Atlantic multidecadal oscillation

The Atlantic Multidecadal Oscillation (AMO) is a climate cycle that affects the sea surface temperature (SST) of the North Atlantic Ocean based on different modes on multidecadal timescales.[1] While there is some support for this mode in models and in historical observations, controversy exists with regard to its amplitude, and in particular, the attribution of sea surface temperature change to natural or anthropogenic causes, especially in tropical Atlantic areas important for hurricane development.[2] The Atlantic multidecadal oscillation is also connected with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations.[3]

AMO Pattern
Atlantic multidecadal oscillation spatial pattern obtained as the regression of monthly HadISST sea surface temperature anomalies (1870-2013).
Atlantic Multidecadal Oscillation
Atlantic Multidecadal Oscillation Index according to the methodology proposed by van Oldenborgh et al. 1880-2018.
Amo timeseries 1856-present
Atlantic Multidecadal Oscillation index computed as the linearly detrended North Atlantic sea surface temperature anomalies 1856-2013.


The Atlantic Multidecadal Oscillation (AMO) was identified by Schlesinger and Ramankutty in 1994.[4]

The AMO signal is usually defined from the patterns of SST variability in the North Atlantic once any linear trend has been removed. This detrending is intended to remove the influence of greenhouse gas-induced global warming from the analysis. However, if the global warming signal is significantly non-linear in time (i.e. not just a smooth linear increase), variations in the forced signal will leak into the AMO definition. Consequently, correlations with the AMO index may mask effects of global warming.[5]

AMO Index

Several methods have been proposed to remove the global trend and El Niño-Southern Oscillation (ENSO) influence over the North Atlantic SST. Trenberth and Shea, assuming that the effect of global forcing over the North Atlantic is similar to the global ocean, subtracted the global (60°N-60°S) mean SST from the North Atlantic SST to derive a revised AMO index.[6]

Ting et al. however argue that the forced SST pattern is not globally uniform; they separated the forced and internally generated variability using signal to noise maximizing EOF analysis.[2]

Van Oldenborgh et al. derived an AMO index as the SST averaged over the extra-tropical North Atlantic (to remove the influence of ENSO that is greater at tropical latitude) minus the regression on global mean temperature.[7]

Guan and Nigam removed the non stationary global trend and Pacific natural variability before applying an EOF analysis to the residual North Atlantic SST.[8]

The linearly detrended index suggests that the North Atlantic SST anomaly at the end of the twentieth century is equally divided between the externally forced component and internally generated variability, and that the current peak is similar to middle twentieth century; by contrast the others methodology suggest that a large portion of the North Atlantic anomaly at the end of the twentieth century is externally forced.[2]

Frajka-Williams et al. 2017 pointed out that recent changes in cooling of the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics, increased the spatial distribution of meridional gradient in sea surface temperatures, which is not captured by the AMO Index.[3]


Based on the about 150-year instrumental record a quasi-periodicity of about 70 years, with a few distinct warmer phases between ca. 1930–1965 and after 1995, and cool between 1900–1930 and 1965–1995 has been identified.[9] In models, AMO-like variability is associated with small changes in the North Atlantic branch of the Thermohaline Circulation.[10] However, historical oceanic observations are not sufficient to associate the derived AMO index to present-day circulation anomalies. Models and observations indicate that changes in atmospheric circulation, which induce changes in clouds, atmospheric dust and surface heat flux, are largely responsible for the tropical portion of the AMO.[11][12]

The Atlantic Multidecadal Oscillation (AMO) is important for how external forcings are linked with North Atlantic SSTs.[13]

Climate impacts worldwide

The AMO is correlated to air temperatures and rainfall over much of the Northern Hemisphere, in particular in the summer climate in North America and Europe.[14][15] Through changes in atmospheric circulation, the AMO can also modulate spring snowfall over the Alps [16] and glaciers' mass variability.[17] Rainfall patterns are affected in North Eastern Brazilian and African Sahel. It is also associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricane activity.[6]

Recent research suggests that the AMO is related to the past occurrence of major droughts in the US Midwest and the Southwest. When the AMO is in its warm phase, these droughts tend to be more frequent or prolonged. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965: The Dust Bowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite—warm AMO, more rainfall.[18]

Climate models suggest that a warm phase of the AMO strengthens the summer rainfall over India and Sahel and the North Atlantic tropical cyclone activity.[19] Paleoclimatologic studies have confirmed this pattern—increased rainfall in AMO warmphase, decreased in cold phase—for the Sahel over the past 3,000 years.[20]

Relation to Atlantic hurricanes

North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone Energy Index 1950–2015
North Atlantic tropical cyclone activity according to the Accumulated Cyclone Energy Index, 1950–2015. For a global ACE graph visit this link.

