Importance of ocean salinity for climate and habitability

Abstract

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...

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets

“…The Earth's thermohaline circulation was recently placed into perspective by Cullum & Stevens (2016) who demonstrated that an increase in mean salinity can cause the haline component to dominate.…”

The ocean's saltiness and its overturning

“…Experiments b – h are identical to a except that the salinity initial conditions and surface forcing are multiplied by a constant factor ξ between 0 and 3. Salinity is shifted in this way to maintain relative salinity differences while changing the mean salinity [ Cullum et al , ].…”

“…Investigating the habitability of exoplanets, Cullum et al [] used a box model to test for the impact of mean salinity on an idealized ocean circulation. As freshwater at surface pressure has a nearly quadratic equation of state with respect to temperature, they used a quadratic function for ρ ( T ) to test the case where .…”

“…Experiments b-h are identical to a except that the salinity initial conditions and surface forcing are multiplied by a constant factor between 0 and 3. Salinity is shifted in this way to maintain relative salinity differences while changing the mean salinity [Cullum et al, 2016]. Experiment i replicates experiment a but uses a different nonlinear equation of state with 12 terms per depth level, to verify that results are not sensitive to the specific implementation of equation of state nonlinearities [Jackett and McDougall, 1995].…”

“…Here we investigate the role of mean salinity on this overturning asymmetry. Mean salinity may have an impact because salinity affects the density‐temperature relationship [ Roquet et al , ], because seawater density is sensitive to salinity at high latitudes where T ≈0°C [ Timmermans and Jayne , ] and because salinity variations scale with mean salinity [ Cullum et al , ] as salinity variations are due to evaporation, precipitation, and other water injection/removal events rather than direct input of salts.…”