Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.3.4.1 Atlantic Meridional Overturning Circulation (AMOC) =====

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The AR5 concluded that there was no evidence of a trend in the AMOC during the period of instrumental observations. However, AR5 also stressed insufficient evidence to support a finding of change in the heat transport of the AMOC. SROCC assessed that there was emerging evidence in sustained observations, both in situ (2004–2017) and revealed from SST-based reconstructions, that the AMOC had weakened during the instrumental era relative to 1850–1900 ( ''medium confidence'' ), although there were insufficient data to quantify the magnitude of the weakening. SROCC also concluded with ''low confidence'' an increase of the Southern Ocean upper cell overturning circulation. SROCC also reported with ''medium confidence'' that the production of Antarctic Bottom Water had decreased since the 1950s consistent with a decreased lower cell overturning circulation, and potentially modulating the strength of the AMOC.

On multi-millennial timescales, proxy evidence indicates that the AMOC varied repeatedly in strength and vertical structure. During the last glacial period, particularly around the LGM, AMOC was estimated to be shallower than present, although there is continued debate about the magnitude of the shoaling ( [[#Lynch-Stieglitz--2007|Lynch-Stieglitz et al., 2007]] ; [[#Gebbie--2014|Gebbie, 2014]] ), and whether this change was associated with a weaker overturning ( [[#Ritz--2013|Ritz et al., 2013]] ; [[#Menviel--2017|Menviel et al., 2017]] ; [[#Muglia--2018|Muglia et al., 2018]] ). There are indications that substantial variations in AMOC were associated with abrupt climate changes during the glacial intervals, including Dansgaard-Oeschger and Heinrich events (14–70 ka) ( [[#McManus--2004|McManus et al., 2004]] ; [[#Böhm--2015|Böhm et al., 2015]] ; [[#Henry--2016|Henry et al., 2016]] ; [[#Lynch-Stieglitz--2017|Lynch-Stieglitz, 2017]] ). During these millennial-scale oscillations, weakened AMOC was associated with dramatic cooling in the NH and warming in the SH ( [[#Buizert--2015|Buizert et al., 2015]] ; [[#Henry--2016|Henry et al., 2016]] ), while hemispheric changes of opposite sign accompanied strengthened AMOC. After the final demise of the Laurentide ice sheet about 8 ka, the mean overall strength of AMOC has been relatively stable throughout the rest of the Holocene compared to the preceding 100 kyr ( [[#Hoffmann--2018|Hoffmann et al., 2018]] ; [[#Lippold--2019|Lippold et al., 2019]] ). There are however indications of episodic variations in AMOC during the Holocene ( [[#Bianchi--1999|Bianchi and McCave, 1999]] ; [[#Oppo--2003|Oppo et al., 2003]] ; [[#Thornalley--2013|Thornalley et al., 2013]] ; [[#Ayache--2018|Ayache et al., 2018]] ), and past interglacial intervals ( [[#Galaasen--2014|Galaasen et al., 2014]] , 2020; [[#Hayes--2014|Hayes et al., 2014]] ; [[#Mokeddem--2014|Mokeddem et al., 2014]] ; H. [[#Huang--2020|]] [[#Huang--2020|Huang et al., 2020]] ). Over the last 3 kyr, there are indications that AMOC variability was potentially linked to decreasing production of Labrador Sea Water (LSW), one of the water masses contributing to AMOC ( [[#Alonso-Garcia--2017|Alonso-Garcia et al., 2017]] ; [[#Moffa-Sánchez--2017|Moffa-Sánchez and Hall, 2017]] ; [[#Moffa-Sánchez--2019|Moffa-Sánchez et al., 2019]] ).

Numerous proxy records collectively imply that AMOC is currently at its weakest point in the past 1.6 ka ( [[#Rahmstorf--2015|Rahmstorf et al., 2015]] ; [[#Caesar--2018|Caesar et al., 2018]] , 2021; [[#Thibodeau--2018|Thibodeau et al., 2018]] ; [[#Thornalley--2018|Thornalley et al., 2018]] ). [[#Caesar--2021|Caesar et al. (2021)]] analyse a compilation of various available indirect AMOC proxies from marine sediments, in situ-based reconstructions and terrestrial proxies, which show a decline beginning in the late 19th century and over the 20th century superimposed by large decadal variability in the second half of the 20th century. Indirect reconstructions of AMOC components based on coastal sea level records in the western North Atlantic ( [[#Ezer--2013|Ezer, 2013]] ; [[#McCarthy--2015|McCarthy et al., 2015]] ; [[#Piecuch--2020|Piecuch, 2020]] ) show an AMOC decline since the late 1950s, with only a short period of recovery during the 1990s.

