Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26&deg;N

Bryden, Harry; Drijfhout, Sybren; Mecking, Jennifer; Hazeleger, Wilco

doi:https://doi.org/10.5194/egusphere-2023-2688

Preprints

https://doi.org/10.5194/egusphere-2023-2688

Preprints

16 Nov 2023

| 16 Nov 2023

Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26°N

Harry Bryden, Sybren Drijfhout, Jennifer Mecking, and Wilco Hazeleger

Abstract. The Coupled Model Intercomparison Project (CMIP) allows assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength by a factor of three, the multi-model mean strength agrees reasonably well with observed estimates from RAPID¹, but this does not hold for its various components. In CMIP6 the present-day AMOC is characterised by a lack of lower North Atlantic Deep Water (lNADW), due to the small-scale of Greenland-Iceland-Scotland Ridge overflow and too much mixing. This is compensated by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared to observations. The future decline in the AMOC to 2100 of 7 Sv under a SSP5-8.5 scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that the wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in the wind-driven gyre transport suggesting a decrease over time in the component of the Gulf Stream originating in the South Atlantic.

¹ RAPID is used here as shorthand for the RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series at 26°N (Moat et al., 2022).

Received: 14 Nov 2023 – Discussion started: 16 Nov 2023

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Journal article(s) based on this preprint

17 Apr 2024

Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26° N

Harry Bryden, Jordi Beunk, Sybren Drijfhout, Wilco Hazeleger, and Jennifer Mecking

Ocean Sci., 20, 589–599, https://doi.org/10.5194/os-20-589-2024,https://doi.org/10.5194/os-20-589-2024, 2024

Short summary

Harry Bryden, Sybren Drijfhout, Jennifer Mecking, and Wilco Hazeleger

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2688', Anonymous Referee #1, 14 Dec 2023

The manuscript is well written and easy to follow. However, I failed to understand the novelty of this study compared to that of Asbjornsen and Arthur (2023). Both use a decomposition of the AMOC to assess its decline under the SSP5-8.5 climate change scenario at 26.5°N using a similar number of CMIP6 models. Asbjorsen and Arthur (2023) do not use RAPID observations, but come to similar conclusions anyway. I do not doubt that both studies were done independently, but a clearer justification of what is new in this study and how its results and methods differ from Asbjornsen and Arthur (2023) is needed before recommending it for publication.
Possible typographical errors:

Line 77 and Table 2: Rapid instead of RAPID

Line 201: historrical instead of historical

Citation: https://doi.org/10.5194/egusphere-2023-2688-RC1
- AC1: 'Reply on RC1', Harry Bryden, 19 Dec 2023
  
  We reference the Asbjørnsen and Årthun paper and we reference it as confirmation that the AMOC decline and western boundary current decline in CMIP6 are robust and not just dependent on the set of specific models we used. We agree that our results and the Asbjørnsen and Årthun results on the AMOC decline in CMIP6 models over the 21st century are similar.
  The 2 principal additions within our paper vis a vis the Asbjørnsen and Årthun paper are:
  First, we compared RAPID components of the overturning for the period 2004-2014 with CMIP6 components for the same period lending credibility to the CMIP6 circulation while pointing out differences particularly in the deep southward flow of Lower North Atlantic Deep Water that is nearly absent in the models.
  Secondly, we discussed the reasons for the decline in AMOC in the CMIP6 models both in terms of the trends in deep water formation and transport within the North Atlantic and also in terms of the decline in the global overturning outside the North Atlantic that appears to be due to smaller diffusive upwelling in the interior Indo-Pacific as reported by Baker et al.
  I (Bryden)) met Helene Asbjørnsen at the Royal Society meeting "Atlantic overturning: new observations and challenges" in December 2022 where we each presented posters on the Asbjørnsen and Årthun work and on the Bryden et al work. Abstracts for these posters were contained in the meeting programme. I talked to Helene at her poster and I remember suggesting that she look at variations in wind stress curl to assess changes in wind-driven circulation over the 21st century, which she subsequently did and included the results in the GRL paper. We think the Asbjørnsen and Årthunpaper is excellent and our results complement their results. Our work adds a comparison between CMIP6 and RAPID components for the period 2004-2014 and discusses the implications of a decline in AMOC on both the AMOC in the North Atlantic and the extra-Atlantic MOC.
  Thank you for pointing out some typographical errors in the submitted text.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC1
RC2:
'Comment on egusphere-2023-2688', Anonymous Referee #2, 15 Dec 2023
The AMOC strength has been observed by the RAPID array in the subtropical Atlantic since 2004. These measurements have greatly increased our understanding of AMOC dynamics and variability. They are also a critical comparator for ocean and climate models. Typically, comparisons between observations and models are of basic metrics such as the maximum and variability of the overturning streamfunction and its vertical distribution. But the observations allow far more mechanistic comparisons to the models and that is what is done here for the latest group of CMIP6 models. This paper identifies how the circulation in observations may be compared to the that in models accounting for the fact models and the real world have different zonal structures due to model resolution particularly at the Florida Straits.

