Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.5.4.1 Atlantic Meridional Overturning Circulation (AMOC) ====

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The Atlantic Meridional Overturning Circulation (AMOC) represents a large-scale flow of warm salty water northward at the surface and a return flow of colder water southward at depth. As such, its mean state plays an important role in transporting heat in the climate system, while its variability can act to redistribute heat (see Sections 2.3.3.4.1 and 9.2.3.1 for more details). Paleo-climatic and model evidence suggest that changes in AMOC strength have played a prominent role in past transitions between warm and cool climatic phases (e.g., [[#Dansgaard--1993|Dansgaard et al., 1993]] ; [[#Ritz--2013|Ritz et al., 2013]] ).

The AR5 concluded that while climate models suggested that an AMOC slowdown would occur in response to anthropogenic forcing, the short direct observational AMOC record precluded it from being used to support this model finding. [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] reports with ''high'' ''confidence'' , a weakening of the AMOC was observed in the mid-2000s to the mid-2010s, while again also noting that the observational record was too short to determine whether this is a significant trend or a manifestation of decadal and multi-decadal variability ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] ). Indirect evidence of AMOC weakening since at least the 1950s is also presented, but confidence in this longer-term decrease was ''low'' [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] ).

Despite the additional six years or so of observations since AR5, the evaluation of the AMOC in models continues to be severely hampered by the geographically sparse and temporally short observational record. The longest continuous observational estimates of the AMOC are based on measurements taken at 26°N by the RAPID-MOCHA array ( [[#Smeed--2018|Smeed et al., 2018]] ). Basic evaluation of the AMOC at 26°N shows that the CMIP5 and CMIP6 multi-model mean overturning strength is comparable with RAPID ( [[#Reintges--2017|Reintges et al., 2017]] ; [[#Weijer--2020|Weijer et al., 2020]] ), but the model range is large (12–29 sverdrups (Sv)) for CMIP5 ( [[#Zhang--2013|Zhang and Wang, 2013]] ); and 10–31 Sv for CMIP6 ( [[#Weijer--2020|Weijer et al., 2020]] ) (Figure 3.30a). It is noted that deviations of AMOC strength in CMIP5 models have been related to global-scale sea surface temperature biases ( [[#Wang--2014|]] [[#Wang--2014|C. Wang et al., 2014]] ). Both coupled and ocean-only models also underestimate the depth of the AMOC cell ( [[#Danabasoglu--2014|Danabasoglu et al., 2014]] ; [[#Weijer--2020|Weijer et al., 2020]] ; Figure 3.30a). Paleo-climatic evidence has also raised questions regarding the accuracy of the representation of the strength and depth of the modelled AMOC during past periods ( [[#Otto-Bliesner--2007|Otto-Bliesner et al., 2007]] ; [[#Muglia--2015|Muglia and Schmittner, 2015]] ). Overall, however, both the CMIP5 and CMIP6 model ensembles simulate the general features of the AMOC mean state reasonably well, but there is a large spread in the latitude and depth of the maximum overturning, and the maximum AMOC strength (Figure 3.30a).

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[[File:0f7a5f938dc405d7e735382d89fd7f4b IPCC_AR6_WGI_Figure_3_30.png]]

