Dakar Niño under global warming investigated by a high-resolution regionally coupled model

Koseki, Shunya; Vázquez, Rubén; Cabos, William; Gutiérrez, Claudia; Sein, Dmitry V.; Bachèlery, Marie-Lou

doi:https://doi.org/10.5194/esd-15-1401-2024

Articles | Volume 15, issue 6

https://doi.org/10.5194/esd-15-1401-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esd-15-1401-2024

© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 6

Research article

|

08 Nov 2024

Research article |

| 08 Nov 2024

Dakar Niño under global warming investigated by a high-resolution regionally coupled model

Shunya Koseki, Rubén Vázquez, William Cabos, Claudia Gutiérrez, Dmitry V. Sein, and Marie-Lou Bachèlery

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Final revised paper (published on 08 Nov 2024)
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Preprint (discussion started on 01 Dec 2023)
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Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2494', Anonymous Referee #1, 02 Feb 2024

Review for “Dakar Niño variability under global warming investigated by a high resolution regionally coupled model” by Koseki et al.
The study by Koseki et al. investigates how sea surface temperature (SST) variability at the Senegalese-Mauritania coast associated with so called Dakar Niños might change in the future. Utilizing a regionally high-resolution coupled climate model they find that the Dakar Niño variability increases under the RCP8.5 scenario. This is explained by an increase in the wind variability and higher ocean stratification under global warming.
The manuscript is mostly well structured and written and provides interesting results on the future of SST variability in the Northeastern Tropical Atlantic. However, I find that it is sometimes hard to follow the presentation of the results and that the investigation of the processes needs some work. Also, there are some recent studies dealing with similar questions that should be taken into account. I summarize my major comments and list minor points and specific comments below.
Major comments:
A) Definition of Dakar Nino
It is not really clear to me what exactly this study regards as Dakar Niño variability. Is it all interannual SST variability in the region considered here (i.e. from 7ºN to 21ºN)? Everything related to the position of the front (line 37/38)? Or just SST anomalies occurring in the Dakar Niño Index box which is much more confined? Is the peak in SSTA variability around 20ºN in boreal summer also related to Dakar Niños? Please provide a definition and make sure to be consistent throughout the manuscript. Also, please indicate the Dakar Niño box in Figure 4.
B) Processes behind Dakar Niño variability
I believe that more analysis is needed on the processes by which Dakar Niños and Niñas are driven in the simulations and how they change in the future. For example, Oettli et al. (2016) argue that heat fluxes are important for the generation of Dakar Niños but they are not considered here. The same goes for changes in the depth of the mixed layer.
It is also not clear to me why stronger stratification should lead to higher variability. Stratification is also increasing in the equatorial Atlantic but there, the argument is that this hinders subsurface-surface coupling and thus leads to weaker variability.
It is further argued that correlation between the DNI and ocean temperatures at greater depth means stronger SST variability (line 189). Why would that be?
In the conclusions “stronger vertical velocity variability” (279) is mentioned. Where is this shown?
C) Literature to be taken into account
There are some recent studies dealing with the future of the variability in the Dakar Niño region that should be cited here:
Yang et al. (2021) find increasing variability in the Northeastern Tropical Atlantic under global warming. Chang et al. (2023) look at changes in temperature and wind patterns in eastern boundary upwelling regions in HighResMip simulations. Also, a study on the future of Benguela Niños was recently published (Prigent et al., 2023) and could be mentioned in line 142.
Minor points:
1) Model validation
