Salinity as the driving force for deep circulation
Ocean salinity varies from about 32 ppt in estuaries to 35-37 ppt in open oceans. Higher salinity directly increases water density, allowing denser water to sink beneath lighter water. This salinity gradient is the engine of thermohaline circulation. In the North Atlantic, evaporation increases salinity in the Mediterranean, and cold winter temperatures in high northern latitudes create the densest water on Earth. This water sinks to the ocean bottom and begins its journey southward as deep Atlantic water.
Salinity changes are slow compared to temperature, so a region's density is often determined as much by salinity as by temperature. Ice melt and river discharge freshen surface waters, making them lighter and causing them to float above denser seawater below. This density stratification can suppress mixing and modify local circulation patterns.
Ocean conveyor mechanics and climate implications
The thermohaline circulation is sometimes called the ocean conveyor belt because it moves water slowly but continuously across the globe. Warm, salty surface water flows northward in the Atlantic, loses heat and gains salt through evaporation, becomes dense enough to sink in the Nordic Seas, and returns southward at depth. The trip takes roughly 1000 years, meaning water that sinks now surfaces elsewhere a millennium later.
Freshwater input from melting ice sheets can disrupt this circulation by freshening the sinking regions and preventing dense-water formation. This 'conveyor shutdown' is a possibility under strong warming scenarios and could trigger abrupt climate changes in the North Atlantic region. Observing salinity and temperature changes in key sinking regions is a priority for climate monitoring because circulation changes can alter regional climate rapidly.