the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Multidecadal sea level rise hiatus in the tropical Atlantic margin off northwest Africa
Abstract. Satellite and reanalysis data sets are used to find and explain the drivers of a multidecadal sea level rise hiatus in the tropical North Atlantic margin off northwest Africa. The study domain sea level was rising as far back as 1986, and the hiatus began around 2010 and stopped in 2019. Mean sea level anomaly during a rising period (1996–2004) was compared to the hiatus period (2010–2018). Results show that domain-wide seawater expansion owing to shifts in density structure (steric shifts) contributed 74 % of the multidecadal sea level shift and increase in mass contributed 22 %. There are, however, regional differences in the steric and mass shift patterns. In the northern subdomain, the steric shift is dominated by salinity-driven (halosteric) expansion, whereas in the southern subdomain the steric shift is dominated by temperature-driven (thermosteric) expansion. Stronger anticyclonic circulation shift in the Guinea Dome, a permanent upwelling region where isotherms are displaced upwards, and the necessary adjustment of horizontal flow toward this anticyclonic sink, enable accumulation of low-salinity water in the northern subdomain and precipitation in the southern subdomain. The low-salinity water influx to the northern subdomain is linked to a shift in the southward-flowing Canary Current. This current was freshened by subpolar North Atlantic waters that reached the northwest African coast via two pathways: an open ocean path that is consistent with the Azores current, and a Western Europe coastal ocean path. These results highlight a multidecadal linkage between subpolar salinity anomalies and tropical sea level anomalies in the North Atlantic, with a transit period of about 5.5 years.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2546', Anonymous Referee #1, 06 Aug 2025
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AC1: 'Reply on RC1', Hamed Ibrahim, 19 Sep 2025
Dear Reviewer,
Thank you for reviewing our manuscript and for providing us with very insightful comments. We have responded to all the comments as documented in the attached pdf file.
Regards,
Hamed,
- AC4: 'Reply on AC1', Hamed Ibrahim, 19 Sep 2025
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AC1: 'Reply on RC1', Hamed Ibrahim, 19 Sep 2025
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RC2: 'Comment on egusphere-2025-2546', Anonymous Referee #2, 08 Aug 2025
This study provides a valuable analysis of the 2010–2018 sea level rise hiatus off northwest Africa using satellite/reanalysis data (1993–2018). The identification of steric dominance (74%) and subpolar-tropical salinity linkages is novel and impactful. Methodological rigor and validation with independent datasets are key strengths. However, clarification of physical mechanisms and data limitations is needed for full acceptance. I recommend acceptance after minor revisions.
- Eq. 6 oversimplifies vertical-horizontal coupling. Could you show decadal trends of vertical velocity (w) in the dome core using ORAS5 data?
- Please discuss how anticyclonic circulation modulates downwelling against background upwelling in more detail.
- While mentioned in Section 2.2, error quantification is missing. Could the authors add error bars in Fig. 4?
- The 2010–2018 hiatus (Fig. 2a) coincides with subdomain A freshening (Fig. 7a2) but the causal sequencing is unclear. Please add lead-lag correlation analysis between subpolar salinity and SLA.
Citation: https://doi.org/10.5194/egusphere-2025-2546-RC2 -
AC2: 'Reply on RC2', Hamed Ibrahim, 19 Sep 2025
Dear Reviewer,
Thank you for reviewing our manuscript and for providing insightful comments that have improved our work. We have responded to all the comments as documented in the attached pdf file.
Regards,
Hamed
- AC3: 'Reply on AC2', Hamed Ibrahim, 19 Sep 2025
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- 1
This manuscript aims at understanding the causes of the multidecadal sea level rise hiatus that occurred in 2010-2019 in the eastern north tropical Atlantic margin off northwest Africa. To do so, it mainly uses the ocean reanalysis ORAS5, performs an empirical orthogonal function analysis to define subdomains, and decomposes the sea level anomaly into its different components.
