the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
05 Feb 2021
05 Feb 2021
On the Role of Westerly Wind Anomalies in the Development of the 1982–1983 El Niño
- National Oceanography Centre, Southampton SO14 3ZH, U.K.
- National Oceanography Centre, Southampton SO14 3ZH, U.K.
Abstract. A recent study of two strong El Niños highlighted the potential importance of a region of low sea level that developed in the western equatorial Pacific prior to the El Niños of 1982–1983 and 1997–1998. Here the cause of the low sea level in 1982 is investigated using a series of runs of a global ocean model with different wind fields and initial conditions.
The results indicate that the low sea level was due to the increased wind shear that developed just north of the Equator during 1982. This generated Ekman divergence at the latitudes of the North Equatorial Trough, raising the underlying density surfaces and increasing the depth of the trough. This also increased the strength of the North Equatorial Counter Current which lies on the southern slope of the trough.
The anomalous westerly winds associated with Madden Julian Oscillations are often held responsible for triggering El Niños through the generation of westerly wind bursts and the resulting equatorial Kelvin waves in the ocean. However if Webb (2018) is correct, the present results imply that a different physical process was involved in which Ekman divergence due to the same winds, increased the heat transported by the North Equatorial Counter Current early in the year and ultimately caused the strong 1982–1983 El Niño.
David J. Webb
Status: final response (author comments only)
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CC1: 'Comment on os-2021-5', D. Wang, 11 Mar 2021
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AC1: 'Reply to review by D. Wang', David Webb, 13 Mar 2021
Many thanks for your comments on my paper. In response to your main points:
1. I am intrigued by your comment that it is more like a report than a research paper and am not sure whether, given the extensive length of many research papers, I should take it as a compliment. I always think of a report as a short cold dispassionate thing, which is also good to find in a research paper.
But maybe it is a response to the small number of citations. I know these are important for researchers and that I am bad at the game.
I also have a problem in that the model validation and the run of model tests was a hard slog and inevitably means trying to get a large amount of data across - which is difficult to do without getting repetitive. However I hope that the reader finds the final conclusion worth the effort.
2. On the question of figures, in the two column version of the manuscript (see supplement), many of the key figures lie side by side. Unfortunately this is not possible in the single column version used by Ocean Science for reviews. However the suggestions for reducing the number of figures are helpful and I will investigate the possibilities.
3. On the question of discussing mechanisms, I did this a lot in the previous two papers, so reduced the amount of discussion here. I could say a bit more about the hypotheses which were disproved. However for the final conclusions where Ekman divergence appears to be operating over a period of many months I think a lot more work is needed.
The problem here arises because of my idea that on oceanic time scales, the atmosphere acts as a source of random noise (possibly red, i.e. with an excess of low frequencies) plus some systematic biases (like the westerlies or the trades). To generate anomalous Ekman suction over a period of order 6 months, something has to be organising the atmosphere, and I suspect this has to be some unusual ocean feature.
The simplest might be that it was just a warmer than normal NECC in the western Pacific. But it also could be say a weaker than normal convective regime in the Coral Sea, the resulting weak cross equatorial flows in the atmosphere allowing a stronger than normal ITCZ in the western pacific.
Anyway more hypotheses are needed and more tests.
4. Thanks for the references. I will read (and reread) them and consider.
5. As far as I know the only sea level measurements available for 1982-83 would be from tide gauges on island stations. This is really a task for another paper. For the 1997-98 El Nino, comparisons with satellite data were made in the Webb, Coward and Snaith (2020) paper - which showed very good agreement between the NEMO model and observations. For that reason I think that for the purposes of this paper comparison with the NEMO sea levels is a good start. However there is nothing to stop someone else looking at the tide gauge records.
6. My meteorological colleagues claim that MJOs are associated with westerly wind bursts and so cause El Ninos. I'll find some references.
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AC1: 'Reply to review by D. Wang', David Webb, 13 Mar 2021
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RC1: 'Comment on os-2021-5', Anonymous Referee #1, 12 Mar 2021
General comments:
This study investigates the origin of western equatorial Pacific sea level anomalies that seem to be related to the strong El Niño 1982/83 via modulating the North Equatorial Counter Current. This is motivated by a previous study about the importance of these sea level anomalies in driving the strong El Niño 1982/83. Using a global ocean general circulation model forced by different atmospheric wind fields, the author rules out different possibilities for causing the anomalously low sea level in the western equatorial Pacific, such as the annually occurring Rossby wave or a remote wind field, coming to the conclusion that it is the local wind anomaly field that drives the sea level anomalies driven by Ekman divergence.
The study is interesting as it sheds light on further processes that seem to be relevant in driving overly strong El Niño events beyond the conventional processes that are typically addressed in this context.
