Joint observation–model mixed-layer heat and salt budgets in the eastern tropical Atlantic

Ngakala, Roy Dorgeless; Alory, Gaël; Da-Allada, Casimir Yélognissè; Kom, Olivia Estelle; Jouanno, Julien; Rath, Willi; Baloïtcha, Ezinvi

doi:https://doi.org/10.5194/os-19-535-2023

Articles | Volume 19, issue 3

https://doi.org/10.5194/os-19-535-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-19-535-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 3

Research article

|

05 May 2023

Research article |

| 05 May 2023

Joint observation–model mixed-layer heat and salt budgets in the eastern tropical Atlantic

Roy Dorgeless Ngakala, Gaël Alory, Casimir Yélognissè Da-Allada, Olivia Estelle Kom, Julien Jouanno, Willi Rath, and Ezinvi Baloïtcha

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Final revised paper (published on 05 May 2023)
Preprint (discussion started on 16 Nov 2022)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1245', Anonymous Referee #1, 03 Dec 2022

This paper examines the mean seasonal cycle heat and salt balances in the eastern equatorial Atlantic using an observational climatology (PREFCLIM) and a high resolution ocean general circulation model (NEMO). The observational and model results are compared for consistently. In addition, the model is subsampled as in the observed climatology and the heat budget terms recomputed (offline) to examine the importance of submesocale variability in the model that is not represented in PREFCLIM. The results are generally consistent with previous budget analyses in the region, which builds confidence in our understanding of the key processes involved.

The paper is suitable for publication after major revision taking account of the comments below.

My most serious concern is that there is no discussion of the importance of tropical instability waves in the heat balance of the eastern equatorial Atlantic. The effect of TIWs is hidden in the lateral diffusion term (D-sub-L) as well as in the vertical turbulent diffusion term (because of the effects of TIWs on vertical shear, e.g. Heukamp et al, 2022). These important processes are not discussed at all and yet there is a long history of describing their role in the near equatorial heat balance (e.g., Weisberg and Weingartner, 1998; Grodsky et al, 2005; Lee et al, 2014; and many more). I’m guessing the authors have ignored this issue because TIWs have periods of ~30 days whereas PREFCLIM is a monthly climatology (although its temporal resolution is never specified—see below). Thus, PREFCLIM does not resolve TIWs. However, the model has no such limitation in terms of temporal resolution. The model is used to assess what is lost in terms of the effects of submesoscale variability in computing the surface layer heat balance from PREFCLIM. The model can be used in a very similar way to assess the effects of mesoscale TIWs that are not resolved by PREFCLIM.

Other.

Introduction. Somewhere around lines 50-70, the paper by Scannell and McPhaden (2018) should be cited and discussed.

Line 85. The PIRATA acronym should be defined and a reference provided (e.g. Bourles et al, 2019).

Section 2.1.1. The temporal resolution of PREFCLIM is not specified. What is it?

Figure 3. The Angola and Equatorial boxes contain areas of very high variability and very low variability. Presumably, the areal averages are therefore representative of much smaller regions within the boxes where the variability is high. This bias should be noted and discussed.

Line 275. What are the implications of the off-line advection correlating better with The Lagrangian estimate of advection? Similar question regarding statement in lines 298-99 comparing offline/online to PREFCLIM.

Figures 5 and 8. I do not see full dotted lines in these figures.

The Rath and Dengler (2016) reference seems incomplete.

References

Bourlès, B.M. Araujo, M.J. McPhaden, P. Brandt, G. Foltz, R. Lumpkin, H. Giordani, F. Hernandez, N. Lefèvre, P. Nobre, E. Campos, R. Saravanan, J. Trotte-Duhà, M. Dengler, J. Hahn, R. Hummels, J. Lübbecke, M. Rouault, L. Corim, A. Sutton, M. Jochum, and R. Perez, 2019: PIRATA: A sustained observing system for tropical Atlantic Climate research and forecasting. Earth Space Sci., 6, 577-616. https://doi.org/10.1029/2018EA000428.

Grodsky, A. S., J. A. Carton, C. Provost, J. Servain, J. A. Lorenzzetti, and M. J. McPhaden (2005), Tropical instability waves at 0N, 23W in the Atlantic: A case study using Pilot Research Moored Array in the Tropical Atlantic (PIRATA) mooring data, J. Geophys. Res., 110, C08010, doi:10.1029/2005JC002941.