The AMOC metric is a zonally integrated view of the circulation. In this paper key mechanisms are identified: Florida Straits transport, Antilles Current and several key water mass layers. These are all mechanistically interesting being relatable to both wind forcing and thermohaline forcing, particularly the wind forcing of course.

For the period 2004 to 2014 the observations and models are compared. Based on insights gained from this comparison model trends 2015-2100 are then considered and discussed.

Some important results emerge:
The observational community needs to refine its data products to allow better comparison with the models. Specifically, careful consideration needs to be given to how the thermocline wind-driven circulation and the western boundary current (Florida Current and Antilles Current) are separated. This is because the coarse model resolutions do not resolve the narrow Florida Straits and their western boundary currents are the sum of Florida and Antilles Currents.

The observations and models when carefully compared in the period 2004 to 2014 have similar mean transports overall. There are a number of caveats relating to the depth structure of the North Atlantic deep-water layers, which are considered a result of model inadequacies in representing exchanges across the Greenland-Scotland Ridge.

The slowing of the AMOC in the 21^stcentury is quantified as a 30% reduction in the mean. Florida and Antilles current transport reduces by 11Sv. The wind-driven Sverdrup transport in the thermocline reduces by 4.4Sv so the wind-driven upper ocean circulation reduces by 6.6Sv. This reduction is driven by a 14% reduction of the wind-stress curl in the CMIP models. The additional necessary southward reduction in transport to achieve a mass balance across the section is 6.4Sv is in the NADW layer (in the model the reduction is mainly in upper North Atlantic Deep Water, whereas the reduction seen in observations in the early part of the 21^stcentury occurs in the lower North Atlantic Deep Water).

This rather nice third result diagnosing the buoyancy forced reduction in AMOC is discussed with reference to Baker et al (2023). Baker show that in CMIP models this reduction is supported by reductions in the Indo-Pacific upwelling of North Atlantic Deep Water.

While the AMOC functioning is important it is perhaps more important to additionally understand how heat transport is affected by the changing wind and buoyancy driven circulations. This is briefly mentioned in the discussion and suggests another detailed model observation comparison study on this would be valuable.

This is a nice, short, straightforward paper that has some long-needed analysis and hopefully points a fruitful way forward for more detailed comparisons between AMOC observations and CMIP models. The attempt to separate wind and buoyancy forced changes is nice.

Minor Comments

Lines 95-120: I think these definitions of water masses only apply to the model and not to the observations? It is not clear.

Line 95-102: What is the typical longitude of the eastern edge of the Antilles Current?

Line 104-105: Why was a temperature boundary chosen – is this to do with being able to compare the correct water masses between observations and models? Why 8°C which is 900m deep in the west and 800m deep in the east? The 7°C isotherm looks flatter and at ~950m deep would have more thermocline flow?

Overall a few more explanatory lines justifying the choices would be helpful.

