the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jul 2023
Persistent Climate Model Biases in the Atlantic Ocean's Freshwater Transport
Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is considered to be one of the most dangerous climate tipping elements. From idealised model studies, it is known that the tipping behaviour is caused by a positive salt-advection feedback, which is strongly connected to the Atlantic Ocean's freshwater transport. In earlier model studies, using climate models of the Coupled Model Intercomparison Projects (phase 3 and phase 5), biases in this freshwater transport have been identified. Here, we show that these biases persist in CMIP phase~6 models, as well as in a climate model with an eddying ocean, and provide a more detailed analysis of the origin of the biases. The most important model bias is in the Atlantic Surface Water properties, which arises from deficiencies in the surface freshwater flux over the Indian Ocean. The second largest bias is in the properties in the North Atlantic Deep Water and arises through deficiencies in the freshwater flux over the Atlantic Subpolar Gyre region. Due to the biases, the Atlantic Ocean's freshwater transport is not in agreement with available observations and the strength of the salt advection feedback is underestimated. To better project future AMOC behaviour, an urgent effort is needed to reduce biases in the atmospheric components of current climate models.
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Notice on discussion status
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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Preprint
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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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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1502', Anonymous Referee #1, 23 Aug 2023
Review of: “Persistent Climate Model Biases in the Atlantic Ocean’s Freshwater Transport”, by Van Westen and Dijkstra.
In this paper the authors study biases in the AMOC stability metric Fov, by analyzing two CESM simulations with different resolutions, as well as a large number of CMIP6 simulations. The authors conclude that the biases that existed in CMIP3 and CMIP5 persist in CMIP6. Furthermore, they point to several biases in the freshwater budget as likely culprits for these biases in Fov.
This is a very thorough analysis, and I commend the authors for the work they have done. That said, the depth of the analysis has gone at the expense of the readability of the manuscript; I have to admit --with some embarrassment-- that I have not been able to get past the first pages of the Results section, despite several attempts. In my mind, the information density is far too high to make this a comfortable read. To illustrate this point, page 4 alone refers to Fig. 1 (4 panels plus 7 insets), Fig. 2 (8 panels, each with two insets), and 5 figures in Supplemental. The total number of panels + insets covered on page 4 is 70. That is a lot of information to get one’s head around in the space of 30 lines.
I hope that the authors will reconsider simplifying the paper and improve its readability. The paper can be slowed down significantly, simply by taking more time to develop the material. Not by adding more information, but by more carefully walking the reader through the argumentation following the key results. I understand that it is a challenging task, but the authors should make an effort to boil down the figures to those that are most critical to the storyline. Relegating more figures to Supplemental would be an option, but it only works if they are indeed treated as being of secondary importance, with limited referencing in the main text to avoid distracting from the main storyline. Although the insets might be useful in some cases (after careful study, Fig. 2 started to make sense), in others they are definitely a distraction (Figs. 1, 3). The insets that are critical to the narrative deserve their own figure and should be described and referenced in the proper order.
It is possible that there is simply too much ground to cover for one paper, in which case the authors might consider splitting it up in two companion papers.
I apologize that I don’t have anything more substantial to offer at this point.
Citation: https://doi.org/10.5194/egusphere-2023-1502-RC1 -
AC1: 'Reply on RC1', René van Westen, 25 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1502/egusphere-2023-1502-AC1-supplement.pdf
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RC2: 'Reply on RC1', Anonymous Referee #2, 31 Oct 2023
Review of ‘Persistent Climate Model Biases in the Atlantic Ocean’s Freshwater Transport’
In this manuscript van Westen and Dijkstra take a detailed look into what is causing the model biases in Fov in the Atlantic at 34S. They investigate the bias by looking at the different water masses at 34S and how they change immediately after the model spins up. These responses are compared in both a high (0.1 degree) and low (1 degree) CESM model and later compared to CMIP6 models and changes in future projections. In the CESM models the surface fresh bias can be related to the impact the Indian Ocean has on the Atlantic Surface waters, while slightly deeper the North Atlantic Deep Water biases are related to issues with surface fluxes in the North Atlantic Subpolar gyre. The manuscript furthermore investigates Fov in CMIP6 models and how it changes in future climate projections.
Having begun the review of this manuscript after reviewer 1 posted their response I agree with them on the manuscript. The authors have approached the Fov bias issue from a from the perspective of different water masses, which is a very nice and informative way of investigating the model bias. Therefore, I believe this work is of interest to the community. They have also completed and presented a large amount of analysis. However, there is a large amount of information packed densely into one manuscript and it would benefit from streamlining it and/or splitting the manuscript. Similarly with the figures, some panels could be combined instead of having separate panels for the two models allowing a few of the insets to become their own panels, as opposed to small postage stamps.
A few smaller points:
- The introduction seems short and could benefit from being expanded, discussion of the usefulness of Fov as an indicator would nice. See Yin and Stouffer 2007 and Mecking et al. 2016 for a discussion on using the divergence around the subtropical gyre. Also, the role of bias correction using flux adjustment (i.e. Liu et al. 2014, Liu et al. 2017, Jackson 2013). The paper Mignac et al. 2019 also worth mentioning.
