Atlantic multidecadal oscillation

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Atlantic multidecadal oscillation spatial pattern obtained as the regression of monthly HadISST sea surface temperature anomalies (1870-2013). AMO Pattern.png
Atlantic multidecadal oscillation spatial pattern obtained as the regression of monthly HadISST sea surface temperature anomalies (1870–2013).
Atlantic Multidecadal Oscillation Index according to the methodology proposed by van Oldenborgh et al. 1880-2018. Atlantic Multidecadal Oscillation.svg
Atlantic Multidecadal Oscillation Index according to the methodology proposed by van Oldenborgh et al. 1880–2018.
Atlantic Multidecadal Oscillation index computed as the linearly detrended North Atlantic sea surface temperature anomalies 1856-2022. Amo timeseries 1856-present.svg
Atlantic Multidecadal Oscillation index computed as the linearly detrended North Atlantic sea surface temperature anomalies 1856–2022.

The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), [1] is the theorized variability of the sea surface temperature (SST) of the North Atlantic Ocean on the timescale of several decades. [2]

Contents

While there is some support for this mode in models and in historical observations, controversy exists with regard to its amplitude, and whether it has a typical timescale and can be classified as an oscillation. There is also discussion on the attribution of sea surface temperature change to natural or anthropogenic causes, especially in tropical Atlantic areas important for hurricane development. [3] The Atlantic multidecadal oscillation is also connected with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations. [4]

Definition and history

Evidence for a multidecadal climate oscillation centered in the North Atlantic began to emerge in 1980s work by Folland and colleagues, seen in Fig. 2.d.A. [5] That oscillation was the sole focus of Schlesinger and Ramankutty in 1994, [6] but the actual term Atlantic Multidecadal Oscillation (AMO) was coined by Michael Mann in a 2000 telephone interview with Richard Kerr, [7] as recounted by Mann, p. 30 in The Hockey Stick and the Climate Wars: Dispatches from the Front Lines (2012).

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, as per Mann, Steinman and Miller, [8] which also provides a more detailed history of the science development.

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. [9]

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. [3]

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. [10]

Guan and Nigam removed the non stationary global trend and Pacific natural variability before applying an EOF analysis to the residual North Atlantic SST. [11]

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. [3]

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. [4]

Mechanisms

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. [12] In models, AMO-like variability is associated with small changes in the North Atlantic branch of the Thermohaline Circulation. [13] However, historical oceanic observations are not sufficient to associate the derived AMO index to present-day circulation anomalies.[ citation needed ] 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. [14] [15]

The Atlantic Multidecadal Oscillation (AMO) is important for how external forcings are linked with North Atlantic SSTs. [16]

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. [17] [18] Through changes in atmospheric circulation, the AMO can also modulate spring snowfall over the Alps [19] and glaciers' mass variability. [20] 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. [9]

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. [21]

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. [22] Paleoclimatologic studies have confirmed this pattern—increased rainfall in AMO warmphase, decreased in cold phase—for the Sahel over the past 3,000 years. [23]

Relation to Atlantic hurricanes

North Atlantic tropical cyclone activity according to the Accumulated Cyclone Energy Index, 1950-2015. For a global ACE graph visit this link Archived 2018-11-02 at the Wayback Machine . North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone Energy Index 1950-2015.png
North Atlantic tropical cyclone activity according to the Accumulated Cyclone Energy Index, 1950–2015. For a global ACE graph visit this link Archived 2018-11-02 at the Wayback Machine .

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." [24] 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. [21] 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. [25]

However, Mann and Emanuel had found in 2006 that "anthropogenic factors are responsible for long-term trends in tropic Atlantic warmth and tropical cyclone activity" and "There is no apparent role of the AMO." [26]

In 2014 Mann, Steinman and Miller [8] showed that warming (and therefore any effects on hurricanes) were not caused by the AMO, writing: "certain procedures used in past studies to estimate internal variability, and in particular, an internal multidecadal oscillation termed the "Atlantic Multidecadal Oscillation" or "AMO", fail to isolate the true internal variability when it is a priori known. Such procedures yield an AMO signal with an inflated amplitude and biased phase, attributing some of the recent NH mean temperature rise to the AMO. The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming."

