Planetary climate under extremely high vertical diffusivity

Abstract

Aims. Planets with large moon(s) or those in the habitable zone of low-mass stars may experience much stronger tidal force and tide-induced ocean mixing than that on Earth. Thus, the vertical diffusivity (or, more precisely, diapycnal diffusivity) on such planets, which represents the strength of vertical mixing in the ocean, would be greater than that on Earth. In this study, we explore the effects of extremely high diffusivity on the ocean circulation and surface climate of Earth-like planets in one asynchronous rotation orbit. Methods. The response of planetary climate to 10 and 100 times greater vertical diffusivity than that found on Earth is investigated using a fully coupled atmosphere–ocean general circulation model. In order to perform a clear comparison with the climate of modern Earth, Earth’s orbit, land–sea configuration, and present levels of greenhouse gases are included in the simulations. Results. We find that a larger vertical diffusivity intensifies the meridional overturning circulation (MOC) in the ocean, which transports more heat to polar regions and melts sea ice there. Feedback associated with sea ice, clouds, and water vapor act to further amplify surface warming. When the vertical diffusivity is 10 (100) times the present-day value, the magnitude of MOC increases by ≈3 (18) times, and the global-mean surface temperature increases by ≈4 °C (10 °C). This study quantifies the climatic effect of an extremely strong vertical diffusivity and confirms an indirect link between planetary orbit, tidal mixing, ocean circulation, and surface climate. Our results suggest a moderate effect of varying vertical ocean mixing on planetary climate.

Thermocline Depth on Water-rich Exoplanets

“…In this study, the effect of varying wind stresses is tested without considering the full interaction between the atmosphere and the ocean and the imposed wind stresses are steady, so an ocean-atmosphere coupling model can be utilized to investigate the thermocline depth in future work. The highest vertical diffusivity for the interior ocean we tested is 10 −3 m 2 s −1 , or 100 times that on Earth, which is the estimated strength of vertical mixing on potentially habitable asynchronous rotating exoplanets around M dwarfs (Si et al 2021). However, this estimation is obtained based on simple scalings, and more rigorous calculations of the background vertical diffusivity using high-resolution ocean models are required for further investigations.…”

“…Also, a case with stronger and wider easterly winds and equatorward winds in the tropics is carried out (see the dashed lines in Figures 2(a The strength of mixing is quantified as vertical eddy diffusivity, which can range from about 10 −5 (background level in the ocean interior) to 10 −3 m 2 s −1 (enhanced mixing due to such as bottom topography) on Earth (Waterhouse et al 2014). Si et al (2021) use simple scalings and obtain the possible strength of vertical mixing on asynchronous rotating Comparisons of the depth dependence of density for pure water between four different EOSs, including a linear EOS (density varies with potential temperature and salinity linearly but does not depend on pressure), the original "JMD95Z," modified "JMD95Z" under a conserved potential temperature of 285 K, and "IAPWS-95" under a fixed in situ temperature of 285 K. Below we show that different EOSs will not significantly affect the simulation results. habitable exoplanets around M dwarfs, which is roughly 100 times the mean value on Earth.…”