Atlantic meridional overturning circulation

File:thermohaline_circulation.svg|lang=am|thumb|link=|AMOC in relation to the global thermohaline circulation ([ animation]) The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean 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. 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). 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. 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. 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. ==Role in the climate system==

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 (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. D-O events are best known for the rapid warming of between 8 °C (15 °F) and 15 °C (27 °F) that occurred in Greenland over several decades. The warming of the northern hemisphere would have caused ice-sheet melting and many D-O events appear to have been ended by Heinrich events, in which massive streams of icebergs broke off from the then-present Laurentide ice sheet. As the icebergs melted in the ocean, the ocean water would have become fresher, weakening the circulation and stopping the D-O warming. There is not yet a consensus explanation for why AMOC would have fluctuated so much, and only during this glacial period. Common hypotheses include cyclical patterns of salinity change in the North Atlantic or a wind-pattern cycle due to the growth and decline of the region's ice sheets, which are large enough to affect wind patterns. making the Last Glacial Period a "sweet spot" for such oscillations. For instance, some research suggests changes in sea-ice cover initiated the D-O events because they would have affected water temperature and circulation through Ice–albedo feedback. data and replicated in simulations, is consistent with significant AMOC strengthening. D-O events are numbered in reverse order; the largest numbers are assigned to the oldest events. It was named after the two sites in Denmark with vegetation fossils that could only have survived during a comparatively warm period in the northern hemisphere. The onset of the interstadial also caused a period of sea level rise from ice-sheet collapse that is designated Meltwater pulse 1A. The Bølling and Allerød stages of the interglacial were separated by two centuries of the opposite pattern – northern hemisphere cooling, southern hemisphere warming – which is known as the Older Dryas because the Arctic flower Dryas octopetala became dominant where forests were able to grow during the interglacial. This happened due to an abrupt slowing of the AMOC, Major changes in the precipitation regime, such as the shift of the Intertropical Convergence Zone to the south, increased rainfall in North America, and the drying of South America and Europe, occurred. Global temperatures again barely changed during the Younger Dryas and long-term, post-glacial warming resumed after it ended. == Stability and vulnerability ==

may eventually reverse this shift and re-establish the Pacific circulation after the AMOC shuts down. In the 1960s, Henry Stommel did much of the research into the AMOC with what later became known as the Stommel Box model, which introduced the idea of Stommel Bifurcation in which the AMOC could exist either in a strong state like the one throughout recorded history or effectively collapse to a much weaker state and not recover unless the increased warming and/or freshening that caused the collapse is reduced. The warming and freshening could directly cause the collapse or weaken the circulation to a state in which its ordinary fluctuations (noise) could push it past the tipping point. Modeling AMOC collapse s (GCMs) that represent the "gold standard" for simulating the entire climate but often have to simplify certain interactions. GCMs typically show the AMOC has a single equilibrium state and that it is difficult or impossible for it to collapse. Researchers have raised concerns this modeled resistance to collapse only occurs because GCM simulations tend to redirect large quantities of freshwater toward the North Pole, where it would no longer affect the circulation, a movement that does not occur in nature. of per year, and well above any level considered plausible. According to the researchers, those unrealistic conditions were intended to counterbalance the model's unrealistic stability and the model's output should not be regarded as a prediction but rather as a high-resolution representation of the way currents would start changing before a collapse. Some research indicates classic EMIC projections are biased toward AMOC collapse because they subject the circulation toward an unrealistically constant flow of freshwater. In one study, the difference between constant and variable freshwater flux delayed collapse of the circulation in a typical Stommel's Bifurcation EMIC by over 1,000 years. The researchers said this simulation is more consistent with reconstructions of the AMOC's response to Meltwater pulse 1A 13,500–14,700 years ago and indicates a similarly long delay. In 2022, a paleoceanographic reconstruction found a limited effect from massive freshwater forcing of the final Holocene deglaciation ~11,700–6,000 years ago, when the sea level rise was around . It suggested most models overestimate the effects of freshwater forcing on the AMOC. If the AMOC is more dependent on wind strength – which changes relatively little with warming – than is commonly understood, then it would be more resistant to collapse. According to some researchers, the less-studied Southern Ocean overturning circulation (SOOC) may be more vulnerable to collapse than the AMOC. Some research also suggests the Southern Ocean overturning circulation may be more prone to collapse than the AMOC. In October 2024, 44 climate scientists published an open letter, claiming that according to scientific studies in the past few years, the risk of AMOC collapse has been greatly underestimated, it can occur in the next few decades, with devastating impacts especially for Nordic countries. They called on Nordic countries to ensure the implementation of the Paris Agreement to prevent it. A 2026 preprint study utilizing high-resolution modeling indicates that an AMOC collapse may be significantly more likely than suggested by previous simulations, citing a higher sensitivity to meltwater influx than previously captured. == Trends ==

