the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jun 2023
Ocean 2D eddy energy fluxes from small mesoscale processes with SWOT
Abstract. We investigate ocean dynamics at different scales in the Agulhas current system, a region of important interocean exchange of heat and energy. While ocean observations and some of the most advanced climate models capture the larger mesoscale dynamics (>100 km), the smaller-scale fronts and eddies are underrepresented. The recently launched NASA-CNES Surface Water and Ocean Topography (SWOT) wide-swath altimeter mission observes the smaller ocean geostrophic scales down to 15 km in wavelength globally. Here we will analyze different eddy diagnostics in the Agulhas Current region and quantify the contributions from the larger mesoscales observable today, and the smaller scales to be observed with SWOT. Surface geostrophic diagnostics of eddy kinetic energy, strain, and energy cascades are estimated from modelled sea surface height fields of the MITGCM / LLC4320 simulation subsampled at 1/10°. In this region, the smaller scales (<150 km) have a strong signature on the horizontal geostrophic strain rate, and for all eddy diagnostics in the western boundary current and along the meandering Agulhas Extension. We investigate the horizontal cascade of energy using a coarse-graining technique, and we observe that the wavelength range where the inverse cascade occurs is biased towards larger mesoscale wavelengths with today’s altimetric sampling. We also calculate the projected sampling of the eddy diagnostics under the SWOT swaths built with the CNES-NASA simulator to include the satellite position and realistic noise. For the swaths, a neural network noise mitigation method is implemented to reduce the residual SWOT random error before calculating eddy diagnostics. In terms of sea surface height (SSH), observable wavelengths of 15 to 20 km are retrieved after neural network noise mitigation, as opposed to wavelengths larger than 40 km before the noise reduction.
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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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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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CC1: 'Comment on egusphere-2023-1124', Zhiwei Zhang, 25 Jun 2023
General comments
In this paper, through analyzing outputs from a submesoscale permitting ocean simulation, the authors investigte different contributions from the larger mesoscales observable today and the smaller scales to be observed with SWOT in the region of the Agulhas current system. They found that the smaller-scale mesoscale eddies (<150 km) have a strong signature on the horizontal geostrophic strain rate and eddy diagnostics in the study region. They also pointed out that the wavelength of inverse cascade occurs is biased towards larger mesoscale wavelengths with today’s altimetric sampling. Overall, this paper provides some useful implications for the application of SWOT data to study smaller mesoscale eddies and the associated energetics issues in the future. Although I think this paper deserves to be published in Egusphere, the following major and minor issues have to be clarified before the acceptance.
Major comments
1. Lines 260-290: The methods described here aim to show the potential of the SWOT mission to observe smaller mesoscale eddies (some times called larger submesoscale eddies) and the associated energy cascade. However, in addition to the instrumental noises, the SWOT-derived SSH at mesoscale also contain large signals of unbalanced internal tides and internal gravity waves. So, you can not directly calculate the geostrophic current and thus the EKE from the SWOT data based on geostrophic relation. Actually, how to remove the “noise”signals of unblanced motions from the SWOT SSH data is still a large challenge for the community. So, I suggest you explain it more clearly here to what degeree can you data and methods represent the context and potential of SWOT data in reality.
2. Lines 307-315: The residual and smaller scales of EKE and strain rate you defined here (i.e. difference between the results of LLC10 and the DUACS) also contain the contributions of the cross terms. You should clarify this point more explicitly.
3. Lines 393-395: Coluld you explain the possible reasons accounting for the difference between your results and those in Sasaki et al. (2014)?By the way, I also suggest the authors briefly discuss the generation mechanisms of the smaller mesoscale eddies (sometime called larger submesoscale eddies) here. Are they generated by mixed-layer baroclinic instability?
Minor comments
(1). Lines 21-30: I also recommend reading Zhang et al. (2023), which highlighs the important role of submesoscale inverse energy cascade in modulating the meridional heat transport in the SO using LLC4320 data (Title: Submesoscale inverse energy cascade enhances Southern Ocean eddy heat transport).
