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Past, Present, and Future Changes in the Atlantic Meridional Overturning Circulation

TL;DR: In this paper, the current understanding of past, present, and future changes in the Atlantic meridional overturning circulation and the effects of such changes on climate are reviewed, as well as the outstanding challenges and possible future directions for AMOC research are outlined.
Abstract: Observations and numerical modeling experiments provide evidence for links between variability in the Atlantic meridional overturning circulation (AMOC) and global climate patterns. Reduction in the strength of the overturning circulation is thought to have played a key role in rapid climate change in the past and may have the potential to significantly influence climate change in the future, as noted in the last two Intergovernmental Panel on Climate Change (IPCC) assessment reports (Houghton et al.; Solomon et al.). Both IPCC reports also highlighted the significant uncertainties that exist regarding the future behavior of the AMOC under global warming. Model results suggest that changes in the AMOC can impact surface air temperature, precipitation patterns, and sea level, particularly in areas bordering the North Atlantic, thus affecting human populations. Here, the current understanding of past, present, and future changes in the AMOC and the effects of such changes on climate are reviewed. The focus is on observations of the AMOC, how the AMOC influences climate, and in what way the AMOC is likely to change over the next few decades and the twenty-first century. The potential for decadal prediction of the AMOC is also discussed. Finally, the outstanding challenges and possible future directions for AMOC research are outlined.

Summary (1 min read)

How will the AMOC change over the next few decades and the twenty-first century?

  • The climate models used in the assessment have relatively low ocean resolution O(1°) and do not include all relevant physical processes (e.g., Greenland melting; Swingedouw et al.
  • This uncertainty, together with the potential climatic impacts of AMOC changes, has stimulated attempts to predict changes in the AMOC on decadal time scales.
  • They find that the potential predictability of the heat transport in the subtropical gyre is closely linked to the potential predictability of the AMOC, which is consistent with the high correlation of the two in the 26.5°N observations (Johns et al. 2011) .
  • If the AMOC transports freshwater southward across the section, then the system is in a bistable regime, because an assumed AMOC decrease would cause a reduction of this freshwater export and thus an overall freshening of the Atlantic, potentially causing a further weakening of the AMOC and thereby constituting a destabilizing feedback.

C O N C L u S I O N S A N d F u t u r e

  • The key conclusions from the above are as follows: the importance of the AMOC for the climate is paramount; there is a pressing need for sustained observations of the AMOC and associated heat transport; and the potential predictability of the AMOC and therefore of its climate impacts needs further study.
  • Where and how to deploy observing systems in the subpolar North Atlantic and the subtropical South Atlantic; and How to take advantage of new technologies, such as gliders and Argo floats.
  • The third challenge is one for the longer term, as at present gliders have an operating limit of 1,000 m and Argo floats of 2,000 m, which severely restricts their ability to measure the deep circulation.
  • With regard to initialization, the continually improving blend of observations (e.g., from Argo and satellite altimetry) and ocean state estimation should lead to better initial conditions for decadal forecasts of the AMOC, heat transport, and the climate impacts.

