the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Dec 2023
Dakar Niño variability under global warming investigated by a high-resolution regionally coupled model
Abstract. In this study, we investigated the interannual variability of sea surface temperature (SST) along the northwest African coast and the strong Dakar Niño and Niña events, and their potential changes under the highest emission scenario RCP8.5 of global warming using a high-resolution regional coupled model. Our model accurately reproduces the SST seasonal cycle along the northwest African coast and its interannual variability in terms of amplitude, timing, and position of the maximum variability. Comparing the Dakar Niño variability between the 1980–2010 and 2069–2099 periods, we found that its variability intensifies under a warmer climate without changing its location and timing. The intensification is more pronounced during the Dakar Niñas (cold SST events) than during Niños (warm SST events) and the ocean temperature variability is connected more deeply with the Dakar Niño variability (vertical motion is more deeply correlated with Dakar Niño variability). The increase of Dakar Niño variability can be explained by the larger variability in meridional wind stresses, which is likely to be amplified in the future by enhanced land-sea thermal contrast and associated sea-level pressure anomalies elongated from the Iberian-Mediterranean area. In addition, the ocean temperature is warmed more effectively above 40m depth, where the temperature anomaly is maximum, i.e., the stratification is reinforced around 40m depth. This enhanced stratification may also lead to an increase in the amplitude of the Dakar Niño/Niña events.
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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.
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AC1: 'Reply on RC1', Shunya Koseki, 25 Mar 2024
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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), ROMp (middle), and ROMF (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 ROMp) and “Future” (ROMF) 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.
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