Figure 3.30 | '''Observed and CMIP6 simulated AMOC mean state, variability and long-term trends. (a)''' AMOC meridional stream function profiles at 26.5°N from the historical CMIP5 (1860–2004) and CMIP6 (1860–2014) simulations compared with the mean maximum overturning depth (horizontal grey line) and magnitude (vertical grey line) from the RAPID observations (2004–2018). The distributions of model ranges of AMOC maximum magnitude and depth are respectively displayed near the x- and y-axis. '''(b)''' Distributions of overlapping eight-year AMOC trends from individual CMIP6 historical simulations (pink box plots) are plotted along with the combined distributions of all available CMIP5 (blue boxplot) and CMIP6 (red boxplot) models. For reference, the observed eight-year trend calculated between 2004 and 2012 is also shown as a horizontal grey line (following [[#Roberts--2014|Roberts et al., 2014]] ). '''(c)''' Distributions of interannual AMOC variability from individual CMIP6 model historical simulations, along with the combined distributions of all available CMIP5 and CMIP6 models. Interannual variability in models and observations is estimated as annual mean (April–March) differences, and the horizontal grey line is the observed value for 2009/2010 minus 2008/2009 (following [[#Roberts--2014|Roberts et al., 2014]] ). '''(d–f)''' Distributions of linear AMOC trends calculated over various time periods (see panel titles) in CMIP6 simulations forced with: greenhouse gas forcing only (GHG), natural forcing only (NAT), anthropogenic aerosol forcing only (AER) and all forcing combined (Historical; HIST). (a–f) Boxes indicate the 25th to 75th percentile range, whiskers indicate 1st and 99th percentiles in (a-c) and 5th and 95th percentile in (d-f), and dots indicate outliers, while the horizontal black line is the multi-model mean trend. In (d–f) the multi-model mean trend is also written above each distribution. The multi-model distributions in (a–c) were produced with one historical ensemble member per model for which the AMOC variable was available (listed), while those in (d–f) were produced with the detection and attribution simulation datasets utilized by [[#Menary--2020|Menary et al. (2020)]] . Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

The short length of the observed time-series (RAPID has measured the AMOC since 2004), sparse observations, observational uncertainties ( [[#Sinha--2018|Sinha et al., 2018]] ), as well as significant observed variability on interannual and longer time scales, makes comparison with modelled AMOC variability challenging. RAPID observations show that the overturning at 26°N was 2.9 Sv weaker in the multi-year average of 2008–2012 relative to 2004–2008 and 2.5 Sv weaker in 2012–2017 relative to 2004–2008 ( [[#Smeed--2014|Smeed et al., 2014]] , [[#Smeed--2018|2018]] ) (see also ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] ). As expected, this weakening was accompanied by a significant reduction in northward heat transport ( [[#Bryden--2020|Bryden et al., 2020]] ). CMIP5 and CMIP6 models produce a forced weakening of the AMOC over the 2012–2017 period relative to 2004–2008, but at 26°N the multi-model mean response is substantially weaker than the observed AMOC decline over the same period. The discrepancy between the modelled multi-model mean (i.e., the forced response) and the RAPID observed AMOC changes has led studies to suggest that the observed weakening over 2004–2017 is largely due to internal variability ( [[#Yan--2018|Yan et al., 2018]] ). However, comparison of observed RAPID AMOC variability with modelled variability also reveals that most CMIP5 models appear to underestimate the interannual and decadal time scale AMOC variability ( [[#Roberts--2014|Roberts et al., 2014]] ; [[#Yan--2018|Yan et al., 2018]] ), and, although the overall variance is larger in CMIP6 than in CMIP5, similar results are found analysing the CMIP6 models (Figure 3.30b,c). It is currently unknown why most models underestimate this AMOC variability, or whether they are underestimating the internal or externally forced components. This underestimation of AMOC variability may also have potential implications for detection and attribution, the relationship between AMOC and AMV (see [[#3.7.7|Section 3.7.7]] ), and near-term predictions. There is also emerging evidence, based on analysis of freshwater transports, that the AMOC in CMIP5-era models is too stable, largely due to systematic biases in ocean salinity (W. [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ; [[#Mecking--2017|Mecking et al., 2017]] ). Such a systematic bias may potentially be linked with the underestimation of both simulated AMOC internal variability through eddy-mean flow interactions that are poorly represented in standard CMIP-class model resolution ( [[#Leroux--2018|Leroux et al., 2018]] ), and externally forced change.