(2). Line 76: Delete “geostrophic” in “fine-scale geostrophic sea surface height” because the SSH also contain signals of unbalanced motions.
(3). Line 113: Delete the second “(Rocha et al., 2016)”.
(4). Line 115: “maximum model depth of 7000 m”. It should be 6760 m?
(5). Section 3.3 Geographical distribution of the energy cascade: In addition to geographical distribution, it is also meaningful to discuss the seasonal variations. Does the energy cascade has a clear seasonal cycle due to the enhanced mixed-layer eddies in winter ?
(6). Section 5 Conclusions: This part also contains a certain contents of discussions. So, I suggest changing the section title to “5 Conclusions and Discussions”.
Citation: https://doi.org/10.5194/egusphere-2023-1124-CC1 -
AC1: 'Reply on CC1', Elisa Carli, 11 Aug 2023
Dear Dr. Zhang, thank you for your insightful comments. Indeed, this study is of particular interest to the scientific community working in the Agulhas Current region or other western boundary currents, in showing what the SWOT satellite will add to today’s knowledge and observability with altimetry. It shows that the finer 2D scales (15 - 150 km) introduced by SWOT will greatly improve our knowledge of the region’s dynamics, particularly of the energy fluxes transferring between different scales. Please see the PDF to find the specific answers to your comments.
-
AC1: 'Reply on CC1', Elisa Carli, 11 Aug 2023
-
RC1: 'Comment on egusphere-2023-1124', Anonymous Referee #1, 04 Jul 2023
Review of "Ocean 2D eddy energy fluxes from small mesoscale processes
with SWOT"
By E. Carli, R. Morrow, O. Vergara, R. Chevrier and L. Renault
This article diagnoses the potential improvements in the restitution of geostrophic eddy dynamics
in the Agulhas current system from the new SWOT satellite mission.
It uses subsampled outputs of the MITgcm 1/48deg LLCC4320 global ocean model (LLC10) with the SWOT simulator
to compares the eddy dynamics representation when using current altimetry products and SWOT.
The Agulhas current region is used as one of the most turbulent regions of the world oceans.
An innovative artificial intelligence processing (U-Net) trained with a model of the North Atlantic
appears promising for the reduction of random noise in the Agulhas Current system.
Observable wavelengths of 15 to 20 km are retrieved for sea surface height
and also for high-order eddy diagnostics such as EKE and strain.
A specific attention is given on key processes of the region:
turbulence in the Agulhas retroflection, Natal Pulses, frontal eddies...
A coarse graining technique is used to investigate the energy cascades for two scales (60km and 150 km).This is a very good article which uses the opportunity of the SWOT mission to revisit
important eddy processes in the Agulhas region.
It shows the limitations in the current altimetry products and the potential improvements for the new mission.
Even not trained in the region, the U-Net denoising method appears promising
in mitigating the KarIn random error.Specific comments
Line 113: "(Rocha et al., 2016). (Rocha et al., 2016)"
citation doubledLine 125: "Arbic et al. (2022)"
citet instead of citepLine 201: "KarIn random noise"
please define KarIn in the textLine 250: "which the noise to signal ratio (NSR) reaches one."
please refer to equation (1)Equation (6):
can you defined how the bars are computed ?Line 301: "mean surface geostrophic EKE over this 1-year period,"
One year is a bit short for a mean surface geostrophic EKE here.
This is why EKE pattern in Figure 3 is quite patchy.
... but I think this is the limitation of using LLC10 ?Line 350: "these smaller-scale features"
They are actually called "submesoscale frontal eddies" or "shear eddies"Figure 5: What is the definition of the cross-terms ?
Table 1: Values of strain for PseudoDUACS are quite large compared to Figure 3.