The manuscript has rather the form of a report-type publication rather than a classical research article. However, this does not degrade the fact that it is informative. The methodology, the analytical reasoning and the writing are adequate. The overall quality of the current stage of the manuscript is however such that it is partly built too complicated, especially regarding the way the figures are presented (see specific comments). There are some typos and some phrases that are difficult to understand (see technical comments).
Specific comments:
- I suggest to rephrase the title of the manuscript to “On the origin of western equatorial Pacific sea level anomalies prior to the 1982/83 El Niño”.
- I strongly encourage the author to provide a few more references about driving mechanisms of the strong El Ninos and the role of anomalous wind fields in the introductory section, especially more recent ones.
- A lot of figures can be merged to reduce the total number of figures and also ease the comparison among them. For example, one could merge figures 1, 2, 7 and 8; figures 3, 4, 9 and 10; figures 5, 6, 11 and 12; figures 13 and 16; figures 14, 15, 17 and 18; figures 19 and 20; figures 25 and 26. The author may find a more reasonable order of merging some figure, but I think some merging should be done in a way that it reduces the total amount of figures and such that it puts those figures side-by-side that should be compared to one another.
- The author may add the observed sea level and temperature field for that time period to the supplement to complement the model validation.
Technical corrections:
- There are some typos such that double-occuring words: “from from”, “that that” – please search for these occurrences and correct.
- I also suggest to add the information of the physical units to the figure captions.
- I also think that is called “Hovmoeller” rather than “Hovmuller” diagrams.
- 147-148: Please check structure and meaning of the text - unclear
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AC2: 'Response to Referee #1', David Webb, 13 Mar 2021
Many thanks for reviewing this paper and for your comments. In response to your main points:
1. On the question of the figures, I agree about the problem with the review version of the manuscript. You may find the two column version better structured - a copy is available as a supplementary document in my response to the first reviewer. The layout of the final published version could be similar.
2. Change of title. Fine.
3. I take your point. The trouble here is that I did not realise the importance of the local wind field when the study started. It might seem a bit odd to focus on the local wind field in the introduction and then to forget about it until the last few sections of the paper. Maybe a bit at the beginning and more near the end would be better.
4. I accept you point about the figures in the review manuscript. However if you examine the two column version you will see that they are sensibly grouped - but not quite in the way you suggest. Further feedback would be welcome.
5. As in my response for the first reviewer good satellite data is available for the 1997-98 episode. The comparison of key features during that period show good agreement between the NEMO model and observations. It was on this basis that I used the NEMO model results as the check datasets in the model validation section.
Unfortunately I know of no comparable set of sea level and sea surface temperature datasets covering the 1982-1983 period.
Minor comments;
1. Typos. Thanks.
2. Good point.
3. Ditto.
4. Will correct.
Regards,
David Webb.
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RC2: 'Comment on os-2021-5', Anonymous Referee #2, 05 Apr 2021
This manuscript offers description of a series of forced ocean model results in which different ocean initial conditions and applied wind forcing, representative of 1981 and 1982, are paired to help explain why the model ocean developed lower sea surface height in 1982 than 1981 over the northwestern tropical Pacific region of interest; approximately 6N, 140E-170E. The motivation for this question was provided by a previous paper by the author, which hypothesized that increased transport of warm water by the North Equatorial Counter Current (NECC) was the main cause of the 1982-83 El Nino event. The present manuscript is focused on the question of what causes the modeled NECC change in 1982. It reaches the conclusion that local wind forcing caused the countercurrent to increase in 1982 compared to 1981.
Evaluation of wind variability components and ocean processes responsible for large El Nino events such as that of 1982-83 remains relevant to ENSO and climate science. The hypothesis that the NECC might play a role in El Nino development is not new (as stated in this manuscript), nor is the finding, in the forced ocean model experiment context, that the annual wind stress applied to the tropical Pacific mainly determines that year’s El Nino-related oceanic development. The pairing of the two in the previous paper Webb (2018) does not represent a widely held belief. In that sense, the context for the present paper can be considered somewhat novel. However, the present paper is focused on explaining modeled NECC variability.
It is difficult to say that the conclusions reached here have been adequately supported. There is no observational evidence offered in this manuscript to validate the model that is used in the experiments presented. Model validation is instead attempted by comparing one set of model results to another. This is an insufficient method for evaluating the capability of the model to simulate the observed behavior that is ultimately in question. The degree to which the forced model can accurately hindcast the observed space-time variability of SST, SSH, and surface currents in 1981 and 1982, along with northwest Pacific isopycnal depth (as in Fig. 25), is fundamental to the conclusions reached in this manuscript. These aspects have not been adequately evaluated but should be.