While I like the comparisons shown in Fig. 2, they could be improved and further complemented with line plots. The axis labels are very small and the color scale of (e) to (h) is hard to interpret (this also applies to Fig. 7). I would also find it more instructive to show SST in ºC. In addition, one could show a line plot with SST and its standard deviation averaged over certain latitude bands overlaid for the different data sets and model simulations. This would facilitate an easier comparison in terms of timing and amplitude. I believe that it would probably show that the season of highest variability in the model is shifted towards later in the year (April instead of March) which makes the focus on March later in the analysis a bit questionable.
Further, in line 120, it is stated that the model has a warm bias. Where? What is the general pattern of SST difference to observations?
2) Seasonality of SST in the region
The seasonal cycle in SST shown in Figure 2 is mainly explained in terms of a meridional movement of the front driven by meridional currents. However, I believe that it is largely impacted by the upwelling season in the southern part of the region, i.e. cooler waters are upwelled to the surface in February to April.
Regarding the seasonal migration of the Mauritania current (line 105), is there a reference for this? Does it fit with the findings by Klenz et al. (2018)?
3) Role of remote forcing
It is stated (lines 42 to 44, also 116 to 118) that Benguela Niño events are stronger than Dakar Niños because of the additional role of remote forcing from the Equator. Is this something that has actually been shown (in this case, please provide a reference) or just an assumption of the authors?
4) Correlation with Dakar Niño index (Figure 4)
How do the correlations in Figure 4 change when they are based on April instead (see comment 1 above)? Also, I am not sure the propagating signal described is really propagating (associated with a Rossby wave). Couldn’t the SST anomaly just decrease first at the coast due to coastal processes and linger around for longer offshore?
Specific comments:
- At several instances, “reinforced” is used when probably “strengthened” or “increased” is meant (e.g. line 23, 206, 217, 226, 269,…)
- line 17/18 and 186 to 191: Here, it is not clear whether the correlation gets stronger or whether high correlation is extending deeper in the ocean. Please rephrase to make this clear.
- line 33: “feature” instead of “exhibit”
- line 41: “driven by”
- line 52: In contrast to what?
- line 53: “even in” instead of “until”
- line 58: “recently” instead of “timely”
- Figure 1: What is shown by the shading?
- line 105: What is “inversely” referring to? When are the trade winds relaxing?
- line 111: “second maximum” in terms of timing or location?
- line 116: “has larger” (without “more”)
- line 128: “focus region”
- line 138/139 “K” or “ºC” instead of “degree”
- line 149: ORA-S5 is a reanalysis product.
- line 152: Please rephrase this sentence.
- caption to Fig. 4: “January” (typo)
- line 173: “south of” instead of “below”
- line 177: What does “somewhat simulated well” mean? Please rephrase.
- line 178: “is correlated”, “SST anomalies develop”
- line 179: “compared to ERA5”
- line 181: Should be “S2”
- line 199: “1 standard deviation”
- line 284: “is” instead of “can be also”
References:
Chang, P., G. Xu, J. Kurian et al., 2023: Uncertain future of sustainable fisheries environment in eastern boundary upwelling zones under climate change, Comm. Earth & Environment, https://doi.org/10.1038/s43247-023-00681-0
Klenz, T., Dengler, M., and Brandt, P., 2018: Seasonal variability of the Mauritania Current and hydrography at 18°N. Journal of Geophysical Research: Oceans,123, 8122–8137. https://doi.org/10.1029/2018JC014264
Prigent, A., Imbol Koungue, R., Lübbecke, J.F. et al. Future weakening of southeastern tropical Atlantic Ocean interannual sea surface temperature variability in a global climate model. Clim Dyn (2023). https://doi.org/10.1007/s00382-023-07007-y
Yang, Y., L. Wu, Y. Guo et al., 2021: Greenhouse warming intensifies north tropical Atlantic climate variability, Sci. Adv. 2021;7:eabg9690