As reported in [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] , estimates of AMOC since at least 1950, which are generated from observed surface temperatures or sea surface height, suggest the AMOC weakened through the 20th century ( ''low confidence'' ) ( [[#Ezer--2013|Ezer et al., 2013]] ; [[#Caesar--2018|Caesar et al., 2018]] ). Over the same period, the CMIP5 multi-model mean showed no significant net forced response in AMOC ( [[#Cheng--2013|Cheng et al., 2013]] ). However, a significant forced change is simulated in the CMIP6 multi-model mean, where a clear increase of the AMOC is seen over the 1940–1985 period (Figure 3.30e; [[#Menary--2020|Menary et al., 2020]] ). Although there is general agreement that the influence of greenhouse gases acts to a weaken the modelled AMOC ( [[#Delworth--2006|Delworth and Dixon, 2006]] ; [[#Caesar--2018|Caesar et al., 2018]] ), changes in solar, volcanic and anthropogenic aerosol emissions can lead to temporary changes in AMOC on decadal- to multi-decadal time scales ( [[#Delworth--2006|Delworth and Dixon, 2006]] ; [[#Menary--2013|Menary et al., 2013]] ; [[#Menary--2014|Menary and Scaife, 2014]] ; [[#Swingedouw--2017|Swingedouw et al., 2017]] ; [[#Undorf--2018b|Undorf et al., 2018b]] ). As such, the simulated net forced response in AMOC is a balance between the different forcing factors (Section 9.2.3.1; [[#Delworth--2006|Delworth and Dixon, 2006]] ; [[#Menary--2020|Menary et al., 2020]] ). The differing AMOC response of CMIP5 and CMIP6 models during the historical period has been associated with stronger aerosol effective radiative forcing in the CMIP6 models ( [[#Menary--2020|Menary et al., 2020]] ), such that the aerosol-induced AMOC increase during the 1940–1985 period overcomes the greenhouse gas induced decline (Figure 3.30e). However, models simulate a range of anthropogenic aerosol effective radiative forcing and a range of historical AMOC trends in CMIP6 ( [[#Menary--2020|Menary et al., 2020]] ) and there remains considerable uncertainty over the realism of the CMIP6 AMOC response during the 20th century (Figure 3.30d–f) due to disagreement among the differing lines of evidence. For example, ocean reanalysis ( [[#Jackson--2019|Jackson et al., 2019]] ) and forced ocean model simulations ( [[#Robson--2012|Robson et al., 2012]] ; [[#Danabasoglu--2016|Danabasoglu et al., 2016]] ), which show AMOC changes that are broadly consistent with the CMIP6 response, appear to disagree with observational estimates of AMOC over the historical period ( [[#Ezer--2013|Ezer et al., 2013]] ; [[#Caesar--2018|Caesar et al., 2018]] ). It is noted, however, that the relatively short length of the forced ocean simulations and ocean reanalysis precludes a comparable assessment of 20th century trends. Furthermore, despite the similar AMOC evolution seen in forced ocean model simulations and the CMIP6 models, it is unclear whether the same underlying mechanisms are responsible for the changes.

In summary, models do not support robust assessment of the role of anthropogenic forcing in the observed AMOC weakening between the mid-2000s and the mid-2010s, which is assessed to have occurred with ''high confidence'' in [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4.1|Section 2.3.3.4.1]] , as the changes are outside of the range of modelled AMOC trends (regardless of whether they are forced or internally generated) in most models. Thus, we have ''low confidence'' that anthropogenic forcing has influenced the observed changes in AMOC strength in the post-2004 period. In addition, there remains considerable uncertainty over the realism of the CMIP6 AMOC response during the 20th century due to disagreement among the differing lines of observational and modelled evidence (i.e., historical AMOC estimates, ocean reanalysis, forced ocean simulations and historical CMIP6 simulations). Thus, we have ''low confidence'' that anthropogenic forcing has had a significant influence on changes in AMOC strength during the 1860–2014 period.

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