A 2008 study correlated the Atlantic Multidecadal Mode (AMM), with HURDAT data (1851–2007), and noted a positive linear trend for minor hurricanes (category 1 and 2), but removed when the authors adjusted their model for undercounted storms, and stated "If there is an increase in hurricane activity connected to a greenhouse gas induced global warming, it is currently obscured by the 60 year quasi-periodic cycle."[21] With full consideration of meteorological science, the number of tropical storms that can mature into severe hurricanes is much greater during warm phases of the AMO than during cool phases, at least twice as many; the AMO is reflected in the frequency of severe Atlantic hurricanes.[18] Based on the typical duration of negative and positive phases of the AMO, the current warm regime is expected to persist at least until 2015 and possibly as late as 2035. Enfield et al. assume a peak around 2020.[22]

Since 1995, there have been nine Atlantic hurricane seasons considered "extremely active" by Accumulated Cyclone Energy - 1995, 1996, 1998, 1999, 2003, 2004, 2005, 2010 and 2017.

Periodicity and prediction of AMO shifts

There are only about 130–150 years of data based on instrument data, which are too few samples for conventional statistical approaches. With the aid of multi-century proxy reconstruction, a longer period of 424 years was used by Enfield and Cid–Serrano as an illustration of an approach as described in their paper called "The Probabilistic Projection of Climate Risk".[23] Their histogram of zero crossing intervals from a set of five re-sampled and smoothed version of Gray et al. (2004) index together with the maximum likelihood estimate gamma distribution fit to the histogram, showed that the largest frequency of regime interval was around 10–20 year. The cumulative probability for all intervals 20 years or less was about 70%.

There is no demonstrated predictability for when the AMO will switch, in any deterministic sense. Computer models, such as those that predict El Niño, are far from being able to do this. Enfield and colleagues have calculated the probability that a change in the AMO will occur within a given future time frame, assuming that historical variability persists. Probabilistic projections of this kind may prove to be useful for long-term planning in climate sensitive applications, such as water management.

Assuming that the AMO continues with a quasi-cycle of roughly 70 years, the peak of the current warm phase would be expected in c. 2020,[24] or based on a 50–90 year quasi-cycle, between 2000 and 2040 (after peaks in c. 1880 and c. 1950).[22]

A 2017 study predicts a continued cooling shift beginning 2014, and the authors note, "..unlike the last cold period in the Atlantic, the spatial pattern of sea surface temperature anomalies in the Atlantic is not uniformly cool, but instead has anomalously cold temperatures in the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics. The tripole pattern of anomalies has increased the subpolar to subtropical meridional gradient in SSTs, which are not represented by the AMO index value, but which may lead to increased atmospheric baroclinicity and storminess."[3]