Citation: https://doi.org/10.5194/egusphere-2023-2494-RC1
- AC1: 'Reply on RC1', Shunya Koseki, 25 Mar 2024
  
  We greatly appreciate the reviewer for the very constructive comments. As we uploaded a file, we replied to all the comments point-by-point.
  In the attached file, you will find our responses to the questions in blue characters to facilitate distinction.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2494-AC1
RC2:
'Comment on egusphere-2023-2494', Anonymous Referee #2, 07 Feb 2024
Review of “Dakar Niño variability under global warming investigated by a high-resolution regionally coupled model”
by Koseki, S., Vázquez, R., Cabos, W., Gutiérrez, C., Sein, D. V., and Bachèlery, M.-L.
General comments
The aim of this work is to characterize the future of the coastal upwelling regions off the Senegalese coast, under the IPCC’ RCP8.5 pathway (i.e., highest emission scenario), using a high-resolution regionally-coupled model. This is an important topic, because the future of the upwelling regions around the world is still under investigation and subject to debates. In Benguela and California systems, a consensus for a positive trend in upwelling-favorable winds seems to exist (Sydeman et al., 2014), the signal is less clear for the Canary region. Particularly the response of marine ecosystems to climate change/global warming is still uncertain (e.g., Xiu et al., 2018). Also, the upwelling regions in eastern tropical oceans are known to be not well represented in climate models (Richter, 2015). So a better knowledge of their variability and underlying mechanisms is still needed.
However, the current study presents several issues, some major, some minor, that need to be addressed before publication. The detail of these issues is given below. I therefore recommend major revisions.
Major comments
In this section, the major issues are detailed.
Dakar Niño
This coastal Niño phenomenon is, like the ENSO or other coastal Niños, is a recurring climate pattern, characterized by sea-surface temperatures (SST) warmer by a few degrees than normal, with only a few occurrences in a decade. This is different from the definition given at L.37-38.
This is also different from the Dakar Niño Index (DNI) which depicts the temporal variability of the SST, averaged over the 21°–17°W, 9°–14°N region. In this perspective, the title could be changed to reflect either the exploration of the variability of the DNI under RCP8.5 or the future of Dakar Niño under RCP8.5. This is particularly true regarding Figs. 5 and 6. The former is using the DNI, while the latter is focusing on Dakar Niño and Dakar Niña.
The existence of a Dakar Niño (or Niña) is the result of complex land-sea-atmosphere interaction system. According to Oettli et al. (2016), the coastal, alongshore, wind variability, the mixed-layer depth anomaly, and the modulation of the mixed-layer temperature (mostly due to the shortwave radiation variations), are the necessary components to develop a Dakar Niño. The current work is mainly focusing on the coastal winds and some atmospheric variables (sea-level pressure, 2m temperature,…), forgetting about the other components of the dynamical system. It would be preferable, in this work, to also discuss about the evolution of the costal ocean mixed-layer, and its heat budget, under the RCP8.5. At L.257, there is a mention of the land-sea thermal contrast anomalies. It would be interesting to discuss how the contrast helps to maintain, or not, the favorable alongshore winds.
A time series of the DNI is also needed for the periods 1980-2010 and 2069-2099, to provide the reader with the number and intensity of the events. This is also particularly important to understand Fig. 4.
Target of the study
There is some confusions throughout the main text. The title indicates that this work is focusing on the Dakar Niño, which is a coastal, phenomenon with specific characteristics (see previous sub-section), but the Senegalese–Mauritania Frontal Zone (SMFZ) is also often referred, introducing some sort equivalence in the reader’s mind, between two different phenomena.
Also, this study discusses the intensity and the location changes of the Dakar Niño between present and future periods, but doesn’t tackle the frequency change. It would be informative to also present if, under RCP8.5, the number of Dakar Niño events is likely to increase/decrease.
The Bakun hypothesis (or Upwelling intensification hypothesis, Cropper et al., 2014) is introduced at L.235. This is an important topic to discuss when it comes to the future of tropical upwelling regions, because several studies have been dedicated to corroborate or contradict the Bakun hypothesis. See for example Oettli et al. (2021, p.255–256) for a discussion on this. I would recommend to better highlight how this study seems to corroborate the upwelling intensification hypothesis.
Clarity
The global structure of the text is not clear and needs to be rethought, because it is often difficult to understand what the authors are describing and putting emphasis on.
Throughout the text sometimes appears some vague statements which need to be clarified:
L.64-68: The apparent opposite conclusion on model resolution between Sylla et al. (2022) and Vázquez et al. (2022) should be better emphasized and explained, because of its implication for the current work.

L.158: What is the meaning of this anomaly pattern for the Dakar Niño?

L.188-189: “This deeper connection of ocean temperature in ROMF can be indicative of the stronger SST variability”. This is unclear what the authors are saying here. This needs to be clarified.

L. 228: “[...] indicating that the wind variability might be more relevant due to local effects”. This also is unclear. What are those local effects?

L.278-280: How? What would be the mechanism?

L.283-285: Again, how? What would be the mechanism?

Figs. 5 and 6: The mix between DNI and Niño/Niña events makes things difficult to follow.

L.292-293: This statement is not clear. What are the other climate modes? Please clarify.

