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# The Atlantic Meridional Overturning Circulation and its Hypothetical Collapse

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Collapse of the Atlantic Meridional Overturning Circulation (AMOC) is often invoked as an explanation of major past climate changes and as a major risk for future climate. Many of these arguments appear, from an observer’s point of view, as far-more definitive than is warranted. In the hypothetical event of a future collapse, the implications may be much less severe than those from many other elements of global change already underway. The Gulf Stream system, and its required return flow of mass, implies that changed circulations will nonetheless continue to carry significant amounts of heat, carbon etc., poleward even without any AMOC.
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LETTER TO THE
EDITOR
CORRESPONDING AUTHOR:
Fabien Roquet
Department of Marine Sciences,
University of Gothenburg,
Gothenburg, Sweden
fabien.roquet@gu.se
KEYWORDS:
AMOC; Gulf Stream;
Thermohaline circulation;
collapse
TO CITE THIS ARTICLE:
Roquet, F and Wunsch, C.
2022. The Atlantic Meridional
Overturning Circulation and its
Hypothetical Collapse. Tellus
A: Dynamic Meteorology and
Oceanography, 74(1): 393–398.
DOI: https://doi.org/10.16993/
tellusa.679
The Atlantic Meridional
Overturning Circulation and
its Hypothetical Collapse
FABIEN ROQUET
CARL WUNSCH
*Author afﬁliations can be found in the back matter of this article
ABSTRACT
Collapse of the Atlantic Meridional Overturning Circulation (AMOC) is often invoked as an
explanation of major past climate changes and as a major risk for future climate. Many
of these arguments appear, from an observer’s point of view, as far-more deﬁnitive
than is warranted. In the hypothetical event of a future collapse, the implications may
be much less severe than those from many other elements of global change already
underway. The Gulf Stream system, and its required return ﬂow of mass, implies that
changed circulations will nonetheless continue to carry signiﬁcant amounts of heat,
carbon etc., poleward even without any AMOC.
394Roquet and Wunsch Tellus A: Dynamic Meteorology and Oceanography DOI: 10.16993/tellusa.679
THE AMOC IN THE CLIMATE SYSTEM
The Atlantic Meridional Overturning Circulation (AMOC) is
a complex system of oceanic currents carrying surface
waters northward across the Atlantic basins—plunging in
high latitudes and forming the North Atlantic Deep Water
which ﬂows back southward (Buckley and Marshall, 2016).
As a major component of the global ocean circulation,
acting as a conduit for the movement of climatological
heat, carbon, and other important properties, it is widely
believed that any changing AMOC would have profound
climatic impacts. As such, the AMOC is an important
focus of research on both the modern climate system
(Frajka-Williams et al., 2019) and as a nearly all-purpose
explanation for inferred paleo-climate states (Cronin,
2009), (Lynch-Stieglitz, 2021). Its collapse could, in
the literature, arise from a number of possible causes,
generally connected with suppression of high latitude
convective exchange between upper and lower oceans.
Although understanding the science of the AMOC is
undeniably important, what is perhaps surprising is the
way in which its existence and possible change have
captured the imagination not only of the ﬂuid dynamics
community, but also scientists working on the edges of
ﬂuid oceanography, and, somewhat disturbingly, the
popular media, including a widely seen 2004 movie, “The
Day After Tomorrow”. More recently, a New York Times
article (https://www.nytimes.com/2021/08/05/us/gulf--
stream-collapse.html) made prominent a recent paper
(Boers, 2021) suggesting that the AMOC was nearing a
point of collapse, with perhaps dire consequences. To
a great extent, the emphasis on the AMOC stems from
a cartoon picture of the ocean circulation of the “Great
Ocean Conveyor” and the invocation of a zoomorphic
attribute “the climate is an angry beast…”; (Broecker,
1987), recently reproduced as part of the New York Times
story. Intense research in the past 30 years demonstrates
however that such a sweeping sketch of the AMOC fails to
capture the complex, intrinsically fully turbulent, three-
dimensional nature of the real ﬂow ﬁeld as portrayed in
observational studies (Ferrari and Wunsch, 2009).