Is this due to the specific locations of the boxes ?Line 370 : "sub-mesoscale barotropic anomalies"
no. these are (baroclinic) submesoscale frontal eddies generated by barotropic instabilities.Figure 7: if the figure is dominated by the propagagtion of Natal Pulses, one possibility would
be to do the statistics in absence of Natal Pulses (as in Tedesco et al., 2019).Line 545:
"direct cascade ad scales"
"direct cascade at scales"Citation: https://doi.org/10.5194/egusphere-2023-1124-RC1 -
AC2: 'Reply on RC1', Elisa Carli, 11 Aug 2023
Dear Sir/Madam,
Thank you for your insightful comments. This article is indeed very important for the community as it explores the potential of SWOT to observe mesoscale dynamics that are still missing in current altimetry products. We show that the smaller scale dynamics that SWOT will detect contribute to stronger eddy kinetic energy, strain rate and inverse energy cascade in the Agulhas Current region compared to what is observed today. Please refer to the PDF for the specific responses to your comments.
-
AC2: 'Reply on RC1', Elisa Carli, 11 Aug 2023
-
RC2: 'Comment on egusphere-2023-1124', Anonymous Referee #2, 17 Jul 2023
This paper investigates ocean dynamics at different scales in the Agulhas current system, a region of important interocean exchange of heat and energy. The authors analyze different eddy diagnostics in the Agulhas Current region and quantify the contributions from the larger mesoscales observable today, and the smaller scales to be observed with the recently launched NASA-CNES Surface Water and Ocean Topography (SWOT) wide-swath altimeter mission. The paper presents surface geostrophic diagnostics of eddy kinetic energy, strain, and energy cascades estimated from modeled sea surface height fields of the MITGCM / LLC4320 simulation subsampled at 1/10 degree. The authors also used a coarse-graining technique to investigate the horizontal cascade of energy and observed that the wavelength range where the inverse cascade occurs is biased towards larger mesoscale wavelengths with today's altimetric sampling. To calculate the projected sampling of the eddy diagnostics under the SWOT swaths, the authors used the CNES-NASA simulator to include the satellite position and realistic noise. They also implemented a neural network noise mitigation method to reduce the residual SWOT random error before calculating eddy diagnostics. The paper presents the observability of the ocean eddy diagnostics before and after the U-net de-noising treatment of the residual KarIn random error.
This paper presents valuable results of what SWOT could bring to the scientific community in terms of small-scale altimetric observation and it would benefit from some examination in the methodology and discussion. I recommend the minor revision before acceptance for publication.
Major comments
L210: Systematic errors such as satellite roll, and phase errors, could be much bigger sources of error on SSH than Karin random error. Can the authors elaborate on the reason why they choose to focus on Karin random error on SSH data and observable wavelengths in SSH, eddy kinetic energy, and strain rate?
L391-L395 The paper's findings don't seem to align with Sasaki et al. (2014)'s research regarding the energy feeding dynamics between different mesoscale EKE during various seasons. If the paper doesn't adequately address this discrepancy, it could be a point of critique.
L233-244 U-Net's General Application: The authors show that U-net performs well even with different models and in different zones, leading to similar Root Mean Square Error (RMSE) and variance of SSH residuals. This indicates the potential universal applicability of the U-net method across various regions and circumstances. While the authors show U-Net's promise, the discussion lacks a thorough analysis of why U-Net performs similarly across various regions and models. It’s not clear how U-Net achieves this consistency, which might lead to skepticism about the broad application of U-Net in other environments. Moreover, the potential improvements that could be achieved by training U-Net specifically for different regions could be discussed in more detail.
Minor comments
L70: “multi-multi mission” “multi-mission information”?
L125: … and an offset on the atmosphere forcing fields (by 6 hours) (Arbic et al., 2022).
L290 define how the u bar and v bar are calculated.
L383 The study found that Natal Pulses, despite being the strongest events in terms of Eddy Kinetic Energy (EKE), had only a small impact on the strain, particularly on the pseudo-DUACS strain. This could be criticized if it contradicts the existing understanding of the effects of Natal Pulses on ocean dynamics or if the paper does not provide a sufficient explanation for this finding.