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". [27] 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%.[ citation needed ]

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.

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." [4]

Criticism

In a 2021 study by Michael Mann and others, it was shown that the periodicity of the AMO in the last millennium was driven by volcanic eruptions and other external forcings, and therefore that there is no compelling evidence for the AMO being an oscillation or cycle. [28] There was also a lack of oscillatory behaviour in models on time scales longer than El Niño Southern Oscillation; the AMV is indistinguishable from red noise, a typical null hypothesis to test whether there are oscillations in a model. [29] Referring to the 2021 study, Michael Mann, the originator of the term AMO, put it more succinctly in a blog post on the matter: "my colleagues and I have provided what we consider to be the most definitive evidence yet that the AMO doesn't actually exist." [30]

Related Research Articles

<span class="mw-page-title-main">North Atlantic Current</span> Current of the Atlantic Ocean

The North Atlantic Current (NAC), also known as North Atlantic Drift and North Atlantic Sea Movement, is a powerful warm western boundary current within the Atlantic Ocean that extends the Gulf Stream northeastward.

<span class="mw-page-title-main">El Niño–Southern Oscillation</span> Climate phenomenon that periodically fluctuates

El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variations in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO is not predictable. It affects the climate of much of the tropics and subtropics, and has links (teleconnections) to higher-latitude regions of the world. The warming phase of the sea surface temperature is known as "El Niño" and the cooling phase as "La Niña". The Southern Oscillation is the accompanying atmospheric oscillation, which is coupled with the sea temperature change.

The North Atlantic Oscillation (NAO) is a weather phenomenon over 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.

<span class="mw-page-title-main">Sea surface temperature</span> Water temperature close to the oceans surface

Sea surface temperature is the temperature of ocean water close to the surface. The exact meaning of surface varies in the literature and in practice. It is usually between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Sea surface temperatures greatly modify air masses in the Earth's atmosphere within a short distance of the shore. The thermohaline circulation has a major impact on average sea surface temperature throughout most of the world's oceans.

<span class="mw-page-title-main">Pacific decadal oscillation</span> Recurring pattern of climate variability

The Pacific decadal oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.

<span class="mw-page-title-main">Western Hemisphere Warm Pool</span>

The Western Hemisphere Warm Pool (WHWP) is a region of sea surface temperatures (SST) warmer than 28.5 °C that develops west of Central America in the spring, then expands to the tropical waters to the east.

<span class="mw-page-title-main">North Equatorial Current</span> Current in the Pacific and Atlantic Oceans

The North Equatorial Current (NEC) is a westward wind-driven current mostly located near the equator, but the location varies from different oceans. The NEC in the Pacific and the Atlantic is about 5°-20°N, while the NEC in the Indian Ocean is very close to the equator. It ranges from the sea surface down to 400 m in the western Pacific.

<span class="mw-page-title-main">Indian Ocean Dipole</span> Climatic and oceanographic cycle affecting Southeast Asia, Australia and Africa

The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.

<span class="mw-page-title-main">Polar amplification</span>

Polar amplification is the phenomenon that any change in the net radiation balance tends to produce a larger change in temperature near the poles than in the planetary average. This is commonly referred to as the ratio of polar warming to tropical warming. On a planet with an atmosphere that can restrict emission of longwave radiation to space, 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. The poles will experience the most cooling when the global-mean temperature is lower relative to a reference climate; alternatively, the poles will experience the greatest warming when the global-mean temperature is higher.

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.

The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean. It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation. The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin. For this reason, the Atlantic Niño is often called the little brother of El Niño. The Atlantic Niño usually appears in northern summer, and is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole across the equator and operates more during northern spring. The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to rainfall variability over the surrounding continents, especially in West African countries bordering the Gulf of Guinea. Therefore, understanding of the Atlantic Niño has important implications for climate prediction in those regions. Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.

