Wind-Driven Ocean Circulation on Water-Rich Exoplanets

Abstract:

Exoplanet missions uncover a new type of planet that is water-rich and the ocean depth can reach tens of to hundreds of kilo-meters with no exposed continent. In this study, we investigate the features of wind-driven ocean circulation on water-rich planets, using the global ocean model MITgcm. Due to the lack of continents, neither western boundary current nor subtropical or subpolar gyre can form in the simulations, in contrast to that on Earth. The large-scale wind-driven ocean circulation is dominated by overturning cells through Ekman pumping and subduction and by circum-longitudinal currents that flowing from west to east or from east to west, similar to the Antarctic Circumpolar Current. We focus on the sensitivity of the ocean circulation to various parameters, including sea surface temperature, wind stress, planetary rotation rate, viscosity coefficient, diffusion coefficient, bottom drag, the equation of state for seawater, and the parameterization scheme of mesoscale eddies. In all the simulations, the depth of the ocean circulation is within about 2 km, which is mainly constrained by the vertical stratification of the ocean, i.e., the ocean density increases with depth. Below the level of 2 km, that the ocean is quite calm. The shallowness of the ocean circulation limits the nutrient and energy exchanges between the upper ocean and the deep ocean, so that in this aspect water-rich planets are less habitable than planets with shallower ocean(s) like Earth. Future work is required for investigating thermohaline circulation on water-rich planets.