Atlantic meridional overturning circulation

The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean. It helps shape Earth's climate. The AMOC includes currents at the surface and deep down. These currents move because of changes in weather, temperature, and salinity. They are part of the world's large-scale ocean flow, called the global thermohaline circulation, with the other part happening in the Southern Ocean overturning circulation.

Topographic map of the Nordic Seas and subpolar basins with surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures.

The AMOC works like a giant conveyor belt. Warm, salty water flows north near the surface. Cold, less salty water flows south deep in the ocean. This movement helps move heat, oxygen, carbon, and nutrients around the ocean. It is important for sea life and for storing carbon. When the warm water cools, it becomes heavier and sinks, keeping the system moving. This links areas like the Nordic Seas and the Southern Ocean.

Because of climate change, the AMOC might be getting weaker. More heat in the ocean and extra freshwater from melting ice sheets could slow it down. Some scientists think the AMOC is already weaker than it was before the Industrial Revolution. If it weakens too much, it could change temperatures and weather patterns, especially in places like Scandinavia, Great Britain, and Ireland. It could also raise sea levels near North America.

Overall structure

The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean. It is part of the world's thermohaline circulation. This system connects all the oceans like a giant conveyor belt of moving water.

Warm, less salty water stays near the surface. Colder, denser, saltier water stays deep below. These differences in temperature and saltiness make the water move around the planet.

The Atlantic Ocean has saltier water than the Pacific Ocean. This is because the Atlantic loses a lot of water through evaporation, which leaves salt behind. Also, when sea ice forms near the Arctic Circle, it pushes salt into the surrounding water. This salty water becomes dense and sinks to form deep water called the North Atlantic Deep Water. This deep water is not the very deepest layer. The densest water is called Antarctic bottom water. As this deep water moves, it eventually rises again in different parts of the ocean, continuing the cycle.

Role in the climate system

The Atlantic Ocean helps move heat around the world. Warm water near the equator flows north, bringing heat to places like northwest Europe. This keeps those areas warmer than they would be otherwise.

Heat transfer from the ocean to atmosphere (left) and an increase in Atlantic Ocean heat content (right) observed when the AMOC is strong

The ocean also helps control carbon in the air. Cold water that rises brings nutrients for tiny plants called phytoplankton to grow. This process uses carbon from the air. The ocean also takes in carbon when cold water rises and sinks back down.

Abrupt changes during the Late Pleistocene

Stability and vulnerability

Further information: Multiple equilibria in the Atlantic meridional overturning circulation

In the classic Stommel box models, AMOC tipping occurs either because of a large increase in freshwater volumes which makes the circulation impossible (B-tipping), or because of a lower increase which makes it possible for circulation's own variability to push it to collapse (N-tipping). As freshwater input increases, probability of N-tipping increases. If the probability is at 100%, B-tipping occurs

The AMOC is the main ocean current system in the Atlantic Ocean. It has not always existed. For much of Earth's history, a similar current flowed in the North Pacific. About 34 million years ago, the current shifted to the Atlantic when a gateway between the Arctic and Atlantic closed.

Today, climate change is affecting the AMOC. It is warming the surface water and adding fresh water from melting ice, mainly from Greenland. These changes make it harder for the AMOC to keep moving.

In the 1960s, a scientist named Henry Stommel studied the AMOC. He suggested it could exist in two states: a strong state, like today, or a much weaker state if warming and fresh water continue. Some models show the AMOC might become weaker, while others suggest it is more stable. Scientists are still trying to understand how and when this might happen. There is some uncertainty, and some scientists think the AMOC might become weaker than earlier models suggested, especially if warming continues.

Trends

Until 2024, scientists were not sure if a big ocean current in the Atlantic Ocean was slowing down or staying the same. In November 2024, a study tried to answer this question. The scientists used special computer models to learn more about the ocean. The study found that the current has been slowing down since 1950.

Direct measurements of this ocean current have been available since 2004. These measurements help scientists see how the current is changing. Some earlier smaller measurements suggested parts of the current were weaker, but later data showed some recovery. By 2014, there was enough data to show a decline, but scientists were not sure if this was because of climate change or natural changes over many years.

1992–2002 altimeter data from NASA Pathfinder indicated a slowing (red) in the subpolar gyre region. This was used as a proxy for the AMOC before the initiation of RAPID, and before subsequent research demonstrated the subpolar gyre often behaves separately from the larger circulation.

Scientists also measure the current by tracking changes in heat movement. In 2017 and 2019, satellite and float data suggested less heat was moving, indicating a fairly stable flow with some changes over many years. The strength of another current near Florida has been stable over the last four decades.

