Thermohaline circulation (THC) is part of the large ocean currents that move water around the world. These currents are created by differences in water density, which are caused by temperature and salt content. The word "thermohaline" comes from "thermo-" (meaning heat) and "haline" (meaning salt). Together, these factors affect how dense seawater is.
Surface ocean currents, like the Gulf Stream, move from the equator toward the poles. As these currents travel, they cool and sink, eventually becoming part of deep ocean water called North Atlantic Deep Water. This deep water then flows into the ocean basins. While most deep water rises in the Southern Ocean, the oldest deep water (which has been traveling for about 1,000 years) rises in the North Pacific. As water moves between ocean basins, it mixes, reducing differences in density and creating a global ocean system. This movement carries heat and dissolved materials, such as gases and salts, around the world. The way this circulation works has a major effect on Earth’s climate.
Thermohaline circulation is sometimes called the "ocean conveyor belt" or "global conveyor belt." This term was first used by scientist Wallace Smith Broecker. It is also known as the meridional overturning circulation (MOC), which describes how temperature and salt differences drive currents. However, not all ocean currents are part of a single global system, as other forces like wind and tides also influence them.
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 research has focused on the AMOC because most people live in the Northern Hemisphere. However, the SMOC is also important for the global climate. Evidence shows that both systems are slowing down because of climate change. This happens as melting ice changes salt levels in Antarctic waters. If either circulation stops working well, it could lead to long dry periods in one part of the world and more rain in another. Ocean ecosystems might receive less nutrients and have less oxygen. In the Northern Hemisphere, if the AMOC weakens, parts of Europe could get colder, and the east coast of North America might experience faster rising sea levels. Scientists believe such changes are unlikely for at least a century, but predictions about them are uncertain.
History of research
It has been known for a long time that wind moves water on the ocean's surface. In the 19th century, some scientists thought that heat movement might also create currents deeper in the ocean. In 1908, Johan Sandström did experiments at a research station in Bornö, Sweden. His work showed that currents driven by heat can exist, but only if heating happens deeper in the ocean than cooling. Usually, the opposite happens because the Sun warms the ocean from above. This makes surface water less dense, causing it to float above cooler, denser water below. This separation of water layers is called ocean stratification. However, wind and tides mix these layers. For example, tidal currents cause mixing between layers, which allows water to move between ocean layers and creates deep ocean currents.
In the 1920s, scientists built on Sandström's work by adding the role of salt in forming ocean layers. Salt is important because, like temperature, it affects water density. Water becomes less dense when it warms because its molecules spread apart. However, water becomes denser when salt is added because the salt increases its mass. Freshwater is most dense at 4°C, but seawater keeps getting denser as it cools until it reaches its freezing point. The freezing point of seawater is lower than that of freshwater because of salt. Depending on salt levels and pressure, seawater can freeze below −2°C.
Structure
Density differences caused by temperature and salinity divide ocean water into separate groups, such as North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). These two water masses are the main forces behind ocean circulation, a system discovered in 1960 by scientists Henry Stommel and Arnold B. Arons. Each water mass has unique chemical, temperature, and isotopic signs (like Pa/Th ratios) that help track their movement, measure their speed, and estimate their age.
NADW forms in the North Atlantic because evaporation there removes more water than rainfall adds. Evaporation leaves salt behind, making surface water in this region unusually salty. The North Atlantic is already cool, and evaporation further lowers the temperature. This dense, cold water sinks in the Norwegian Sea, fills the Arctic Ocean, and flows south through gaps in the Greenland-Scotland Ridge, which connects Greenland, Iceland, and Great Britain. It cannot enter the Pacific Ocean due to the narrow Bering Strait but moves slowly into the deep parts of the South Atlantic.
In the Southern Ocean, strong winds from Antarctica push newly formed sea ice away, creating open water areas called polynyas near the Weddell and Ross Seas, Adélie Coast, and Cape Darnley. Without sea ice protection, the ocean cools rapidly. As sea ice reforms, salt is left behind, increasing the salinity of surface water and making it denser. This process, called brine rejection, creates cold, salty water that sinks to the ocean floor. This dense Antarctic Bottom Water (AABW) 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, this cold, salty water warms and becomes less salty 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 difference creates a slow flow of warmer, fresher water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago, replacing the cold, salty AABW. This process, called haline forcing, involves freshwater added at high latitudes and evaporation at low latitudes. The warm, fresh water from the Pacific rises through the South Atlantic, cools near Greenland, and sinks to the ocean floor, maintaining 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. Some of this water then rises, a process called upwelling. The speed of this rising water is very slow, even compared to the movement of water near the ocean floor. Because of this, it is hard to find where upwelling happens by measuring current speeds, 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 areas. Wallace Broecker used computer models to suggest that most deep upwelling happens in the North Pacific, based on high levels of silicon in the water. Other scientists have not found such clear evidence.
Computer models of ocean currents increasingly show that most deep upwelling occurs 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 made 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 from temperature and salinity data to track the full-depth, basin-wide movement of ocean water. However, the program has only been active since 2004, which is too short a time to fully understand the circulation, as it operates over centuries.
Effects on global climate
The thermohaline circulation helps move heat to polar regions, which affects how much sea ice forms there. However, most heat movement toward the poles happens through the atmosphere, not the ocean. Changes in this circulation may influence Earth's energy balance.
Large amounts of fresh water from Lake Agassiz and melting ice in North America are believed to have changed deep water formation in the North Atlantic. This shift contributed 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. Scientists have "high confidence" that changes to the AMOC could be reversed if global warming stops. However, the report had "medium confidence" that the AMOC would avoid a collapse before the end of the century, a change from earlier reports. This shift in confidence may be due to studies showing that models might overestimate the AMOC's stability.
The IPCC report summarized that the AMOC is likely to weaken throughout the 21st century for all climate scenarios. However, a sudden collapse before 2100 is considered unlikely. If such an event occurred, it could cause sudden changes in weather patterns, such as shifting tropical rain belts and affecting ecosystems and human activities.
As of 2024, scientists have not agreed whether the AMOC has already slowed. However, they believe it will slow further if climate change continues. The IPCC predicts that a weaker AMOC could lead to less rain in mid-latitude regions, more intense rainfall in the tropics and Europe, and stronger storms along the North Atlantic. Research from 2020 suggests a weakened AMOC might slow Arctic sea ice loss and create weather patterns similar to those during the Younger Dryas, such as shifting the Intertropical Convergence Zone southward. These changes would be more extreme under high-emissions scenarios.
A weaker AMOC could speed up sea level rise along the U.S. East Coast. One event linked to temporary AMOC slowing has already been observed. This happens because warmer coastal waters expand, reducing heat transfer to Europe. This is why sea level rise along the East Coast is expected to be three to four times higher than the global average.
The Southern Annular Mode (SAM), which influences Southern Hemisphere weather, has increasingly stayed in its positive phase due to climate change and ozone depletion. This pattern strengthens westerly winds, increasing ocean rainfall and freshening the Southern Ocean. Scientists disagree on whether the 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 could strengthen by about 20% by 2100. Uncertainty remains due to poor representation 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 years.
The AMOC is also affected by warming and meltwater from the Greenland ice sheet. Scientists are unsure whether the AMOC will only weaken or could collapse entirely, becoming a tipping point in the climate system. Evidence from past climates shows the AMOC was weaker during warmer and colder periods than today. However, the Southern Ocean circulation has received less attention than the AMOC, despite its importance. Research on when the Southern Ocean circulation might collapse is less developed than for the AMOC. Some estimates suggest collapse could occur between 1.7°C and 3°C of global warming, but these predictions are less certain than others.