A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

Emma L. Worthington1, Ben I. Moat2, David A. Smeed2, Jennifer V. Mecking2, Robert Marsh1, and Gerard D. McCarthy3

  • 1University of Southampton, European Way, Southampton, SO14 3ZH, UK
  • 2National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK
  • 3ICARUS, Department of Geography, Maynooth University, Maynooth, Co. Kildare, Ireland

Received: 16 Jul 2020 – 

Discussion started: 14 Aug 2020 – 

Revised: 09 Dec 2020 – 

Accepted: 21 Dec 2020 – 

Published: 15 Feb 2021

Abstract

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26 N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline.How to cite. Worthington, E. L., Moat, B. I., Smeed, D. A., Mecking, J. V., Marsh, R., and McCarthy, G. D.: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline, Ocean Sci., 17, 285–299, https://doi.org/10.5194/os-17-285-2021, 2021.1 Introduction

In the Northern Hemisphere, the Atlantic meridional overturning circulation (AMOC) carries as much as 90 % of all the heat transported poleward by the subtropical Atlantic Ocean (Johns et al.2011), with the associated release of heat to the overlying air helping to maintain north-western Europe’s relatively mild climate for its latitude. The AMOC also transports freshwater towards the Equator, and the associated deep water formation moves carbon and heat into the deep ocean (Kostov et al.2014Winton et al.2013McDonagh et al.2015). A significant change in AMOC circulation is thus likely to have an impact on the climate of north-western Europe and further afield, with possible influences on global hydrological and carbon cycles. Although the Intergovernmental Panel on Climate Change (IPCC) says that it is unlikely that the AMOC will stop this century, they state with medium confidence that a slowdown by 2050 due to anthropogenic climate change is very likely (Stocker et al.2013).

The importance of the AMOC means that since 2004 it has been observed by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) mooring array at 26 N. The resulting observations have highlighted the great variability in AMOC transport on a range of timescales (Kanzow et al.2010Cunningham et al.2007), including a decline in AMOC strength between 2004 and 2012 (Smeed et al.2014). This reduced state persisted in 2017 (Smeed et al.2018). The decrease is more likely to be internal variability rather than a long-term decline in response to anthropogenic forcing (Roberts et al.2014), which the time series is currently too short to detect. Although the AMOC has been well-observed at 26 N since 2004, prior to this, estimates of AMOC strength were restricted to instances of transatlantic hydrographic sections along 24.5 N in 1957, 1981, 1992, 1998, and 2004, which provided only snapshots of the overturning circulation strength (Bryden et al.2005). There are extensive additional hydrographic data around 26 N, particularly at the western boundary, but these are insufficient to reconstruct the AMOC conventionally (Longworth et al.2011). Due to the limited availability of hydrographic data, proxies have been used to reconstruct the AMOC time series earlier than 2004.

In one proxy reconstruction, Frajka-Williams (2015) used sea-surface height from satellite altimetry to estimate trans-basin baroclinic transport at 26 N between 1993 to 2014. In another, Longworth et al. (2011) used temperature anomaly at the western boundary as a proxy for geostrophic transport within the upper 800 m, or thermocline layer, finding the temperature anomaly at 400 dbar explained 53 % of the variance in thermocline transport. However, both Longworth et al. (2011) and Frajka-Williams (2015) used single-layer models that do not account for the variable depth structure of the AMOC in the subtropics.

At 26 N, the dynamics of the AMOC involve multiple water masses flowing in opposite directions in different layers, driven by the changing density structure with depth (Fig. 1a). Within the permanent thermocline layer, which reaches as deep as 800 m on the western boundary and 600 m on the eastern, isopycnals rise towards the eastern boundary, indicative of southward flow (Hernández-Guerra et al.2014). Below the thermocline, isopycnals deepen towards the east, and the resulting transport profile (Fig. 1c) shows a small northward transport centred around 1000 m sandwiched between southward transports above and below. Although referred to by RAPID as Antarctic Intermediate Water (AAIW), both AAIW and Mediterranean Water are observed between 700–1600 m on the eastern boundary, with the relative contribution of each varying seasonally (Fraile-Nuez et al.2010Machín and Pelegrí2009Hernández-Guerra et al.2003). The transport profile also shows North Atlantic Deep Water (NADW), which has two distinct layers: Upper (UNADW) above 3000 m, primarily formed in the Labrador Sea (Talley and McCartney1982), and Lower (LNADW) below 3000 m, which has its origins in the overflows from the Nordic Seas (Pickart et al.2003). Changes observed in one NADW layer are not necessarily observed in another. Smeed et al. (2014) found that the reduction in AMOC strength between 2004 and 2012 was seen in LNADW but not UNADW, while Bryden et al. (2005) found that LNADW transport estimated from transatlantic hydrographic sections at 25 N decreased from −15 Sv in 1957 to less than −7 Sv in 1998 and 2004 but the UNADW transport remained between −9 and −12 Sv. Below the NADW layers, there is a small northward transport below 5000 m, Antarctic Bottom Water (AABW), that flows along the western side of the Mid-Atlantic Ridge. The partition between the upper southward and deep southward transports defines the strength of the overturning circulation: a weak AMOC is associated with a greater recirculation within the upper layers of the thermocline and weaker deep return flow; a stronger AMOC is associated with weaker thermocline recirculation and stronger deep NADW transport. For an empirical model to more fully represent AMOC dynamics, in particular lower-frequency changes, we suggest that it must represent these deeper layers. A layered-model interpretation of the density structure and the associated water mass transports is shown in Fig. 1b.

