Thermohaline circulation (THC) is a part of the large-scale movement of ocean water caused by differences in water density. These density differences are created by changes in temperature and salt content at the ocean's surface. The term "thermohaline" comes from "thermo-" (meaning temperature) and "haline" (meaning salt), which together influence the density of seawater.

Wind-driven surface currents, like the Gulf Stream, move from the equator toward the poles in the Atlantic Ocean. As the water travels, it cools and becomes denser, eventually sinking to form deep ocean water that flows into the ocean basins. Most of the thermohaline water rises up in the Southern Ocean, but the oldest water (which has traveled for about 1,000 years) rises in the North Pacific. Mixing between ocean basins reduces density differences, creating a connected global ocean system. This movement carries heat, dissolved solids, and gases across the planet. As a result, the circulation strongly affects Earth's climate.

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

This global system has two main parts: the Atlantic meridional overturning circulation (AMOC), which occurs in the North Atlantic Ocean, and the Southern Ocean overturning circulation (SMOC), which happens near Antarctica. Since most people live in the Northern Hemisphere, more research has focused on the AMOC. However, the SMOC is equally important for Earth's climate. Evidence shows both systems are slowing due to climate change, as melting ice sheets add more freshwater to the ocean, reducing the salt content of Antarctic bottom water. If either circulation weakens significantly, it could lead to long-term changes in weather patterns, such as more droughts in one hemisphere and increased rainfall in the other. Marine life might receive fewer nutrients and face lower oxygen levels. In the Northern Hemisphere, a collapse of the AMOC could cause colder temperatures in Europe and faster sea-level rise along the eastern coast of North America. While scientists believe such changes might take more than a century to happen, predictions remain uncertain.

History of research

Wind has been known to drive ocean currents, but only on the surface. In the 19th century, some oceanographers proposed that heat movement could create deeper 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 at a greater depth than cooling." Usually, the opposite occurs because sunlight warms ocean water from above, making it less dense. This causes the surface layer to float above cooler, denser layers, forming ocean layers. However, wind and tides mix these layers. Mixing caused by tidal currents is one example of this process. This mixing allows heat to move between ocean layers, creating 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 most dense at 4°C, but seawater continues to get denser as it cools until it reaches its freezing point. The freezing point of seawater is lower than that of freshwater due to salinity and can be below −2°C, depending on salinity and pressure.

Structure

Differences in water density caused by temperature and salinity create separate groups of ocean water, such as 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 Henry Stommel and Arnold B. Arons. Each has unique chemical, temperature, and isotopic markers, such as Pa/Th ratios, which scientists use to track their movement, calculate their speed, and determine their age. NADW forms in the North Atlantic, where evaporation, which increases salinity, is greater than precipitation, which adds fresh water. Evaporation leaves salt behind, making surface water in this region very salty. The North Atlantic is already cool, and evaporation also lowers water temperature further. This dense water sinks in the Norwegian Sea, fills the Arctic Ocean, and flows south through the Greenland-Scotland Ridge. It cannot reach the Pacific Ocean due to the narrow Bering Strait but instead moves into the deep southern Atlantic.

In the Southern Ocean, strong winds from Antarctica blow sea ice away, creating open water areas called polynyas near places like the Weddell and Ross Seas. Without sea ice, the ocean cools rapidly. As sea ice reforms, salt is left behind, increasing the salinity of surface water. This makes the water very dense. When sea ice forms, saltier brine remains in the ice, lowering the freezing point of seawater. This brine melts the ice beneath it and eventually sinks, a process called brine rejection. The dense Antarctic Bottom Water (AABW) then flows north and east. AABW from the Weddell Sea fills the Atlantic and Indian Ocean basins, while AABW from the Ross Sea moves toward the Pacific. In the Indian Ocean, cold, salty water from the Atlantic mixes with warmer, fresher water from the tropical Pacific, a process called overturning. In the Pacific, cold, salty water from the Atlantic warms and becomes fresher more quickly.

