Nonlinear and nonmonotonic effect of ocean tidal mixing on exoplanet climates and habitability

Tides play an important role in the circulation and mean state of the Earth's oceans through inducing significant mixing. On other planets, tidal forcings could be highly amplified compared to Earth, such as planets orbiting relatively close to low-mass host stars, or planets having massive and/or close moons. The former scenario is especially important as, due to their abundance and their observational advantages, low-mass stars offer the best chance of finding habitable planets through sheer numbers. By varying the magnitude of tidal forcing over several orders of magnitude in a coupled atmosphere–ocean global circulation climate model, we find that key climatic quantities, such as heat transport intensity and both surface and deep ocean temperature, change with tidal strength in a nonlinear and nonmonotonic manner. We find an optimum value of tidal mixing, approximately 100 times that of Earth's oceans, which minimizes climatic thermal gradients across the planet. In particular, we show that such planets are habitable for stellar flux values at which oceans with weaker or stronger tidal mixing freeze globally, suggesting an important role for ocean tidal mixing in planetary habitability.