Here we seek to provide some perspective on the AMOC
and its role in climate. Much discussion of the inﬂuence of
the changing ocean on past climate states, has invoked
the idea of a collapse of the AMOC (Cronin, 2009), brought
on by suppression of the vertical convection—by differing
mechanisms. This idea has been translated to the study
of present and future climates, motivating research on
the potential occurrence of an AMOC collapse in a more
or less distant future (Rahmstorf, 2000), triggered by
anthropogenic climate change. The literature on this topic
is abundant, and it is not the goal of this letter to provide
a comprehensive review, but see for example (Weijer
et al., 2019). Representations of the AMOC in numerical
ocean simulations suffer from important biases (Lee
et al., 2019) and they have often shown a stable response
incompatible with the idea of a collapse (Stouffer et al.,
2006). Recent studies may however give the impression
that new observations are now conﬁrming unequivocally
the decline of the AMOC (Caesar et al., 2021) and a large
potential for collapse (Boers, 2021).
DEFINING THE AMOC
A general deﬁnition, applying to any ocean, zonally
bounded or otherwise, is the meridional overturning
circulation (MOC) or the sum of the mass ﬂux from a
western to an eastern longitude of the ocean, to some
speciﬁed depth (not the bottom) at some speciﬁed
latitude of the northward and southward going ﬂows.1
Thus the MOC is the net ﬂow going north or south
above the integration depth (often taken as a ﬁxed
depth or bounded by a surface of constant potential
density). If an ocean basin is closed e.g., at the north,
the integral (sum) from top to bottom has to vanish,
and thus the MOC is normally deﬁned in terms of
some ﬁnite depth (or density), perhaps varying with
longitude, and deﬁnitely varying with latitude. Basin
scale spatial averages, such as long zonal means,
often do display many of the elements of classical
physical oceanography, including boundary currents,
gyres, equatorial ﬂows etc., but masking the observed
three-dimensional, intensely time-varying ﬂow that
comprises the apparent average.
In the Atlantic Ocean, various deﬁnitions of the
AMOC exist, generally all referring to the net northward
movement of mass above depths of order 1000 m
from the western to the eastern boundary, over greatly
varying time-averages. The major, permanent, feature of
the North Atlantic Ocean is the powerful, warm, largely
wind-driven, poleward ﬂow on the western side, known
as the Gulf Stream—a western boundary current (WBC)
that is a fundamental phenomenon of all ocean basins
bounded on the west. The Gulf Stream is a dominating
part of the AMOC, but should not be confused with the
AMOC itself.2 The North Atlantic is nearly closed at its
northernmost reaches (a weak mass input exists there
from the Arctic Sea) and the far larger amount of water
headed northwards in the Gulf Stream at e.g., about
35 × 106m3s–1 at 30°N, and deﬁnable with different
numbers and different averaging times at other latitudes
(Richardson, 1985), must return southward in the ocean
further east or at depth.
Historically, the conventional view was that the
dominant northward WBC mass transport would be
compensated largely by a southward return ﬂow over
the entire ocean to the east, in what is known as Sverdrup
balance, one conﬁned primarily to the upper layers of
the ocean and driven by the large-scale distribution of
winds. Superimposed upon this circulation would be an
additional, meridional overturning, directly involving the
395Roquet and Wunsch Tellus A: Dynamic Meteorology and Oceanography DOI: 10.16993/tellusa.679
deep ocean, also returning strongly cooled water at high
latitudes through a convectively driven very cold deep
western boundary current (Gordon, 1986). Transports
of heat, carbon, oxygen, and other tracers result from
the differing properties of the massive northward and
southward-going ﬂows. In the last three decades
however, this laminar and nearly-steady picture has been
replaced in observations by one of an ocean effectively
turbulent on all measurable time and space scales
(although envisioned much earlier (Stommel, 1948)).