The movement of cold, salty water from the Atlantic to the Pacific lowers the Atlantic’s sea level slightly and increases its salinity compared to the Pacific. This creates a slow flow of warmer, fresher water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago, replacing the cold, salty Antarctic Bottom Water. This process is called "haline forcing" (a balance between freshwater gained at high latitudes and evaporation at low latitudes). The warmer, fresher Pacific water moves into the South Atlantic, cools, and sinks near Greenland, continuing the thermohaline circulation.

Upwelling

As deep water moves into the ocean basins, it pushes aside older deep-water masses. These older masses become less dense over time because of mixing in the ocean. This movement causes some water to rise, a process 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, as other wind-driven processes also affect the surface ocean. Deep water has a unique chemical makeup, created by the breakdown of small particles that fall into it over long periods. Scientists have tried using these chemical clues to find where upwelling happens. Wallace Broecker used simplified models and found that most deep upwelling occurs in the North Pacific, based on high levels of silicon in these waters. Other researchers have not found such clear evidence.

Computer models of ocean movement increasingly suggest that most deep upwelling happens in the Southern Ocean, where strong winds blow between South America and Antarctica. Direct measurements of the strength of the thermohaline circulation have also been taken at 26.5°N in the North Atlantic by the UK-US RAPID program. This program combines direct measurements of ocean movement using instruments and underwater cables with estimates of geostrophic currents based on temperature and salinity data. These methods provide ongoing, full-depth measurements of the ocean's meridional overturning circulation. However, the program has only been active since 2004, which is too short a time to fully understand processes that occur over centuries.

Effects on global climate

The thermohaline circulation helps move heat toward 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 may affect Earth’s overall energy balance.

Large amounts of low-density meltwater from Lake Agassiz and ice melting in North America are believed to have changed deep water formation in the North Atlantic, leading 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 changes to the AMOC could be reversed over centuries if global warming stops. Unlike the Fifth Assessment Report, the Sixth had "medium confidence" rather than "high confidence" that the AMOC would avoid collapse by the end of the century. This change in confidence may be due to studies showing that models might overestimate the stability of the AMOC.

The IPCC report summarized that the AMOC is likely to weaken by the end of the century, but a sudden collapse is not expected before 2100. If such an event happened, it could cause sudden changes in weather patterns, such as shifting tropical rain belts and affecting ecosystems and human activities.

As of 2024, scientists are not certain if the AMOC has already slowed, but they agree it will likely weaken if climate change continues. The IPCC says future AMOC weakening could reduce rainfall in mid-latitude regions, change precipitation patterns in the tropics and Europe, and increase storm intensity 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.

A weaker AMOC could also increase sea level rise along the U.S. East Coast. This is because warmer coastal waters expand, trapping more heat near the coast instead of moving it toward Europe. Sea level rise along this coast is expected to be three to four times higher than the global average.

In the Southern Hemisphere, the Southern Annular Mode (SAM) influences climate patterns. Climate change and ozone depletion have caused SAM to stay in its positive phase more often, leading to stronger winds, more ocean warming, and fresher Southern Ocean waters. Scientists are unsure if Southern Ocean circulation will continue to respond to SAM changes as it does now. Current models suggest the lower part of the circulation may weaken, while the upper part might strengthen by about 20% by the end of the century. Uncertainty remains due to inconsistent modeling of ocean layers in climate models. Antarctic meltwater plays a major role in Southern Ocean circulation, but predictions about Antarctic ice loss have been uncertain for many years.

Similar to the AMOC, the Southern Ocean circulation is affected by warming and meltwater from the Greenland ice sheet. Both circulations might weaken further or collapse under continued warming and freshening, which could be difficult to reverse and represent climate tipping points. Evidence from Earth’s past shows that ocean circulation was weaker during warmer and colder periods than today. However, the Southern Ocean receives less attention than the AMOC, and research on its potential collapse is limited. Some studies suggest the Southern Ocean circulation might collapse if global warming reaches 1.7 to 3 degrees Celsius, but this estimate is less certain than for other tipping points.

Other sources

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