Oceanic Super-rotation on Tidally Locked Terrestrial Planets

In searching for life beyond the solar system, terrestrial planets in the habitable zone around M dwarfs are the primary targets. These planets are likely to be in tidally locked orbits, which indicates very uneven radiation heating. The climates of tidally locked planets have been studied in recent years (e.g., Hu & Yang, 2014; Del Genio et al. 2019; Yang et al. 2019 ). Due to the effect of ocean dynamics, the spatial pattern of the substellar open ocean would be in a "lobster" mode rather than an "eyeball" mode, associated with Rossby and Kelvin waves in the ocean (Hu & Yang, 2014).

We use the fully coupled global climate model CCSM3 (active atmosphere, ocean, land, and sea ice) to simulate the climate on a tidally locked terrestrial aqua-planet with ocean depth of 1000m . This planet has similar parameters to earth, with a rotation period (= orbit period) of 30 Earth days.

In our experiment, the lobster-like mode pattern (Gill 1980 ) can be observed at the ocean surface (Fig. 1). Besides of the waves, the dominated feature of the ocean circulation on 1:1 tidally locked planets is a strong westerly jet with velocity of ~1m/s in the tropics, may be called "oceanic super-rotation".

Ocean currents are mainly driven by the wind stress. The atmospheric super-rotation causes the oceanic super-rotation, and the Ekman transport convergence/divergence causes the equatorial downwelling/upwelling. Besides, the eddy momentum transport is also important, which causes the transition from double-jet pattern in the atmosphere to single-jet pattern in the ocean.

This study helps to better understand the mechanism of the ocean circulation on tidally locked planets, which is of great importance to more accurately simulating the edges of the habitable zone in the near future.

Figure: Oceanic super-rotation on tidally locked planets. a, sea surface temperature (color shading) and wind stress (vector). b, sea surface height (color shading) and ocean surface current (vector). c, zonal-mean ocean zonal velocity for lat-depth diagram. d, equatorial onal-mean ocean zonal velocity for lon-depth diagram.