  Planets with large moon(s) or those in the habitable zone of low-mass stars
may experience much stronger tidal force and tide-induced ocean mixing than
that on Earth. Thus, the vertical diffusivity (or, more precisely, diapycnal
diffusivity) on such planets, which represents the strength of vertical mixing
in the ocean, would be greater than that on Earth. In this study, we explore
the effects of extremely high diffusivity on the ocean circulation and surface
climate of Earth-like planets in one asynchronous rotation orbit. The response
of planetary climate to 10 and 100 times greater vertical diffusivity than that
found on Earth is investigated using a fully coupled atmosphere-ocean general
circulation model. In order to perform a clear comparison with the climate of
modern Earth, Earth's orbit, land-sea configuration, and present levels of
greenhouse gases are included in the simulations. We find that a larger
vertical diffusivity intensifies the meridional overturning circulation (MOC)
in the ocean, which transports more heat to polar regions and melts sea ice
there. Feedback associated with sea ice, clouds, and water vapor act to further
amplify surface warming. When the vertical diffusivity is 10 (100) times the
present-day value, the magnitude of MOC increases by $\approx$3 (18) times, and
the global-mean surface temperature increases by $\approx$4$^{\circ}$C
(10$^{\circ}$C). This study quantifies the climatic effect of an extremely
strong vertical diffusivity and confirms an indirect link between planetary
orbit, tidal mixing, ocean circulation, and surface climate. Our results
suggest a moderate effect of varying vertical ocean mixing on planetary
climate.

Earth and Planetary Astrophysics