The importance of ocean salinity for climate and habitability

Modelling studies of terrestrial extrasolar planetary climates are now including the effects of ocean circulation due to a recognition of the importance of oceans for climate; indeed the peak equator-pole ocean heat transport on Earth peaks at almost half that of the at- mosphere. However, such studies have made the assumption that fundamental oceanic properties, such as salinity, temperature, and depth, are similar to Earth. This results in Earth-like circulations; a meridional overturning with warm water moving poleward at the surface, being cooled, sinking at high latitudes, and travelling equa- torward at depth. Here it is shown that an exoplanetary ocean with a different salinity can circulate in the opposite direction; an equa- torward flow of polar water at the surface, sinking in the tropics, and filling the deep ocean with warm water. This results in a dramatic warming in the polar regions, demonstrated here using both a con- ceptual model and an ocean general circulation model. These results highlight the importance of ocean salinity for exoplanetary climate and consequent habitability, and the need for its consideration in future studies.