Specific comments
L.25: The region of the Senegalese–Mauritania Frontal Zone (SMFZ) looks similar to the Senegalo-Mauritanian Upwelling System (SMUS) used in Sylla et al. (2019) or the Mauritania-Senegalese Upwelling Region (MSUR) used in Vázquez et al. (2023). Please clarify what is the SMFZ compared to SMUS and MSUR.
L.26: “[…] one of the most pronounced oceanic frontal zones generated along the eastern boundary current system”. Source?
L.28: Please remove the second left bracket.
L.36: Please remove the second left bracket.
L.43: “[…] stronger Benguela Niño events compared to Dakar Niño events”. Source? (Probably L.116-117).
L.47: Local or multi-fleet fishery? The former is certainly more sustainable, but also more sensitive than the latter. The latter has probably more impact on worldwide economy than the former. Please clarify.
L.64: “Sylla et al. (2022)” is missing in the references.
L.65: “Resolution” instead of “Resoliution”.
L.86: Please explain what RCP8.5 is by adding what is said at L.138 (Which then can be shorten as “Under RCP8.5”).
L.119-120: What is the source of the poor representation of coastal upwelling in ERA5?
L.120: “[…] and ERA5 has a warm bias (Vázquez et al., 2022).”
L.134: “The meridional SST gradient greater than 0.5K/100km is shown in blue”.
L.134: Why between 21° and 16°W when the DNI is defined between 21° and 17°W? Is it a typo?
L.135: “[…] respectively (bottom).”.
L.135: “deviation” instead of “devitation”.
L.136: “kelvin” instead of “Kelvin”.
L.143: March is the peak phase of the Dakar Niño.
L.148: “isotherm” instead of “of temperature”.
L.165: “21°–17°W, 9°–14°N”
L.167: “SST over 21°–17°W, 9°–14°N”.
L.168: How is the significance of the correlations evaluated. And for the wind stress, because there are two components (zonal and meridional), both can be significant, as well as only one over two. Does the figure only shows correlations when significant in both directions? Please clarify.
L.193: “section” instead of “seciton”.
L.196: “Niñas” instead of “Niña”.
L.210: “section” instead of “seciton”.
L.212: Is 16°W also a typo?
L.253-254: “Composite anomalies of the 2m temperature during Dakar Niño (left) and Niña (right) events in ERA5 (top), ROM_p (middle), and ROM_F (bottom) in March.”
L.277: Unclear what is the variability is referring to. Is it among warm (cold) events?. Is it in terms of number of warm (cold) events. Please rewrite and clarify.
L.324-524: The references doesn’t follow the Copernicus Publications guidelines. Please revise according to them.
Figure 3: In order to understand Fig.4, we need to know the mean state from January to May for the SST and the wind stress.
Figure 5: The land mask should be in a different color than 0 (gray for example, similar to the mask in Fig. 7)
Figure 6: Same as Fig. 5
Figure 7: Is the standard deviation calculated for all the months of March in “Present” (ERA5 and ROM_p) and “Future” (ROM_F) periods? If it’s the case, why not doing a composite with/without Dakar Niño/Niña?
References used in this review
Cropper, T. E., Hanna, E., and Bigg, G. R.: Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981–2012, Deep Sea Res. Part I Oceanogr. Res. Pap., 86, 94–111, https://doi.org/10.1016/j.dsr.2014.01.007, 2014.
Oettli, P., Morioka, Y., and Yamagata, T.: A regional climate mode discovered in the North Atlantic: Dakar Niño/Niña, Sci. Rep., 6, 18782, https://doi.org/10.1038/srep18782, 2016.
Oettli, P., Yuan, C., and Richter, I.: The other coastal Niño/Niña—the Benguela, California, and Dakar Niños/Niñas, in: Tropical and Extratropical Air-Sea Interactions, edited by: Behera, S. K., Elsevier, 237–266, https://doi.org/10.1016/B978-0-12-818156-0.00010-1, 2021.
Richter, I.: Climate model biases in the eastern tropical oceans: Causes, impacts and ways forward, WIREs Clim. Change, 6, 345–358, https://doi.org/10.1002/wcc.338, 2015.
Sydeman, W. J., García-Reyes, M., Schoeman, D. S., Rykaczewski, R. R., Thompson, S. A., Black, B. A., and Bograd, S. J.: Climate change and wind intensification in coastal upwelling ecosystems, Science, 345, 77–80, https://doi.org/10.1126/science.1251635, 2014.
Sylla, A., Mignot, J., Capet, X., and Gaye, A. T.: Weakening of the Senegalo–Mauritanian upwelling system under climate change, Clim Dyn, 53, 4447–4473, https://doi.org/10.1007/s00382-019-04797-y, 2019.
Vázquez, R., Parras-Berrocal, I. M., Koseki, S., Cabos, W., Sein, D. V., and Izquierdo, A.: Seasonality of coastal upwelling trends in the Mauritania-Senegalese region under RCP8.5 climate change scenario, Sci. Total Environ., 898, 166391, https://doi.org/10.1016/j.scitotenv.2023.166391, 2023.
Xiu, P., Chai, F., Curchitser, E. N., and Castruccio, F. S.: Future changes in coastal upwelling ecosystems with global warming: The case of the California Current System, Sci Rep, 8, 1–9, https://doi.org/10.1038/s41598-018-21247-7, 2018.
Citation: https://doi.org/10.5194/egusphere-2023-2494-RC2
- AC2: 'Reply on RC2', Shunya Koseki, 25 Mar 2024
  