Results

Table 2 is quite confusing, and it needs a much better caption to explain it and also state the units of Sv. Also note use of upper case in Table that is referenced by lower case in the text. 1. The Antilles Current transport of 5Sv is a result from Meinen et al. 2019 and should be referenced. Note also that the Meinen result for the Antilles Transport is ±4.7±7.5Sv not 5±10Sv as stated in the results; 2. Western Boundary Current (FS+AC) does equal 31.3+5=36.3Sv. But Thermocline Recirculation+AC -18.6+5=-13.6Sv does not equal -23.6Sv as in paper. The number you want in the table is the -23.6Sv but you need to read the results section to carefully understand your argument for separating the thermocline and Antilles transports. It needs an explanatory note in the caption or you need to write Recirculation-AC; The line Western Boundary Current+Ekman+Model TR should be written as AMOC= Western Boundary Current+Ekman+Model TR so it is obvious it can be compared directly to AMOC=FS+Ekman+IW+TR for the AMOC northward upper limb. The Deep Water part of the table is much simpler especially as the AMOC deep limb appears on the same line!
A recent paper discusses the Gulf Stream weakening which the authors might reference (Piecuch, C. G. & Beal, L. M. Robust Weakening of the Gulf Stream During the Past Four Decades Observed in the Florida Straits. Geophysical Research Letters 50 (2023). https://doi.org:10.1029/2023gl105170)
Citation: https://doi.org/10.5194/egusphere-2023-2688-RC2
- AC2: 'Reply on RC2', Harry Bryden, 19 Dec 2023
  
  Thank you for your critical review emphasizing many of the points made in our manuscript.
  With respect to Minor Comments, we will try to make clear the definitions of water masses, the longitude of the eastern edge of the Antilles Current and the reason for the choice of temperature boundary and we will add a reference to Piecuch and Beal in the revised manuscript to be prepared and submitted once all reviews and discussions are finished. We agree Table 2 is confusing and the comments you provide will be helpful for revising the text and layout of Table 2 in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC2
RC3:
'Comment on egusphere-2023-2688', Anonymous Referee #3, 19 Dec 2023

Review of Bryden et al. “Comparing observed and modelled components of the AMOC at 26ºN”
This paper presents a comparison of CMIP6 model output vs RAPID observations, with transports decomposed similar to RAPID. I found the paper easy to follow, had some nice results. It’s quite a short paper for OS but I appreciate it came about in an unusual way with the MSc student not pursueing it.
The Figure 2 is very informative, especially l182–183: this is a very nice finding. Very striking WBC decline and the decomposition of this into wind-driven components is very interesting (l212). I think one more figure should be added: a figure that simplifies Fig 2 to show MOC decline broken into wind-driven and non-wind components.
I confess I haven’t read Asbjornsen and Arthun but their abstracts sounds very similar to your paper. Can you clarify the differences and unique insights from this study?
Overall, I think it’s an interesting study that can be published following minor corrections.
Minor comments:
l36: add a line on importance of the global overturning circulation

l48: the latest RAPID data are available to 2022 at time of writing. Please explain why your analyses ends in 2018 and any issues that might arise because of this shortening.

l52. If you’re not going to state the mean transport in the climate models, then you need to report the decline differently i.e. not a percentage. Weijer also reports the decline as 1 Sv/decade (I think).

l64. I think you can bring in findings from a few more papers than just Weijer here:

Robson, Jon, et al. "The role of anthropogenic aerosol forcing in the 1850–1985 strengthening of the AMOC in CMIP6 historical simulations." Journal of Climate 35.20 (2022): 6843-6863.

McCarthy, Gerard D., and Levke Caesar. "Can we trust projections of AMOC weakening based on climate models that cannot reproduce the past?." Philosophical Transactions of the Royal Society A 381.2262 (2023): 20220193.

l83. Why monthly?

l87. How was the one ensemble member chosen? Was it the first one?

l91. Can you show the range of the net transport through the section? There is surely quite a range around -1Sv.

l104–118. Why use isotherms rather than depth as RAPID does?

l120. Is Ekman recalculated in the models from their winds? Or taken as very near surface ageostrophic transport?