4.2 SWOT Observability: The authors employ the U-net noise mitigation technique and assess its impact on different models. Despite the efficacy of the U-net method, it does slightly underestimate the signal amplitude. Also, there is no clear discussion on the limitations of using the U-net technique and the reasons behind its efficacy.
Citation: https://doi.org/10.5194/egusphere-2023-1124-RC2 -
AC3: 'Reply on RC2', Elisa Carli, 11 Aug 2023
Dear Sir/Madam,
Thank you for your insightful comments. We do believe that this article is valuable to the community as it explores the new mesoscale dynamics observable with SWOT, and it also demonstrates the potential of the latest noise mitigation techniques to retrieve the SSH signal at 15-20 km, which will greatly improve our knowledge of the dynamics of the region. Please refer to the PDF for the specific responses to your comments.
-
AC3: 'Reply on RC2', Elisa Carli, 11 Aug 2023
Interactive discussion
Status: closed
-
CC1: 'Comment on egusphere-2023-1124', Zhiwei Zhang, 25 Jun 2023
General comments
In this paper, through analyzing outputs from a submesoscale permitting ocean simulation, the authors investigte different contributions from the larger mesoscales observable today and the smaller scales to be observed with SWOT in the region of the Agulhas current system. They found that the smaller-scale mesoscale eddies (<150 km) have a strong signature on the horizontal geostrophic strain rate and eddy diagnostics in the study region. They also pointed out that the wavelength of inverse cascade occurs is biased towards larger mesoscale wavelengths with today’s altimetric sampling. Overall, this paper provides some useful implications for the application of SWOT data to study smaller mesoscale eddies and the associated energetics issues in the future. Although I think this paper deserves to be published in Egusphere, the following major and minor issues have to be clarified before the acceptance.
Major comments
1. Lines 260-290: The methods described here aim to show the potential of the SWOT mission to observe smaller mesoscale eddies (some times called larger submesoscale eddies) and the associated energy cascade. However, in addition to the instrumental noises, the SWOT-derived SSH at mesoscale also contain large signals of unbalanced internal tides and internal gravity waves. So, you can not directly calculate the geostrophic current and thus the EKE from the SWOT data based on geostrophic relation. Actually, how to remove the “noise”signals of unblanced motions from the SWOT SSH data is still a large challenge for the community. So, I suggest you explain it more clearly here to what degeree can you data and methods represent the context and potential of SWOT data in reality.
2. Lines 307-315: The residual and smaller scales of EKE and strain rate you defined here (i.e. difference between the results of LLC10 and the DUACS) also contain the contributions of the cross terms. You should clarify this point more explicitly.
3. Lines 393-395: Coluld you explain the possible reasons accounting for the difference between your results and those in Sasaki et al. (2014)?By the way, I also suggest the authors briefly discuss the generation mechanisms of the smaller mesoscale eddies (sometime called larger submesoscale eddies) here. Are they generated by mixed-layer baroclinic instability?
Minor comments
(1). Lines 21-30: I also recommend reading Zhang et al. (2023), which highlighs the important role of submesoscale inverse energy cascade in modulating the meridional heat transport in the SO using LLC4320 data (Title: Submesoscale inverse energy cascade enhances Southern Ocean eddy heat transport).
(2). Line 76: Delete “geostrophic” in “fine-scale geostrophic sea surface height” because the SSH also contain signals of unbalanced motions.
(3). Line 113: Delete the second “(Rocha et al., 2016)”.
(4). Line 115: “maximum model depth of 7000 m”. It should be 6760 m?
(5). Section 3.3 Geographical distribution of the energy cascade: In addition to geographical distribution, it is also meaningful to discuss the seasonal variations. Does the energy cascade has a clear seasonal cycle due to the enhanced mixed-layer eddies in winter ?
(6). Section 5 Conclusions: This part also contains a certain contents of discussions. So, I suggest changing the section title to “5 Conclusions and Discussions”.