  We greatly appreciate the reviewer for the very constructive comments. As we uploaded a file, we replied to all the comments point-by-point.
  In the attached file, you will find our responses to the questions in blue characters to facilitate distinction.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2494-AC2

Peer review completion

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

ED: Reconsider after major revisions (28 Mar 2024) by Ben Kravitz

AR by Shunya Koseki on behalf of the Authors (10 Jun 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Jun 2024) by Ben Kravitz

RR by Anonymous Referee #1 (27 Jun 2024)

Suggestions for revision or reasons for rejection

Review for revised version of “Dakar Niño under global warming investigated by a high-resolution regionally coupled model” by Koseki et al.

I appreciate the authors’ effort in addressing the issues raised in my previous review. I find the revised manuscript to be improved, but there are still a number of issues that need to be taken care of before publication, in particular regarding the clarity of the presentation and the processes driving Dakar Niños.

Major comments:

A) I find it still very hard to follow the manuscript and its argumentation. Please make sure that is always clear to the reader why certain analyses are done and what is shown in the plots before getting to what the results are. As an example, in the paragraph starting from line 190, it would be instructive to first state that in order to describe the evolution of Dakar Niños and Niñas and to evaluate how well it is simulated in the model and how it might change in the future (if that is the purpose of the figure - I am actually not sure!), correlation of the Dakar Niño index with the large scale wind stress and SST field are performed. Then you can get to where these correlations are high and what this means.
As another example, for Fig. 6, there is an abrupt transition from “climatology of SST and wind stress is given” to “In ROMP, the significant positive correlation concentrates between the surface and 40m depth and decreases to 100m depth, which is about 0.4 (Fig. 6a).” Please state first what is looked at in Fig. 6 and why before getting to what one can see, i.e. that the Dakar Niño index is well correlated with the ocean temperature over the upper 40m of the water column.

B) Related to point A, the discussion of the correlation figures is rather confusing. Please note that correlations can be high or low and variables can be highly/strongly or weakly correlated, but not “deeply”.
In particular for Fig. 6, I cannot make sense of what is suggested here. The plots indicate that there are higher subsurface temperatures during Dakar Niños and weaker upwelling. To me, this implies that, in the model, weaker winds lead to reduced upwelling and turbulent mixing, warming the subsurface and subsequently the surface ocean. This process appears to get more important in the future. It does not mean that “Dakar Niños would influence deeper ocean in the future” (as stated in lines 207/208).

C) Again related to point A, the newly added heat budget calculation appears rather disconnected from the rest of the manuscript. While in lines 235/236, it is stated that “According to Oettli et al. (2016), the Dakar Niño is mainly driven by changes in alongshore local surface wind”, this section (from line 294) starts with stating that “According to Oettli et al. (2016), surface heat flux is responsible for generating Dakar Niño events”. This needs at least some more context and explanation as the previous section has not mentioned whether the wind is important because of its impact on latent heat flux. By showing vertical velocities and subsurface temperatures, it is rather implied that it is the effect of wind changes on upwelling that are important.
Surprisingly, the equation used for the heat budget does not even contain a term for the surface heat fluxes that, however, show up in Fig. 11 without ever being introduced before. This Figure then suggests that it is the horizontal advection that is mainly driving Dakar Niños and Niños, another process that has not really been introduced so far.

