the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jul 2024
The formation and ventilation of an Oxygen Minimum Zone in a simple model for latitudinally alternating zonal jets
Abstract. An advection-diffusion model coupled to a simple dynamical ocean model is used to illustrate the formation and ventilation of an oxygen minimum zone. The advection-diffusion model carries a tracer mimicking oxygen, and the dynamical model is a non-linear 1 ½ layer reduced-gravity model. The latter is forced by an annually oscillating mass flux confined to the near-equatorial band that, in turn, leads to the generation of mesoscale eddies and latitudinally alternating zonal jets at higher latitudes. The model uses North Atlantic geometry and develops a tracer minimum zone remarkably similar in location to the observed oxygen minimum zone in the Eastern Tropical North Atlantic (ETNA). This is despite the absence of wind forcing and the shadow zone predicted by the ventilated thermocline theory. Although the model is forced only at the annual period, the model nevertheless exhibits decadal and multidecadal variability in its spun-up state. The associated trends are comparable to observed trends in oxygen within the ETNA oxygen minimum zone. Notable exceptions are the multi-decadal decrease in oxygen in the lower oxygen minimum zone, and the sharp decrease in oxygen in the upper oxygen minimum zone between 2006 and 2013.
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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-2024-2007', Anonymous Referee #1, 12 Aug 2024
Review of "The formation and ventilation of an Oxygen Minimum Zone in a simple model for latitudinally alternating zonal jets" by Köhn et al
In this manuscript, the authors present an analysis of an elegantly simple 1.5 layer reduced gravity model of the Tropical Atlantic Ocean to show that a tracer released at the western boundary does not propagate eastward very well close to the Equator, creating a 'shadow zone' of very low tracer concentration that qualitatively resembles the observed thermocline oxygen concentration. The authors argue that the eddies generated in the model, and the zonal jets they lead to, are thus an explanation for the Oxygen Minimum Zone, in contrast to the ventilated thermocline theory that requires wind forcing.
This is a very clear, nicely succinct, model study that I have read with great interest and pleasure. In the plethora of full-complexity, fine-resolution model studies, it is nice to see an interesting and thought-provoking idealised study. I thus think that the physical oceanographic community could certainly be inspired by this work.
However, I do have some suggestions for improvements before I can recommend publication. My most important ones are
1. While the model is quite suited for the question, I am not sure how directly interpretable the tracer is as oxygen. The model does not include biogeochemical processes such as remineralisation, which are also important for the formation of the oxygen minimum zones. While I do not think the authors should try extending their model with biogeochemistry, I do think that they should be more explicit (especially in the abstract and introduction) that they only consider the advective effect of an idealised tracer, and that care should be taken when interpreting the tracer as a proxy for oxygen.
2. I don't fully understand why the model is 'only' 0.1 degrees resolution. This is barely eddy-resolving, and indeed the eddies in Figure 2 look conspicuously circular and uniform. For what seems to be a quite efficient model (no vertical dimension), why not increase the resolution and also resolve more of the submesoscale processes?
3. One of the most interesting results to me is that the tracer spreading is eastward; against the (I assume) westward propagation of the eddies. Can the authors more carefully discuss _how_ the eddies would spread tracer in the opposite direction as their translation?
4. The authors nicely contrast the role of eddies with the ventilated thermocline theory, but then stop short of arguing which is more important in the real ocean. Can they not explore the relevance of both methods a bit more? That would greatly help the community
I also have some minor comments:
- lines 2 and 14: The abstract and introduction both start a bit sudden with the method; it is typical to first motivate the study by introducing its relevance?
- Figure 1 (panel an and caption): Why is this field given at ~500? Why not provide the exact depth?
- End of introduction: It is interesting that the bathymetry apparently does _not_ play a role in the organisation of the zonal jets. I had seen other studies in the past (I think?) that argued that the location of zonal jets was 'pinned' by bathymetry? Perhaps the authors want to reflect why their model without bathymetric effects (except coastlines) still has zonal jets?
- line 73: be explicit in which direction the mass flux is?
- line 80: give a motivation/reference why the tracer diffusivity can be set equal to the eddy viscosity?
- line 90: why is the consumption rate only half? Is there an intuitive reason?
- line 112: give an intuitive explanation why the eddy-induced velocity here is analogous to Stokes drift?
- line 116: maybe I missed it, but what are the boundary conditions on the open boundaries for the dynamic model?
- line 135: Can 'remarkably similar' here be quantified?
- line 143: This role of rectification of short Rossby waves could be tested/assessed? It doesn't need to be assumed?
