Connecting the dots III: Night side cooling and surface friction affect climates of tidally locked terrestrial planets

TL;DR

This study explores how night side cooling and surface friction influence climate states and circulation patterns on tidally locked Earth-like planets across various rotation periods, revealing significant impacts on temperature distribution and atmospheric dynamics.

Contribution

It provides new insights into the effects of surface friction and night side cooling on climate states and circulation patterns of tidally locked planets, highlighting the role of Rossby waves and thermal forcing.

Findings

Strong surface friction leads to standing Rossby waves and specific jet patterns.

Weak surface friction causes decoupling of surface temperatures and circulation, with significant night side cooling.

Enhanced night side cooling can reduce night side temperatures by up to 100 K.

Abstract

We investigate how night side cooling and surface friction impact surface temperatures and large scale circulation for tidally locked Earth-like planets. For each scenario, we vary the orbital period between $P_{rot}=1-100$~days and capture changes in climate states. We find drastic changes in climate states for different surface friction scenarios. For very efficient surface friction ($t_{s,fric}=$ 0.1 days), the simulations for short rotation periods ($P_{rot} \leq$ 10 days) show predominantly standing extra tropical Rossby waves. These waves lead to climate states with two high latitude westerly jets and unperturbed meridional direct circulation. In most other scenarios, simulations with short rotation periods exhibit instead dominance by standing tropical Rossby waves. Such climate states have a single equatorial westerly jet, which disrupts direct circulation. Experiments with weak surface friction ($t_{s,fric}=~10 -100$ days) show decoupling between surface temperatures and circulation, which leads to strong cooling of the night side. The experiment with $t_{s,fric}= 100$ days assumes climate states with easterly flow (retrograde rotation) for medium and slow planetary rotations $P_{rot}= 12 - 100$~days. We show that an increase of night side cooling efficiency by one order of magnitude compared to the nominal model leads to a cooling of the night side surface temperatures by 80-100~K. The day side surface temperatures only drop by 25~K at the same time. The increase in thermal forcing suppresses the formation of extra tropical Rossby waves on small planets ($R_P=1 R_{Earth}$) in the short rotation period regime ($P_{rot} \leq$ 10 days).