Figure 1: I’m surprised at the small spread of the climate models mean AMOC. Was there any selection of the models based on mean AMOC strength?

Citation: https://doi.org/10.5194/egusphere-2023-2688-RC3
- AC3: 'Reply on RC3', Harry Bryden, 08 Jan 2024
  
  Response to Reviewer 3.
       Thank you for your careful review of our manuscript. We have addressed the similarities and differences between our study and the Asbjørnsen and Årthun paper in our response to Reviewer 1.
        With respect to the suggestion that we add a new figure with the circulation separated into wind and non-wind components, we will consider producing such a figure. However the central thrust of our manuscript is to compare CMIP6 and RAPID components. Neither RAPID nor CMIP6 analyses (to date) separate wind and non-wind components. Bryden (2021) does interpret the RAPID time series in wind-driven and thermohaline components but this involves a separation of Florida Straits transport into a wind-driven component that is balanced by mid-ocean thermocline recirculation and a thermohaline component defined by the difference between Florida Straits transport and thermocline recirculation. We could do the same with the CMIP6 analyses. But our initial reaction is to maintain the emphasis on CMIP6 and RAPID components in the manuscript without further (arguable) interpretations.
       We will address the Minor Comments in a revised manuscript and we will certainly add references to relevant, recently published papers as the Reviewer suggests.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC3
AC4: 'Comment on egusphere-2023-2688', Harry Bryden, 16 Jan 2024

In revising our manuscript, we are going to add Jordi Beunk as second author. Jordi was the Masters student who did much of the original CMIP6 analysis for the manuscript. When he finished the Masters, Jordi said he did not want to be involved in publication of the work and he effectively disappeared from climate science. Last week Wilco Hazeleger 'found' Jordi via social media and Jordi has now agreed to participate in the publishing of the manuscript. Thus, we intend to add Jordi Beunk as an author for the final publication. It is our intention that the author list become
Bryden, Beunk, Drijfhout, Hazeleger, Mecking

Citation: https://doi.org/10.5194/egusphere-2023-2688-AC4

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2688', Anonymous Referee #1, 14 Dec 2023

The manuscript is well written and easy to follow. However, I failed to understand the novelty of this study compared to that of Asbjornsen and Arthur (2023). Both use a decomposition of the AMOC to assess its decline under the SSP5-8.5 climate change scenario at 26.5°N using a similar number of CMIP6 models. Asbjorsen and Arthur (2023) do not use RAPID observations, but come to similar conclusions anyway. I do not doubt that both studies were done independently, but a clearer justification of what is new in this study and how its results and methods differ from Asbjornsen and Arthur (2023) is needed before recommending it for publication.
Possible typographical errors:

Line 77 and Table 2: Rapid instead of RAPID

Line 201: historrical instead of historical

Citation: https://doi.org/10.5194/egusphere-2023-2688-RC1
- AC1: 'Reply on RC1', Harry Bryden, 19 Dec 2023
  