Figure 1(a) World Ocean Circulation Experiment (WOCE) North Atlantic A05 section of neutral density γn (kg m−3) at 24∘ N, July or August 1992. From the WOCE Atlantic Ocean Atlas Vol. 3. (Koltermann et al., 2011). (b) Schematic of four dynamic layers to be represented within the regression model by density anomalies at the western and eastern boundaries at a depth within each layer. The density anomalies are represented by the circular markers. (c) Profile of RAPID-estimated mean mid-ocean transport and the resulting northward and southward layer transports. Mean AMOC depth is around 1100 m.
Figure 1(a) World Ocean Circulation Experiment (WOCE) North Atlantic A05 section of neutral density γn (kg m−3) at 24 N, July or August 1992. From the WOCE Atlantic Ocean Atlas Vol. 3. (Koltermann et al.2011). (b) Schematic of four dynamic layers to be represented within the regression model by density anomalies at the western and eastern boundaries at a depth within each layer. The density anomalies are represented by the circular markers. (c) Profile of RAPID-estimated mean mid-ocean transport and the resulting northward and southward layer transports. Mean AMOC depth is around 1100 m.

Here, we revisit the approach of Longworth et al. (2011) by using linear regression models to represent the AMOC and develop the method further to include additional layers representative of the deep circulation. Section 2 describes how we trained and validated our statistical model using the RAPID dataset and how we selected historical hydrographic data to apply to the model. Section 3 describes how these hydrographic data were used to create an extended time series of AMOC strength from 1982 to 2016. In Sects. 4 and 5, we discuss the implications of creating the longest observational time series of AMOC strength that incorporates variability in the deep NADW layers and acknowledge the limitations of using an empirical model.

Read the full paper here.

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Duker
February 26, 2021 10:46 pm

What it says in the abstract isn’t whats being spun in the news about another study from Copenhagen
https://www.voanews.com/science-health/new-study-suggests-atlantic-ocean-circulation-system-could-collapse

alastair gray
Reply to  Duker
February 26, 2021 11:11 pm

Yes and Thursday in The Times yet another paper by Thornalley of London University published in Nature Geoscience was referenced –

https://www.thetimes.co.uk/article/cold-stormy-weather-forecast-for-uk-as-ocean-current-slows-k2d039509

In the Times article by Tom Whipps Science Editor claiming doom and gloom it is Claimed that AMOC is weaker than at any time in the last 1000 years and a Dr Laura Jackson is quoted as saying “This paper adds to evidence that the AMOC has been weakening over the historical period”

Well is Historical period the last 30 years – Maybe not and maybe it has weakened over a 1000 year period – clearly a long term cycle that has nothing to do with AGW Here we have 3 different papers. The Southhampton article suggesting that there has been little or no decline since 1981..
It is just as well that the science is settled otherwise we might be a tad uncertqin about what is actually happening to AMOC

Duker
Reply to  alastair gray
February 27, 2021 3:58 pm

Im wondering if word about the Southhampton study got around during peer review and the usual suspects worked together to publish ‘some other stuff’ to cut the ground from under Worthington et al. Its certainly how it works in politics…..

Rod Evans
Reply to  Duker
February 26, 2021 11:25 pm

Any report or supposed study that is presented with the preposition “could” is useless.
I could win the lottery, but it is very unlikely.
I could become PM but it is very unlikely.
I could find something positive to say about Climate Alarmists, I am searching every day.

lee
Reply to  Duker
February 26, 2021 11:30 pm

The models duke it out. Peanuts on special. Popcorn futures rise.

Krishna Gans
Reply to  Duker
February 27, 2021 2:08 am

That’s a study Rahmstorf participated ?
Since he was in cinema for “The day after tomorrow” he has a AMIC obsession, created even a site at PIK about the “realities” in that well done piece of SF. (I posted the link some days ago in an other threat)

But nobody else will follow him, allways rebutted.

