the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2024
Interannual variability of Sea Surface Salinity in North-Eastern Tropical Atlantic: influence of freshwater fluxes
Abstract. In tropical regions, the fresh water flux entering the ocean originates primarily from precipitation and, to a lesser extent when considering basin scale averages, from continental rivers. Nevertheless, at regional scale, river flows can have a significant impact on the surface ocean dynamics. Riverine fresh water modifies salinity, and therefore density, stratification and circulation. With its particular coastline, relatively high cumulative river discharge, and the vicinity of Inter Tropical Convergence Zone (ITCZ), the eastern Southern North Tropical Atlantic (e-SNTA) region off Northwest Africa is a particularly interesting location to study the linkage between precipitations, river outflows and Sea Surface Salinity (SSS). Here we focus on the regional e-SNTA SSS seasonal cycle and interannual variability. We quantify the impact of river runoff and precipitation on SSS by means of regional simulations forced by different interannual and climatological river runoffs and precipitation products. The simulated SSS are compared with the Climate Change Initiative (CCI) SSS, in situ SSS from Argo, ships and a coastal mooring, and the GLORYS reanalysis SSS. The analysis of the salinity balance in the mixed layer is conducted to explore the dynamics influencing the SSS variability. Overall, the simulations reproduce well the seasonal cycle and interannual variability despite a positive mean model bias north of 15N. The seasonal cycle is impacted by the phasing of the different runoff products. The mixed layer SSS decrease during the rainy season is mainly driven by precipitation followed by runoff by means of horizontal advection and partly compensated by vertical mixing. In terms of interannual anomalies, river runoffs have a more direct impact on SSS than precipitation. This study highlights the importance of properly constraining river runoff and precipitation to simulate realistic SSS, and the importance of observing SSS in coastal regions to validate such constraints.
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RC1: 'Comment on egusphere-2024-818', Anonymous Referee #1, 03 May 2024
Review for “Interannual variability of Sea Surface Salinity in North-Eastern Tropical Atlantic: influence of freshwater fluxes” by Thouvenin-Masson et al.
General comments:
This study investigates seasonal and interannual variations of sea surface salinity (SSS) in the North-Eastern Tropical Atlantic by means of several observational products and high-resolution regional model simulations forced by different runoff and precipitation data sets. It provides a thorough comparison between these different products and highlights how the choice in forcing data sets can impact the simulation of surface salinity in ocean models.
The manuscript is generally well written and provides a number of interesting insights into the factors impacting surface salinity in this region. I find, however, that it reads rather technical and that the value of the study lies mainly in exploring differences between various precipitation and runoff data sets and their impact on simulations of salinity while the dynamical understanding of the salinity variability remains rather vague.
Specific comments:
Major:
1) I am missing a bit more of a motivation that is then revisited in the conclusions. Why are interannual variations in salinity important?
2) I find it very hard to bring together the first part of the results section (3.1) with the corresponding figure in the Appendix. Instead of just referring to Appendix B, it would be helpful to refer to specific subplots and lines (something like “red line in Fig. B1(a)”). Also, the legend entries are hard to interpret and don’t seem to always match the figure caption.
3) While the case studies of years with strong robust SSS anomalies in section 3.3 is very interesting, the 2018 case remains rather inconclusive. It didn’t become clear to me what actually caused this anomaly.
4) One of the main conclusions of the study is that in the SSS budgets, the precipitation term is largely compensated by the entrainment term. What drives this compensation, i.e. why does entrainment react to the surface freshwater input ?
Minor:
a) I would suggest to reword the title to “Influence of Freshwater Fluxes on the Interannual Variability of Sea Surface Salinity in the North-Eastern Tropical Atlantic”
b) The region considered here can just be called “North-Eastern Tropical Atlantic” as in the title. There is no need to add an extra “southern” (line 13, 50 and elsewhere).
c) As there are several units for salinity (psu, pss, g/kg,…) it would be good to comment on the unit used here.
d) Please specify the time period of all the used data sets.
e) Section 2.2.4: I guess the model uses more than one baroclinic mode.
f) In line 301, it should probably read “deeper” instead of “thinner mixed layer”?
g) In Figure 6, it would be helpful to also show SSS anomalies.
h) Looking at Figure 5, I wouldn’t say that that interannual variations are ”very consistent” between model simulations and observations.
i) There is a huge number of subsections, and I believe some of them could be merged. This applies to section 4.3 and 4.5. in particular.
Technical corrections:
- line 28: As many waves are wind-forced themselves, waves shouldn’t be lumped together with wind as a forcing.
- line 30: Not sure what is meant by “exogenous” here.
- line 31: “they lower the density” instead of “they make the density decrease”
- line 139: “August” (with capital A)
- line 278: “linked to the salinity budget”
- line 298: I am not sure “attenuates” is a good expression in this context.
- line 377: “band-pass filtered” instead of “band-passed”
- line 413: “lower magnitude”
- line 456: There are no brown lines in Figure 7.
- line 553: It is not clear here whether “maximum difference” refers to the seasonal range or difference between products.
Citation: https://doi.org/10.5194/egusphere-2024-818-RC1 -
RC2: 'Comment on egusphere-2024-818', Anonymous Referee #2, 29 May 2024
Review of Interannual variability of Sea Surface Salinity in North-Eastern Tropical Atlantic: influence of freshwater fluxes by Thouvenin-Masson et al.
