Thermohaline circulation (THC) is a type of large ocean current that moves water around the world. It is powered by differences in water density, which are caused by changes in temperature and salt levels. The word "thermohaline" comes from "thermo-" (meaning heat) and "haline" (meaning salt), which together affect how dense seawater is.

Wind-driven surface currents, like the Gulf Stream, move water from the equator toward the poles. As the water travels, it cools and becomes heavier, eventually sinking to form deep ocean water, such as North Atlantic Deep Water. This deep water then flows into other parts of the ocean. While most of this deep water rises back to the surface in the Southern Ocean, the oldest water (which has been traveling for about 1,000 years) rises in the North Pacific. Mixing between different ocean basins helps balance their densities, creating a connected global ocean system. This movement of water carries heat and dissolved materials, such as gases and salts, around the world. As a result, the circulation plays a major role in shaping Earth's climate.

Thermohaline circulation is sometimes called the "ocean conveyor belt" or "global conveyor belt," a term created by scientist Wallace Smith Broecker. It is also known as the meridional overturning circulation (MOC), which describes how water moves due to temperature and salt differences. However, not all ocean currents are part of a single global system, as some are influenced by other factors like wind and tides.

The global circulation has two main parts: the Atlantic meridional overturning circulation (AMOC), which is centered in the North Atlantic Ocean, and the Southern Ocean overturning circulation (SMOC), which is near Antarctica. Most people live in the Northern Hemisphere, so more research has focused on the AMOC. However, the SMOC is also important for Earth's climate. Evidence shows that both circulations are slowing down because of climate change, as melting ice adds more freshwater to the ocean, which lowers the salt levels in Antarctic bottom water. If either circulation were to collapse completely, it could cause major changes in climate. For example, one hemisphere might experience more droughts, while the other becomes wetter. Ocean ecosystems could also suffer from less oxygen and fewer nutrients. In the Northern Hemisphere, a collapse of the AMOC might lead to colder temperatures in Europe and faster rising sea levels along the eastern coast of North America. Scientists believe such changes are unlikely for at least a century, but predictions are uncertain.

History of research

Wind has long been known to drive ocean currents, but only at the surface. In the 19th century, some oceanographers proposed that heat movement could create deeper ocean currents. In 1908, Johan Sandström conducted experiments at the Bornö Marine Research Station, showing that currents driven by heat transfer can exist, but only if heating happens deeper than cooling. Usually, the opposite occurs because the Sun warms ocean water from above, making it less dense. This causes the surface layer to float above cooler, denser layers, leading to ocean stratification. However, wind and tides mix these layers. For example, tidal currents cause mixing between different water layers, which allows convection between ocean layers and creates deep water currents.

In the 1920s, Sandström’s ideas were expanded to include the role of salinity in ocean layer formation. Salinity is important because, like temperature, it affects water density. Water becomes less dense when its temperature increases, as molecules spread apart. However, water becomes denser when salinity increases because more salt dissolves in it. Freshwater is densest at 4°C, but seawater becomes denser as it cools until it reaches the freezing point. The freezing point of seawater is lower than that of freshwater due to its salt content and can be below −2°C, depending on salinity and pressure.

Structure

Differences in water density caused by temperature and salinity help divide ocean water into separate groups, such as the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). These two water groups are the main causes of ocean circulation, a system first described in 1960 by scientists Henry Stommel and Arnold B. Arons. These waters have unique chemical, temperature, and isotopic markers (such as Pa/Th ratios), which scientists can use to track their movement, measure their speed, and estimate their age.

NADW forms in the North Atlantic because this region is rare in the ocean where evaporation, which adds salt to the water, is greater than precipitation, which adds fresh water. When water evaporates, it leaves salt behind, making the surface water in the North Atlantic very salty. The North Atlantic is also already cool, and evaporation further lowers the water temperature. This dense, cold, and salty water sinks in the Norwegian Sea, fills the Arctic Ocean, and moves south through cracks in underwater ridges that connect Greenland, Iceland, and Great Britain. It cannot flow into the Pacific Ocean because of the narrow and shallow Bering Strait, but it slowly moves into the deep ocean floor of the South Atlantic.

In the Southern Ocean, strong winds from Antarctica blow sea ice away, creating open water areas called polynyas in places like the Weddell and Ross Seas, near the Adélie Coast and Cape Darnley. Without sea ice to protect it, the ocean cools rapidly. As sea ice reforms, salt is left behind in the water, making it even saltier and denser. When sea ice forms, pure water freezes first, leaving behind saltier brine. This brine lowers the freezing point of the surrounding water, causing it to melt the ice beneath it and eventually sink. This process is called brine rejection. The dense Antarctic Bottom Water (AABW) sinks and flows north and east. AABW from the Weddell Sea mainly fills the Atlantic and Indian Ocean basins, while AABW from the Ross Sea flows toward the Pacific Ocean. In the Indian Ocean, cold and salty water from the Atlantic mixes with warmer and fresher water from the tropical Pacific in a process called overturning. In the Pacific Ocean, the cold and salty water from the Atlantic warms and becomes fresher more quickly.