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Past, present and future change in the
Atlantic meridional overturning circulation
Article
Published Version
Srokosz, M., Baringer, M., Bryden, H., Cunningham, S.,
Delworth, T., Lozier, S., Marotzke, J. and Sutton, R. (2012)
Past, present and future change in the Atlantic meridional
overturning circulation. Bulletin of the American Meteorological
Society, 93 (11). pp. 1663-1676. ISSN 1520-0477 doi:
https://doi.org/10.1175/BAMS-D-11-00151.1 Available at
https://centaur.reading.ac.uk/27753/
It is advisable to refer to the publishers version if you intend to cite from the
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To link to this article DOI: http://dx.doi.org/10.1175/BAMS-D-11-00151.1
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The AMOC is a major component of Earth's climate system,
due to its transport of heat, but its future behavior is uncertain.
PAST, PRESENT, AND FUTURE CHANGES
IN THE ATLANTIC MERIDIONAL
OVERTURNING CIRCULATION
by M. SrokoSz, M. baringer, H. bryden, S. CunningHaM, T. delworTH,
S. lozier, J. MaroTzke, and r. SuTTon
T
he future of the global climate system is
uncertain and depends on the anthro-
pogenic input of CO
2
into the atmosphere
(Solomon et al. 2007). One of the significant
areas of uncertainty highlighted in the most
recent Intergovernmental Panel on Climate
Change’s (IPCC) report, the Fourth Assess-
ment Report, is the future behavior of the
Atlantic Ocean’s meridional overturning cir-
culation [MOC
1
; see Fig. 10.15 in Solomon et
al. (2007)]. The Atlantic MOC (AMOC) con-
sists of a near-surface, warm northward flow,
compensated by a colder southward return
flow at depth. Heat loss to the atmosphere at
high latitudes in the North Atlantic makes
the northward-flowing surface waters denser,
causing them to sink to considerable depths.
These waters constitute the deep return flow
of the overturning circulation (see Fig. 1). The
AMOC is unusual in the worlds oceans, as it
transports heat northward across the equa-
tor. The maximum northward oceanic heat
transport occurs at 24°–26°N and is 1.3 PW
Fi g . 1. A simplified schematic of the AMOC showing both
the overturning and gyre recirculation components. Warm
water flows north in the upper ocean (red), gives up heat to
the atmosphere (atmospheric flow gaining heat represented
by the changing color of broad arrows), sinks, and returns
as a deep cold flow (blue). Latitude of the 26.N AMOC
observations is indicated. Note that the actual flow is more
complex. For example, see Bower et al. (2009, their Fig. 1) for
the intermediate depth circulation in the vicinity of the Grand
Banks and Biastoch et al. (2008, their Fig. 2) for the middepth
circulation around South Africa, showing the importance of
eddies in transferring heat and salt from the Indian Ocean to
the Atlantic Ocean.
1
The MOC has at times been referred to as the ther-
mohaline circulation (THC); that is, that part of the
ocean circulation determined by changes in tem-
perature and salinity—the two are not synonymous.
The MOC is what can be determined in practice, as
a zonal integral of the meridional velocity, whereas
the THC is not directly measurable but is related to
one of the mechanisms involved in the overturning
(see Kuhlbrodt et al. 2007).
1663
NOVEMBER 2012AMERICAN METEOROLOGICAL SOCIETY
|

(1 PW = 10
15
W) and accounts for ~25% of the total
(atmosphere and ocean) poleward heat transport at
those latitudes (Hall and Bryden 1982; Trenberth and
Caron 2001; Johns et al. 2011). As this oceanic heat is
advected poleward, there is a strong transfer of heat
from the ocean to the atmosphere at midlatitudes,
contributing to the temperate climate of northwest
Europe. Future changes in the AMOC could there-
fore have significant climatic impacts. In addition,
such changes could affect the North Atlantic sink for
CO
2
(Schuster and Watson 2007), the position of the
intertropical convergence zone (ITCZ), the Atlantic
storm track, rainfall (Vellinga and Wood 2002), and
marine ecosystems (Schmittner 2005).
Despite its importance, and the uncertainty about
its future behavior, the AMOC has not been well
observed until recently. The traditional approach
for measuring the AMOC was using synoptic trans-
ocean basin ship-based estimates of geostrophic
velocities, calculated from density, in turn obtained
from temperature and salinity. This approach led to
the most highly sampled part of the AMOC being
a section at ~24°N, with occupations in 1957, 1981,
1992, 1998, and 2004 (Bryden et al. 2005). A further
occupation of this section occurred in 2010 (Atkinson
et al. 2012; Frajka-Williams et al. 2011). Such serious
undersampling means that any conclusions drawn
about the past behavior of the AMOC are subject to
considerable uncertainty (Cunningham et al. 2007;
Kanzow et al. 2010). This paper will discuss the fol-
lowing: the past and present behavior of the AMOC
in light of more recent observations; the possible
impacts of future changes; the potential for predicting
future changes, particularly on decadal time scales;
and future directions for AMOC research. Further
background on the AMOC may be found in the re-
views of Kuhlbrodt et al. (2007, 2009), Lozier (2010,
2012) and special issue of Deep-Sea Research (2011,
Vol. 58, Nos. 17 and 18). Kuhlbrodt et al. (2007) dis-
cuss the driving processes of the AMOC—surface
heat and freshwater fluxes, vertical mixing processes
in the ocean interior, wind-induced upwelling in
the Southern Ocean—so readers are referred to that
review for more on those topics.
What do we know about present and past changes in
the AMOC? In addition to the uncertainties regarding
the future behavior of the AMOC, a spur to investi-
gate the role of the AMOC in climate has been the
paleoclimate record, as captured in ice cores and
ocean sediments. Past rapid (in this context, on the
order of a decade) changes in the climate have been
linked to changes in the AMOC, leading to Broecker’s
(1991) characterization of the global MOC as the
“great ocean conveyor” [see reviews of Clark et al.
2002; Rahmstorf 2002; Alley 2007; Lynch-Stieglitz
et al. 2007; see special issue of Global and Planetary
Change, 2011, Vol. 79, Nos. 3 and 4, containing a range
of results from the Rapid Climate Change (RAPID)
program paleostudies]. That the circulation might
have more than one stable state has been known
since Stommel’s (1961) paper (see also Longworth
et al. 2005), and potentially this could allow rapid
switching between ocean circulation states under
external forcing (see the “How will the AMOC
change over the next few decades and the twenty-first
century?” section).
A paper that bridges the gap between paleo obser-
vations and modern ones is that of Boessenkool et al.
(2007), which uses the paleocurrent proxy of “sort-
able” silt from a core on the Reykjanes Ridge to exam-
ine the flow of Iceland–Scotland overflow water—one
of the sources of the deep limb of the AMOC—over
the last 230 years. The authors show that the flow cor-
relates well with modern observations of salinity and
with the North Atlantic Oscillation (NAO) on decadal
time scales. The relationship between the NAO and
the AMOC via the deep overflows is one that remains
to be determined, as the link between high-latitude
deep flows and the AMOC is complex (Lozier 2012).
The behavior of the AMOC even farther back in
time has been examined using a variety of paleo-
proxies [as discussed in detail by Alley (2007)]. In
particular, in addition to the possible “on/off” modes
characterized by Stommel (1961), paleoevidence
suggests that there might have been three modes of
AMOC operation during the last glacial period. These
AFFILIATIONS: SrokoSz, bryden, and CunningHaMNational
Oceanography Centre, Southampton, Southampton, United
Kingdom; baringerAtlantic Oceanographic and Meteorological
Laboratory, Miami, Florida; delworTH Geophysical Fluid
Dynamics Laboratory, Princeton, New Jersey; lozierNicholas
School of the Environment, Duke University, Durham, North
Carolina; MaroTzke—Max Planck Institute for Meteorology,
Hamburg, Germany; SuTTonDepartment of Meteorology,
University of Reading, Reading, United Kingdom
CORRESPONDING AUTHOR: M. Srokosz, National
Oceanography Centre, Southampton, Empress Dock,
Southampton SO14 3ZH, United Kingdom
E-mail: mas@noc.ac.uk
The abstract for this article can be found in this issue, following the
table of contents.
DOI:10.117 5 / B A M S - D -11- 0 0151.1
In final form 16 March 2012
©2012 American Meteorological Society
1664
november 2012
|