This study uses a combination of model data along with reanalysis, in situ and satellite observations to understand the seasonal and interannual variability of sea surface salinity (SSS) along the Senegalese coast in the north eastern tropical Atlantic Ocean. Sensitivity runs from CROCO model forced with different precipitation and river runoff datasets are analyzed to infer the impact of different model forcings on the seasonal and interannual variations of SSS in the region. A detailed description of the model data validation against the in situ, satellite and reanalysis SSS is provided. The study finds that the modelled interannual SSS variability off the Senegalese coast is more sensitive to river runoff forcing rather than precipitation. The seasonal cycle in SSS however remains unaffected by the different model forcings of precipitation and river runoff.
The manuscript is generally well written with decent quality figures. However, the manuscript needs some re-organization with more clear captions for the figures including the ones in the Appendices. The novelty of this study lies in exploring the impact of different model forcings on the e-NTA coastal SSS rather than analysis of the processes contributing to the SSS variability. This needs to be highlighted and stated in the Introduction section clearly. The manuscript may be considered for publication after the authors have addressed the major and minor comments listed below.
Major comments:
- Motivation and the main objectives of the study need to be clearly stated towards the end of Introduction section. The focus of the study is on understanding the impact of different types of river runoff and precipitation model forcings on the seasonal and interannual variability of coastal SSS in e-NTA ocean. The discussion related to processes impacting the SSS variability on seasonal and interannual timescales using salt balance seems very descriptive and lacks physical understanding of the processes.
- There are too many subsections which can be merged (especially in sections 2 and 4). The discussion related to salt balance figures in the appendix is vague and not easy to understand. It was really difficult to go back and forth from the appendix to main article while reading the salt balance part. I suggest moving the salt balance figures in appendices B and C to the main article and the model validation plots to the appendix.
- All figures’ captions need to be written more clearly. The labels are not captioned in a chronological order. For example, Figure 6 caption includes text related to panels (a, d, g), (b,e,h), (c,f,i). It was difficult to navigate through the panels while reading the caption. Also add product name and variable as text inside each panel (‘CROCO SSS’ or ‘CCI SSS’) to make it easy for reader to understand what is plotted. This applies for other figures as well.
- The discussion related to salt balances in Figure 6 for the 2011, 2015 and 2018 episodes is not clear. In 2011, the positive SSS anomaly is attributed to advection but the forcing term also shows the same sign and has magnitude comparable to the advection term (Fig. 6a). For 2018 positive SSS anomaly case, the rate term is negative (Fig. 6g). This needs to be checked. The negative entrainment (or residual) term in Fig.6a,g doesn’t make physical sense as you would expect SSS to increase if there is entrainment of deeper saltier water to the surface.
- In Fig. 5, the 2010 negative SSS anomaly event is interesting. This event could also be analyzed in addition to the 2011, 2015 and 2018 events, if that’s easy. Also, can you comment on why the freshwater forcing terms estimated from GLOFAS and ISBA have huge differences in 2010, 2017 and 2018 (Fig. 5c)?
- Figure 7 needs modification. There are no brown lines plotted in the figure (Line 456). For each case study, can you add spatial plots of SSS, currents with box regions marked for e-NTA and south of e-NTA? Select the period during which you observe the advection of freshwater from the southern region to the north. How is the lag determined? Is the correlation coefficient maximum at this lag period? Include a plot of the correlation coefficient as function of lag in appendix if possible.
Minor comments:
- The title needs to be modified to make it relevant to the main results presented in the study.
- Eastern Southern North Tropical Atlantic (e-SNTA) is confusing. I suggest changing it to east northern tropical Atlantic (e-NTA).
- Mark the 2011, 2015 and 2018 periods in Figure 7 as well.
- Cite the appendix figure number instead of just saying Appendix in the main article. For example, Fig. D.1 instead of Appendix D in line 591. Same applies elsewhere.
- Font size of axes labels and legend in Fig. 6, Fig. B.1 needs to be increased.
- Remove the label for zero line in the legend of figures 5 and 6.
Line 13 – “relatively high cumulative river discharge” – relative to what?
Line 14 – precipitations - precipitation
Line 28 – Forcing does not create mixed layer but impacts the mixed layer depth and dynamics.
Line 30 – What does flows exogenous to ocean mean?
Citation: https://doi.org/10.5194/egusphere-2024-818-RC2 -
RC3: 'Comment on egusphere-2024-818', Anonymous Referee #3, 05 Jun 2024
I agree with the two previous reviewers that this manuscript is more about runoff products validation. Because for interannual variability of mixed layer salinity in such a big regions, the datasets (observations & model) cited in this manuscript are good enough. There's no need for such complex high resolution sets of simulations. Therefore, the title, introduction, objectives, methodology needs to be reviewed substantially.
The study region needs better reasoning. Currently the dashed black box in Figure 1 does not include the impacts of the whole merged catchment.
The datasets description lacks important specifications such as data period, temporal and spatial resolution.
The figures in the appendix should be included in a separated supplement document with increasing numbering order.
Citation: https://doi.org/10.5194/egusphere-2024-818-RC3
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