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  8. ^ Guan, Bin; Sumant Nigam (2009). "Analysis of Atlantic SST Variability Factoring Interbasin Links and the Secular Trend: Clarified Structure of the Atlantic Multidecadal Oscillation". J. Climate. 22 (15): 4228–4240. Bibcode:2009JCli...22.4228G. doi:10.1175/2009JCLI2921.1.
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  12. ^ Yuan, T.; L. Oreopoulos; M. Zalinka; H. Yu; J. R. Norris; M. Chin; S. Platnick; K. Meyer (2016). "Positive low cloud and dust feedbacks amplify tropical North Atlantic Multidecadal Oscillation". Geophys. Res. Lett. 43 (3): 1349–1356. Bibcode:2016GeoRL..43.1349Y. doi:10.1002/2016GL067679.
  13. ^ Mads Faurschou Knudsen; Bo Holm Jacobsen; Marit-Solveig Seidenkrantz & Jesper Olsen (25 February 2014). "Evidence for external forcing of the Atlantic Multidecadal Oscillation since termination of the Little Ice Age". Nature. 5: 3323. Bibcode:2014NatCo...5E3323K. doi:10.1038/ncomms4323. PMC 3948066. PMID 24567051.
  14. ^ Ghosh, Rohit; Müller, Wolfgang A.; Baehr, Johanna; Bader, Jürgen (2016-07-28). "Impact of observed North Atlantic multidecadal variations to European summer climate: a linear baroclinic response to surface heating". Climate Dynamics. 48 (11–12): 3547. Bibcode:2017ClDy...48.3547G. doi:10.1007/s00382-016-3283-4. ISSN 0930-7575.
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  19. ^ Zhang, R.; Delworth, T. L. (2006). "Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes". Geophys. Res. Lett. 33 (17): L17712. Bibcode:2006GeoRL..3317712Z. doi:10.1029/2006GL026267.
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  24. ^ Curry, Judith A. (2008). "Potential Increased Hurricane Activity in a Greenhouse Warmed World". In MacCracken, Michael C.; Moore, Frances; Topping, John C. Sudden and disruptive climate change. London: Earthscan. pp. 29–38. ISBN 978-1-84407-478-5. Assuming that the AMO continues with a 70-year periodicity, the peak of the next cycle would be expected in 2020 (70 years after the previous 1950 peak).

Further reading

External links


The 2020s (pronounced as "twenty-twenties" or "two thousand (and) twenties”) is the next decade in the Gregorian Calendar. It will begin on January 1, 2020 and will end on December 31, 2029.

Amy C. Clement

Amy C. Clement is an atmospheric and marine scientist studying and modeling global climate change. Her research focuses on cloud albedo feedbacks, ocean circulation patterns, and the El Niño Southern Oscillation (ENSO). She is currently a professor at University of Miami's Rosenfield School.

Antarctic oscillation

The Antarctic oscillation (AAO, to distinguish it from the Arctic oscillation or AO) is a low-frequency mode of atmospheric variability of the southern hemisphere. It is also known as the Southern Annular Mode (SAM). It is defined as a belt of westerly winds or low pressure surrounding Antarctica which moves north or south as its mode of variability. In its positive phase, the westerly wind belt that drives the Antarctic Circumpolar Current intensifies and contracts towards Antarctica, while its negative phase involves this belt moving towards the Equator. Winds associated with the Southern Annular Mode cause oceanic upwelling of warm circumpolar deep water along the Antarctic continental shelf, which has been linked to ice shelf basal melt, representing a possible wind-driven mechanism that could destabilize large portions of the Antarctic Ice Sheet.In 2014, Dr Nerilie Abram used a network of temperature-sensitive ice core and tree growth records to reconstruct a 1000-year history of the Southern Annular Mode. This work suggests that the Southern Annular Mode is currently in its most extreme positive phase over at least the last 1000 years, and that recent positive trends in the SAM are attributed to increasing greenhouse gas levels and later stratospheric ozone depletion.

Arctic dipole anomaly

The Arctic dipole anomaly is a pressure pattern characterized by high pressure on the arctic regions of North America and low pressure on those of Eurasia. This pattern sometimes replaces the Arctic oscillation and the North Atlantic oscillation. It was observed for the first time in the first decade of 2000s and is perhaps linked to recent climate change. The Arctic dipole lets more southern winds into the Arctic Ocean resulting in more ice melting. The summer 2007 event played an important role in the record low sea ice extent which was recorded in September. The Arctic dipole has also been linked to changes in arctic circulation patterns that cause drier winters in Northern Europe, but much wetter winters in Southern Europe and colder winters in East Asia, Europe and the eastern half of North America.