- line 216: I see that the data of the model is available via a dot, but could not easily locate the modelcode itself. In the spirit of open and reproducible science, I wonder whether the code of the model is available somewhere?Citation: https://doi.org/10.5194/egusphere-2024-2007-RC1 -
AC1: 'Reply on RC1', Richard Greatbatch, 05 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2007/egusphere-2024-2007-AC1-supplement.pdf
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AC1: 'Reply on RC1', Richard Greatbatch, 05 Sep 2024
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RC2: 'Comment on egusphere-2024-2007', Anonymous Referee #2, 22 Aug 2024
Review of “The formation and ventilation of an Oxygen Minimum Zone in a simple model for latitudinally alternating zonal jets” submitted to Ocean Science.
This paper presents an idealized model of the ocean circulation physics and oxygen dynamics of the thermocline in the Atlantic basin that yields some thought-provoking results regarding the dynamics of the oxygen minimum zones. The physical model is a 1 ½ layer nonlinear reduced gravity numerical model at 0.1 deg resolution with realistic coastlines forced by an idealized annually oscillating mass flux near the equator. The oxygen is modeled as a single tracer subject to advection-diffusion dynamics with a source between 55-65 W (the western boundary in the northern hemisphere) formulated as a rapid restoring and a constant loss rate. The physical model produces alternating zonal jets and mesoscale eddies, as expected. More surprisingly, the idealized oxygen tracer evolves to exhibit oxygen minimum zones that are qualitatively consistent with observations. This is surprising, because these zones are often described using ventilated thermocline theory that suggests the oxygen minimum zones are unventilated shadow zones that are a consequence of the basin geometry and the wind forcing, which is absent in these simulations. Although the simplicity of the models implies many caveats, which the discussion carefully enumerates, I think the results are intriguing and should be published with minor revisions.
Suggested revisions include:
- Explain better the physical interpretation of the annual mass flux driving the physical model. Does this implicitly represent the seasonal cycle of wind forcing in some way? Be more explicit about how the forcing differs in the conception of the OMZ as a consequence of ventilated thermocline theory. How might the results differ with climatological wind stress forcing instead?
- Explain the rationale behind the oxygen source term better. Perhaps make a comparison between Fig3b and Fig1a more explicitly.
- Consider revising the beginning of the introduction. I appreciate that the paper is concise, but starting the paper with “Figure 1a…” is a bit over the top.
Citation: https://doi.org/10.5194/egusphere-2024-2007-RC2 -
AC2: 'Reply on RC2', Richard Greatbatch, 05 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2007/egusphere-2024-2007-AC2-supplement.pdf
-
EC1: 'Comment on egusphere-2024-2007', Karen J. Heywood, 22 Aug 2024
I am very grateful to both reviewers for their helpful, thorough and constructive reviews. I invite the authors to respond to both reviews here in the open forum (you do not have to have submitted the revised paper at that stage). You will then get the opportunity to upload a revised manuscript and point-by-point responses to all of the reviewers' suggestions and comments. If I consider that you have suitably addressed all their concerns on my own reading of the revisions and responses, it may not be necessary to send the paper out for re-review.
Citation: https://doi.org/10.5194/egusphere-2024-2007-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-2007', Anonymous Referee #1, 12 Aug 2024
Review of "The formation and ventilation of an Oxygen Minimum Zone in a simple model for latitudinally alternating zonal jets" by Köhn et al
In this manuscript, the authors present an analysis of an elegantly simple 1.5 layer reduced gravity model of the Tropical Atlantic Ocean to show that a tracer released at the western boundary does not propagate eastward very well close to the Equator, creating a 'shadow zone' of very low tracer concentration that qualitatively resembles the observed thermocline oxygen concentration. The authors argue that the eddies generated in the model, and the zonal jets they lead to, are thus an explanation for the Oxygen Minimum Zone, in contrast to the ventilated thermocline theory that requires wind forcing.
This is a very clear, nicely succinct, model study that I have read with great interest and pleasure. In the plethora of full-complexity, fine-resolution model studies, it is nice to see an interesting and thought-provoking idealised study. I thus think that the physical oceanographic community could certainly be inspired by this work.
However, I do have some suggestions for improvements before I can recommend publication. My most important ones are
1. While the model is quite suited for the question, I am not sure how directly interpretable the tracer is as oxygen. The model does not include biogeochemical processes such as remineralisation, which are also important for the formation of the oxygen minimum zones. While I do not think the authors should try extending their model with biogeochemistry, I do think that they should be more explicit (especially in the abstract and introduction) that they only consider the advective effect of an idealised tracer, and that care should be taken when interpreting the tracer as a proxy for oxygen.
2. I don't fully understand why the model is 'only' 0.1 degrees resolution. This is barely eddy-resolving, and indeed the eddies in Figure 2 look conspicuously circular and uniform. For what seems to be a quite efficient model (no vertical dimension), why not increase the resolution and also resolve more of the submesoscale processes?
