How does thermohaline circulation move water around the global ocean?

Thermohaline circulation moves water throughout the global ocean by relying on horizontal differences in temperature and salinity, which create variations in water density.12

The process functions as follows:

• Sinking: In high-latitude polar regions, such as the northern North Atlantic and near Antarctica, surface water becomes cold and salty—and thus dense—due to atmospheric cooling or evaporation. This dense water sinks through the water column via convection until it reaches a depth where its density matches the surrounding water.123
• Spreading: Once at depth, these water masses spread slowly across the seafloor into the global ocean basins.12
• Upwelling and Return: To replace the sinking surface water, deep water gradually upwells in other regions, such as the Southern Ocean, to feed a slow return flow back toward the sinking regions.12

This sluggish, deep-reaching circulation transports vital heat, nutrients, and other materials across vast distances, playing a critical role in moderating the global climate.1