Deep ocean currents are driven by differences in water density, which is controlled by two properties: temperature and salinity. When seawater becomes cold enough or salty enough, it grows denser than the water around it and sinks, sometimes all the way to the ocean floor. This sinking sets massive slow-moving currents in motion thousands of meters below the surface, circulating water around the entire planet in a process scientists call thermohaline circulation.
How Temperature and Salinity Create Density
The word “thermohaline” combines the Greek roots for heat (thermo) and salt (haline), pointing to the two factors that matter most. Cold water is denser than warm water, and salty water is denser than fresh water. When ocean water becomes both cold and salty at the same time, its density increases dramatically, and gravity pulls it downward. That downward plunge is what generates deep ocean currents.
This process begins in the polar regions, where ocean water loses heat to the frigid atmosphere. As surface water cools, some of it freezes into sea ice. Here’s the critical detail: when seawater freezes, the salt doesn’t freeze with it. Instead, the salt gets expelled into the surrounding liquid water, a process called brine rejection. The leftover seawater becomes significantly saltier, and therefore denser, than it was before. That extra-dense water sinks rapidly, displacing the water beneath it and pushing it along the ocean floor.
Where Deep Water Forms
Not every polar coastline produces deep water. The sinking happens at a few specific sites where conditions align just right.
In the Northern Hemisphere, cold dense water forms in two areas of the North Atlantic: the Labrador Sea (between Greenland and Canada) and the GIN Sea (between Greenland, Iceland, and Norway). Water from these regions is collectively known as North Atlantic Deep Water. The two sources actually create distinct layers. Water from the Labrador Sea is slightly warmer, around 2.5 to 5°C, so it’s a bit less dense and flows southward above the colder GIN Sea water, which sits at 0 to 2.5°C. These two layers stack on top of each other as they move through the deep Atlantic.
In the Southern Hemisphere, the densest water on Earth forms along the margins of Antarctica. Antarctic Bottom Water is colder, fresher, and denser than its North Atlantic counterpart. Its formation relies heavily on brine rejection: as vast quantities of sea ice form on the Antarctic continental shelf, the expelled salt makes the surrounding water extremely dense. That water mixes with ambient seawater at the shelf break, then slides down the continental slope into the deepest parts of the ocean. A secondary mechanism involves intense cooling at the ocean surface combined with brine release, particularly in the Weddell Sea. Because Antarctic Bottom Water is denser than North Atlantic Deep Water, it settles beneath it, hugging the very bottom of the ocean floor.
The Ocean Floor Shapes the Flow
Once water sinks, it doesn’t just spread out freely. The shape of the seafloor acts as a system of walls, channels, and gates that directs where deep currents can and can’t go. Underwater mountain ranges like the Mid-Atlantic Ridge, which rises to about 2,500 meters below the surface, physically block deep water from crossing from one side of the Atlantic to the other along most of its length. Currents must steer around these features or squeeze through narrow gaps called fracture zones.
These constrictions matter enormously. Where a ridge has a gap or a basin has a shallow sill, water is forced through a tight passage, which controls how much deep water can move between ocean basins and increases mixing in those bottleneck areas. The Indonesian seas, for example, contain a series of underwater sills and barriers that regulate how deep Pacific water flows into the Indian Ocean. Even the shape of the sill itself, whether it’s U-shaped or V-shaped, influences the volume of water that can pass through. Because deep currents tend to flow as coherent vertical columns, the topography that blocks water at depth can also influence the path of currents closer to the surface.
The Global Conveyor Belt
All of these regional sinking events connect into a planet-spanning loop often called the global conveyor belt. Cold, dense water sinks in the North Atlantic and around Antarctica, then creeps along the ocean floor toward the equator and beyond. As it travels, it gradually mixes with warmer water, slowly rises back toward the surface in the Indian and Pacific Oceans, and eventually flows back toward the Atlantic as surface currents to complete the circuit. A single parcel of water takes roughly 1,000 years to make the full journey.
This conveyor belt moves staggering volumes of water. Ocean scientists measure deep current flow in sverdrups, where one sverdrup equals one million cubic meters of water per second. The Atlantic portion of this system, called the Atlantic Meridional Overturning Circulation (AMOC), redistributes enormous amounts of heat from the tropics toward higher latitudes, which is why Western Europe has a milder climate than its latitude would otherwise suggest.
Why Deep Currents Matter for Climate
Deep ocean currents are one of the planet’s primary mechanisms for moving heat and carbon dioxide around the globe. Water that sinks in the North Atlantic carries dissolved carbon from the atmosphere down into the deep ocean, effectively storing it for centuries. The heat transported northward by the AMOC warms the air over Northern Europe by several degrees compared to what it would be without the circulation.
Climate change threatens to weaken this system. As polar ice sheets melt, they pour fresh water into the ocean, reducing surface salinity and making the water less dense. Less dense water is less likely to sink, which could slow the conveyor belt. A 2025 study from a team including University of Washington researchers projected that the AMOC will weaken by around 18 to 43% by the end of the 21st century, though the researchers noted this is less drastic than some earlier projections had suggested. Even a limited decline could shift rainfall patterns across Africa and South America, alter monsoon systems, and accelerate sea level rise along the U.S. East Coast.
The deep ocean’s circulation is slow and invisible from the surface, but it connects every ocean basin on Earth. The same basic physics, cold salty water sinking under the force of gravity, has been driving this system for millions of years. What changes it, ultimately, is anything that alters the temperature or saltiness of surface water in those critical polar sinking zones.