Termohaline

Termohaline circulation, often spelled thermohaline circulation, is a component of Earth's global ocean circulation driven by density differences arising from temperature (thermo) and salinity (haline) variations. It describes a basin-scale overturning system in which surface waters are chilled and, in some regions, become denser due to salinity changes, causing them to sink and spread as deep currents, while less dense waters rise elsewhere to complete the loop.

The circulation links the surface and the deep ocean and plays a major role in heat and freshwater transport, as well as the distribution of nutrients and carbon. A key part of the system is the formation of dense water in polar regions, notably North Atlantic Deep Water and Antarctic Bottom Water, which flow along the ocean basins before upwelling and returning to the surface. The result is a global overturning circulation that connects all major oceans.

Climate relevance and variability are central to its study. The strength and pattern of the thermohaline circulation influence regional climates and the storage of heat and carbon in the ocean. Freshwater input from increased rainfall, ice melt, or river discharge can reduce surface salinity and density, potentially weakening the circulation. Conversely, cooling and salinification in high latitudes can enhance it. Observations from hydrographic surveys, moorings, and Argo floats, along with numerical models and paleoclimate records, are used to monitor and understand its variability.

Overall, the termohaline circulation is a key component of the global ocean system, operating over long timescales and shaping climate and biogeochemical cycles.