Lee, T., G. Lagerloef, H.-Y. Kao, M. J. McPhaden, J. Willis, and M. M. Gierach, 2014: The influence of salinity on tropical Atlantic instability waves and eddies. J. Geophys. Res., 41, 7933–7941.

Heukamp, F.O., P. Brandt, M. Dengler, F.P. Tuchen, M.J. McPhaden, and J.N. Moum, 2022: Tropical instability waves and wind-forced cross-equatorial flow in the central Atlantic Ocean. Geophys. Res. Lett., https://doi.org/10.1029/2022GL099325.

Scannell, H.A. and M.J. McPhaden, 2018: Seasonal mixed layer temperature balance in the southeastern tropical Atlantic. J. Geophys. Res., 123, 5557-5570. https://doi.org/10.1029/2018JC014099.

Weisberg, R. H., and T. J. Weingartner (1988), Instability waves in the equatorial Atlantic Ocean, J. Phys. Oceanogr., 18, 1641– 1657.

Citation: https://doi.org/10.5194/egusphere-2022-1245-RC1
- AC2: 'Reply on RC1', Roy Dorgeless Ngakala, 02 Mar 2023
  
  We thank the anonymous reviewer #1 for these constructive comments to improve our manuscript. We also appreciate the recommended papers on the importance of tropical instability waves (TIWs) in the eastern equatorial Atlantic heat budget, which we found useful. In the attached file, we have tried to provide responses (blue text) to the reviewer's comments (black text).
  
  Citation: https://doi.org/10.5194/egusphere-2022-1245-AC2
RC2:
'Comment on egusphere-2022-1245', Anonymous Referee #2, 22 Dec 2022

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1245/egusphere-2022-1245-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2022-1245-RC2
- AC3: 'Reply on RC2', Roy Dorgeless Ngakala, 02 Mar 2023
  
  We thank anonymous reviewer #2 for his availability to review our manuscript and his suggestions will be taken into account. In the attached file, our responses and the reviewer's comments are represented by blue and black text, respectively.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1245-AC3
EC1:
'Comment on egusphere-2022-1245', Karen J. Heywood, 03 Jan 2023

The two reviewers have posted helpful and constructive reviews to improve the paper. I look forward to you posting your responses.

Citation: https://doi.org/10.5194/egusphere-2022-1245-EC1
- AC1: 'Reply on EC1', Roy Dorgeless Ngakala, 10 Jan 2023
  
  Dear Editor,
  I hereby request a deadline extension to respond to the reviewer's comments on OS preprint (egusphere-2022-1245).
  This is because most of the co-authors were on Christmas holiday and were unable to provide input to the responses provided.
  In addition, one of the analyses required by the reviewers was to be addressed by a co-author who is still on holiday.
  He will resume next week and we will therefore be able to submit the responses to the reviewer's comments.
  Therefore, I would like to ask for a deadline extension.
  I would appreciate it if you take my request into consideration
  Yours sincerely
  
  Roy
  
  Citation: https://doi.org/10.5194/egusphere-2022-1245-AC1

Peer review completion

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

AR by Roy Dorgeless Ngakala on behalf of the Authors (02 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Mar 2023) by Karen J. Heywood

RR by Anonymous Referee #1 (22 Mar 2023)

ED: Publish subject to minor revisions (review by editor) (22 Mar 2023) by Karen J. Heywood

AR by Roy Dorgeless Ngakala on behalf of the Authors (31 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (31 Mar 2023) by Karen J. Heywood

AR by Roy Dorgeless Ngakala on behalf of the Authors (07 Apr 2023) Author's response Manuscript

Short summary

Surface heat flux is the main driver of the heat budget in the Senegal, Angola, and Benguela regions but not in the equatorial region. In the Senegal and Benguela regions, freshwater flux governs the salt budget, while in equatorial and Angola regions, oceanic processes are the main drivers. Results from numerical simulation show the important role of mesoscale advection for temperature and salinity variations in the mixed layer. Nonlinear processes unresolved by observations play a key role.

Joint observation–model mixed-layer heat and salt budgets in the eastern tropical Atlantic

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