Sensitivity of the Atmospheric Water Cycle within the Habitable Zone of a Tidally-Locked, Earth-like Exoplanet

TL;DR

This study investigates how the atmospheric water cycle on tidally-locked, Earth-like exoplanets responds to small increases in stellar radiation, highlighting the role of energy transport and atmospheric stratification in shaping climate changes.

Contribution

It provides new insights into the sensitivity of the water cycle and energy transport mechanisms on tidally-locked exoplanets under slight stellar flux variations.

Findings

Water cycle enhancement with increased stellar irradiance

Day-to-night energy transport is crucial for precipitation changes

Nightside precipitation is weak due to energy and emission balance

Abstract

Synchronously orbiting, tidally-locked exoplanets with a dayside facing their star and a permanently dark nightside orbiting dim stars are prime candidates for habitability. Simulations of these planets often show the potential to maintain an Earth-like climate with a complete hydrological cycle. Here, we examine the sensitivity of the atmospheric water cycle to changes in stellar flux and describe the main underlying mechanisms. In a slowly-rotating, tidally-locked Earth-like atmospheric model, the response to a small (about 10%) increase in stellar irradiance from a habitable-zone control simulation is examined. The water cycle is enhanced in response to the increased stellar irradiance. While the evaporation increase behaves similarly to the stellar radiation increase, the day-to-night energy transport by the mean circulation is critical to the planet's precipitation changes. Increased efficiency of the energy transport in a warmer climate shapes the substellar precipitation increase. On the nightside, precipitation changes are weak as a result of the large cancellation between the increased energy transport and the increased longwave emission. The day-to-night energy transport efficiency is sensitive to the variation of the atmosphere's vertical stratification. Due to weak temperature gradients in upper troposphere and a moist adiabat maintained in the substellar region, variations in the substellar surface temperature and specific humidity govern the increase of the planet's stratification with warming. This suggests a scaling of nightside's precipitation based on the substellar surface thermodynamic changes, a sensitivity that holds over a wider range of stellar irradiance changes.