are characterized by Rahmstorf
(2002, his Fig. 2) as “warm,” “cold,
and “off.” Warm corresponds to the
current AMOC configuration, off
has no northward warm water flow
at the surface, while cold is a mode in
which the AMOC exists but the sur-
face warm waters do not penetrate as
far north as the Nordic Seas, rather
they sink and form a shallower re-
turn flow south of Iceland.
Most of the effort in paleostud-
ies of the AMOC has focused on
periods covered by the Greenland
and Antarctic ice core records (e.g.,
Barker et al. 2011). Prior to the
Holocene (the last ~11,000 years),
which has been relatively stable
climatically, the ice core tempera-
ture records (based on the oxygen-18 isotope proxy)
show large fluctuations on short (decadal) times
scales. Some of these fluctuations are concurrent, to
within dating errors, with changes in proxies found
in ocean sediments and indicative of AMOC changes
(e.g., carbon-13 and carbon-14, cadmium-to-calcium
ratios in planktonic and benthic forminifera; sort-
able silt; Alley 2007). Several of these changes are
linked to so-called Heinrich events during the last ice
age, when icebergs calved from glaciers entered the
North Atlantic and the additional freshwater input
changed the mode of operation of the AMOC (e.g.,
Hemming 2004). Other changes, such as the 8.2-kyr
event during the Holocene and the Younger Dryas
event, are thought to be linked to large outbursts
of freshwater, from ice-dammed lakes in North
America, entering the North Atlantic and disrupting
the AMOC, causing it to shut down (e.g., McManus
et al. 2004; Alley and Ágústsdóttir 2005; Wiersma
and Renssen 2006; Murton et al. 2010). The climatic
impacts of these disruptions of the AMOC can be
felt far afield (see Fig. 2 for the impacts of the 8.2 kyr;
Alley and Ágústsdóttir 2005).
Perhaps the key insight to be gained from paleocli-
matic reconstructions of the AMOCs past behavior is
that it can be highly variable and its mode of opera-
tion can change on short (decadal) time scales with
significant climate impacts. A challenge is whether
the climate models in current use can reproduce such
AMOC behavior (Alley 2003; Valdes 2011).
Both the paleoclimate record and the 2001 IPCC
assessment (Houghton et al. 2001) underline the
need for continuous observations of the AMOC, to
better understand its role in the climate system, to
determine its behavior, and to test climate model
predictions. This need led to the jointly funded UK-
US RAPID AMOC observing system being deployed
along latitude 26.5°N since April 2004.
2
Rayner et
al. (2011) give details of the system, of which the
key components are 1) the Gulf Stream transport
through the Florida Straits measured by seabed cable
(Baringer and Larsen 2001; Meinen et al. 2010); 2)
the Ekman transport calculated from wind stress
[originally from Quick Scatterometer (QuikSCAT)
winds until its demise in 2009; now from European
Centre for Medium-Range Weather Forecasts Interim
Re-Analysis (ERA-Interim) winds (www.ecmwf
.int/research/era/do/get/era-interim)]; 3) midocean
transport measured by arrays of moorings at the east-
ern and western boundaries, and the Mid-Atlantic
Ridge. The first year of observations (Cunningham et
al. 2007; Kanzow et al. 2007) showed that the system
was able to monitor the AMOC on a 10-day basis.
Doubts have been raised about the systems ability to
measure the AMOC because of the impact of meso-
scale variability on the measurements (Wunsch 2008),
but observations and modeling studies by Bryden et
al. (2009) and Kanzow et al. (2009) have demonstrated
that these doubts are unfounded. Figure 3 shows the
time series of the AMOC obtained to date. Analysis of
the first 4 yr of data (Kanzow et al. 2010) showed that
the AMOC at 26.5°N had a mean strength of 18.7 Sv
(1 Sv = 10
6
m
3
s
−1
) with fluctuations of 4.8 Sv rms. The
AMOC also showed a pronounced seasonal cycle with
an estimated peak-to-peak amplitude of 6.7 Sv. The
study revealed that, contrary to the accepted view,
Fig. 2. Climate anomalies, determined from paleoproxies, associ-
ated with the so-called 8.2 kyr event (also known as 8 kyr event) that
occurred approximately 8,200 yr ago; paleoevidence suggests that
the AMOC was disrupted by a freshwater outburst into the North
Atlantic from an ice-dammed lake in North America (after Fig. 1 of
Alley and Ágústsdóttir 2005).
2
Currently funded until 2014.
1665
november 2012AmerICAn meTeoroLoGICAL SoCIeTY
|