These scales range from the full size of the ocean (10,000
km) to order 1 cm, and on time-scales from seconds and
potentially out to the age of the ﬂuid ocean. Eddies and
their variability are fundamental to the ocean circulation
in a way the classical theories could not describe. Thus
the AMOC is in practice the superposition of a myriad of
complex circulations more or less interconnected and
varying—at vastly different time and spatial scales (see
e.g. Bower et al. (2009) or Kostov et al. (2021)). It can be
regarded as a mass residual of the upper ocean gyre with
its return ﬂow. Known physical elements of the variability
at all depths include the spatial and temporal scales of
the boundary currents, balanced and sub-mesoscale
eddies, internal waves, and likely inertial and viscous sub-
ranges. Energy is believed to move both towards larger
and smaller scales relative to the spatial scale of input
(Arbic et al., 2014).
OBSERVING THE AMOC
To observe this complex system is challenging. A useful
AMOC estimate at any latitude must integrate across a
wide-variety of features (Figure 1). Some useful estimates
of the AMOC transport have become available only for
the last 25 years, none of them showing any indication
of signiﬁcant long-term trends (Frajka-Williams et al.,
2019). Localized estimates of the MOC in Nordic Seas are
available for longer time periods, but again with no sign
of any long-term trend (Hansen et al., 2016; Rossby et al.,
2020) within the intense spatial and temporal variability.
Determining the amount of heat transported poleward by
the circulation (the major focus of most AMOC discussions,
albeit usually only implicit) is a complicated matter, one
in which the time required to obtain a stable average
of a quadratic quantity (velocity times the transported
property) is likely to vary greatly depending upon the
property and the latitude. Such accurate calculations lie
beyond any observing system in place before the very
recent past–and one with still remaining issues.
If it is true that a collapse reduces the poleward high
latitude transport of heat by the Atlantic Ocean, one can
expect, at zero-order, that the atmosphere—globally—
will tend to compensate it (Bjerknes, 1964) along with
corresponding shifts in the rest of the world ocean.
Changes can occur elsewhere in the oceanic poleward
Figure 1 19-year average meridional ﬂow at 30°N (Wunsch and Heimbach, 2013) in the Atlantic. The ﬂow ﬁeld was computed using
a dynamically consistent, energy, mass, etc. conserving model, driven by known atmospheric forcing, and adjusted to be consistent
with the great majority of observed data. Model time-step is about 1 hour. The eddy ﬁeld was parameterized and thus is not visually
apparent. As expected, the averaged result shows the known dominant elements of the North Atlantic Ocean circulation including an
intense Gulf Stream, a Deep Western Boundary Current, an interior Sverdrup-like return ﬂow, eastern boundary currents and less well-
documented interior ﬂows over the entire water column associated in large part with the topography. Structures and volume transports
vary considerably with latitude, and also temporally—as suppressed by averaging. Reproduced from Wunsch and Heimbach, 2013.
396Roquet and Wunsch Tellus A: Dynamic Meteorology and Oceanography DOI: 10.16993/tellusa.679
transport, in the atmospheric transport, in the nature
and degree of cloud cover, surface albedo, and the near-
surface return ﬂow, etc (e.g. Nummelin et al., 2017;
Chen and Tung, 2018). Climate change is a fully global
process involving ocean, atmosphere, ice, chemical, and
biological processes.
ON THE RISKS OF A COLLAPSING AMOC
Recent claims of “observation-based” signals for an
ongoing collapse of the AMOC (Boers, 2021) or that
the AMOC is at its weakest point in the last thousand
years (Caesar et al., 2021) were obtained by making
some extreme assumptions about the implications of
existing fragmentary, short-duration, observations of
the modern intensely variable system. Both analyses
assumed a strong correlation between subpolar Atlantic
sea surface temperature and the AMOC, and which is
only weakly supported by observations (Keil et al., 2020;
Li et al., 2021). The proxy-based inferences in (Caesar
et al., 2021) have also been criticized for methodological
reasons (Kilbourne et al., 2022) and they appear to be in
contradiction with evidence for a stable AMOC during the
last century (Fraser and Cunningham, 2021; Latif et al.,
2022). Recognition is needed of the turbulent, complex,
nature of the ocean circulation and of the difﬁculty in
observing its variability (Wunsch et al., 2013). Apart from
a few local exceptions (Hansen et al., 2016; Rossby et al.,
2020), too few direct observations of the AMOC exist to
warrant deﬁnite conclusions about the distant past or
future of the circulation.