- line 80 – see Menary et al. 2020 figure S1 for a comparison between computing own AMOC and AMOC provided by CMIP6 models.
- What are the initial S&T conditions used in this study?
- How are the freshwater transports computed in Fig.2 for the different water masses?
- There isn’t very much mention about Faz in the manuscript despite being defined. Interestingly, looking at the inset in Figure 1b it is clear that Faz also makes a quick adjustment. One thing that is very noticeable in Figure 5 d,e and f is that there is an azonal structure in ASW.
- In figure 5 and A6 it would be nice to see the plots as biases as opposed to absolute values.
- There is no mention in the abstract about the future projection results.
I believe there are several nice results in this manuscript, and I would be happy to provide a more detailed review of this after the above mentioned comments have been considered.
Refs:
Yin, J. and Stouffer, R.J., 2007. Comparison of the stability of the Atlantic thermohaline circulation in two coupled atmosphere–ocean general circulation models. Journal of Climate, 20(17), pp.4293-4315.
Mecking, J.V., Drijfhout, S.S., Jackson, L.C. and Graham, T., 2016. Stable AMOC off state in an eddy-permitting coupled climate model. Climate Dynamics, 47, pp.2455-2470.
Liu, W., Liu, Z. and Brady, E.C., 2014. Why is the AMOC monostable in coupled general circulation models?. Journal of Climate, 27(6), pp.2427-2443.
Liu, W., Xie, S.P., Liu, Z. and Zhu, J., 2017. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances, 3(1), p.e1601666.
Jackson, L.C., 2013. Shutdown and recovery of the AMOC in a coupled global climate model: the role of the advective feedback. Geophysical Research Letters, 40(6), pp.1182-1188.
Mignac, D., Ferreira, D. and Haines, K., 2019. Decoupled freshwater transport and meridional overturning in the South Atlantic. Geophysical Research Letters, 46(4), pp.2178-2186.
Menary, M.B., Robson, J., Allan, R.P., Booth, B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J. and Ringer, M., 2020. Aerosol‐forced AMOC changes in CMIP6 historical simulations. Geophysical Research Letters, 47(14), p.e2020GL088166.
Citation: https://doi.org/10.5194/egusphere-2023-1502-RC2 -
AC2: 'Reply on RC2', René van Westen, 06 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1502/egusphere-2023-1502-AC2-supplement.pdf
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AC1: 'Reply on RC1', René van Westen, 25 Oct 2023
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EC1: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 19 Oct 2023
As editor I suggest the authors post a reply detailing how they plan to address the issued raised by the reviewer 1.
Citation: https://doi.org/10.5194/egusphere-2023-1502-EC1 -
EC2: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 01 Nov 2023
Both reviewers suggest that the paper be streamlined to remove non-critical information or split into two manuscript. The authors reply to reviewers 1 indicate that they will remove information/subplots rather than split the study into two manuscripts. I encourage the authors to undertake the suggested revisions to the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1502-EC2
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1502', Anonymous Referee #1, 23 Aug 2023
Review of: “Persistent Climate Model Biases in the Atlantic Ocean’s Freshwater Transport”, by Van Westen and Dijkstra.
In this paper the authors study biases in the AMOC stability metric Fov, by analyzing two CESM simulations with different resolutions, as well as a large number of CMIP6 simulations. The authors conclude that the biases that existed in CMIP3 and CMIP5 persist in CMIP6. Furthermore, they point to several biases in the freshwater budget as likely culprits for these biases in Fov.
This is a very thorough analysis, and I commend the authors for the work they have done. That said, the depth of the analysis has gone at the expense of the readability of the manuscript; I have to admit --with some embarrassment-- that I have not been able to get past the first pages of the Results section, despite several attempts. In my mind, the information density is far too high to make this a comfortable read. To illustrate this point, page 4 alone refers to Fig. 1 (4 panels plus 7 insets), Fig. 2 (8 panels, each with two insets), and 5 figures in Supplemental. The total number of panels + insets covered on page 4 is 70. That is a lot of information to get one’s head around in the space of 30 lines.
I hope that the authors will reconsider simplifying the paper and improve its readability. The paper can be slowed down significantly, simply by taking more time to develop the material. Not by adding more information, but by more carefully walking the reader through the argumentation following the key results. I understand that it is a challenging task, but the authors should make an effort to boil down the figures to those that are most critical to the storyline. Relegating more figures to Supplemental would be an option, but it only works if they are indeed treated as being of secondary importance, with limited referencing in the main text to avoid distracting from the main storyline. Although the insets might be useful in some cases (after careful study, Fig. 2 started to make sense), in others they are definitely a distraction (Figs. 1, 3). The insets that are critical to the narrative deserve their own figure and should be described and referenced in the proper order.
It is possible that there is simply too much ground to cover for one paper, in which case the authors might consider splitting it up in two companion papers.
I apologize that I don’t have anything more substantial to offer at this point.