Citation: https://doi.org/10.5194/egusphere-2023-1124-CC1 -
AC1: 'Reply on CC1', Elisa Carli, 11 Aug 2023
Dear Dr. Zhang, thank you for your insightful comments. Indeed, this study is of particular interest to the scientific community working in the Agulhas Current region or other western boundary currents, in showing what the SWOT satellite will add to today’s knowledge and observability with altimetry. It shows that the finer 2D scales (15 - 150 km) introduced by SWOT will greatly improve our knowledge of the region’s dynamics, particularly of the energy fluxes transferring between different scales. Please see the PDF to find the specific answers to your comments.
-
AC1: 'Reply on CC1', Elisa Carli, 11 Aug 2023
-
RC1: 'Comment on egusphere-2023-1124', Anonymous Referee #1, 04 Jul 2023
Review of "Ocean 2D eddy energy fluxes from small mesoscale processes
with SWOT"
By E. Carli, R. Morrow, O. Vergara, R. Chevrier and L. Renault
This article diagnoses the potential improvements in the restitution of geostrophic eddy dynamics
in the Agulhas current system from the new SWOT satellite mission.
It uses subsampled outputs of the MITgcm 1/48deg LLCC4320 global ocean model (LLC10) with the SWOT simulator
to compares the eddy dynamics representation when using current altimetry products and SWOT.
The Agulhas current region is used as one of the most turbulent regions of the world oceans.
An innovative artificial intelligence processing (U-Net) trained with a model of the North Atlantic
appears promising for the reduction of random noise in the Agulhas Current system.
Observable wavelengths of 15 to 20 km are retrieved for sea surface height
and also for high-order eddy diagnostics such as EKE and strain.
A specific attention is given on key processes of the region:
turbulence in the Agulhas retroflection, Natal Pulses, frontal eddies...
A coarse graining technique is used to investigate the energy cascades for two scales (60km and 150 km).This is a very good article which uses the opportunity of the SWOT mission to revisit
important eddy processes in the Agulhas region.
It shows the limitations in the current altimetry products and the potential improvements for the new mission.
Even not trained in the region, the U-Net denoising method appears promising
in mitigating the KarIn random error.Specific comments
Line 113: "(Rocha et al., 2016). (Rocha et al., 2016)"
citation doubledLine 125: "Arbic et al. (2022)"
citet instead of citepLine 201: "KarIn random noise"
please define KarIn in the textLine 250: "which the noise to signal ratio (NSR) reaches one."
please refer to equation (1)Equation (6):
can you defined how the bars are computed ?Line 301: "mean surface geostrophic EKE over this 1-year period,"
One year is a bit short for a mean surface geostrophic EKE here.
This is why EKE pattern in Figure 3 is quite patchy.
... but I think this is the limitation of using LLC10 ?Line 350: "these smaller-scale features"
They are actually called "submesoscale frontal eddies" or "shear eddies"Figure 5: What is the definition of the cross-terms ?
Table 1: Values of strain for PseudoDUACS are quite large compared to Figure 3.
Is this due to the specific locations of the boxes ?Line 370 : "sub-mesoscale barotropic anomalies"
no. these are (baroclinic) submesoscale frontal eddies generated by barotropic instabilities.Figure 7: if the figure is dominated by the propagagtion of Natal Pulses, one possibility would
be to do the statistics in absence of Natal Pulses (as in Tedesco et al., 2019).Line 545:
"direct cascade ad scales"
"direct cascade at scales"Citation: https://doi.org/10.5194/egusphere-2023-1124-RC1 -
AC2: 'Reply on RC1', Elisa Carli, 11 Aug 2023
Dear Sir/Madam,
Thank you for your insightful comments. This article is indeed very important for the community as it explores the potential of SWOT to observe mesoscale dynamics that are still missing in current altimetry products. We show that the smaller scale dynamics that SWOT will detect contribute to stronger eddy kinetic energy, strain rate and inverse energy cascade in the Agulhas Current region compared to what is observed today. Please refer to the PDF for the specific responses to your comments.