Bond event

Bond events are North Atlantic ice rafting events that are tentatively linked to climate fluctuations in the Holocene. Eight such events have been identified. Bond events were previously believed to exhibit a quasi c. 1,500-year cycle, but the primary period of variability is now put at c. 1,000 years.Gerard C. Bond of the Lamont–Doherty Earth Observatory at Columbia University was the lead author of the 1997 paper that postulated the theory of 1470-year climate cycles in the Late Pleistocene and Holocene, mainly based on petrologic tracers of drift ice in the North Atlantic. However, more recent work has shown that these tracers provide little support for 1,500-year intervals of climate change, and the reported c. 1,500 ± 500-year period was a statistical artifact. Furthermore, following publication of the Greenland Ice Core Chronology 2005 (GICC05) for the North GRIP ice core, it became clear that Dansgaard–Oeschger events also show no such pattern. The North Atlantic ice-rafting events happen to correlate with episodes of lowered lake levels in the Mid-Atlantic region, USA, the weakest events of the Asian monsoon for at least the past 9,000 years, and also correlate with most aridification events in the Middle East for the past 55,000 years (both Heinrich and Bond events).For reasons that are unclear, the only Holocene Bond event that has a clear temperature signal in the Greenland ice cores is the 8.2 kiloyear event.

Climate change acronyms

The Intergovernmental Panel on Climate Change (IPCC) with the United Nations Framework Convention on Climate Change (UNFCCC) use tens of acronyms and initialisms in documents relating to climate change policy.

Climate oscillation

A climate oscillation or climate cycle is any recurring cyclical oscillation within global or regional climate, and is a type of climate pattern. These fluctuations in atmospheric temperature, sea surface temperature, precipitation or other parameters can be quasi-periodic, often occurring on inter-annual, multi-annual, decadal, multidecadal, century-wide, millennial or longer timescales. They are not perfectly periodic and a Fourier analysis of the data does not give a sharp spectrum.

A prominent example is the El Niño Southern Oscillation, involving sea surface temperatures along a stretch of the equatorial Central and East Pacific Ocean and the western coast of tropical South America, but which affects climate worldwide.

Records of past climate conditions are recovered through geological examination of proxies, found in glacier ice, sea bed sediment, tree ring studies or otherwise.

Climate pattern

A climate pattern is any recurring characteristic of the climate. Climate patterns can last tens of thousands of years, like the glacial and interglacial periods within ice ages, or repeat each year, like monsoons.A climate pattern may come in the form of a regular cycle, like the diurnal cycle or the seasonal cycle; a quasi periodic event, like El Niño; or a highly irregular event, such as a volcanic winter. The regular cycles are generally well understood and may be removed by normalization. For example, graphs which show trends of temperature change will usually have the effects of seasonal variation removed.

Diurnal cycle

A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth around its own axis.

In climatology, the diurnal cycle is one of the most basic forms of climate patterns. The most familiar such pattern is the diurnal temperature variation. Such a cycle may be approximately sinusoidal, or include components of a truncated sinusoid (due to the Sun's rising and setting) and thermal relaxation (Newton cooling) at night.

Diurnal cycles of environmental conditions (light or temperature) can result in similar cycles in dependent biological processes, such as photosynthesis in plants, or clinical depression in humans. Plant responses to environmental cycles may even induce indirect cycles in rhizosphere microbial activities, including nitrogen fixation.A semi-diurnal cycle refers to a pattern that occurs about every twelve hours or about twice a day. Often these can be related to lunar tides, in which case the interval is closer to 12 hours and 25 minutes.

Index of climate change articles

This is a list of climate change topics.

Measurement of sea ice

Measurement of sea ice is important for safety of navigation and for monitoring the environment, particularly the climate. Sea ice extent interacts with large climate patterns such as the North Atlantic oscillation and Atlantic Multidecadal Oscillation, to name just two, and influences climate in the rest of the globe.

The amount of sea ice coverage in the arctic has been of interest for centuries, as the Northwest Passage was of high interest for trade and seafaring. There is a longstanding history of records and measurements of some effects of the sea ice extent, but comprehensive measurements were sparse till the 1950s and started with the satellite era in the late 1970s. Modern direct records include data about ice extent, ice area, concentration, thickness, and the age of the ice. The current trends in the records show a significant decline in Northern hemisphere sea ice and a small but statistically significant increase in the winter Southern hemisphere sea ice.

Furthermore, current research comprises and establishes extensive sets of multi-century historical records of arctic and subarctic sea ice and uses, among others high-resolution paleo-proxy sea-ice records. The arctic sea ice is a dynamic climate-system component and is linked to the Atlantic multidecadal variability and the historical climate over various decades. There are circular changes of sea ice patterns but so far no clear patterns based on modeling predictions.