Until 2024 there was a disagreement between observations showing a slowdown of the circulation and climate models showing a stable circulation. In November 2024, Nature Geoscience published a study which tried to solve the problem. The scientists used "Earth system and eddy-permitting coupled ocean–sea-ice models". Then observations and models corresponded to each other much better. The study found a slowdown of 0.46 sverdrups per decade since 1950. Observations (NAG), which is also known as the Northern Subpolar Gyre (SPG). Measurements taken in 2004 found a 30% decline in the NAG relative to the measurement in 1992; some interpreted this measurement as a sign of AMOC collapse. RAPID data have since shown this to be a statistical anomaly, and observations from 2007 and 2008 have shown a recovery of the NAG. It is now known the NAG is largely separate from the rest of the AMOC and could collapse independently of it. Data up until 2017 showed the decline in 2008 and 2009 was anomalously large but the circulation after 2008 was weaker than it was in 2004–2008. The AMOC is also measured by tracking changes in heat transport that would be correlated with overall current flows. In 2017 and 2019, estimates derived from heat observations made by NASA's CERES satellites and international Argo floats suggested 15–20% less heat transport was occurring than was implied by the RAPID, and indicated a fairly stable flow with a limited indication of decadal variability. The strength of Florida Current has been measured as stable over the last four decades after correction for changes in Earth's magnetic field. Reconstructions Recent past '' study published in 2020 found no significant change in the AMOC circulation compared to that in the 1990s, although substantial changes have occurred across the North Atlantic in the same period. A March 2022 review article concluded while global warming may cause a long-term weakening of the AMOC, it remains difficult to detect when analyzing changes since 1980, including both direct – as that time frame presents both periods of weakening and strengthening – and the magnitude of either change is uncertain, ranging between 5% and 25%. The review concluded with a call for more-sensitive and longer-term research. 20th century reduction in AMOC strength did not occur until 1980, a variation that remains within range of natural variability. Millennial scale and many of the Heinrich events. In 2022, another millennial-scale reconstruction found the Atlantic multidecadal variability strongly displayed increasing "memory", meaning it is now less likely to return to the mean state and instead would proceed in the direction of past variation. Because this pattern is likely connected to the AMOC, it could indicate a "quiet" loss of stability that is not seen in most models. In February 2021, a major study in Nature Geoscience reported the preceding millennium saw an unprecedented weakening of the AMOC, an indication the change was caused by human actions. The study's co-author said the AMOC had already slowed by about 15% and effects now being seen; according to them: "In 20 to 30 years it is likely to weaken further, and that will inevitably influence our weather, so we would see an increase in storms and heatwaves in Europe, and sea level rises on the east coast of the US." Possible indirect signs /NOAA; 20 January 2016). Some researchers have interpreted a range of recently observed climatic changes and trends as being connected to a decline in the AMOC; for instance, a large area of the North Atlantic Gyre Between 2014 and 2016, waters in the area stayed cool for 19 months before warming, and media described this phenomenon as the cold blob. Later research found atmospheric changes, such as an increase in low cloud cover The overall importance of the NAO in the phenomenon is disputed Another possible early indication of a slowing of the AMOC is the relative reduction in the North Atlantic's potential to act as a carbon sink. Between 2004 and 2014, the amount of carbon sequestered in the North Atlantic declined by 20% relative to 1994–2004, which the researchers considered evidence of AMOC slowing. This decline was offset by a comparable increase in the South Atlantic, which is considered part of the Southern Ocean. While the total amount of carbon absorption by all carbon sinks is generally projected to increase throughout the 21st century, a continuing decline in the North Atlantic sink would have important implications. Other processes that were attributed in some studies to AMOC slowing include increasing salinity in the South Atlantic, rapid deoxygenation in the Gulf of St. Lawrence, and an approximately 10% decline in phytoplankton productivity across the North Atlantic over the past 200 years, although this evidence is contested. == Projections ==