However, other studies highlight that proxy records do not show such clear signals ( [[#Moffa-Sánchez--2019|Moffa-Sánchez et al., 2019]] ), and the use of SST- and coastal sea level-based proxies of AMOC places uncertainties on these results ( [[#Little--2019|Little et al., 2019]] ; [[#Jackson--2020|Jackson and Wood, 2020]] ; [[#Menary--2020|Menary et al., 2020]] ). For instance, SSTs are additionally influenced by atmospheric and non-AMOC related ocean variability ( [[#Josey--2018|Josey et al., 2018]] ; [[#Keil--2020|Keil et al., 2020]] ; [[#Menary--2020|Menary et al., 2020]] ), while sea level responds to a variety of factors (e.g., atmospheric pressure and local winds) independent of the AMOC ( [[#Woodworth--2014|Woodworth et al., 2014]] ; [[#Piecuch--2015|Piecuch and Ponte, 2015]] ; [[#Piecuch--2016|Piecuch et al., 2016]] ). Finally, large decadal variability is present in many reconstructions and obscures estimation of the long-term trend over the 20th century ( [[#Ezer--2013|Ezer, 2013]] ; [[#McCarthy--2015|McCarthy et al., 2015]] ; [[#Yashayaev--2016|Yashayaev and Loder, 2016]] ; [[#Thornalley--2018|Thornalley et al., 2018]] ; [[#Caesar--2021|Caesar et al., 2021]] ). It is also noted that the proxy reported AMOC decline, beginning in the late 19th century, is not supported by model-based evidence (Sections 3.5.4.1 and 9.2.3.1).

Since the 1980s, multiple lines of observational evidence for AMOC change exist. Ship-based hydrographic estimates of AMOC as far back as the 1980s show no overall decline in AMOC strength ( [[#Fu--2020|Fu et al., 2020]] ; [[#Worthington--2021|Worthington et al., 2021]] ). Direct indications from in-situ observations report a –2.5 ± 1.4 Sv change between 1993 and 2010 across the OVIDE section, superimposed on large interannual to decadal variability ( [[#Mercier--2015|Mercier et al., 2015]] ). At 41°N and 26°N, a decline of –3.1 ± 3.2 Sv per decade and –2.5 ± 2.1 Sv per decade respectively has been reported over 2004–2016 ( [[#Baringer--2018|Baringer et al., 2018]] ; [[#Smeed--2018|Smeed et al., 2018]] ). However, [[#Moat--2020|Moat et al. (2020)]] report an increase in AMOC strength at 26°N over 2009–2018. Recent time series of moored observations at 11ºS ( [[#Hummels--2015|Hummels et al., 2015]] ), 34°S ( [[#Meinen--2018|Meinen et al., 2018]] ; [[#Kersalé--2020|Kersalé et al., 2020]] ), and between 57 and 60ºN ( [[#Lozier--2019|Lozier et al., 2019]] ) are currently too short to permit robust conclusions about changes. The directly observed AMOC weakening since 2004, while significant, is over too short a period to assess whether it is part of a longer term trend or dominated by decadal‐scale internal variability ( [[#Smeed--2014|Smeed et al., 2014]] ; [[#Collins--2019|Collins et al., 2019]] ; [[#Moat--2020|Moat et al., 2020]] ). Notably an increase and subsequent decline in the 1990s is present in estimates of AMOC and associated heat transport constructed from reanalyses or auxiliary data (Section 9.2.3.1; [[#Frajka-Williams--2015|Frajka-Williams, 2015]] ; [[#Jackson--2016|Jackson et al., 2016]] ; [[#Trenberth--2017|Trenberth and Fasullo, 2017]] ; [[#Jackson--2020|Jackson and Wood, 2020]] ).

Repeated full depth in situ measurements report that deep convection – a major driver for AMOC – has recently returned to the Labrador Sea, particularly in 2015 ( [[#Yashayaev--2016|Yashayaev and Loder, 2016]] ; [[#Rhein--2017|Rhein et al., 2017]] ), and to the Irminger Sea ( [[#de%20Jong--2016|de Jong and de Steur, 2016]] ; [[#Gladyshev--2016|Gladyshev et al., 2016]] ; [[#de%20Jong--2018|de Jong et al., 2018]] ) following an extended period with weak convection since 2000. An associated strengthening of the outflow from the Labrador Sea has not been observed ( [[#Zantopp--2017|Zantopp et al., 2017]] ; [[#Lozier--2019|Lozier et al., 2019]] ), while strengthening of the AMOC is tentative ( [[#Desbruyères--2019|Desbruyères et al., 2019]] ; [[#Moat--2020|Moat et al., 2020]] ). A long-term increase of the upper overturning cell in the Southern Ocean since the 1990s can be assessed with ''low confidence'' , and there is ''medium confidence'' of a decrease in Antarctic bottom water (AABW) volume and circulation, which has potential implications for the strength of the AMOC (Section 9.2.3.2).

In summary, proxy-based reconstructions suggest that the AMOC was relatively stable during the past 8 kyr ( ''medium confidence'' ), with a weakening beginning since the late 19th century ( ''medium confidence'' ), but due to a lack of direct observations, ''confidence'' in an overall decline of AMOC during the 20th century ''is low'' . From the mid-2000s to mid-2010s, the directly observed weakening in AMOC ( ''high confidence'' ) cannot be distinguished between decadal-scale variability or a long-term trend ( ''high confidence'' ).

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