The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean.IPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022. It is a major component of Earth's ocean circulation system and plays an important role in the planet's climate system. The AMOC includes Atlantic currents at the surface and at great depths that are driven by changes in weather, temperature and salinity. Those currents comprise half of the global thermohaline circulation that includes the flow of major ocean currents, the other half being the Southern Ocean overturning circulation.
The AMOC is composed of a northward flow of warm, more saline water in the Atlantic's upper layers and a southward, return flow of cold, less salty, deep water. Warm water from the south is more saline because of the higher evaporation rate in the tropical zone. The warm saline water forms the upper layer of the ocean, but when this layer cools down, the density of the salty water increases, making it sink into the deep. This is an important part of the motor of the AMOC system. The limbs are linked by regions of overturning in the Nordic Seas and the Southern Ocean. Overturning sites are associated with intense exchanges of heat, dissolved oxygen, carbon and other nutrients, and are very important for the ocean's ecosystems and its function as a carbon sink. Changes in the strength of the AMOC can affect multiple elements of the climate system.
Climate change may be weakening the AMOC through increases in ocean heat content and elevated flows of freshwater from melting Greenland ice sheets, compounded by the persistence of the North Atlantic cold blob. Studies using oceanographic reconstructions suggest that , the AMOC was weaker than before the Industrial Revolution. There is debate over the relative contributions of different factors and it is unclear how much of this weakening is due to climate change or the circulation's natural variability over millennia. Climate models predict the AMOC will further weaken during the 21st century.IPCC, 2019: Summary for Policymakers . In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. . This weakening would reduce average air temperatures over Scandinavia, Great Britain, and Ireland, because these regions are warmed by the North Atlantic Current. Weakening of the AMOC would also accelerate sea level rise around North America and reduce primary production in the North Atlantic.
Severe weakening of the AMOC may lead to a collapse of the circulation, which would not be reversible and thus would constitute one of the tipping points in the climate system. A collapse would substantially lower the average temperature and amount of rain and snowfall in Europe. It may also raise the frequency of extreme weather events and have other severe effects.
Overall structure
The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean and is also part of the global thermohaline circulation, which connects the world's oceans with a single "conveyor belt" of continuous water exchange. Normally, relatively warm, less saline water stays on the ocean's surface while deep layers are colder, denser and more saline, in what is known as ocean stratification. Deep water eventually gains heat and/or loses salinity in an exchange with the mixed ocean layer, and becomes less dense and rises towards the surface. Differences in temperature and salinity exist between ocean layers and between parts of the World Ocean, and together they drive the thermohaline circulation. The Pacific Ocean is less saline than the other oceans because it receives large quantities of fresh rainfall. Its surface water is insufficiently saline to sink lower than several hundred meters, meaning deep ocean water must come from elsewhere.
Ocean water in the North Atlantic is more saline than that in the Pacific, partly because extensive evaporation on the surface concentrates salt within the remaining water and partly because sea ice near the Arctic Circle expels salt as it freezes during winter. Even more importantly, evaporated moisture in the Atlantic is swiftly carried away by atmospheric circulation before it can fall back as rain. Trade winds move this moisture across Central America and to the eastern North Pacific, where it falls as rain. Major mountain ranges such as the Tibetan Plateau, the Rocky Mountains and the Andes prevent any equivalent moisture transport back to the Atlantic.
Due to this process, Atlantic surface water becomes salty and therefore dense, eventually downwelling to form the North Atlantic Deep Water (NADW). NADW formation primarily occurs in the Nordic Seas and involves a complex interplay of regional water masses such as the Denmark Strait Overflow Water (DSOW), Iceland-Scotland Overflow Water (ISOW) and Nordic Seas Overflow Water. Labrador Sea Water may play an important role as well but increasing evidence suggests water in Labrador and Irminger Seas primarily recirculates through the North Atlantic Gyre and has little connection with the rest of the AMOC.