  We reference the Asbjørnsen and Årthun paper and we reference it as confirmation that the AMOC decline and western boundary current decline in CMIP6 are robust and not just dependent on the set of specific models we used. We agree that our results and the Asbjørnsen and Årthun results on the AMOC decline in CMIP6 models over the 21st century are similar.
  The 2 principal additions within our paper vis a vis the Asbjørnsen and Årthun paper are:
  First, we compared RAPID components of the overturning for the period 2004-2014 with CMIP6 components for the same period lending credibility to the CMIP6 circulation while pointing out differences particularly in the deep southward flow of Lower North Atlantic Deep Water that is nearly absent in the models.
  Secondly, we discussed the reasons for the decline in AMOC in the CMIP6 models both in terms of the trends in deep water formation and transport within the North Atlantic and also in terms of the decline in the global overturning outside the North Atlantic that appears to be due to smaller diffusive upwelling in the interior Indo-Pacific as reported by Baker et al.
  I (Bryden)) met Helene Asbjørnsen at the Royal Society meeting "Atlantic overturning: new observations and challenges" in December 2022 where we each presented posters on the Asbjørnsen and Årthun work and on the Bryden et al work. Abstracts for these posters were contained in the meeting programme. I talked to Helene at her poster and I remember suggesting that she look at variations in wind stress curl to assess changes in wind-driven circulation over the 21st century, which she subsequently did and included the results in the GRL paper. We think the Asbjørnsen and Årthunpaper is excellent and our results complement their results. Our work adds a comparison between CMIP6 and RAPID components for the period 2004-2014 and discusses the implications of a decline in AMOC on both the AMOC in the North Atlantic and the extra-Atlantic MOC.
  Thank you for pointing out some typographical errors in the submitted text.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC1
RC2:
'Comment on egusphere-2023-2688', Anonymous Referee #2, 15 Dec 2023
The AMOC strength has been observed by the RAPID array in the subtropical Atlantic since 2004. These measurements have greatly increased our understanding of AMOC dynamics and variability. They are also a critical comparator for ocean and climate models. Typically, comparisons between observations and models are of basic metrics such as the maximum and variability of the overturning streamfunction and its vertical distribution. But the observations allow far more mechanistic comparisons to the models and that is what is done here for the latest group of CMIP6 models. This paper identifies how the circulation in observations may be compared to the that in models accounting for the fact models and the real world have different zonal structures due to model resolution particularly at the Florida Straits.

The AMOC metric is a zonally integrated view of the circulation. In this paper key mechanisms are identified: Florida Straits transport, Antilles Current and several key water mass layers. These are all mechanistically interesting being relatable to both wind forcing and thermohaline forcing, particularly the wind forcing of course.

For the period 2004 to 2014 the observations and models are compared. Based on insights gained from this comparison model trends 2015-2100 are then considered and discussed.

Some important results emerge:
The observational community needs to refine its data products to allow better comparison with the models. Specifically, careful consideration needs to be given to how the thermocline wind-driven circulation and the western boundary current (Florida Current and Antilles Current) are separated. This is because the coarse model resolutions do not resolve the narrow Florida Straits and their western boundary currents are the sum of Florida and Antilles Currents.

The observations and models when carefully compared in the period 2004 to 2014 have similar mean transports overall. There are a number of caveats relating to the depth structure of the North Atlantic deep-water layers, which are considered a result of model inadequacies in representing exchanges across the Greenland-Scotland Ridge.

The slowing of the AMOC in the 21^stcentury is quantified as a 30% reduction in the mean. Florida and Antilles current transport reduces by 11Sv. The wind-driven Sverdrup transport in the thermocline reduces by 4.4Sv so the wind-driven upper ocean circulation reduces by 6.6Sv. This reduction is driven by a 14% reduction of the wind-stress curl in the CMIP models. The additional necessary southward reduction in transport to achieve a mass balance across the section is 6.4Sv is in the NADW layer (in the model the reduction is mainly in upper North Atlantic Deep Water, whereas the reduction seen in observations in the early part of the 21^stcentury occurs in the lower North Atlantic Deep Water).

This rather nice third result diagnosing the buoyancy forced reduction in AMOC is discussed with reference to Baker et al (2023). Baker show that in CMIP models this reduction is supported by reductions in the Indo-Pacific upwelling of North Atlantic Deep Water.

While the AMOC functioning is important it is perhaps more important to additionally understand how heat transport is affected by the changing wind and buoyancy driven circulations. This is briefly mentioned in the discussion and suggests another detailed model observation comparison study on this would be valuable.

This is a nice, short, straightforward paper that has some long-needed analysis and hopefully points a fruitful way forward for more detailed comparisons between AMOC observations and CMIP models. The attempt to separate wind and buoyancy forced changes is nice.

Minor Comments

Lines 95-120: I think these definitions of water masses only apply to the model and not to the observations? It is not clear.

Line 95-102: What is the typical longitude of the eastern edge of the Antilles Current?