The movement of cold and salty water from the Atlantic to the Pacific causes the Atlantic’s sea level to be slightly lower than the Pacific’s and makes the Atlantic’s water saltier. This creates a slow but large flow of warmer and fresher water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago to replace the cold and salty Antarctic Bottom Water. This process is called haline forcing, which involves more fresh water near the poles and more evaporation near the equator. This warmer, fresher water from the Pacific rises through the South Atlantic, reaches Greenland, cools, and sinks to the ocean floor, creating a continuous thermohaline circulation.

Upwelling

As deep ocean water moves into ocean basins, it pushes aside older deep-water masses. These older masses become less dense over time because of mixing in the ocean. This process causes some water to rise, a movement called upwelling. Upwelling happens very slowly, even compared to the movement of water near the ocean floor. Because of this, it is hard to find where upwelling occurs by measuring current speeds alone, since other wind-driven processes also affect the surface ocean. Deep water has a unique chemical makeup, created by the breakdown of tiny particles that fall into it over long periods. Scientists have used these chemical clues to try to locate upwelling. Wallace Broecker studied this using box models and suggested that most deep upwelling happens in the North Pacific, based on high levels of silicon found in these waters. Other researchers have not found such clear evidence.

Computer models of ocean circulation now show that much of the deep upwelling occurs in the Southern Ocean, where strong winds blow between South America and Antarctica. Scientists have also measured the strength of the thermohaline circulation in the North Atlantic at 26.5°N through the UK-US RAPID program. This program combines direct measurements of ocean movement using current meters and subsea cables with estimates of geostrophic currents from temperature and salinity data. These methods provide continuous, full-depth measurements of the ocean's meridional overturning circulation. However, the program has only been active since 2004, which is a short time compared to the centuries-long timescale of ocean circulation.

Effects on global climate

The thermohaline circulation helps move heat to the polar regions, which affects the amount of sea ice there. However, most heat movement toward the poles happens in the atmosphere, not the ocean. Changes in this circulation can influence how much heat the Earth absorbs and reflects.

Large amounts of meltwater from Lake Agassiz and the melting of glaciers in North America are believed to have changed deep water formation in the North Atlantic. These changes are linked to a cold climate period in Europe called the Younger Dryas.

In 2021, the IPCC Sixth Assessment Report stated that the Atlantic Meridional Overturning Circulation (AMOC) is "very likely" to weaken during the 21st century. It also said that if global warming stops, changes to the AMOC could be reversed over centuries. Unlike the Fifth Assessment Report, the Sixth had "medium confidence" that the AMOC would avoid collapsing before 2100. This change in confidence may be due to studies showing that models used to predict circulation patterns might not be fully accurate.

The IPCC Sixth Assessment Report concluded that the AMOC is likely to weaken throughout the 21st century. However, a sudden collapse before 2100 is not expected with medium confidence. If such a collapse were to happen, it could cause sudden changes in weather patterns, such as a shift in tropical rain belts, and harm ecosystems and human activities.

As of 2024, scientists have not agreed whether the AMOC has already slowed. However, they are certain it will slow if climate change continues. The IPCC says future AMOC weakening could lead to less rain in mid-latitude regions, more rain in the tropics and Europe, and stronger storms along the North Atlantic. A 2020 study found that a weaker AMOC might slow Arctic sea ice loss and cause weather patterns similar to those during the Younger Dryas, such as a shift in the Intertropical Convergence Zone. These effects would be more extreme under high-emissions scenarios.

A weaker AMOC could increase sea level rise along the U.S. East Coast. This is because warmer ocean water expands, reducing heat transfer to Europe. Sea levels there are expected to rise three to four times faster than the global average.

The Southern Annular Mode (SAM), which influences Southern Hemisphere weather, has spent more time in its positive phase due to climate change and ozone depletion. This leads to stronger winds, more rain over oceans, and further freshening of the Southern Ocean. Scientists are unsure if the Southern Ocean’s circulation will continue to respond to SAM changes as it does now. Models suggest the lower part of the circulation may weaken, while the upper part could strengthen by about 20% by 2100. Uncertainty remains because models struggle to accurately represent ocean layers. Antarctic meltwater plays a major role in Southern Ocean circulation, but predicting future ice loss has been difficult.

The AMOC is also affected by warming and meltwater from Greenland’s shrinking ice sheet. Scientists are unsure if the AMOC and Southern Ocean circulation will only weaken or if they might collapse entirely, becoming irreversible tipping points. Evidence from Earth’s past shows the AMOC was weaker during both warmer and colder periods. However, the Southern Ocean has received less attention than the AMOC, and research on its future is limited. Some studies suggest the Southern Ocean’s circulation might collapse if global warming reaches 1.7°C to 3°C, but this estimate is less certain than for other tipping points.

Other sources

• Apel, JR (1987). Principles of Ocean Physics. Academic Press. ISBN 0-12-058866-8.
• Gnanadesikan, A.; R. D. Slater; P. S. Swathi; G. K. Vallis (2005). "Energy in Ocean Heat Movement." Journal of Climate. 18 (14): 2604–16. Bibcode: 2005JCli…18.2604G. doi: 10.1175/JCLI3436.1.
• Knauss, JA (1996). Introduction to Physical Oceanography. Prentice Hall. ISBN 0-13-238155-9.