Atlantic Meridional Overturning Circulation (AMOC)

Here are the current Papers & Articles under the research topic Atlantic Meridional Overturning Circulation (AMOC). Click on the title of a paper you are interested in to go straight to the full paper. Papers and articles covering the basics (ideal for learning) are shown in Green.

What is the Atlantic Meridional Overturning Circulation?

Physics-based early warning signal shows that AMOC is on tipping course
Published Feb 2024
Abstract:
One of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC), which can potentially collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have so far not been investigated. Here, we show results of the first tipping event in the Community Earth System Model, including the large climate impacts of the collapse. Using these results, we develop a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. Reanalysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping.

Warning of a forthcoming collapse of the Atlantic meridional overturning circulation
Published 25 July 2023
Abstract:
The Atlantic meridional overturning circulation (AMOC) is a major tipping element in the climate system and a future collapse would have severe impacts on the climate in the North Atlantic region. In recent years weakening in circulation has been reported, but assessments by the Intergovernmental Panel on Climate Change (IPCC), based on the Climate Model Intercomparison Project (CMIP) model simulations suggest that a full collapse is unlikely within the 21st century. Tipping to an undesired state in the climate is, however, a growing concern with increasing greenhouse gas concentrations. Predictions based on observations rely on detecting early-warning signals, primarily an increase in variance (loss of resilience) and increased autocorrelation (critical slowing down), which have recently been reported for the AMOC. Here we provide statistical significance and data-driven estimators for the time of tipping. We estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions.

Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation
Published March 2023.
Abstract:
Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

A last millennium perspective on North Atlantic variability: exploiting synergies between models and proxy data
Abstract:
One of the key players in the North Atlantic region is the Atlantic Meridional Overturning Circulation (AMOC), which is associated with sinking due to deep water formation in the Labrador and Nordic Seas. The AMOC is the primary control of the poleward heat transport in the Atlantic region. Therefore, the AMOC is associated with important climate impacts, and plays an active role in various feedback mechanisms with, for example, sea ice (Mahajan et al., 2011) and the atmospheric circulation

Effect of AMOC collapse on ENSO in a high resolution general circulation model
Published June 2017.
Abstract:
We look at changes in the El Niño Southern Oscillation (ENSO) in a high-resolution eddy-permitting climate model experiment in which the Atlantic Meridional Circulation (AMOC) is switched off using freshwater hosing. The ENSO mode is shifted eastward and its period becomes longer and more regular when the AMOC is off. The eastward shift can be attributed to an anomalous eastern Ekman transport in the mean equatorial Pacific ocean state. Convergence of this transport deepens the thermocline in the eastern tropical Pacific and increases the temperature anomaly relaxation time, causing increased ENSO period. The anomalous Ekman transport is caused by a surface northerly wind anomaly in response to the meridional sea surface temperature dipole that results from switching the AMOC off. In contrast to a previous study with an earlier version of the model, which showed an increase in ENSO amplitude in an AMOC off experiment, here the amplitude remains the same as in the AMOC on control state. We attribute this difference to variations in the response of decreased stochastic forcing in the different models, which competes with the reduced damping of temperature anomalies. In the new high-resolution model, these effects approximately cancel resulting in no change in amplitude.

Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM

Published June 2018.
Abstract:
El Niño–Southern Oscillation (ENSO) is known to have profound climate impacts worldwide, but the causes of its complex behaviors are still not fully understood. In this study, we show that the subtropical atmosphere-ocean coupled forcing is a key source of ENSO complexity, whereas the tropical ocean heat content variation acts to reduce ENSO complexity. The subtropical forcing also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño-LaNiña asymmetries. In contrast to the steady strength of the tropical variation throughout the past six decades,the strength of the subtropical forcing has increased since the early 1990s. This may have made ENSO more complex recently and, if this trend does not reverse, possibly into the coming decades. Contemporary climate models overestimate the strength of the tropical ocean heat content variation but underestimate thestrength of the subtropical forcing, which may be a reason why contemporary models produce ENSO behavior that is too regular.

