Modeling 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 equatorpole ocean heat transport on Earth peaks at almost half that of the atmosphere. However, such studies have made the assumption that fundamental oceanic properties, such as salinity, temperature, and depth, are similar to Earth. This assumption results in Earth-like circulations: a meridional overturning with warm water moving poleward at the surface, being cooled, sinking at high latitudes, and traveling equatorward at depth. Here it is shown that an exoplanetary ocean with a different salinity can circulate in the opposite direction: an equatorward flow of polar water at the surface, sinking in the tropics, and filling the deep ocean with warm water. This alternative flow regime results in a dramatic warming in the polar regions, demonstrated here using both a conceptual 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.exoplanet | habitability | planetary climate | ocean circulation W ith the ongoing discovery of exoplanets, it is natural to question their habitability. The most widely accepted definition of a habitable planet is one that can sustain liquid water on its surface (1), which usually implies a range of orbital radii in which a planet receives a level of stellar radiation that enables surface liquid water to be sustained (2, 3). Habitability and climate also depend on heat being transported effectively around the planet, which is achieved through the circulation of the atmosphere and ocean. There are many studies describing simulations of exoplanetary atmospheric circulations and climate (4), but most incorporate either no ocean (5-7) or a heavily simplified slab ocean (8-12). Only recently have studies considered a dynamical ocean (13-16), and even in these studies, the ocean is consistently assumed to have similar properties to the oceans on Earth. Such an assumption may prove to be an oversimplification. More recently changes in ocean properties have begun to be investigated, for example, ocean depth (17), and here, salinity, which has significant influence on density.There is evidence on Earth that the mean ocean salinity has decreased from up to double the present day level since the Archean (18,19). In addition, modeling of water delivery during planetary formation concludes that there could be an extensive range of possible ocean masses (20,21). Planets in the habitable zone may have oceans with a small fraction to hundreds of times the volume of the oceans on Earth, which will have important implications for the geological evolution of the planet and the salinity of any oceans. Ocean salinity can also be influenced by planetary properties, for example, the immense pressures at the floor of a very deep ocean could maintain a layer of ice at...

The climate and, hence, potential habitability of a planet crucially depends on how its atmospheric and ocean circulation transports heat from warmer to cooler regions. However, previous studies of planetary climate have concentrated on modeling the dynamics of atmospheres, while dramatically simplifying the treatment of oceans, which neglects or misrepresents the effect of the ocean in the total heat transport. Even the majority of studies with a dynamic ocean have used a simple so-called aquaplanet that has no continental barriers, which is a configuration that dramatically changes the ocean dynamics. Here, the significance of the response of poleward ocean heat transport to planetary rotation period is shown with a simple meridional barrier--the simplest representation of any continental configuration. The poleward ocean heat transport increases significantly as the planetary rotation period is increased. The peak heat transport more than doubles when the rotation period is increased by a factor of ten. There are also significant changes to ocean temperature at depth, with implications for the carbon cycle. There is strong agreement between the model results and a scale analysis of the governing equations. This result highlights the importance of both planetary rotation period and the ocean circulation when considering planetary habitability.