-
AC2: 'Reply on RC1', Elisa Carli, 11 Aug 2023
-
RC2: 'Comment on egusphere-2023-1124', Anonymous Referee #2, 17 Jul 2023
This paper investigates ocean dynamics at different scales in the Agulhas current system, a region of important interocean exchange of heat and energy. The authors analyze different eddy diagnostics in the Agulhas Current region and quantify the contributions from the larger mesoscales observable today, and the smaller scales to be observed with the recently launched NASA-CNES Surface Water and Ocean Topography (SWOT) wide-swath altimeter mission. The paper presents surface geostrophic diagnostics of eddy kinetic energy, strain, and energy cascades estimated from modeled sea surface height fields of the MITGCM / LLC4320 simulation subsampled at 1/10 degree. The authors also used a coarse-graining technique to investigate the horizontal cascade of energy and observed that the wavelength range where the inverse cascade occurs is biased towards larger mesoscale wavelengths with today's altimetric sampling. To calculate the projected sampling of the eddy diagnostics under the SWOT swaths, the authors used the CNES-NASA simulator to include the satellite position and realistic noise. They also implemented a neural network noise mitigation method to reduce the residual SWOT random error before calculating eddy diagnostics. The paper presents the observability of the ocean eddy diagnostics before and after the U-net de-noising treatment of the residual KarIn random error.
This paper presents valuable results of what SWOT could bring to the scientific community in terms of small-scale altimetric observation and it would benefit from some examination in the methodology and discussion. I recommend the minor revision before acceptance for publication.
Major comments
L210: Systematic errors such as satellite roll, and phase errors, could be much bigger sources of error on SSH than Karin random error. Can the authors elaborate on the reason why they choose to focus on Karin random error on SSH data and observable wavelengths in SSH, eddy kinetic energy, and strain rate?
L391-L395 The paper's findings don't seem to align with Sasaki et al. (2014)'s research regarding the energy feeding dynamics between different mesoscale EKE during various seasons. If the paper doesn't adequately address this discrepancy, it could be a point of critique.
L233-244 U-Net's General Application: The authors show that U-net performs well even with different models and in different zones, leading to similar Root Mean Square Error (RMSE) and variance of SSH residuals. This indicates the potential universal applicability of the U-net method across various regions and circumstances. While the authors show U-Net's promise, the discussion lacks a thorough analysis of why U-Net performs similarly across various regions and models. It’s not clear how U-Net achieves this consistency, which might lead to skepticism about the broad application of U-Net in other environments. Moreover, the potential improvements that could be achieved by training U-Net specifically for different regions could be discussed in more detail.
Minor comments
L70: “multi-multi mission” “multi-mission information”?
L125: … and an offset on the atmosphere forcing fields (by 6 hours) (Arbic et al., 2022).
L290 define how the u bar and v bar are calculated.
L383 The study found that Natal Pulses, despite being the strongest events in terms of Eddy Kinetic Energy (EKE), had only a small impact on the strain, particularly on the pseudo-DUACS strain. This could be criticized if it contradicts the existing understanding of the effects of Natal Pulses on ocean dynamics or if the paper does not provide a sufficient explanation for this finding.
4.2 SWOT Observability: The authors employ the U-net noise mitigation technique and assess its impact on different models. Despite the efficacy of the U-net method, it does slightly underestimate the signal amplitude. Also, there is no clear discussion on the limitations of using the U-net technique and the reasons behind its efficacy.
Citation: https://doi.org/10.5194/egusphere-2023-1124-RC2 -
AC3: 'Reply on RC2', Elisa Carli, 11 Aug 2023
Dear Sir/Madam,
Thank you for your insightful comments. We do believe that this article is valuable to the community as it explores the new mesoscale dynamics observable with SWOT, and it also demonstrates the potential of the latest noise mitigation techniques to retrieve the SSH signal at 15-20 km, which will greatly improve our knowledge of the dynamics of the region. Please refer to the PDF for the specific responses to your comments.
-
AC3: 'Reply on RC2', Elisa Carli, 11 Aug 2023
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Rosemary Morrow
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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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(9245 KB) - Metadata XML