Oceanic Superrotation on Tidally Locked Planets

Oceanic superrotation on a tidally locked aqua-planet. (a): surface ocean velocity (vector) and sea surface height (shading) as a function of longitude and latitude; (b): zonal-mean zonal ocean velocity as a function of latitude and depth; (c): zonal-mean zonal wind stress (red), vertically-averaged zonal ocean velocity (blue), and the zonal velocity following angular momentum conservation for a resting parcel that flows starting from the equator (black dashed); (d): same as (b) but for vertical velocity. In (a), the substellar point is marked with a black dot. In (b), the contour lines show the geostrophic velocity based on the balance between pressure gradient force and the Coriolis force, and the interval is 0.5 m s−1 . In this control experiment, the planetary rotation period (= orbit period) is 30 Earth days, stellar flux is 1400 W m−2 , and CO2 concentration is 300 ppmv.

Is there oceanic superrotation on exoplanets? Atmospheric superrotation, characterized by west-to-east winds over the equator, is a common phenomenon in the atmospheres of Venus, Titan, Saturn, Jupiter, and tidally locked exoplanets.

The stratospheric atmosphere of Earth is also superrotating during the westerly phase of the quasi-biennial oscillation (QBO). However, whether the same phenomenon can occur in ocean is poorly known. Through numerical simulations, here we show that oceanic superrotation does occur on tidally locked terrestrial planets around low-mass stars. Its formation (spun-up from rest) is associated with surface winds, the equatorward momentum convergence by Rossby waves, and the eastward propagation of Kelvin waves in the ocean. Its maintenance is driven by equatorward momentum transports of coupled Rossby-Kelvin waves in the ocean excited from the uneven stellar radiation distribution. The width of the superrotation is mainly constrained by the Rossby deformation radius in the ocean, while its strength is more complex.

Many factors can influence the strength, including planetary rotation rate, stellar flux, greenhouse gas concentration, seawater salinity, bottom drag, and a scaling theory is lack. This work confirms that superrotation can occur on tidally locked terrestrial planets with seawater oceans and suggests that it may also occur on tidally locked hot planets with magma oceans that will possibly be observed in the near future.

Yaoxuan Zeng, Jun Yang

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2101.11784 [astro-ph.EP] (or arXiv:2101.11784v1 [astro-ph.EP] for this version)
Submission history
From: Yaoxuan Zeng
[v1] Thu, 28 Jan 2021 02:39:47 UTC (20,507 KB)

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