North Atlantic oscillation

The North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. Through fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity.The NAO was discovered through several studies in the late 19th and early 20th centuries. Unlike the El Niño-Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.The North Atlantic Oscillation is closely related to the Arctic oscillation (AO) (or Northern Annular Mode (NAM)), but should not be confused with the Atlantic Multidecadal Oscillation (AMO).

North Pacific Oscillation

The North Pacific Oscillation (NPO) is a teleconnection pattern first described by Walker and Bliss and characterized by a north-south seesaw in sea level pressure over the North Pacific.

Rogers, using surface atmospheric temperature from St. Paul, Alaska and Edmonton, identified two phases of the NPO, an Aleutian below (AB) phase that correspond to a deepened and eastward shifted Aleutian low and an Aleutian above (AA) phase that is the opposite.During the positive (AB) phase sea level pressure is enhanced over a large region in the subtropics that extend poleward to 40N° and reduced at higher latitudes, westerlies are enhanced over the central Pacific and winter temperature are mild along much of the North America west coast but cooler than usual over Eastern Siberia and the United States South-West, precipitations are higher than usual over Alaska and the Great Plains.

The NPGO is the oceanic expression of the NPO.

Orbital forcing

Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the orbit (see Milankovitch cycles). These orbital changes change the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 Wm−2 at latitudes of 60 degrees). In this context, the term "forcing" signifies a physical process that affects the Earth's climate.

This mechanism is believed to be responsible for the timing of the ice age cycles. A strict application of the Milankovitch theory does not allow the prediction of a "sudden" ice age (sudden being anything under a century or two), since the fastest orbital period is about 20,000 years. The timing of past glacial periods coincides very well with the predictions of the Milankovitch theory, and these effects can be calculated into the future.


Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. The term vibration is precisely used to describe mechanical oscillation. Familiar examples of oscillation include a swinging pendulum and alternating current.

Oscillations occur not only in mechanical systems but also in dynamic systems in virtually every area of science: for example the beating of the human heart (for circulation), business cycles in economics, predator–prey population cycles in ecology, geothermal geysers in geology, vibration of strings in guitar and other string instruments, periodic firing of nerve cells in the brain, and the periodic swelling of Cepheid variable stars in astronomy.

Polar amplification

Polar amplification is the phenomenon that any change in the net radiation balance (for example greenhouse intensification) tends to produce a larger change in temperature near the poles than the planetary average. On a planet with an atmosphere that can restrict longwave radiation to space (a greenhouse effect), surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict.

In the extreme, the planet Venus is thought to have experienced a very large increase in greenhouse effect over its lifetime, so much so that its poles have warmed sufficiently to render its surface temperature effectively isothermal (no difference between poles and equator). On Earth, water vapor and trace gasses provide a lesser greenhouse effect, and the atmosphere and extensive oceans provide efficient poleward heat transport. Both palaeoclimate changes and recent global warming changes have exhibited strong polar amplification, as described below.

Sea surface temperature

Sea surface temperature (SST) is the water temperature close to the ocean's surface. The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a short distance of the shore. Localized areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass. Warm sea surface temperatures are known to be a cause of tropical cyclogenesis over the Earth's oceans. Tropical cyclones can also cause a cool wake, due to turbulent mixing of the upper 30 metres (100 ft) of the ocean. SST changes diurnally, like the air above it, but to a lesser degree. There is less SST variation on breezy days than on calm days. In addition, ocean currents such as the Atlantic Multidecadal Oscillation (AMO), can effect SST's on multi-decadal time scales, a major impact results from the global thermohaline circulation, which affects average SST significantly throughout most of the world's oceans.

Ocean temperature is related to ocean heat content, an important topic in the debate over global warming.

Coastal SSTs can cause offshore winds to generate upwelling, which can significantly cool or warm nearby landmasses, but shallower waters over a continental shelf are often warmer. Onshore winds can cause a considerable warm-up even in areas where upwelling is fairly constant, such as the northwest coast of South America. Its values are important within numerical weather prediction as the SST influences the atmosphere above, such as in the formation of sea breezes and sea fog. It is also used to calibrate measurements from weather satellites.


Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of kilometers). The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Australia, which defines the Southern Oscillation.

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