Gulf Stream Excursions and Sectional Detachments Generate the Decadal Pulses in the Atlantic Multidecadal Oscillation
Published Jan 2017.
Abstract:
Decadal pulses within the lower-frequency Atlantic Multidecadal Oscillation (AMO) are a prominent but underappreciated AMO feature, representing decadal variability of the subpolar gyre (e.g., the 1970s Great Salinity Anomaly)and wielding notable influence on the hydroclimate of the African and American continents. Here we seek clues into their origin in the spatio temporal developmentof the Gulf Stream’s (GS) meridional excursions and sectional detachments evident in the 1954-2012 record of ocean surface and subsurface salinity and temperature observations. The GS excursions are tracked via meridional displacement of the 15°C isotherm at 200m depth–the GS index–while AMO’s decadal pulses are targeted through the AMO-tendency which implicitly highlights the shorter timescales of the AMO index. We show the GS’s northward shift to be preceded by the positive phase of the low-frequency North Atlantic Oscillation (LF-NAO),and followed by a positive AMO-tendency, by 1.25 and 2.5 years, respectively.The temporal phasing is such that the GS’s northward shift is nearly concurrent with the AMO’s cold decadal phase (cold,fresh subpolar gyre). Ocean-atmosphere processes that can initiate phase-reversal of the gyre state are discussed, starting with reversal of the LF-NAO; leading to a mechanistic hypothesis for decadal fluctuations of the subpolar gyre. According to the hypothesis, the fluctuation timescale is set by the self-feedback of the LF-NAO generated from its influence on SSTs in the seas around Greenland, and by the cross-basin transit of the GS’s detached eastern section; the latter produced by southward intrusion of subpolar water through the Newfoundland Basin, just prior to the GS’s northward shift in the western basin.

Impacts of high-latitude volcanic eruptions on ENSO and AMOC
Published 2010.
Abstract:
Large volcanic eruptions can have major impacts on global climate, affecting both atmospheric and ocean circulation through changes in atmospheric chemical composition and optical properties. The residence time of volcanic aerosol from strong eruptions is roughly 2–3 y. Attention has consequently focused on their short-term impacts, whereas the long-term, ocean-mediated response has not been well studied. Most studies have focused on tropical eruptions; high-latitude eruptions have drawn less attention because their impacts are thought to be merely hemispheric rather than global. No study to date has investigated the long-term effects of high-latitude eruptions. Here, we use a climate model to show that large summer high-latitude eruptions in the Northern Hemisphere cause strong hemispheric cooling, which could induce an El Niño-like anomaly, in the equatorial Pacific during the first 8–9 mo after the start of the eruption.

Impact of the Atlantic Meridional Overturning Circulation on Arctic Surface Air Temperature and Sea Ice Variability
Published 2011.
Abstract:
The simulated impact of the Atlantic meridional overturning circulation (AMOC) on the low-frequency variability of the Arctic surface air temperature (SAT) and sea ice extent is studied with a 1000-year-long segment of a control simulation of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1. The simulated AMOC variations in the control simulation are found to be significantly anticorrelated with the Arctic sea ice extent anomalies and significantly correlated with the Arctic SAT anomalies on decadal time scales in the Atlantic sector of the Arctic. The maximum anticorrelation with the Arctic sea ice extent and the maximum correlation with the Arctic SAT occur when the AMOC index leads by one year. An intensification of the AMOC is associated with a sea ice decline in the Labrador, Greenland, and Barents Seas in the control simulation, with the largest change occurring in winter. The recent declining trend in the satellite-observed sea ice extent also shows a similar pattern in the Atlantic sector of the Arctic in the winter, suggesting the possibility of a role of the AMOC in the recent Arctic sea ice decline in addition to anthropogenic greenhouse-gas-induced warming. However, in the summer, the simulated sea ice response to the AMOC in the Pacific sector of the Arctic is much weaker than the observed declining trend, indicating a stronger role for other climate forcings or variability in the recently observed summer sea ice decline in the Chukchi, Beaufort, East Siberian, and Laptev Seas.