3. One of the most interesting results to me is that the tracer spreading is eastward; against the (I assume) westward propagation of the eddies. Can the authors more carefully discuss _how_ the eddies would spread tracer in the opposite direction as their translation?
4. The authors nicely contrast the role of eddies with the ventilated thermocline theory, but then stop short of arguing which is more important in the real ocean. Can they not explore the relevance of both methods a bit more? That would greatly help the community
I also have some minor comments:
- lines 2 and 14: The abstract and introduction both start a bit sudden with the method; it is typical to first motivate the study by introducing its relevance?
- Figure 1 (panel an and caption): Why is this field given at ~500? Why not provide the exact depth?
- End of introduction: It is interesting that the bathymetry apparently does _not_ play a role in the organisation of the zonal jets. I had seen other studies in the past (I think?) that argued that the location of zonal jets was 'pinned' by bathymetry? Perhaps the authors want to reflect why their model without bathymetric effects (except coastlines) still has zonal jets?
- line 73: be explicit in which direction the mass flux is?
- line 80: give a motivation/reference why the tracer diffusivity can be set equal to the eddy viscosity?
- line 90: why is the consumption rate only half? Is there an intuitive reason?
- line 112: give an intuitive explanation why the eddy-induced velocity here is analogous to Stokes drift?
- line 116: maybe I missed it, but what are the boundary conditions on the open boundaries for the dynamic model?
- line 135: Can 'remarkably similar' here be quantified?
- line 143: This role of rectification of short Rossby waves could be tested/assessed? It doesn't need to be assumed?
- line 216: I see that the data of the model is available via a dot, but could not easily locate the modelcode itself. In the spirit of open and reproducible science, I wonder whether the code of the model is available somewhere?Citation: https://doi.org/10.5194/egusphere-2024-2007-RC1 -
AC1: 'Reply on RC1', Richard Greatbatch, 05 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2007/egusphere-2024-2007-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Richard Greatbatch, 05 Sep 2024
-
RC2: 'Comment on egusphere-2024-2007', Anonymous Referee #2, 22 Aug 2024
Review of “The formation and ventilation of an Oxygen Minimum Zone in a simple model for latitudinally alternating zonal jets” submitted to Ocean Science.
This paper presents an idealized model of the ocean circulation physics and oxygen dynamics of the thermocline in the Atlantic basin that yields some thought-provoking results regarding the dynamics of the oxygen minimum zones. The physical model is a 1 ½ layer nonlinear reduced gravity numerical model at 0.1 deg resolution with realistic coastlines forced by an idealized annually oscillating mass flux near the equator. The oxygen is modeled as a single tracer subject to advection-diffusion dynamics with a source between 55-65 W (the western boundary in the northern hemisphere) formulated as a rapid restoring and a constant loss rate. The physical model produces alternating zonal jets and mesoscale eddies, as expected. More surprisingly, the idealized oxygen tracer evolves to exhibit oxygen minimum zones that are qualitatively consistent with observations. This is surprising, because these zones are often described using ventilated thermocline theory that suggests the oxygen minimum zones are unventilated shadow zones that are a consequence of the basin geometry and the wind forcing, which is absent in these simulations. Although the simplicity of the models implies many caveats, which the discussion carefully enumerates, I think the results are intriguing and should be published with minor revisions.
Suggested revisions include:
- Explain better the physical interpretation of the annual mass flux driving the physical model. Does this implicitly represent the seasonal cycle of wind forcing in some way? Be more explicit about how the forcing differs in the conception of the OMZ as a consequence of ventilated thermocline theory. How might the results differ with climatological wind stress forcing instead?
- Explain the rationale behind the oxygen source term better. Perhaps make a comparison between Fig3b and Fig1a more explicitly.
- Consider revising the beginning of the introduction. I appreciate that the paper is concise, but starting the paper with “Figure 1a…” is a bit over the top.
Citation: https://doi.org/10.5194/egusphere-2024-2007-RC2 -
AC2: 'Reply on RC2', Richard Greatbatch, 05 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2007/egusphere-2024-2007-AC2-supplement.pdf
-
EC1: 'Comment on egusphere-2024-2007', Karen J. Heywood, 22 Aug 2024
I am very grateful to both reviewers for their helpful, thorough and constructive reviews. I invite the authors to respond to both reviews here in the open forum (you do not have to have submitted the revised paper at that stage). You will then get the opportunity to upload a revised manuscript and point-by-point responses to all of the reviewers' suggestions and comments. If I consider that you have suitably addressed all their concerns on my own reading of the revisions and responses, it may not be necessary to send the paper out for re-review.
Citation: https://doi.org/10.5194/egusphere-2024-2007-EC1
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Eike E. Köhn
Richard J. Greatbatch
Peter Brandt
Martin Claus
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(4629 KB) - Metadata XML