This manuscript is interesting, of good scientific quality, well structured, and well-written making it easy to follow the stream of thoughts. It addresses the scientific question of better understanding the drivers of sea level variability in the margin off northwest Africa and how it could be linked to remote processes.
I have two major comments and a few minor remarks which I think should be addressed before publication.
Major comment:
My first major comment concerns the scientific question being asked and which is not aligned with the analyses and results. It seems to me that the analyses presented do not answer the question aimed by the authors (specified lines 43-45) which is to find why the sea level stopped rising over period 2. Instead, by comparing the multi-year mean sea level and its components between the two periods, the authors answer a different scientific question which is: Why is the sea level higher during period 2 than during period 1? The concluding remarks section and the results summarized in the abstract indeed highlight the causes of the higher SLA during period 2 compared to period 1: mostly a halosteric expansion in subdomain A and a thermosteric expansion in subdomain B. The mass of the domain is also greater during period 2 than during period 1 mostly because of net precipitation in subdomain B and a net inflow of freshwater in subdomain A.
In my understanding, the analyses that would have answered the question asked by the authors (which is: why the sea level stopped rising during period 2?) would have been to study separately the two periods and their drivers and then compare them. For example, for the whole domain, the positive sea level trend during period one (fig 4a) seems to be due to a positive steric trend (fig 5a) and a positive manometric trend (fig 4b). On the other hand, during period 2, the stabilisation of the SLA (fig 4a) seems to be due to a positive manometric trend (fig 4b) being counterbalanced by a negative steric trend (fig 5a) which is itself due to a negative thermosteric trend (fig 8a). Therefore, we can interpret the pausing in sea level rise during period 2 as a result of a cooling of the domain.
I suggest that the author adjust their narrative so that the results match the scientific question being asked by either modifying the scientific question or the results presented.
My second major comment is about the potential physical linkage between subdomain A salinity and salinity in the western subpolar north atlantic. In my opinion, the manuscript does not provide enough evidence of the existence of the path 1 and path 2 and the westward flow connecting path 2 and 1. It is not clear to me, to what extent the paths 1 and 2 are already known (please provide citation if so) or if they are an original finding of the authors. These paths could be valid but should be presented with more care (as hypotheses or as potential connections) if they are not demonstrated more convincingly. It would be possible to prove a direct link for example by using water parcels lagrangian backwards tracking in time with particles released in the region of interest.
Minor comments:
Throughout the whole manuscript:
Usage of the word ‘upward’. I found a bit confusing the usage of ‘upward’ to refer to the positive shift in sea level between the two periods. The instinctive understanding of ‘upward’ usually refers to something moved higher in depth (it is used as such lines 9, 29, and 30). I recommend replacing it by ‘positive’ when referring to a shift when values are greater during period 2 than period 1.
Usage of word ‘shift’: This word is overly used and sometimes its meaning is not clear. I suggest replacing it with words like ‘change’, ‘increase’, ‘decrease’ etc. when relevant.
The word ‘hiatus’ is used very early in the manuscript but is never really defined. It is only line 45 than the reader guess than it means ‘pause in the sea level rise’. I recommend defining it early in the introduction. In addition, if the authors decide to modify the scientific question of the manuscript following my major comment, I recommend to not use the word ‘hiatus’ at all as it can be easily replaced by ‘pausing in sea level rise’ or other simple words.
All figures showing time series: It would greatly help the reader to highlight period 1 and 2 on all time series either by lightly color-shading the background or delimiting them using vertical lines.
line 5: The steric and mass contributions do not sum up to 100% which can be confusing as the reader usually expects it to do so. The reader later understands why (table A1) but I suggest clarifying this in the abstract.
Line 20-22: please add a citation to support this sentence.
Line 30: ‘Fig 10’. It seems that the figure number is incorrect. Please verify.
Line 35: ‘temperature shifted upward’. Does the author mean ‘northward’? or increase in temperature? Please clarify.
