Thermocline Depth on Water-rich Exoplanets

TL;DR

This study investigates the thermocline depth in water-rich exoplanets using global ocean modeling, revealing that thermocline depth remains shallow and is influenced by wind and mixing, with scaling theories applicable but modified due to the absence of continents.

Contribution

The paper applies Earth-based thermocline scaling theories to water-rich exoplanets and analyzes how their unique features affect ocean circulation and thermocline depth.

Findings

Thermocline remains within upper ocean layers regardless of ocean depth.

Wind-driven circulation resembles Earth's Antarctic Circumpolar Current.

Scaling relationships are similar to Earth's but adjusted for lack of continents.

Abstract

Water-rich exoplanet is a type of terrestrial planet that is water-rich and its ocean depth can reach tens of to hundreds of kilo-meters with no exposed continents. Due to the lack of exposed continents, neither western boundary current nor coastal upwelling exists, and ocean overturning circulation becomes the most important way to return the nutrients deposited in deep ocean back to the thermocline and to the surface ocean.Here we investigate the depth of the thermocline in both wind-dominated and mixing-dominated systems on water-rich exoplanets using the global ocean model MITgcm. We find that the wind-driven circulation is dominated by overturning cells through Ekman pumping and subduction and by zonal (west--east) circum-longitudinal currents, similar to the Antarctic Circumpolar Current on Earth. The wind-influenced thermocline depth shows little dependence on the ocean depth, and under a large range of parameters, the thermocline is restricted within the upper layers of the ocean. The mixing-influenced thermocline is limited within the upper 10 km of the ocean and can not reach the bottom of the ocean even under extremely strong vertical mixing. The scaling theories for the thermocline depth on Earth are applicable for the thermocline depth on water-rich exoplanets. However, due to the lack of exposed continents, the zonal and meridional flow speeds are not in the same magnitude as that in the oceans of Earth, which results in the scaling relationships for water-rich exoplanets are a little different from that used on Earth.