The role of observational uncertainty has not been but also should be considered in this context. Previously, for example, Harrison et al. (1990) reported on forced ocean model hindcasts of the 1982-83 El Nino event and found that the answers to questions like those being asked presently, for example, concerning the relative importance of local and remote wind forcing to anomalous currents and SST, depended very much on which wind data set was used to force the model. The present manuscript appears to report results based on only one wind data set, which is not described in the text. Given the previous Harrison et al. demonstration of the importance/limitations of observational wind uncertainty in this context, the impact of this wind uncertainty needs to be examined before the reliability of the results presented can be understood.
Notwithstanding the issues raised in the comments above, more precise description of the experiment results would improve their presentation (and facilitate comparison to observations). This manuscript relies mainly on visual inspection of snapshot-maps and time-longitude plots of SST, SSH and currents to support its conclusions about the relative importance of different ocean initial conditions and components of wind variability for causing changes in NECC-related SSH. I suggest defining metrics that quantify the salient model experiment results in relation to the control-hindcast to thereby offer a more streamlined and precise presentation of results.
I suggest that “Westerly Wind Events” be removed from the title. This manuscript does not identify or directly discuss westerly wind events. Something like “On the development of low North Equatorial Pacific sea level pressure during 1982-1983” would be more appropriate.
The last paragraph of the abstract attempts to describe the relationship between westerly wind events, the Madden Julian Oscillation, North Equatorial Counter Current (NECC) and the observed development of the 1982-83 El Nino event based on the NECC-related model experiments presented herein. However, what is presented herein does not sufficiently support conclusions about these relationships because three of these four phenomena (wind events, MJO, observed El Nino development) are not substantially addressed by the results presented in the manuscript. The abstract should be modified to better reflect what has and has not been done here.
Many of the model comparison figures, for example Figs. 3&4, 5&6 etc. can be combined to the benefit of the reader’s ability to make the intended visual comparison. Reducing the total number of figures may also improve the presentation; 24 is perhaps an over-abundance of figures for the scope of this paper.
There has been considerable progress made in understanding El Nino development since Wyrtki (1973, 1974) offered hypotheses about the role of enhanced NECC. This manuscript would benefit from taking into account what has been learned and described in many of the relevant publications since then.
Specific comments.
line 52 stated -> started
Line 53 "depend primarily ON the wind field..."
Line 76 "global topography THAT has previously..."
Line 96. Suggest "all the high" -> "higher" or be more specific about which frequencies are muted by the 5 day averaging.
Figure 1 caption: Hoffmuller diagram OF the sea level...
Also, would be better to match Y-axis labels of (a) and (b)
Line 110. Better to quantify "reasonably small drop"
Line 113. Beginning of this line should read "Figs. 3 to 6"
Paragraph beginning Line 117 and associated Figures. The key, near-equatorial features are difficult to see with latitudes +/- 30 and surface current vectors shown. The author may wish to consider reducing the Y-axis range to facilitate visual inspection of the most import model results.
Figure 13. Caption and text refers to "top" and "bottom” panels although the Figure 13 panels (a) and (b) are side-by-side.
Figure 16's top/bottom description in text and caption does not match its left/right layout.
Line 183. Western Pacific cold pool?
Line 214. an region -> a region
Paragraph beginning on Line 252. Integration smooths any field regardless of whether its variability is characterized by a "long term systematic change", or a more event-like abundance of, for example, equatorial westerly anomalies. Results therefore may not imply what the text claims they do.
Line 300. The observed variability of coupled tropical Pacific system does not support a one-to-one correspondence between equatorial ocean current variability and the observed variables most closely related to deep atmospheric convection activity, such as is implied here. The statement about convection should be sufficiently supported or withdrawn.
Reference:
Harrison, D.E., B.S. Giese, and E.S. Sarachik (1990): Mechanisms of SST change in the equatorial waveguide during the 1982–83 ENSO. J. Climate, 3(2), 173–188.
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AC3: 'Response to Reviewer #2', David Webb, 06 Apr 2021
Response to Reviewer #2
Again many thanks for reviewing this paper. I suspect that you were not all that happy with premise, content and style (I've been there myself), but thanks for continuing.
1. Background
"The pairing of the two [NECC and wind stress] in the previous paper Webb (2018) does not represent a widely held belief. In that sense, the context for the present paper can be considered somewhat novel. However, the present paper is focused on explaining modelled NECC variability."
Yes. I assume that the reader knows the background to the work. The pairing only comes up when the other mechanisms investigated have been discounted. It is in the abstract but I should probably add a bit more in the introduction to prime the reader for what comes later.
2. Model validation
"There is no observational evidence offered in this manuscript to validate the model that is used in the experiments presented."
