Understanding the Atlantic Meridional Overturning Circulation
We are all connected by one ocean. It moves water, heat, nutrients, dissolved gases (like oxygen and carbon dioxide), and microscopic life around Earth through a vast network of currents.
The Atlantic Meridional Overturning Circulation (AMOC) is a complex system of these currents and eddies (circular movements of water). Together, they form a massive vertical loop that spans the entire length of the Atlantic Ocean.
AMOC: The Key Stats
Every second, the AMOC moves 17 million cubic metres of water northwards. That is roughly equivalent to 6,800 Olympic-sized swimming pools.
This movement transfers 1.2 petawatts (PW) of heat. To put that in perspective, it is about 100 times larger than the total global energy production on Earth from all power sources combined.
Spanning over 20 years (and counting), scientists have taken part in expeditions taking place over 794 days at sea - the equivalent to 2.2 years!
The amount of wire deployed while studying AMOC. alongside this, over 300 moorings and over 3,000 individual pieces of equipment have been used.
What Does the AMOC do?
In the Atlantic Ocean, the AMOC drives warm water northwards along the surface and sends cold, deep waters back southwards. This process delivers vital heat and nutrients to colder latitudes and transfers carbon down to the ocean depths.
Because of this heat transport, the AMOC is critical for regulating the climate in Europe. For example, the mild winters we experience in the UK and north-western Europe happen partly because the AMOC brings heat up from southern latitudes.
How Does Climate Change Impact the AMOC?
The atmosphere and the ocean are interconnected. As climate change warms air temperatures, the ocean surface warms up too. This warming leads to more freshwater entering subpolar regions from Greenland and the Arctic due to additional rain and melting ice.
This matters because fresher, warmer water is less dense than cold, salty water. It does not sink as deeply as it would during cooler conditions. If this sinking process slows down, the ocean cannot transfer carbon dioxide from the atmosphere to the deep ocean as quickly. This leaves larger amounts of greenhouse gases in the atmosphere, which accelerates climate change.
Are the AMOC and the Gulf Stream the Same Thing?
It is common to confuse the Gulf Stream with the AMOC. While the Gulf Stream is an important part of the AMOC system, they are not identical.
The Gulf Stream is a surface current partly driven by the wind. Think of the AMOC as a long journey and the Gulf Stream as a motorway for just one part of that trip. It carries warm water from the South Atlantic to the far north. When this 'motorway' ends, the warm water takes different 'A roads' to continue its journey northward.
How Do Scientist Measure the AMOC?
Scientists measure the AMOC using instruments deployed at different latitudes across the North and South Atlantic Ocean. The National Oceanography Centre (NOC) leads two international programmes in the North Atlantic: Rapid Climate Change (RAPID) and UK Overturning in the Subpolar North Atlantic (OSNAP).
Both programmes use moorings. These are sensors attached to wires that reach hundreds to thousands of metres deep. These special sensors measure the speed, temperature, salinity, and direction of the water flowing past them. Scientists use this data to inform climate models. These models help us predict what could happen to the AMOC as our climate changes.
Will the AMOC Slow Down?
With human-caused climate change increasing global temperatures, scientists say it is likely that the AMOC will slow down.
Climate models show a very high probability of a slowdown throughout the 21st century—which would have severe impacts. The potential for abrupt changes is a subject of ongoing research and debate because of the uncertainties in complex climate models. It is classified as a "low-risk, high-impact" tipping point. This makes maintaining our observing arrays critical; we need them to understand and predict these potential changes.
If the AMOC Slows, What Happens?
The AMOC transports warm water from the tropics northward. A slowdown would disrupt this heat transport, possibly leading to a cooling of Northern Europe. While this might seem odd given global warming trends, regional climate effects can differ from the global trend due to complex interactions between ocean currents and the atmosphere.
A slowdown could alter atmospheric circulation. This might change rainfall patterns, storm tracks, and the frequency of extreme weather events.
Changes in the AMOC influence sea levels. For example, in 2009–2010, a short-term reduction in the AMOC caused a 13 cm sea-level rise along the New York coastline. A long-term slowdown could cause sea levels to rise more rapidly in some regions due to the thermal expansion of seawater.
The AMOC transports nutrients and regulates circulation patterns that influence marine ecosystems. A slowdown could disrupt these ecosystems, impacting fisheries and food sources.
A slowdown could affect the distribution of heat and moisture around the world. This could influence global climate patterns such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD).
Could Abrupt Changes Lead to a "Day After Tomorrow" Scenario?
No. In the film The Day After Tomorrow, the AMOC collapses in a matter of days, causing immediate extreme cold.
In real life, such a rapid transition will not happen. If the AMOC does reach a tipping point, it will happen over several decades at least. However, a slowdown of the AMOC will still lead to major changes in regional climate and generate more extreme weather systems that could damage people and property.
Test Your Knowledge
The AMOC is a complex system of ocean currents vital to the climate system. Take the quiz to find out why it plays such a key role.
Start QuizEvery second, the AMOC moves how many cubic metres of water northwards?
How many petawatts (PW) of heat is moved by the AMOC every second?
Mild winters experienced in the UK and north-western Europe are partly due to what?
Which of these does NOT contribute to freshwater entering subpolar regions from Greenland and the Arctic?
The Gulf Stream, commonly confused with the AMOC, is partly driven by what type of weather?
Which of these is NOT an international programme led by NOC in the North Atlantic?
What is the name of the instrument, made up of sensors attached to wires, that scientists use to measure the health of ocean systems like the AMOC?
Which film's central plot line involved the sudden collapse of the AMOC leading to catastrophic climate events?
Key Publications
Irminger Sea is the center of action for subpolar AMOC variability
Chafik, L.; Holliday, N. P. 
ORCID: https://orcid.org/0000-0002-9733-8002; Bacon, S. ORCID: https://orcid.org/0000-0002-2471-9373; Rossby, T..
2022
Irminger Sea is the center of action for subpolar AMOC variability.
Geophysical Research Letters, 49 (17).
10.1029/2022GL099133
2022
Article
Challenges simulating the AMOC in climate models
Jackson, Laura C.; Hewitt, Helene T.; Bruciaferri, Diego; Calvert, Daley; Graham, Tim; Guiavarc’h, Catherine; Menary, Matthew B.; New, Adrian L. ORCID: https://orcid.org/0000-0002-3159-8872; Roberts, Malcolm; Storkey, David.
2023
Challenges simulating the AMOC in climate models.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 381 (2262).
10.1098/rsta.2022.0187
2023
Article
Large diversity in AMOC internal variability across NEMO-based climate models
Dive Deeper: Ocean Circulation
Ocean circulation is vital to life on Earth. It moves heat, carbon, oxygen, and nutrients around the planet, connecting ecosystems and influencing global climate and weather. These vast currents shape ocean health, regulate temperature, and sustain marine life across the globe.