To address these major comments, I would advice the authors to first show that Dakar Niños are related to the wind field (Fig. 5) and that the wind variability is intensifying in the scenario simulation (Fig. 8), explaining the increase in SST variability. Then there could be a section on the different mechanisms that are related to wind variability, namely upwelling, advection and latent heat flux and how each of them is changing.

Minor points:
1) I don’t understand what is meant by lines 17/18 in the abstract (see also major comment B above).

2) Instead of the current Figure 1, I would suggest to show a map for the coupled domain with SST standard deviation in shading and mean SST overlaid in contours as well as the box indicating the Dakar Niño region. Then this area of high SST variability can be referred to early in the manuscript.

3) Related to major comment A above, the transition in line 160 is not very clear. Please state explicitly that you are now referring to future changes.

4) In the discussion of the Dakar Niño index time series (lines 172 to 177), please also refer to the change in standard deviation visible in Fig. S1.

Specific comments:
- line 21: “more important” instead of “more explainable”
- line 25: “Climatologically” instead of “”From a climatological aspect”
- line 29: “northern boundary” instead of “northern end”
- line 30: “joins” (typo), “North Equatorial Current”
- line 59: “studies” or “analysis” instead of “surveys”
- line 59: add recent study on future evolution of Benguela Niños by Prigent et al., (2024)
- line 76: “configuration … is” or “configurations … are”
- line 77: “… atmospheric component, namely the limited-area…”
- line 78: “… and a global oceanic component, which is the Max-Planck Institute Ocean Model…”
- line 85: “this” instead of “those”
- line 99: “shading shows” instead of “shade show” Is the topography height actually relevant for the study? Maybe it would be more instructive to show SST instead.
- line 110/111: Not sure what is meant by “The steep SST gradient is consistent with the SST seasonal cycle”
- line 126: What is meant by “in a whole year”? “during the whole year” or “for all calendar months” maybe?
- line 136: “absolute” (typo)
- line 137: “meridional” (typo)
- line 141/142: This is not a full sentence and it is not clear what is meant. Please rephrase.
- line 146: “focus on”
- line 148: Please remove “for fairness comparison with observations”
- line 149: “averaged over 9ºN - 14ºN”
- line 152: “The yellow box indicates the Dakar Niño index region.”
- line 172: “in ERA5, there are” instead of “ERA5 counts”
- line 197: Part of the sentence seems to be missing.
- line 198: It’s the connection between the Dakar Niño index and the wind field that is not well simulated.
- line 204: Why is the climatology shown here? What is the connection to the preceding or following text?
- line 253/254: “more intensely than the ocean”
- line 266: “In the climatology”
- line 297 and in many following sentences: “ocean mixed layer” instead of “ocean mixing layer”
- line 338: “in contrast to” instead of “in discrepancy against”

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RR by Anonymous Referee #2 (04 Jul 2024)

Suggestions for revision or reasons for rejection

I am not satisfied with the revision and the responses provided by the authors. Particularly, the authors continue to only focus on the alongshore wind, while the mechanism behind the Dakar Niño/Niña is more complex, as already highlighted in the review.

To the credit of the authors, they have performed some heat budget (as suggested in the review) but the results are quite different from those found in Oettli et al. (2016), which is fine. But the differences are quite drastic and seem to invalidate previous studies. This must be thoroughly discussed in the manuscript. Also, the heat budget is not performed on observation/reanalysis. This should be done and shown at least as supporting information, to compare with ROMp heat budget (Figure 11).

Also, the Figure 4 is interesting, but not really discussed. We can see that the years of Dakar Niño/Niña are more or less the same than in Oettli et al. (2016), but using ERA5. Which is an important result. But with ROMp, the results are different. A few years of observed Dakar Niño/Niña are detected (1985, 2003 for example). But there are also important discrepancies that must be thoroughly discussed. What explains the strong 1999 Dakar Niño in ROMp, when it was a strong Dakar Niña? Same applies to 2009. Also, 1988 and 1989 were neutral, but ROMp simulated 2 consecutive Dakar Niños. Finally there are 6 consecutive Dakar Niñas between 2002 and 2007. This doesn't argue in favor of ROM.

