  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.
