on synchronously rotating planets, the water vapor abundance in the nightside upper troposphere depends weakly on planetary rotation, while more water vapor builds up in the nightside lower troposphere on fast-rotating planets. We then discuss how last-saturation statistics can elucidate the multiple moist climate equilibrium states on synchronously and asynchronously rotating arid planets. We show that the multiple moist climate states arise from the cold-trapping competition between the substellar upper atmosphere and cold surface regions. We find that fast synchronously rotating planets tend to trap surface water on the nightside as a result of their weak atmospheric and strong surface cold traps compared to the slow rotating case. These results elucidate the nature of the water cycle on arid rocky exoplanets and will aid interpretation of atmospheric observations in the future.