<span class="mw-page-title-main">Subtropical Indian Ocean Dipole</span> Oscillation of sea surface temperatures

The Subtropical Indian Ocean Dipole (SIOD) is featured by the oscillation of sea surface temperatures (SST) in which the southwest Indian Ocean i.e. south of Madagascar is warmer and then colder than the eastern part i.e. off Australia. It was first identified in the studies of the relationship between the SST anomaly and the south-central Africa rainfall anomaly; the existence of such a dipole was identified from both observational studies and model simulations .

The Tropical Atlantic SST Dipole refers to a cross-equatorial sea surface temperature (SST) pattern that appears dominant on decadal timescales. It has a period of about 12 years, with the SST anomalies manifesting their most pronounced features around 10–15 degrees of latitude off of the Equator. It is also referred to as the interhemispheric SST gradient or the Meridional Atlantic mode.

<span class="mw-page-title-main">Global warming hiatus</span> Period of little Earth temperature change

A global warming hiatus, also sometimes referred to as a global warming pause or a global warming slowdown, is a period of relatively little change in globally averaged surface temperatures. In the current episode of global warming many such 15-year periods appear in the surface temperature record, along with robust evidence of the long-term warming trend. Such a "hiatus" is shorter than the 30-year periods that climate is classically averaged over.

<span class="mw-page-title-main">Medieval Warm Period</span> Time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250

The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from c. 950 to c. 1250. Climate proxy records show peak warmth occurred at different times for different regions, which indicate that the MWP was not a globally uniform event. Some refer to the MWP as the Medieval Climatic Anomaly to emphasize that climatic effects other than temperature were also important.

<span class="mw-page-title-main">Little Ice Age volcanism</span> Massive volcanic activity during the Little Ice Age

Little Ice Age volcanism refers to the massive volcanic activities during the Little Ice Age. Scientists suggested a hypothesis that volcanism was the major driving force of the global cooling among the other natural factors, i.e. the sunspot activities by orbital forcing and greenhouse gas. The Past Global Change (PAGES), a registered paleo-science association for scientific research and networking on past global changes in the University of Bern, Switzerland, suggested that from 1630 to 1850, a total of 16 major eruptions and cooling events had taken place. When a volcano erupts, ashes burst out of the vent together with magma and forms a cloud in the atmosphere. The ashes act as an isolating layer that block out a proportion of solar radiation, causing global cooling. The global cooling effect impacts ocean currents, atmospheric circulation and cause social impacts such as drought and famine. Wars and rebellions were therefore triggered worldwide in the Little Ice Age. It was suggested that the crisis on Ottoman Empire and Ming-Qing Transition in China were typical examples that closely correlated with Little Ice Age.

<span class="mw-page-title-main">Pacific Meridional Mode</span> Climate mode in the North Pacific

Pacific Meridional Mode (PMM) is a climate mode in the North Pacific. In its positive state, it is characterized by the coupling of weaker trade winds in the northeast Pacific Ocean between Hawaii and Baja California with decreased evaporation over the ocean, thus increasing sea surface temperatures (SST); and the reverse during its negative state. This coupling develops during the winter months and spreads southwestward towards the equator and the central and western Pacific during spring, until it reaches the Intertropical Convergence Zone (ITCZ), which tends to shift north in response to a positive PMM.

Rong Zhang is a Chinese-American physicist and climate scientist at the National Oceanic and Atmospheric Administration. Her research considers the impact of Atlantic meridional overturning circulation on climate phenomena. She was elected Fellow of the American Meteorological Society in 2018 and appointed their Bernhard Haurwitz Memorial Lecturer in 2020.

<span class="mw-page-title-main">Thomas L. Delworth</span> American oceanic climate scientist

Thomas L. Delworth is an atmospheric and oceanic climate scientist and Senior Scientist at the Geophysical Fluid Dynamics Laboratory (GFDL), part of NOAA. He also serves on the faculty of Oceanic Science at Princeton University.

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