Ocean density, temperature, salinity, deep currents, global conveyor belt, water masses, polar sinking, heat transport, carbon storage, oxygen, nutrients, AMOC, climate regulation, freshwater, sea ice, and ocean circulation
Thermohaline circulation
Thermohaline circulation is the slow movement of deep ocean water driven by density differences caused mainly by temperature and salinity.
What it is
Thermohaline circulation is a large-scale part of ocean circulation driven by density. Water becomes denser when it gets colder or saltier, and dense water can sink below lighter water. That sinking helps set deep currents in motion.
Temperature and salinity
Ocean density depends strongly on heat and salt. Warm water tends to be lighter; cold water tends to be heavier. Freshwater from rain, rivers, or melting ice lowers salinity, while evaporation and sea-ice formation can leave surrounding seawater saltier.
Where deep water forms
Important sinking regions occur in cold high-latitude seas, especially parts of the North Atlantic and around Antarctica. There, surface water can cool, become dense, sink, and begin a long journey through deeper ocean basins.
The conveyor idea
Thermohaline circulation is often nicknamed the global conveyor belt. The image is useful because it shows connection: water masses sink, spread, mix, rise elsewhere, and eventually return toward the surface. Still, the real ocean is messier than a single neat belt.
Heat and climate
Deep circulation works with winds and surface currents to redistribute heat. In the Atlantic, overturning circulation helps carry warm surface water northward and return colder deep water southward, which affects regional climate patterns.
Carbon, oxygen, and nutrients
When surface water sinks, it can carry dissolved gases and carbon into the deep ocean. Over time, deep water also accumulates nutrients from decomposing organic matter. When deep water later rises, it can influence productivity, oxygen conditions, and ocean chemistry.
Sensitivity to change
Freshwater from melting ice or increased rainfall can make surface water less salty and less dense, which may affect sinking in some regions. Scientists watch these changes closely, especially in the Atlantic Meridional Overturning Circulation.
Why it matters
Thermohaline circulation is slow, but slow does not mean unimportant. It links polar seas with tropical oceans, the surface with the abyss, and today’s climate with water masses that may not return to the surface for generations.