Line 104-105: Why was a temperature boundary chosen – is this to do with being able to compare the correct water masses between observations and models? Why 8°C which is 900m deep in the west and 800m deep in the east? The 7°C isotherm looks flatter and at ~950m deep would have more thermocline flow?

Overall a few more explanatory lines justifying the choices would be helpful.

Results

Table 2 is quite confusing, and it needs a much better caption to explain it and also state the units of Sv. Also note use of upper case in Table that is referenced by lower case in the text. 1. The Antilles Current transport of 5Sv is a result from Meinen et al. 2019 and should be referenced. Note also that the Meinen result for the Antilles Transport is ±4.7±7.5Sv not 5±10Sv as stated in the results; 2. Western Boundary Current (FS+AC) does equal 31.3+5=36.3Sv. But Thermocline Recirculation+AC -18.6+5=-13.6Sv does not equal -23.6Sv as in paper. The number you want in the table is the -23.6Sv but you need to read the results section to carefully understand your argument for separating the thermocline and Antilles transports. It needs an explanatory note in the caption or you need to write Recirculation-AC; The line Western Boundary Current+Ekman+Model TR should be written as AMOC= Western Boundary Current+Ekman+Model TR so it is obvious it can be compared directly to AMOC=FS+Ekman+IW+TR for the AMOC northward upper limb. The Deep Water part of the table is much simpler especially as the AMOC deep limb appears on the same line!
A recent paper discusses the Gulf Stream weakening which the authors might reference (Piecuch, C. G. & Beal, L. M. Robust Weakening of the Gulf Stream During the Past Four Decades Observed in the Florida Straits. Geophysical Research Letters 50 (2023). https://doi.org:10.1029/2023gl105170)
Citation: https://doi.org/10.5194/egusphere-2023-2688-RC2
- AC2: 'Reply on RC2', Harry Bryden, 19 Dec 2023
  
  Thank you for your critical review emphasizing many of the points made in our manuscript.
  With respect to Minor Comments, we will try to make clear the definitions of water masses, the longitude of the eastern edge of the Antilles Current and the reason for the choice of temperature boundary and we will add a reference to Piecuch and Beal in the revised manuscript to be prepared and submitted once all reviews and discussions are finished. We agree Table 2 is confusing and the comments you provide will be helpful for revising the text and layout of Table 2 in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC2
RC3:
'Comment on egusphere-2023-2688', Anonymous Referee #3, 19 Dec 2023

Review of Bryden et al. “Comparing observed and modelled components of the AMOC at 26ºN”
This paper presents a comparison of CMIP6 model output vs RAPID observations, with transports decomposed similar to RAPID. I found the paper easy to follow, had some nice results. It’s quite a short paper for OS but I appreciate it came about in an unusual way with the MSc student not pursueing it.
The Figure 2 is very informative, especially l182–183: this is a very nice finding. Very striking WBC decline and the decomposition of this into wind-driven components is very interesting (l212). I think one more figure should be added: a figure that simplifies Fig 2 to show MOC decline broken into wind-driven and non-wind components.
I confess I haven’t read Asbjornsen and Arthun but their abstracts sounds very similar to your paper. Can you clarify the differences and unique insights from this study?
Overall, I think it’s an interesting study that can be published following minor corrections.
Minor comments:
l36: add a line on importance of the global overturning circulation

l48: the latest RAPID data are available to 2022 at time of writing. Please explain why your analyses ends in 2018 and any issues that might arise because of this shortening.

l52. If you’re not going to state the mean transport in the climate models, then you need to report the decline differently i.e. not a percentage. Weijer also reports the decline as 1 Sv/decade (I think).

l64. I think you can bring in findings from a few more papers than just Weijer here:

Robson, Jon, et al. "The role of anthropogenic aerosol forcing in the 1850–1985 strengthening of the AMOC in CMIP6 historical simulations." Journal of Climate 35.20 (2022): 6843-6863.