Coeur de Lion
February 27, 2021 12:24 am

Noticed two things. Reference to ‘carbon’ not CO2 = an agenda or lying .
“ capturing changes ….. is key to detecting an anthropogenic-climate-changing-related AMOC’,
Isn’t CO2 wonderful!

Carlo, Monte
Reply to  Coeur de Lion
February 27, 2021 6:45 am

More powerful than a locomotive!
Faster than a speeding bullet!
Able to turn mighty ocean currents!
Look, up in the air!
It’s Carbon Dioxide!

alastair gray
February 27, 2021 12:35 am

Any suggestion of a climatic cooling as indeed is possible with a variation in the AMOC fills me with dread in the light of our unreliable UK energy policy.-+. Where will Fossil fuels be when you really need them@

Ed Zuiderwijk
Reply to  alastair gray
February 27, 2021 1:24 am

We will burn the idiot politicians to keep us warm.

Chaswarnertoo
Reply to  Ed Zuiderwijk
February 27, 2021 1:27 am

They’re too wet.

Abolition Man
Reply to  Chaswarnertoo
February 27, 2021 11:07 am

Chaswarnertoo,
With a seemingly endless supply, we should be able to develop a sustainable technology to dry them out enough to burn readily! Perhaps a cooling climate will allow them to be freeze-dried; but I’m very concerned about the secondary effects! Can you imagine what the smell would be like downwind?

Last edited 1 month ago by Abolition Man
otsar
Reply to  Abolition Man
February 27, 2021 3:27 pm

Brings back memories of a neighbor that had an incinerating loo.

February 27, 2021 12:47 am

Nice model

model handbag.PNG
Vuk
February 27, 2021 1:42 am

Some might say ‘it’s the sun stup.d’
Sunspot observer from Croatia captures plane fly pass across the AR2804 active region
https://youtu.be/FKH6ei9I4JE?t=15

Last edited 1 month ago by Vuk
Peta of Newark
February 27, 2021 2:02 am

Quote:
“”direct observations are sparse prior to 2004“”

If that doesn’t tell me, you, any & everyone that This Thing is a:

  • Mirage
  • Red herring
  • Wild goose/imagining
  • Unicorn fart
  • Waste-of-time
  • Total nada
  • Thing of Completely Zero Consequence

…. then wtf does?

Izaak Walton
Reply to  Peta of Newark
February 27, 2021 1:24 pm

Peta,
I imagine the dinosaurs would have said the same thing when they saw a new comet in the sky.

Tom Abbott
Reply to  Peta of Newark
February 27, 2021 5:15 pm

The alarmists are trying to come up with a new narrative where CO2 causes problems with the ocean circulation which causes the weather to get colder, so they have another way to explain why CO2 is making things colder, not warmer.

Ron Long
February 27, 2021 2:30 am

What the authors are really saying, inadvertently, is that the science isn’t settled.

Tom Abbott
Reply to  Ron Long
February 27, 2021 5:16 pm

That’s what every article that appears here says.

observa
February 27, 2021 4:04 am

Well that clears that up-
Climate change has made the Gulf Stream its weakest in 1,000 years (msn.com)

Whatever happens there’s no escaping the dooming unless you follow the Fearless Leader/s and their next Great Leap Forward with suitable remuneration of course.

Reply to  observa
February 27, 2021 8:12 am

Come on man….you know the thing….Joey is offering a New Deal….a New Great Society…..based on Woke Principles….and Demrat Values ….it’s gonna be….a Utopia…. and free at last of CO2.

Alan M
February 27, 2021 4:40 am

Holy goodness gracious that’s one hell of a long-winded abstract, probably represents the rest of the paper

commieBob
Reply to  Alan M
February 27, 2021 8:53 am

The abstract is just the first paragraph. At the end of the paragraph is the word ‘Introduction’. It’s supposed to be a heading, which becomes apparent when you click on the link to see the whole paper.

Jim Gorman
February 27, 2021 4:42 am

Am I foggy this morning (lost of fog here) and missed seeing any causal measurements in this study that implicated CO2 as a determining factor in the AMOC? I think I see a lot of insinuation that CO2 increased temps somewhere might be affecting it but nothing that legitimately ties everything together.

observa
Reply to  Jim Gorman
February 27, 2021 5:09 am

Don’t you worry about that as these bloodhounds can sniff out CO2 anywhere and everywhere but sensitive equipment don’t come cheap.