Most current climate models show that the conse-
quences of AMOC collapse, although non-negligible,
would remain limited compared to the global effects
that anthropogenic greenhouse gases already have on
the climate system. Even in the most extreme scenarios
for the AMOC, the global mean temperature would
continue to increase (Sgubin et al., 2017). A variety of
regional impacts are expected, some through cooling of
the North Atlantic region and a shift in the mean latitude
of the Inter-tropical Convergence Zone (Bellomo et al.,
2021). Curiously, the invocation of an AMOC collapse,
as a general explanation for anomalous climate states,
implies that the old, classical, understanding built upon
gyres and Sverdrup balance, again becomes directly
applicable (Pedlosky, 1996)—but one with all of its own
variability and complexities. Even in a state with no AMOC,
massive amounts of ﬂuid would still be moving north and
south, conveying not just mass, but also net amounts of
heat, freshwater, carbon etc.
Dramatic proclamations of major shifts to take place
in the ongoing ocean circulation may serve the useful
purpose of alerting the public to the dangers of climate
change; nonetheless, they should be as scientiﬁcally
defensible as possible and should not divert attention
from the immediate dangers posed by increasing
greenhouse gas emissions—global warming, sea-level
rise, loss of biodiversity etc. Continued monitoring in
the decades to come of the entire ocean-atmosphere
coupled system, will be required to assess the true risks
of a collapsing AMOC, yet no evidence of the imminence
or predominance of such danger exists to date.
NOTES
1 The equivalent volume flux in the Boussinesq approximation.
2 Sadly, this confusion is frequent in much media coverage, partly
because of scientific miscommunication (e.g. Potsdam Institute
for Climate Impact, 2021). Short of a planetary-scale collision,
no known physics permits the stopping of the Gulf Stream and
other WBCs for hundreds of millions of years into the future.
ACKNOWLEDGEMENT
Comments by P. Huybers, B. Arbic, K.K. Tung, D. Meltzer, L.
Chaﬁk and D. Ferreira were helpful. Partial support came
from the Cecil and Ida Green Physical Oceanography
Chair at MIT (USA).
COMPETING INTERESTS
The authors have no competing interests to declare.
AUTHOR AFFILIATIONS
Fabien Roquet orcid.org/0000-0003-1124-4564
Department of Marine Sciences, University of Gothenburg,
Gothenburg, Sweden
Carl Wunsch orcid.org/0000-0001-6808-3664
Department of Earth and Planetary Sciences, Harvard
University, Cambridge, MA 02138, USA
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TO CITE THIS ARTICLE:
Roquet, F and Wunsch, C. 2022. The Atlantic Meridional Overturning Circulation and its Hypothetical Collapse. Tellus A: Dynamic
Meteorology and Oceanography, 74(1): 393–398. DOI: https://doi.org/10.16993/tellusa.679
Submitted: 23 August 2022 Accepted: 29 August 2022 Published: 07 November 2022
COPYRIGHT:
© 2022 The Author(s). This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0
International License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original
author and source are credited. See http://creativecommons.org/licenses/by/4.0/.
Tellus A: Dynamic Meteorology and Oceanography is a peer-reviewed open access journal published by Stockholm University Press.
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Article
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There is debate about slowing of the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global climate system. Some focus is on the sea surface temperature (SST) slightly cooling in parts of the subpolar North Atlantic despite widespread ocean warming. Atlantic SST is influenced by the AMOC, especially on decadal timescales and beyond. The local cooling could thus reflect AMOC slowing and diminishing heat transport, consistent with climate model responses to rising atmospheric greenhouse gas concentrations. Here we show from Atlantic SST the prevalence of natural AMOC variability since 1900. This is consistent with historical climate model simulations for 1900–2014 predicting on average AMOC slowing of about 1 Sv at 30° N after 1980, which is within the range of internal multidecadal variability derived from the models’ preindustrial control runs. These results highlight the importance of systematic and sustained in-situ monitoring systems that can detect and attribute with high confidence an anthropogenic AMOC signal. The Atlantic Meridional Overturning Circulation (AMOC) is predicted to slow with climate change. Sea surface temperature data and climate model analysis show that since 1900 natural variability has been dominant in AMOC changes; anthropogenic forcing is not yet reliably detectable by this method.
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