Individual models scenario in which levels more than double from 2015 values by 2100 from around 400 parts per million (ppm) to over 850 ppm, they found it declined by over 50% by 2100. The CMIP6 models are not yet capable of simulating North Atlantic Deep Water (NADW) without errors in its depth, area or both, reducing confidence in their projections. that was integrated with an advanced ocean physics module. Due to the module, the AMOC was subjected to four-to-ten times more freshwater when compared to the standard run. It simulated for RCP 4.5 very similar results to those of the 2016 study while below RCP 8.5, the circulation declines by two-thirds soon after 2100 but does not collapse past that level. In 2023, a statistical analysis of output from multiple intermediate-complexity models suggested an AMOC collapse would most likely happen around 2065 (updated from 2057 in August 2025) with 95% confidence of a collapse between 2037 and 2109. This study received a lot of attention and criticism because intermediate-complexity models are considered less reliable in general and may confuse a major slowing of the circulation with its complete collapse. The study relied on proxy temperature data from the Northern Subpolar Gyre region, which other scientists do not consider representative of the entire circulation, believing it may be subject to a separate tipping point. Some scientists have described this research as "worrisome" and noted it can provide a "valuable contribution" once better observational data is available but there was widespread agreement among experts the paper's proxy record was "insufficient". In these runs, a collapse of deep winter convection across subpolar basins preceded the northern overturning shutdown by roughly three decades on average, consistent with a sequence in which the breakdown of deep mixing destabilizes the overturning via Welander (mixing) and Stommel (salt-advection) feedbacks. When the IPCC Fifth Assessment Report was published in 2014, a rapid transition of the AMOC was considered "very unlikely" and this assessment was offered at a high level of confidence. In 2021, the IPCC Sixth Assessment Report again said the AMOC is "very likely" to decline within the 21st century and that there was a "high confidence" changes to it would be reversible within centuries if warming was reversed. and simplified ocean-modelling studies suggesting the AMOC may be more vulnerable to abrupt change than larger-scale models suggest. The synthesis report of the IPCC Sixth Assessment Report summarized the scientific consensus as follows: "The Atlantic Meridional Overturning Circulation is very likely to weaken over the 21st century for all considered scenarios (high confidence), however an abrupt collapse is not expected before 2100 (medium confidence). If such a low probability event were to occur, it would very likely cause abrupt shifts in regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and large impacts on ecosystems and human activities." In 2022, an extensive assessment of all potential climate tipping points identified 16 plausible climate tipping points, including a collapse of the AMOC. It said a collapse would most likely be triggered by of global warming but that there is enough uncertainty to suggest it could be triggered at warming levels of between and . The assessment estimates once AMOC collapse is triggered, it would occur between 15 and 300 years, and most likely at around 50 years. The assessment also treated the collapse of the Northern Subpolar Gyre as a separate tipping point that could tip at between degrees and , although this is only simulated by a fraction of climate models. The most likely tipping point for the collapse of Northern Subpolar Gyre is and once triggered, the collapse of the gyre would occur between 5 and 50 years, and most likely at 10 years. The loss of this convection is estimated to lower the global temperature by while the average temperature in Europe would decrease by around . There would also be substantial effects on regional precipitation levels. UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for a much colder northern Europe and greater sea-level rise along the U.S. East Coast." In February 2025, a study published in Nature concluded that the AMOC is resilient to extreme greenhouse gas and North Atlantic freshwater forcings across 34 climate models, suggesting that an AMOC collapse is unlikely in the 21st century. == Effects of AMOC slowdown ==