Abolition Man
Reply to  observa
February 27, 2021 10:55 am

Apparently petulant Swedish teenagers are hard to come by and transport as they are allergic to modern technologies like flight! Maybe the Zhou Bai Den Regime has some super CO2 sleuths in training to go after SUVs and BBQs that they can lend to the effort!

garboard
February 27, 2021 5:42 am

someone correct me if I’m wrong but the trade winds create , drive and determine the Gulf Stream . any changes in the trade winds are what would most effect the Gulf Stream . the Atlantic Ocean is considerably higher (feet higher ) where it reaches Central America than it is off Africa . because of the trade winds . so if the slow moving benthic thermohaline current of the sub arctic slows , where else can all that Gulf Stream water go ? Coriolis force would still spin it off towards Europe . seems like all these stories way over estimate the thermohaline component and ignore the primary forces at work ….??? as long as the trade winds blow and the earth spins the Gulf Stream will continue to do what its doing

Last edited 1 month ago by garboard
Krishna Gans
Reply to  garboard
February 27, 2021 9:49 am

As long as earth turns and the winds blow…..
But AMOC is thermohaline

Joel O’Bryan
Reply to  garboard
February 27, 2021 1:15 pm

between 2 to 3 feet higher IIRC.

Richard M
February 27, 2021 6:32 am

Thirumalai et al 2018 shows us the Gulf Stream has been warming for about 400 years. It also shows the salinity increasing which may very well be the reason for the warming.

https://www.nature.com/articles/s41467-018-02846-4

CO2 IR is so weak it can only penetrate a few microns of the surface. Since this would enhance evaporation it is a cooling effect. It’s quite possible our CO2 emissions are countering the natural warming which shows up in the SST data.

There has been strong cooling over the past year that continued into January due to La Nina. As a result we are likely to see UAH drop again in February and March. The SST values are already back to the 2001 levels with no sign of turning around.

Ian W
Reply to  Richard M
March 1, 2021 6:41 am

The UAH cooling below what it was will be difficult for the spinners as will the continual monotonic rise in atmospheric CO2 even during a close to total world shut down.

This is where the billionaires of the WEF may find things getting difficult as discussions go into areas that they have zero understanding.

AndyHce
Reply to  Richard M
March 6, 2021 9:40 pm

I don’t think CO2 IR being “weak” is a thing and definitely not responsible for its low penetration into water. The mechanism here is that IR absorption by water is so devastatingly complete that all IR is absorbed in the first 10 microns of water depth.

John Miller
February 27, 2021 7:48 am

From the abstract: “…the Intergovernmental Panel on Climate Change (IPCC) says that it is unlikely that the AMOC will stop this century, they state with medium confidence that a slowdown by 2050 due to anthropogenic climate change is very likely (Stocker et al.2013).”

So, medium confidence slowdown due to anthropogenic climate change is very likely. Where do these “scientists” get their understanding of verbal logic? Popper would barf.

commieBob
Reply to  John Miller
February 27, 2021 9:15 am

75% of the population would agree with you 19 times out of 20.

Dave Fair
Reply to  John Miller
February 27, 2021 9:56 am

In UN IPCC-speak, “medium confidence” means “about as likley as not;” 50/50. We may then use a coin toss to make trillion dollar decisions.

Ian W
Reply to  Dave Fair
March 1, 2021 11:14 am

There is a psychological aspect to this.

50% chance is seen as almost certain in some areas where some observers say that’s just a coin toss. From a mathematical approach they are both the same from a risk analysis approach they are hugely psychologically different.

So would anyone play Russian Roulette with every other chamber of the revolver loaded? The risk is seen as not acceptable even though that is also ‘only’ 50% risk.

Meteorologists giving weather briefings to flight crew face this all the time.

Dave Fair
Reply to  Ian W
March 1, 2021 11:45 am

I’m not sure as to your point, Ian. Anyway, the chance of something happening is always 50/50; it either happens or it doesn’t. [s/]

Abolition Man
Reply to  John Miller
February 27, 2021 11:02 am

More than making Popper barf, I would love to see how many of our great scientists from across the ages would react to the dithering and wincing practitioners of Scientism!
I’d like to think that many of them, even from the Middle Ages, would go medieval on them; but that may be only projection on my part!

Herbert
Reply to  Ozonebust
February 28, 2021 2:16 am

So the Guardian article cites Caesar et al (2021) which is contradicted by the paper discussed here.
The Guardian begrudgingly admits the relevant UN IPCC paper says it is “very unlikely”the AMOC will collapse this century but watch out in the 22nd Century.
Also,The Guardian hastens to add the fact that although “ tipping points” may be near with the AMOC,(but nothing imminent for us)this should not lead us to “despair and inaction”!
How do these people write this apocalyptic alarmism so relentlessly without one of them repeating that famous line,“what do you think they will do to us if we are wrong?”
There are real world consequences for this nonsense as Extinction Rebellion and its dazed and weeping followers show.

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