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  • ...6 Sv (Baringer et al., 2013; Srokosz et al., 2012), based on 8 years (2004– 2011) of data for an in situ mooring array (Rayner et al....

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  • ...AMOC strength (Sv) at 28N in five ensemble members and their mean (heavy black line) for the 31 A1B GHG scenario and for that scenario plus ice melt in both hemispheres with 10-year doubling time 32 reaching a maximum 5 m contribution to sea level....

    [...]

  • ...Massive ice rafting (“Heinrich”) events are often associated with decreased NADW production and shutdown or slowdown of the Atlantic meridional overturning circulation (AMOC) (Broecker, 2002; Barreiro et al., 2008; Srokosz et al., 2012)....

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3,291 citations

Journal ArticleDOI
TL;DR: In this article, a suite of climate models are used to predict changes in surface air temperature on decadal timescales and regional spatial scales, and it is shown that the uncertainty for the next few decades is dominated by model uncertainty and internal variability that are potentially reducible through progress in climate science.
Abstract: Faced by the realities of a changing climate, decision makers in a wide variety of organizations are increasingly seeking quantitative predictions of regional and local climate. An important issue for these decision makers, and for organizations that fund climate research, is what is the potential for climate science to deliver improvements—especially reductions in uncertainty—in such predictions? Uncertainty in climate predictions arises from three distinct sources: internal variability, model uncertainty, and scenario uncertainty. Using data from a suite of climate models, we separate and quantify these sources. For predictions of changes in surface air temperature on decadal timescales and regional spatial scales, we show that uncertainty for the next few decades is dominated by sources (model uncertainty and internal variability) that are potentially reducible through progress in climate science. Furthermore, we find that model uncertainty is of greater importance than internal variability. Our findin...

2,052 citations

Journal ArticleDOI
22 Apr 2004-Nature
TL;DR: It is found that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500’yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago.
Abstract: The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies1, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning2. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.

1,875 citations