; This effect would be caused by increased warming and thermal expansion of coastal waters, which would transfer less of their heat toward Europe; it is one of the reasons sea level rise along the U.S. East Coast is estimated to be three-to-four times higher than the global average. Some scientists believe a partial slowing of the AMOC would result in limited cooling of around in Europe. Other regions would be differently affected; according to 2022 research, 20th-century winter-weather extremes in Siberia were milder when the AMOC was weakened. According to one assessment, a slowing of the AMOC is one of the few climate tipping points that are likely to reduce the social cost of carbon, a common measure of economic impacts of climate change, by −1.4% rather than increasing it, because Europe represents a larger fraction of global GDP than the regions that will be negatively affected by the slowing. This study's methods have been said to have underestimating climate impacts in general. According to some research, the dominant effect on an AMOC slowdown would be a reduction in oceanic heat uptake, leading to increased global warming, but this is a minority opinion. A 2021 study said other well-known tipping points, such as the Greenland ice sheet, the West Antarctic Ice Sheet and the Amazon rainforest would all be connected to the AMOC. According to this study, changes to the AMOC alone are unlikely to trigger tipping elsewhere but an AMOC slowdown would provide a connection between these elements and reduce the global-warming threshold beyond which any of those four elements – including the AMOC itself – could be expected to tip, rather than the thresholds that have been established from studying those elements in isolation. This connection could cause a cascade of tipping over several centuries. == Effects of an AMOC shutdown ==

Cooling in this scenario (middle). The collapse of the entire Atlantic Meriditional Overturning Circulation (bottom). A complete collapse of the AMOC will be largely irreversible A shutting down of the AMOC is expected to trigger substantial cooling in Europe, particularly in Britain and Ireland, France and the Nordic countries. In 2002, research compared AMOC shutdown to Dansgaard–Oeschger events – abrupt temperature shifts that occurred during the Last Glacial Period. According to that paper, local cooling of up to would occur in Europe. In 2022, a major review of tipping points concluded an AMOC collapse would lower global temperatures by around while regional temperatures in Europe would fall by between and . It found within Great Britain an average temperature drop of after the effect of warming was subtracted from collapse-induced cooling. A collapse of the AMOC would also lower rainfall during the growing season by around , which would in turn reduce the area of land suitable for arable farming from 32% to 7%. The net value of British farming would decline by around £346 million per year – over 10% of its value in 2020. In 2024, one study that modeled the effect of an AMOC collapse on a pre-industrial world, predicted a more severe cooling in Europe. It predicted the average sea surface temperatures in northwest Europe falling and the average February temperatures on land falling between and within a century in northern and western Europe. This change would result in sea ice reaching into the territorial waters of the British Isles and Denmark during winter while Antarctic sea ice would diminish. These findings do not include the counteracting warming from climate change, and the modeling approach used by the paper is controversial. This paper has also been controversial. Other Several studies have investigated the effect of a collapse of the AMOC on the El Niño–Southern Oscillation (ENSO); results have ranged from no overall impact to an increase in ENSO strength, and a shift to a dominant La Niña conditions with an about 95% reduction in El Niño extremes but more-frequent extreme rainfall in eastern Australia, and intensified droughts and wildfire seasons in the southwestern U.S. A 2021 study used a simplified modeling approach to evaluate the effects of an AMOC collapse on the Amazon rainforest, and its hypothesized dieback and transition to a savanna state in some climate-change scenarios. This study found an AMOC collapse would increase rainfall in the southern Amazon due to the shift of an Intertropical Convergence Zone, and this would help to counter the dieback and potentially stabilize the southern part of the rainforest. A 2024 study found the seasonal cycle of the Amazon could reverse with dry seasons becoming wet and vice versa. A 2015 study simulated global ocean changes under AMOC slowing and collapse scenarios, and found these events would greatly decrease dissolved oxygen content in the North Atlantic, although dissolved oxygen would slightly increase globally due to greater increases across other oceans. == See also ==