Citation: https://doi.org/10.5194/egusphere-2023-1502-RC1 -
AC1: 'Reply on RC1', René van Westen, 25 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1502/egusphere-2023-1502-AC1-supplement.pdf
-
RC2: 'Reply on RC1', Anonymous Referee #2, 31 Oct 2023
Review of ‘Persistent Climate Model Biases in the Atlantic Ocean’s Freshwater Transport’
In this manuscript van Westen and Dijkstra take a detailed look into what is causing the model biases in Fov in the Atlantic at 34S. They investigate the bias by looking at the different water masses at 34S and how they change immediately after the model spins up. These responses are compared in both a high (0.1 degree) and low (1 degree) CESM model and later compared to CMIP6 models and changes in future projections. In the CESM models the surface fresh bias can be related to the impact the Indian Ocean has on the Atlantic Surface waters, while slightly deeper the North Atlantic Deep Water biases are related to issues with surface fluxes in the North Atlantic Subpolar gyre. The manuscript furthermore investigates Fov in CMIP6 models and how it changes in future climate projections.
Having begun the review of this manuscript after reviewer 1 posted their response I agree with them on the manuscript. The authors have approached the Fov bias issue from a from the perspective of different water masses, which is a very nice and informative way of investigating the model bias. Therefore, I believe this work is of interest to the community. They have also completed and presented a large amount of analysis. However, there is a large amount of information packed densely into one manuscript and it would benefit from streamlining it and/or splitting the manuscript. Similarly with the figures, some panels could be combined instead of having separate panels for the two models allowing a few of the insets to become their own panels, as opposed to small postage stamps.
A few smaller points:
- The introduction seems short and could benefit from being expanded, discussion of the usefulness of Fov as an indicator would nice. See Yin and Stouffer 2007 and Mecking et al. 2016 for a discussion on using the divergence around the subtropical gyre. Also, the role of bias correction using flux adjustment (i.e. Liu et al. 2014, Liu et al. 2017, Jackson 2013). The paper Mignac et al. 2019 also worth mentioning.
- line 80 – see Menary et al. 2020 figure S1 for a comparison between computing own AMOC and AMOC provided by CMIP6 models.
- What are the initial S&T conditions used in this study?
- How are the freshwater transports computed in Fig.2 for the different water masses?
- There isn’t very much mention about Faz in the manuscript despite being defined. Interestingly, looking at the inset in Figure 1b it is clear that Faz also makes a quick adjustment. One thing that is very noticeable in Figure 5 d,e and f is that there is an azonal structure in ASW.
- In figure 5 and A6 it would be nice to see the plots as biases as opposed to absolute values.
- There is no mention in the abstract about the future projection results.
I believe there are several nice results in this manuscript, and I would be happy to provide a more detailed review of this after the above mentioned comments have been considered.
Refs:
Yin, J. and Stouffer, R.J., 2007. Comparison of the stability of the Atlantic thermohaline circulation in two coupled atmosphere–ocean general circulation models. Journal of Climate, 20(17), pp.4293-4315.
Mecking, J.V., Drijfhout, S.S., Jackson, L.C. and Graham, T., 2016. Stable AMOC off state in an eddy-permitting coupled climate model. Climate Dynamics, 47, pp.2455-2470.
Liu, W., Liu, Z. and Brady, E.C., 2014. Why is the AMOC monostable in coupled general circulation models?. Journal of Climate, 27(6), pp.2427-2443.
Liu, W., Xie, S.P., Liu, Z. and Zhu, J., 2017. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances, 3(1), p.e1601666.
Jackson, L.C., 2013. Shutdown and recovery of the AMOC in a coupled global climate model: the role of the advective feedback. Geophysical Research Letters, 40(6), pp.1182-1188.
Mignac, D., Ferreira, D. and Haines, K., 2019. Decoupled freshwater transport and meridional overturning in the South Atlantic. Geophysical Research Letters, 46(4), pp.2178-2186.
Menary, M.B., Robson, J., Allan, R.P., Booth, B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J. and Ringer, M., 2020. Aerosol‐forced AMOC changes in CMIP6 historical simulations. Geophysical Research Letters, 47(14), p.e2020GL088166.
Citation: https://doi.org/10.5194/egusphere-2023-1502-RC2 -
AC2: 'Reply on RC2', René van Westen, 06 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1502/egusphere-2023-1502-AC2-supplement.pdf
-
AC1: 'Reply on RC1', René van Westen, 25 Oct 2023
-
EC1: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 19 Oct 2023
As editor I suggest the authors post a reply detailing how they plan to address the issued raised by the reviewer 1.
Citation: https://doi.org/10.5194/egusphere-2023-1502-EC1 -
EC2: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 01 Nov 2023
Both reviewers suggest that the paper be streamlined to remove non-critical information or split into two manuscript. The authors reply to reviewers 1 indicate that they will remove information/subplots rather than split the study into two manuscripts. I encourage the authors to undertake the suggested revisions to the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1502-EC2
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Henk A. Dijkstra
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(136833 KB) - Metadata XML