Climate reconstructions help scientists understand the past state of the ocean current. In February 2021, a study combining recent data with older records found no overall decline in the current over the past 30 years. Other studies have found changes in the North Atlantic but no clear long-term decline since the 1990s. Some reconstructions suggest the current has weakened since the late 1930s, with an especially weak period between 1975 and 1995, followed by a limited recovery.

Some researchers have linked recent climate changes to a possible decline in the ocean current. For example, a large area of the North Atlantic near Greenland has cooled, which some think might be connected to the current slowing. However, other factors like changes in cloud cover also play a role. There have also been observations of changes in carbon absorption and other ocean properties that some link to the current’s possible slowdown, though these findings are debated.

RAPID tracks both the AMOC itself (third line from the top, labelled MOC) as well as its separate components (three lower lines) and the AMOC flow combined with the subpolar gyre and/or the western boundary current flow (upper two lines) AMOC flow during 2004–2008 appears stronger than afterwards.

Main article: Atlantic meridional overturning circulation

Main articles: Ocean circulation, Thermohaline circulation

Further information: Climate change and the Atlantic Ocean

A 2021 comparison of the post-2004 RAPID observations with the 1980–2004 reconstructed AMOC trend had indicated no real change across 30 years.

Projections

Scientists use special computer programs to study how Earth’s climate might change. These programs show that a big ocean current in the Atlantic, called the AMOC, stays strong even when the planet gets warmer. For example, when scientists tested what would happen if the amount of a certain gas in the air suddenly doubled, the AMOC got weaker but did not completely stop.

Climate models are often calibrated by comparing their simulations after CO2 concentrations have been suddenly quadrupled. Under those conditions, older fifth-generation climate models (top) simulate substantially smaller declines in AMOC strength than the sixth generation (bottom). Years relative to 2021.

More recent studies suggest that the AMOC might weaken more than we thought. Some newer computer programs show that by the year 2100, this current could lose more than half of its strength. However, these programs also have some problems, so scientists are still learning.

To get better results, some scientists adjust their computer programs. In one test, the AMOC completely stopped after 300 years. Other studies show that by the year 2100, the AMOC might weaken by about 18% to 37%, depending on how much the Earth warms.

In 2023, a study suggested the AMOC might collapse around the year 2065, but many scientists think this might not be accurate.

New long-duration CMIP6 projections

In 2025, scientists looked far into the future using advanced computer programs. They found that under a scenario where the Earth warms a lot, the deep part of the AMOC could almost stop by the year 2100. This would change the way water moves in the North Atlantic.

Big reviews of many studies help scientists understand what might happen to the AMOC. Around 2001, scientists thought the AMOC would get weaker but not stop. By 2014, they still thought a sudden stop was very unlikely.

In 2021, a major report said the AMOC is very likely to get weaker this century, but it probably won’t collapse before 2100. However, scientists were less sure about this than before.

In 2022, a study looked at many possible climate changes and said the AMOC could collapse if the Earth gets very warm. If this happens, it could change weather patterns.

A report in 2025 suggested that melting ice and warming waters might be slowing the AMOC. Another study published in February 2025 found that, across many models, the AMOC is strong enough to handle large changes and is unlikely to collapse this century.

Effects of AMOC slowdown

Scientists are not sure if a big ocean current in the Atlantic Ocean, called the AMOC, has slowed down. But they agree that if the Earth keeps changing climate, this slowdown is very likely to happen. If the AMOC weakens, it could change how much rain falls in many places, especially in middle parts of the world and in Europe. It might also make storms stronger along the North Atlantic coast.

A weaker AMOC could also make sea levels rise more quickly along the U.S. East Coast. This happens because the ocean water near the coast gets warmer and expands.

Some scientists think a slower AMOC might make Europe a little cooler, about 1 °C (1.8 °F). Other places, like Siberia, might feel different effects. For example, research shows that when the AMOC was weaker in the past, winters in Siberia were milder.

One study from 2021 suggested that a slowdown of the AMOC could connect other important parts of the Earth's climate system, like the ice sheets in Greenland and West Antarctica, and the Amazon rainforest. While the AMOC slowdown alone might not cause these systems to change suddenly, it could lower the temperature thresholds at which these systems might change dramatically.

Effects of an AMOC shutdown

If the AMOC, the main ocean current in the Atlantic, were to stop completely, it would be very hard to start again and might take thousands of years. This would make temperatures much cooler in Europe, especially in places like Britain, Ireland, France, and the Nordic countries.

The change could also make farming harder in Great Britain because of colder temperatures and less rain. Severe weather might become more common, with stronger winter storms and more snow. Scientists are still learning how these changes might affect other parts of the world, like the Amazon rainforest and weather in the Pacific Ocean.