McCarthy, Gerard D., and Levke Caesar. "Can we trust projections of AMOC weakening based on climate models that cannot reproduce the past?." Philosophical Transactions of the Royal Society A 381.2262 (2023): 20220193.

l83. Why monthly?

l87. How was the one ensemble member chosen? Was it the first one?

l91. Can you show the range of the net transport through the section? There is surely quite a range around -1Sv.

l104–118. Why use isotherms rather than depth as RAPID does?

l120. Is Ekman recalculated in the models from their winds? Or taken as very near surface ageostrophic transport?

Figure 1: I’m surprised at the small spread of the climate models mean AMOC. Was there any selection of the models based on mean AMOC strength?

Citation: https://doi.org/10.5194/egusphere-2023-2688-RC3
- AC3: 'Reply on RC3', Harry Bryden, 08 Jan 2024
  
  Response to Reviewer 3.
       Thank you for your careful review of our manuscript. We have addressed the similarities and differences between our study and the Asbjørnsen and Årthun paper in our response to Reviewer 1.
        With respect to the suggestion that we add a new figure with the circulation separated into wind and non-wind components, we will consider producing such a figure. However the central thrust of our manuscript is to compare CMIP6 and RAPID components. Neither RAPID nor CMIP6 analyses (to date) separate wind and non-wind components. Bryden (2021) does interpret the RAPID time series in wind-driven and thermohaline components but this involves a separation of Florida Straits transport into a wind-driven component that is balanced by mid-ocean thermocline recirculation and a thermohaline component defined by the difference between Florida Straits transport and thermocline recirculation. We could do the same with the CMIP6 analyses. But our initial reaction is to maintain the emphasis on CMIP6 and RAPID components in the manuscript without further (arguable) interpretations.
       We will address the Minor Comments in a revised manuscript and we will certainly add references to relevant, recently published papers as the Reviewer suggests.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2688-AC3
AC4: 'Comment on egusphere-2023-2688', Harry Bryden, 16 Jan 2024

In revising our manuscript, we are going to add Jordi Beunk as second author. Jordi was the Masters student who did much of the original CMIP6 analysis for the manuscript. When he finished the Masters, Jordi said he did not want to be involved in publication of the work and he effectively disappeared from climate science. Last week Wilco Hazeleger 'found' Jordi via social media and Jordi has now agreed to participate in the publishing of the manuscript. Thus, we intend to add Jordi Beunk as an author for the final publication. It is our intention that the author list become
Bryden, Beunk, Drijfhout, Hazeleger, Mecking

Citation: https://doi.org/10.5194/egusphere-2023-2688-AC4

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Harry Bryden on behalf of the Authors (05 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (21 Feb 2024) by Agnieszka Beszczynska-Möller

AR by Harry Bryden on behalf of the Authors (06 Mar 2024)

Journal article(s) based on this preprint

17 Apr 2024

Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26° N

Harry Bryden, Jordi Beunk, Sybren Drijfhout, Wilco Hazeleger, and Jennifer Mecking

Ocean Sci., 20, 589–599, https://doi.org/10.5194/os-20-589-2024,https://doi.org/10.5194/os-20-589-2024, 2024

Short summary

Harry Bryden, Sybren Drijfhout, Jennifer Mecking, and Wilco Hazeleger

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Dec 2023	83	19	13	115
Jan 2024	52	14	5	71
Feb 2024	20	19	3	42
Mar 2024	27	16	0	43
Apr 2024	7	7	2	16
May 2024	0
Jun 2024	0
Jul 2024	0
Aug 2024	0
Sep 2024	0

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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There is widespread interest in whether the Gulf Stream will decline under global warming. We analyse 19 coupled climate model projections of the AMOC over the 21^st century. Model consensus is that the AMOC will decline by about 40 %. The decline is due to reductions in northward Gulf Stream transport and southward deep boundary current transport. Whilst the wind-driven Gulf Stream decreases by 4 Sv, most of the decrease in Gulf Stream is due to reduction of 7 Sv in its thermohaline component.


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Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26°N

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