The NADW is not the deepest water layer in the Atlantic Ocean; the Antarctic bottom water (AABW) is always the densest, deepest ocean layer in any basin deeper than . As the upper reaches of the AABW flow upwells, it melds into and reinforces the NADW. The formation of the NADW is also the beginning of the lower cell of the circulation. The downwelling that forms the NADW is balanced by an equal amount of upwelling. In the western Atlantic, Ekman transport, the increase in ocean-layer mixing caused by wind activity, results in strong upwelling in the Canary Current and the Benguela Current, which are located on the northwest and southwest coasts of Africa. , upwelling is substantially stronger around the Canary Current than the Benguela Current, though an opposite pattern existed until the closure of the Central American Seaway during the late Pliocene. In the Eastern Atlantic, significant upwelling occurs only during certain months of the year because this region's deep thermocline means it is more dependent on the state of sea surface temperature than on wind activity. There is also a multi-year upwelling cycle that occurs in synchronization with the El Niño/La Niña cycle.
At the same time, the NADW moves southward and at the southern end of the Atlantic transect, around 80% of it upwells in the Southern Ocean, connecting it with the Southern Ocean overturning circulation (SOOC). After upwelling, the water is understood to take one of two pathways. Water surfacing close to Antarctica will likely be cooled by Antarctic sea ice and sink back into the lower cell of the circulation. Some of this water will rejoin the AABW but the rest of the lower-cell flow will eventually reach the depths of the Pacific and Indian oceans. Water that upwells at lower, ice-free latitudes moves further northward due to Ekman transport and is committed to the upper cell. The warm water in the upper cell is responsible for the return flow to the North Atlantic, which occurs mainly around the coast of Africa and through the Indonesian archipelago. Once this water returns to the North Atlantic, it becomes cooler and denser, and sinks, feeding back into the NADW.
Role in the climate system
Equatorial areas are the hottest part of the globe; due to thermodynamics, this heat moves towards the poles. Most of this heat is transported by atmospheric circulation but warm, surface ocean currents play an important role. Heat from the equator moves either northward or southward; the Atlantic Ocean is the only ocean in which the heat flow is northward. Much of the heat transfer in the Atlantic occurs due to the Gulf Stream, a surface current that carries warm water northward from the Caribbean. While the Gulf Stream as a whole is driven by winds alone, its northernmost segment, the North Atlantic Current, obtains much of its heat from thermohaline exchange in the AMOC. Thus, the AMOC carries up to 25% of the total heat toward the northern hemisphere, and plays an important role in the climate around northwest Europe.
Because atmospheric patterns also play a large role in heat transfer, the idea the climate in northern Europe would be as cold as that in northern North America without heat transport via ocean currents (i.e. up to colder) is generally considered incorrect. While one modeling study suggested collapse of the AMOC could result in Ice Age-like cooling, including sea-ice expansion and mass glacier formation, within a century, the accuracy of those results is questionable. There is a consensus the AMOC keeps northern and western Europe warmer than it would be otherwise, with the difference of and depending on the area. For instance, studies of the Florida Current suggest the Gulf Stream was around 10% weaker from around 1200 to 1850 due to increased surface salinity, and this likely contributed to the conditions known as Little Ice Age.
The AMOC makes the Atlantic Ocean into a more effective carbon sink in two major ways. Firstly, the upwelling that takes place supplies large quantities of nutrients to the surface waters, supporting the growth of phytoplankton and therefore increasing marine primary production and the overall amount of photosynthesis in the surface waters. Secondly, upwelled water has low concentrations of dissolved carbon because the water is typically 1,000 years old and has not been exposed to anthropogenic increases in the atmosphere. This water absorbs larger quantities of carbon than the more-saturated surface waters and is prevented from releasing carbon back into the atmosphere when it is downwelled. While Southern Ocean is by far the strongest ocean carbon sink, The North Atlantic is the largest single carbon sink in the northern hemisphere.
Abrupt changes during the Late Pleistocene
Because the Atlantic meridional overturning circulation (AMOC) is dependent on a series of interactions between layers of ocean water of varying temperature and salinity, it is not static but experiences small, cyclical changes and larger, long-term shifts in response to external forcings. Many of those shifts occurred during the Late Pleistocene (126,000 to 11,700 years ago), which was the final geological epoch before the current Holocene. It also includes the Last Glacial Period, which is colloquially known as the "last ice age". Twenty-five abrupt temperature oscillations between the hemispheres occurred during this period; these oscillations are known as Dansgaard–Oeschger events (D-O events) after Willi Dansgaard and Hans Oeschger, who discovered them by analyzing Greenland ice cores in the 1980s.