Thermohaline Circulation
Daily Earth Science Environment Hydrosphere Repost

Thermohaline circulation

The wind drives currents at the surface of the ocean, but what drives the deep ocean currents far below the surface? The answer is density, gravity, and geography.  The engine moving these deep currents is thermohaline circulation. “Thermo” since cold water is denser than warm water, and “haline” because saltwater is denser than freshwater. 

So cold, salty water has a high density and sinks to the ocean floor. But the story doesn’t stop there. Gravity and geography (or geomorphology) then come into play. Imagine a water tank with a sloped bottom, and I place a drop of mercury on the surface of the water. The mercury sinks to the bottom of the tank since it is much denser than the water. But once it reaches the floor of the container, it continues to move down the sloped bottom to the lowest point in the tank. Gravity and the tank shape have worked together, creating this outcome.  

If we take this simple example and apply it to the oceans, we find the same principles at work with thermohaline circulation. Cold saline water sinks to the ocean bottom. Gravity then takes over, and the water moves downslope, always seeking the lowest elevation. If the supply of dense water is continuous, then the gravity-driven flow of water becomes a deep ocean current.

The simple physics driving this process creates a series of deep ocean currents around the world. Water can travel for up to 1,600 years in these deep ocean currents before it surfaces again.  

Polar ice

There are two primary ways that seawater can increase in salinity – evaporation and polar ice. Deep ocean currents that originate in the North Atlantic ocean are influenced by dense water created during the formation of polar sea ice. Sea ice is not salty, so when sea ice forms, the salt is forced out of the freezing water in a process called “brine exclusion.” The net effect is that the remaining seawater becomes saltier, hence denser. 

These salty waters are already as cold as they can get without freezing, and the cold, dense water sinks. In the Arctic, this dense water makes its way into the North Atlantic Ocean. The bottom water is blocked by seafloor geography from flowing into the Pacific Ocean. The passageway from the Arctic to the Pacific Ocean is through the Bering Strait. But the seafloor is shallow at this location, and the dense bottom water cannot flow uphill to the Pacific Ocean. Downslope movement takes the Arctic water to the Atlantic side.

Flow from the tropics

When dense seawater departs from the polar seas, then a mass balance problem arises. Since water is a fluid, a net loss in one area requires that water flow in from somewhere else to replace the lost volume. In the North Atlantic, this need is fulfilled by the Gulf Stream and North Atlantic Current. These two surface currents allow warm water from the tropics to flow northward, thus filling the void left by the departing Arctic seawater.

These warm waters are a blessing to western Europe, and they keep the British Isles and coastal nations from freezing over every winter. Europe would have considerably colder winters, were it not for the Gulf Stream and North Atlantic Current. Summers would be cooler also.

Now, the environment in the tropics is hot and sunny. When sunshine, heat, and water mix, then we get evaporation. The evaporation process removes freshwater leaving behind saltier water, hence denser water. Once this saltier surface water arrives in the North Atlantic, it gives up its heat to Europe. Then the cold salty water sinks and becomes part of the engine driving deep ocean currents.     

Thermohaline circulation affects weather and climatic conditions around the globe. Also, the overturn of surface water to deep ocean water keeps the deep oceans oxygenated. If these circulation patterns slow or disappear, then dramatic changes are in store for the world’s ecosystems.


Riding the global conveyor belt (Source:ArcheanWeb) – Also:

The Antarctic Circumpolar Current: An Ouroboros (Source: ArcheanWeb) – Also:

Gulf Stream and the Little Ice Age (Source: ArcheanWeb) – Also:


Major study uncovers ‘sea change’ in world’s understanding of Atlantic conveyor belt (By Daisy Dunne; CarbonBrief) – Also:

Ocean on the Move: Thermohaline Circulation (Source: UCAR – Center for Science Education) – Also:

Feature Image: Thermohaline Circulation (Source: NASA Earth Observatory) –  – This file is in the public domain in the United States because it was solely created by NASA. NASA copyright policy states that “NASA material is not protected by copyright

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.