Impacts of the North Atlantic Warming Hole in Future Climate Projections: Mean Atmospheric Circulation and the North Atlantic jet (Note: this paper is currently behind a paywall, but an easy-to-read summary here in this Science Daily article).
Published May 2019.
Abstract:
In future climate simulations there is a pronounced region of reduced warming in the subpolar gyre of the North Atlantic Ocean known as the North Atlantic warming hole (NAWH). This study investigates the impact of the North Atlantic warming hole on atmospheric circulation and midlatitude jets within the Community Earth System Model (CESM). A series of large-ensemble atmospheric model experiments with prescribed sea surface temperature (SST) and sea ice are conducted, in which the warming hole is either filled or deepened. Two mechanisms through which the NAWH impacts the atmosphere are identified: a linear response characterized by a shallow atmospheric cooling and increase in sea level pressure shifted slightly downstream of the SST changes, and a transient eddy forced response whereby the enhanced SST gradient produced by the NAWH leads to increased transient eddy activity that propagates vertically and enhances the midlatitude jet. The relative contributions of these two mechanisms and the details of the response are strongly dependent on the season, time period, and warming hole strength. Our results indicate that the NAWH plays an important role in midlatitude atmospheric circulation changes in CESM future climate simulations.

Intrinsic and atmospherically forced variability of the AMOC : insights from a large-ensemble ocean hindcast
Published Mar 2017.
Abstract:
This study investigates the origin and features of interannual–decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/48) ocean–sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960–2015. The magnitude, spatio temporal scales, and patterns of both the atmospherically forced and intrinsic–chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 408N are largely driven by the atmospheric variability,which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin,but it reaches up to 100% of the latter around 358S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 258N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 408N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the ‘‘pure in-trinsic’’ interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere.

Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review
Published Jan 2016.
Abstract:
This is a review about the Atlantic Meridional Overturning Circulation (AMOC), its meanstructure, temporal variability, controlling mechanisms, and role in the coupled climate system. The AMOCplays a central role in climate through its heat and freshwater transports. Northward ocean heat transportachieved by the AMOC is responsible for the relative warmth of the Northern Hemisphere compared tothe Southern Hemisphere and is thought to play a role in setting the mean position of the IntertropicalConvergence Zone north of the equator. The AMOC is a key means by which heat anomalies are sequesteredinto the ocean’s interior and thus modulates the trajectory of climate change. Fluctuations in theAMOC have been linked to low-frequency variability of Atlantic sea surface temperatures with a host ofimplications for climate variability over surrounding landmasses. On intra-annual timescales, variability inAMOC is large and primarily reflects the response to local wind forcing; meridional coherence of anomaliesis limited to that of the wind field. On interannual to decadal timescales, AMOC changes are primarilygeostrophic and related to buoyancy anomalies on the western boundary. A pacemaker region for decadalAMOC changes is located in a western “transition zone” along the boundary between the subtropicaland subpolar gyres. Decadal AMOC anomalies are communicated meridionally from this region. AMOCobservations, as well as the expanded ocean observational network provided by the Argo array and satellitealtimetry, are inspiring efforts to develop decadal predictability systems using coupled atmosphere-oceanmodels initialized by ocean data

The role of Atlantic overturning circulation in the recent decline of Atlantic major hurricane frequencyPublished Nov 2017.
Abstract:
Observed Atlantic major hurricane frequency has exhibited pronounced multidecadal variability since the 1940s. However, the cause of this variability is debated. Using observations and a coupled earth system model (GFDL-ESM2G), here we show that the decline of the Atlantic major hurricane frequency during 2005–2015 is associated with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) inferred from ocean observations. Directly observed North Atlantic sulfate aerosol optical depth has not increased (but shows a modest decline) over this period, suggesting the decline of the Atlantic major hurricane frequency during 2005–2015 is not likely due to recent changes in anthropogenic sulfate aerosols. Instead, we find coherent multidecadal variations involving the inferred AMOC and Atlantic major hurricane frequency, along with indices of Atlantic Multidecadal Variability and inverted vertical wind shear. Our results provide evidence for an important role of the AMOC in the recent decline of Atlantic major hurricane frequency.