It should be noted thath there are a lot of issues with the citations throughout the text (i.e, open parenthesis not correctly placed). This makes the text difficult to read.

I therefore recommend major revisions.

Some comments (non-exhaustive list):
L.145: "To assess the Dakar Niño...". Not sure what the authors mean here. Is it to assess the existence of the phenomenon? To assess the simulation of Dakar Niño in ROM? Please elaborate.
L.235: "[...] the Dakar Nino is driven mainly by changes in alongshore local surface wind". No, it is not, as already said in the review.
L.312-315: Does it mean that ROM generates abnormal coastal SST warming/cooling by horizontal advection? How about ERA5 (hence the mandatory heat budget). Could this explain the differences between ERA5 and ROMp described above?
L.287: "The SLP anomaly gradient runs along..." The gradient is across. It is part of the generation process.

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ED: Reconsider after major revisions (09 Jul 2024) by Ben Kravitz

AR by Shunya Koseki on behalf of the Authors (17 Aug 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (21 Aug 2024) by Ben Kravitz

RR by Anonymous Referee #2 (04 Sep 2024)

RR by Anonymous Referee #1 (04 Sep 2024)

Suggestions for revision or reasons for rejection

Review for revised version of “Dakar Niño under global warming investigated
by a high-resolution regionally coupled model” by Koseki et al.

After the second revision, I find the manuscript to be substantially improved, especially regarding the readability. However, the discussion of the relevant processes still needs more work.

Major comments:

There are several processes for the generation of Dakar Niños and Niñas discussed in the paper, namely local winds, upwelling (although this is somehow only touched on), surface heat fluxes and horizontal advection. It needs to become clear that and how these processes are connected. This starts as early as lines 40/41 in the introduction and continues throughout the manuscript.
Changes in the local winds can drive changes in upwelling, horizontal advection, latent heat loss and mixed layer depth but there are also components not (directly) related to local winds such as temperature gradients and shortwave radiation. At the moment, it is not clear to me what the main role of the winds is for generating SST anomalies and which part of the net surface heat flux is actually important.

Specifically, I would suggest to start the heat budget analysis (Section 4.2) by saying that
a) variations in local winds are important for the generation of Dakar Niños and Niños (as shown in the previous section)
b) these wind variations can be connected to a number of processes, namely upwelling, ocean currents, and latent heat loss
c) that you perform a mixed layer heat budget analysis to determine which of these is most important
It is a bit confusing to start the section by pointing out the importance of heat fluxes as found by Oettli et al. (2016).

Given the high correspondence between SSTs and subsurface temperatures over at least the upper 100m of the water column during Dakar Niño and Niña events (Fig. 6, 7), I find it very hard to believe that vertical processes are not important for the generation of the events as stated later with respect to the heat budget analysis. I rather suspect that a large part of the vertical term (especially mixing) is buried in the residual which is probably quite large. How much of dT/dt do the advection and heat flux terms actually account for, i.e. how large is the residual term in the heat budget?

I find the discussion of the heat budget analysis (Section 4.2) partly confusing and I am not sure if there maybe was a mix-up of the legends. If the black lines show ROMp, horizontal advection appears much more important than the surface heat fluxes (in contrast to what is stated in line 330). The black lines also show a stronger heat flux damping in April than the grey line (in contrast to what is stated in lines 358/359).

Also the components of the net surface heat flux need to be looked at individually. If shortwave radiation would be dominant (as indicated in line 336), how does this fit with the importance of the local winds?

