the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Regional mapping of energetic short mesoscale ocean dynamics from altimetry: performances from real observations.
Florian Le Guillou
Lucile Gaultier
Maxime Ballarotta
Sammy Metref
Clément Ubelmann
Emmanuel Cosme
Marie-Helène Rio
Abstract. For over 25 years, satellite altimetry has provided invaluable information about the ocean dynamics at many scales. In particular, gridded Sea Surface Height (SSH) maps allow to estimate the mesoscale geostrophic circulation in the ocean. However, conventional interpolation techniques rely on static optimal interpolation schemes, hence limiting the estimation of non linear dynamics at scales not well sampled by altimetry (i.e. below 150–200 km at mid latitudes). To overcome this limitation in the resolution of small-scale SSH structures (and thus small-scale geostrophic currents), a Back and Forth Nudging algorithm combined with a Quasi-Geostrophic model, a technique called BFN-QG, has been successfully applied on simulated SSH data in Observing System Simulation Experiments (OSSEs), showing a significant reduction in interpolation error and an improvement of space-time resolutions of the experimental gridded product compared to operational products. In this study, we propose to apply the BFN-QG to real altimetric SSH data in a highly turbulent region spanning a part of the Agulhas current. The performances are evaluated within Observing System Experiments (OSEs) that use independent data (such as independent SSH, Sea Surface Temperature and drifter data) as ground-truth. By comparing the mapping performances to the ones obtained by operational products, we show that the BFN-QG improves the mapping of short, energetic mesoscale structures and associated geostrophic currents both in space and time. In particular, the BFN-QG improves (i) the spatial effective resolution of the SSH maps by a factor of 20 %, (ii) the zonal and (especially) the meridional geostrophic currents and (iii) the prediction of Lagrangian transport for lead times up to 10 days. Unlike the results obtained in the OSSEs, the OSEs reveal more contrasting performances in low variability regions that are discussed in the paper.
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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.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Florian Le Guillou et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-509', Anonymous Referee #1, 26 Apr 2023
The study is novel and could be worth publication after minor-to-major revisions.
General comments:
The presentation of the major findings and results as associated with the key figures should be more adequately outlined and addressed. Now it often appears too brief as it rushes from one figure and relative short explanation and discussion to the next. This should be improved.
Does the choice of the 1.5 layer QG model impact the results in any way? In other words, are there other modes than the first baroclinic mode that might be influecing the dynamics and variability in the greater Agulhas Current region? Is it possible to find discussion of this in the literature? Need some more consideration.
Line 210 comments on the Agulhas Current area and claim that it is characterized by weak advection of the frontal structures. However, in view of the area (Figure 9) the presence of Natal pulses can propagate southwestward on the nearshore side of the Agulhas Current core while highly rich and complicated dynamics occur in the retroflection area where eddies are often shed and also sometimes reintegrated into the retroflection. These areas are characterized by advection and sometimes rapid occurrances and closing of frontal structures. So the statement must be refined to better balance these conditions.
In Figure 10: The black line in the 3 panels seems to be contours of the land-ocean boundary. Must be explained. A bit strange to see values within boxes that are completely inland.
In the discussion and conclusion it would be worthwhile to comment on possible use of SWOT and Sentinel-1 image data to further strengthen this study approach.
Specific editorial comments:
The submitted paper is attached with red-markings suggesting update of the text.
Additional edisotrial comments:
Agulhas Current should always be with captial C.
Be consistent in use of reference to figures (Fig., Figure, figure, I prefer Figure).
Equation 10: The expressions in the exponent should have larger font. Difficult to see properly.
The space scale symbol D (in equation 10) must for consistency also be used in Table 1 (nudging space scale ).
- AC1: 'Reply on RC1', Florian Le Guillou, 21 Jul 2023
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RC2: 'Comment on egusphere-2023-509', Anonymous Referee #2, 28 Jun 2023
This paper considers a technique for obtaining geostrophic circulation from SSH maps (BFN-QG) and applies it to real altimetric SSH data. This method is compared to another method (DUACS) and shown to improve the mapping of short energetic mesoscale structures, producing finer scale coherent structures. While the method improves the mapping of smaller scale high energy structures where there is high variability, it does not perform well for larger structures when the variability is low. Overall this paper is scientifically sound, well written and merits publication in this journal. I have not found any major issues with the paper, and believe it is almost ready for publication, but below I list a couple of minor points that should be taken onboard before final submission.
Minor comments:
page 3 - equation (4): The authors might consider using a different symbol for SSH when applying it in the mathematical formulas, for example maybe using H for SSH, and then make subscripts for when referring to the MDT and SLA part, like H_{MDT}, H_{SLA}.page 4 - equations (7a) and (7b): You have switched the symbol for the Coriolis parameter from f to f_0. I presume we are considering the f-plane here, i.e. f=constant, this should be stated in the text.
page 7 - table 1: Typo - nudging space scale should be denoted as D instead of tau which is used for time scale.
page 7 - equation 11: confusing use of the square brackets containing the letter i, would be better to apply i as a subscript, i.e. SLA_i, and then have brackets all around the delta SLA term, then apply square, i.e. [\Delta SLA_i]^2.
page 11 - equation for flow crossing: This equation is not well defined, what is v[P_i] (presumably the velocity vector) and what is the angled symbol on the RHS inside the cosine term, even if referenced the equation variables needs to be defined here.