The Central Role of Ocean Dynamics in Connecting the NAO to the Extratropical Component of the AMO
Published Jan 2017.
Abstract:
The relationship between the North Atlantic Oscillation (NAO) and Atlantic sea surface temperature(SST) variability is investigated using models and observations. Coupled climate models are used in which the ocean component is either a fully dynamic ocean or a slab ocean with no resolved ocean heat transport. On time scales less than 10 yr, NAO variations drive a tripole pattern of SST anomalies in both observations and models. This SST pattern is a direct response of the ocean mixed layer to turbulent surface heat flux anomalies associated with the NAO. On time scales longer than 10 yr, a similar relationship exists between the NAO andthe tripole pattern of SST anomalies in models with a slab ocean. A different relationship exists both for the observations and for models with a dynamic ocean. In these models, a positive (negative) NAO anomaly leads, after a decadal-scale lag, to a monopole pattern of warming (cooling) that resembles the Atlantic multidecadal oscillation (AMO), although with smaller-than-observed amplitudes of tropical SST anomalies.Ocean dynamics are critical to this decadal-scale response in the models. The simulated Atlantic meridional overturning circulation (AMOC) strengthens (weakens) in response to a prolonged positive (negative) phaseof the NAO, thereby enhancing (decreasing) poleward heat transport, leading to broad-scale warming(cooling). Additional simulations are used in which heat flux anomalies derived from observed NAO variations from 1901 to 2014 are applied to the ocean component of coupled models. It is shown that ocean dynamics allow models to reproduce important aspects of the observed AMO, mainly in the Subpolar Gyre.

The variability of the Atlantic Meridional Circulation Since 1980, as Hindcast by a Data-driven Nonlinear Systems Model
Published May 2018.
Abstract:
The Atlantic Meridional Overturning Circulation (AMOC), an important component of the climate system, has only been directly measured since the RAPID array’s installation across the Atlantic at 26oN in 2004. This has shown that the AMOC strength is highly variable on monthly timescales, however, after an abrupt, short-lived, halving of the strength of the AMOC early in 2010, its mean has remained ~ 15% below its pre-2010 level. To attempt to understand the reasons for this variability, we use a control systems identification approach to model the AMOC, with the RAPID data of 2004-2017 providing a trial and test data set. After testing to find the environmental variables, and systems model, that allow us to best match the RAPID observations, we reconstruct AMOC variation back to 1980. Our reconstruction suggests that there is inter-decadal variability in the strength of the AMOC, with periods of both weaker flow than recently, and flow strengths similar to the late 2000s, since 1980. Recent signs of weakening may therefore not reflect the beginning of a sustained decline. It is also shown that there may be predictive power for AMOC variability of around 6 months, as ocean density contrasts between the source and sink regions for the North Atlantic Drift, with lags up to 6 months, are found to be important components of the systems model.

Underestimated AMOC Variability and Implications for AMV and Predictability in CMIP Models
Published April 2018.
Abstract:
The Atlantic Meridional Overturning Circulation (AMOC) has profound impacts on various climate phenomena. Using both observations and simulations from the Coupled Model Intercomparison Project Phase 3 and 5, here we show that most models underestimate the amplitude of low-frequency AMOC variability. We further show that stronger low-frequency AMOC variability leads to stronger linkages between the AMOC and key variables associated with the Atlantic multidecadal variability (AMV), and between the subpolar AMV signal and northern hemisphere surface air temperature. Low-frequency extratropical northern hemisphere surface air temperature variability might increase with the amplitude of low-frequency AMOC variability. Atlantic decadal predictability is much higher in models with stronger low-frequency AMOC variability and much lower in models with weaker or without AMOC variability. Our results suggest that simulating realistic low-frequency AMOC variability is very important, both for simulating realistic linkages between AMOC and AMV-related variables and for achieving substantially higher Atlantic decadal predictability.