Minor points:

a) Please indicate units in Figure 1.
b) In lines 107 to 110 the seasonal migration of the SST front is described as a result of the seasonal cycle of the Canary Current and changes in the location of upwelling. Doesn’t it also reflect the seasonality of the upwelling strength?
c) CMIP models are coupled climate models, not Earth system models which typically include more components such as biology (e.g. line 134)
d) In Figure S2(b) there is some weird color shading at the shelf.
e) In Figure 4, please use the same y-axis for all subplots.
f) I noticed some curious clustering of events in ROMp (Fig. 4b) with three consecutive Dakar Niños followed by five consecutive Dakar Niñas. Any idea what this might be related to?
g) Related to my major comments above, the relation between upwelling, subsurface temperatures and SSTs needs to be explained before getting to Figure 5.
h) Somewhere in the discussion of Fig. 5, please state that that this means that Dakar Niños are related to weakened Northeasterly trade winds.
i) The word “penetrates” in line 234 suggests that the warming comes from the surface and is transported downward. Is this the case? It could also be the other way around - a subsurface anomaly that is impacting the surface by vertical advection and mixing.
j) To compare the heat budget analysis from the model to ORA-S5 reanalysis, vertical velocities can be calculated from the divergence of the horizontal velocity field (as done in many studies).
k) Explain how variations in mixed layer depth are related to the importance of surface heat fluxes (line 356/357).
l) In the Discussion, Chang et al. (2023) is cited. Please comment on how their results align with what you find.

Specific comments:
- line 20: “wind stress, which is”
- line 29: “The cold water of the southward flowing Canary Current and the…”
- line 86: There is no red rectangular in Fig. 1.
- line 101: “panel”
- line 146: “identical” instead of “identically”
- line 147: Please also refer to Fig. S1.
- line 149: “enhancement” instead of “reinforcement”
- line 163: no comma after especially
- line 171/172: either “we investigate” or “are investigated”, not both
- line 176: “agrees well” or “is consistent with”
- line 177/178: Please specify the length of the time series again here.
- line 195: “using a reanalysis product” (maybe specify which product)
- line 195: “Dakar Niños are…”
- line 197: Please rephrase, e.g. “In January, SST anomalies from ERA5 averaged over the Dakar Niño box are positively correlated with SSTs along the west African coast as well as southwesterly surface wind anomalies. The correlation strengthens in March which is the peak of the event.”
- line 200: “significant” or “pronounced” instead of “more dominant”
- line 201, 210: “The area positively correlated with SST shifts westward…”
- line 214: A word is missing after “coastal”.
- line 218/219: “decreases to 0.4 at 100m depth (Fig. 6a).
- line 219: “deeper in the water column” instead of “more deeply”
- line 248: The increase from ROMp to ROMf is actually strongest in April.
- line 259: “In contrast” instead of “Inversely”
- line 260: I would say that it actually decreases.
- line 274: “becomes” (typo)
- line 275: “of the cool anomaly”
- line 276: “the land-surface”
- line 276 and 278: “towards the west” instead of “more westward”
- line 279: A word is missing after “much”
- line 289: “can also be seen”
- line 290: “the SLP anomalies show”
- line 293: “in particular in the case of…”
- line 311: “investigate” or “examine” instead of “question”
- line 314/315: either "we considered" or "is estimated", not both
- line 319: “the bottom of the ocean mixed layer”
- line 320: “and just below”
- line 321: “Q is the net…”
- line 324: “missing” instead of “missed”
- line 347: It’s not clear what “almost identical” is referring to. Present and future? Please specify.
- line 349/350: What does “remains secondary” mean? Does this refer to the future simulation?
- line 357/358: What is meant by “more persisting”?
- line 364: “an intensification of interannual SST variability in the…”
- line 365: “increase in the amplitude of…”
- line 367: “is in contrast”
- line 369: “coastal” instead of “coast”
- line 381: A part of the sentence seems to be missing.
- line 385: “modes such as”
- line 389: please rephrase “will be desired”
- line 399: I guess “performed” is meant (instead of demonstrated)

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ED: Publish subject to minor revisions (review by editor) (04 Sep 2024) by Ben Kravitz

AR by Shunya Koseki on behalf of the Authors (11 Sep 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (17 Sep 2024) by Ben Kravitz

AR by Shunya Koseki on behalf of the Authors (18 Sep 2024) Manuscript

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Short summary

Using a high-resolution regionally coupled model, we suggest that Dakar Niño variability will be reinforced under the Representative Concentration Pathway (RCP) 8.5 scenario. This may be induced by intensified surface heat flux anomalies and, secondarily, by anomalies in horizontal and vertical advection. Increased sea surface temperature (SST) variability can be associated with stronger wind variability, attributed to amplified surface temperature anomalies between ocean and land.