Citation: https://doi.org/10.5194/egusphere-2023-509-RC2 -
AC2: 'Reply on RC2', Florian Le Guillou, 21 Jul 2023
We are grateful to the reviewer for the minor comments. The manuscript has been revised to comply with the reviewer’s suggestions.
Citation: https://doi.org/10.5194/egusphere-2023-509-AC2
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AC2: 'Reply on RC2', Florian Le Guillou, 21 Jul 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-509', Anonymous Referee #1, 26 Apr 2023
The study is novel and could be worth publication after minor-to-major revisions.
General comments:
The presentation of the major findings and results as associated with the key figures should be more adequately outlined and addressed. Now it often appears too brief as it rushes from one figure and relative short explanation and discussion to the next. This should be improved.
Does the choice of the 1.5 layer QG model impact the results in any way? In other words, are there other modes than the first baroclinic mode that might be influecing the dynamics and variability in the greater Agulhas Current region? Is it possible to find discussion of this in the literature? Need some more consideration.
Line 210 comments on the Agulhas Current area and claim that it is characterized by weak advection of the frontal structures. However, in view of the area (Figure 9) the presence of Natal pulses can propagate southwestward on the nearshore side of the Agulhas Current core while highly rich and complicated dynamics occur in the retroflection area where eddies are often shed and also sometimes reintegrated into the retroflection. These areas are characterized by advection and sometimes rapid occurrances and closing of frontal structures. So the statement must be refined to better balance these conditions.
In Figure 10: The black line in the 3 panels seems to be contours of the land-ocean boundary. Must be explained. A bit strange to see values within boxes that are completely inland.
In the discussion and conclusion it would be worthwhile to comment on possible use of SWOT and Sentinel-1 image data to further strengthen this study approach.
Specific editorial comments:
The submitted paper is attached with red-markings suggesting update of the text.
Additional edisotrial comments:
Agulhas Current should always be with captial C.
Be consistent in use of reference to figures (Fig., Figure, figure, I prefer Figure).
Equation 10: The expressions in the exponent should have larger font. Difficult to see properly.
The space scale symbol D (in equation 10) must for consistency also be used in Table 1 (nudging space scale ).
- AC1: 'Reply on RC1', Florian Le Guillou, 21 Jul 2023
-
RC2: 'Comment on egusphere-2023-509', Anonymous Referee #2, 28 Jun 2023
This paper considers a technique for obtaining geostrophic circulation from SSH maps (BFN-QG) and applies it to real altimetric SSH data. This method is compared to another method (DUACS) and shown to improve the mapping of short energetic mesoscale structures, producing finer scale coherent structures. While the method improves the mapping of smaller scale high energy structures where there is high variability, it does not perform well for larger structures when the variability is low. Overall this paper is scientifically sound, well written and merits publication in this journal. I have not found any major issues with the paper, and believe it is almost ready for publication, but below I list a couple of minor points that should be taken onboard before final submission.
Minor comments:
page 3 - equation (4): The authors might consider using a different symbol for SSH when applying it in the mathematical formulas, for example maybe using H for SSH, and then make subscripts for when referring to the MDT and SLA part, like H_{MDT}, H_{SLA}.page 4 - equations (7a) and (7b): You have switched the symbol for the Coriolis parameter from f to f_0. I presume we are considering the f-plane here, i.e. f=constant, this should be stated in the text.
page 7 - table 1: Typo - nudging space scale should be denoted as D instead of tau which is used for time scale.
page 7 - equation 11: confusing use of the square brackets containing the letter i, would be better to apply i as a subscript, i.e. SLA_i, and then have brackets all around the delta SLA term, then apply square, i.e. [\Delta SLA_i]^2.
page 11 - equation for flow crossing: This equation is not well defined, what is v[P_i] (presumably the velocity vector) and what is the angled symbol on the RHS inside the cosine term, even if referenced the equation variables needs to be defined here.
Citation: https://doi.org/10.5194/egusphere-2023-509-RC2 -
AC2: 'Reply on RC2', Florian Le Guillou, 21 Jul 2023
We are grateful to the reviewer for the minor comments. The manuscript has been revised to comply with the reviewer’s suggestions.
Citation: https://doi.org/10.5194/egusphere-2023-509-AC2
-
AC2: 'Reply on RC2', Florian Le Guillou, 21 Jul 2023
Peer review completion
Journal article(s) based on this preprint
Florian Le Guillou et al.
Florian Le Guillou et al.
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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.
- Preprint
(2839 KB) - Metadata XML