Line 81-84: It is not clear to me how the vertical land displacement is not taken into account in the study. Is it simply neglected? Is it included in the ORAS5 reanalysis or not? How does this impact the comparison with the altimetry data and grace data?
Typos and misspelling:
First paragraph: spaces are missing in units presentation. This would increase readability.
Line 83: tide gauge is misspelled ‘guage’.
Line 162: ‘gauge’ river is misspelled.
Figure 11: ‘and’ is duplicated in the caption.
Line 379: ‘here’ to replace by ‘there’
Methods:
Line 102: it is possible to simplify the sentence by removing ‘taking the difference between two satellite measurements (i.e.,’.
Line 111: replace ‘bathymetry’ by ‘seabed’ or ‘ ocean floor’
Line 153: The agreement is good between ORAS5 G and Grace data for most of the time series but not in 2003-2005 and from 2015. It would be nice to have some comments on this especially because these years are part of periods one and two.
Figure 3: panels a1 and a2 are not referred nor commented on in the text. What is the relevance of these panels?
Results:
Line 202-203: Can you elaborate more to explain why the interpretation is consistent with the SLA pattern in the EOF analysis? This is not evident to me.
Lines 208-220: These paragraphs were a bit difficult to read. I suggest reorganizing them to separate the interpretation for domain A and for domain B.
Figure 7: I am wondering why analysing the temperature and salinity instead of analysing directly the thermosteric and halosteric sea level. Is it because it is not possible to compute the thermosteric and halosteric sea level for the four layers?
Figure 9 and 12: It would help the reader to place the domain, subdomain A, and subdomain B on these maps when they are part of the plot to help to locate them. On figure 9 a1, it would also help to have a schematic arrow to represent the guinea done cyclonic circulation.
Line 273-274: It is said that layers 2 and 3 have ‘comparatively large salinity shifts’. I don’t understand. It is compared to what? To period 1? To the other layers?
Figure 10: panel a. It would be useful to show the time lag corresponding to the maximum correlation shown on panel a. This would strengthen the hypothesis of the potential pathways to link the subpolar North Atlantic and domain A.
Line 281-282: the currents located around 45N that would link path 2 to path 1 is in my opinion not visible from fig 11a.
Figure 10b: I am wondering about the relevance of this map of regression coefficients. Are the regression coefficients computed with a time lag (I understand it is not but please clarify)? I also noticed that it is barely used in the manuscript. To me the paths drawn on top of the map are not hinted at by the map itself. In other words, in my opinion, this map does not add any evidence to the possible existence of the paths indicated by the arrows. In addition, from a methodological point of view, it is not clear to me why the linear regression during period 2 between salinity averaged in domain A and the other grid points in the Atlantic would prove a causal link between the two, especially because (as it is said in the manuscript) it would take some time for the salinity in one place to be reflected in the other.
Line 287-288: Could you provide a justification to why the purple box chosen in the western subpolar north Atlantic is representative?
Line 291 and following: I am not particularly knowledgeable on potential vorticity. Is the conservation of potential vorticity a common way to try to trace water masses? If so please provide citations.
Line 303-305: To me the paths 1 and 2 are not visually clear in figure 11b.
Line 307-308: How is this lagged correlation computed? Is it from the time series of figure 7 a3? In that case it is natural that this is consistent with figure 7 a3.
Section 5: It seems that for the demonstration of this section the vertical velocities are very important but they are not analysed. Why not also analyse the vertical velocities?
Line 361-364: the shift of the northern edge of the countercurrent is not clear to me.Maybe it would be useful to indicate on the figure.
Line 364: ‘vertical velocity’. The shift of the vertical velocities is not clearly demonstrated but inferred. Please correct the wording.
Concluding section:
Point 5: Can you elaborate more on this point? Do you have examples of work that has been done using numerical simulations in that region or elsewhere? Can you hint at what model would be a good option to do this?