This is probably best answered by adding more references to the use of OCCAM elsewhere. OCCAM has been around for nearly thirty years, being one of the first high-resolution global ocean models. A quick search on google advanced scholar search for 'occam ocean model' produced 58 papers using OCCAM results out of the first 70 references listed. I do not have a full count but it probably runs into many hundreds and includes many intercomparisons with the models and observations. There will be even more if you add David Stevens' SEA and Kristofer Doos' Baltic Sea models which are related.
As far as I know none of the papers have shown any major issues with the model. In the Pacific I have used it previously to study the zonal jets south of the Equator and the Indonesian Throughflow. In neither case were there any significant problems.
"The degree to which the forced model can accurately hindcast the observed space-time variability of SST, SSH, and surface currents in 1981 and 1982, along with northwest Pacific isopycnal depth (as in Fig. 25), is fundamental to the conclusions reached in this manuscript. These aspects have not been adequately evaluated but should be."I would argue that the track record of the NEMO model and the comparison paper with Andrew Coward and Helen Snaith (which covers SST and SSH) give grounds to suggest, not only that the model is as good as (or better than) any other present day ocean model but, more importantly, that its performance is more than sufficient for the present research project. I would accept that the OCCAM model, being older and less developed, is not so good. However there is nothing in previous research and publications using the model to indicate that it should not be used for the present study.
The changing depth of the near surface iso-pycnals depends on the Ekman divergence. This depends on the surface wind stress and, although you may query the quality of the wind stress field, it would be difficult to construct an ocean model which generated the wrong divergence without also showing other major flaws.
Of course if anyone has doubts, it is always open for them to prove that something is inadequate or wrong and always essentially impossible prove that it is right.
3. Observational Uncertainty"The role of observational uncertainty has not been but also should be considered in this context. Previously, for example, Harrison et al. (1990) reported on forced ocean model hindcasts of the 1982-83 El Nino event and found that the answers to questions like those being asked presently, for example, concerning the relative importance of local and remote wind forcing to anomalous currents and SST, depended very much on which wind data set was used to force the model. The present manuscript appears to report results based on only one wind data set, which is not described in the text. Given the previous Harrison et al. demonstration of the importance/limitations of observational wind uncertainty in this context, the impact of this wind uncertainty needs to be examined before the reliability of the results presented can be understood."
This is a valid point, the results do depend on the wind field. However, as reported in Webb (2018), the NEMO model was forced with the Drakker DFS5.2 atmospheric fields which are themselves based on the ECMWF reanalyses. I should probably include a similar statement in the present paper.
The forcing dataset has been widely used and has a good reputation. The comparison carried out with Coward and Snaith is essentially a test of both the model and the forcing dataset, the agreement with observations in the equatorial Pacific indicating that neither has significant errors.
I accept your point about the differences in the Harrison forcing datasets but modern reanalyses, like that from the ECMWF, appear to have overcome many of the earlier problems. The work could be repeated with another reanalysis dataset but this would take a considerable amount of work and computer time and it seems unlikely that it could both show agreement with the satellite data and show that different mechanisms dominated the physics.
4. Metrics
"Notwithstanding the issues raised in the comments above, more precise description of the experiment results would improve their presentation (and facilitate comparison to observations). This manuscript relies mainly on visual inspection of snapshot-maps and time-longitude plots of SST, SSH and currents to support its conclusions about the relative importance of different ocean initial conditions and components of wind variability for causing changes in NECC-related SSH. I suggest defining metrics that quantify the salient model experiment results in relation to the control-hindcast to thereby offer a more streamlined and precise presentation of results."
Good point. I am not sure what metrics you had in mind but one possibility would be one or more t-tests using pairs of the height changes seen in figure 23. This could supplant many of the other figures by summarising the changes in height in the west Pacific. However I am unwilling to loose the other figures (they are packed better in the 2-column version of the m/s included in my response to the first reviewer) because they give the reader a chance to think about how different parts of the ocean are connected physically - something you do not get with plots of NINO 3.5 anomalies or a table of statistics.
5. Title
"I suggest that “Westerly Wind Events” be removed from the title. This manuscript does not identify or directly discuss westerly wind events. Something like “On the development of low North Equatorial Pacific sea level pressure during 1982-1983” would be more appropriate."
Maybe. Certainly westerly wind anomalies are involved - but even your Harrison et al reference shows that they are not just anomalies.
6. Relationships
"The last paragraph of the abstract attempts to describe the relationship between westerly wind events, the Madden Julian Oscillation, North Equatorial Counter Current (NECC) and the observed development of the 1982-83 El Nino event based on the NECC-related model experiments presented herein. However, what is presented herein does not sufficiently support conclusions about these relationships because three of these four phenomena (wind events, MJO, observed El Nino development) are not substantially addressed by the results presented in the manuscript. The abstract should be modified to better reflect what has and has not been done here."
I take your point but I think that the connections need to be made - to stimulate discussion and further research. When I have talked to an expert from the UK Met Office they were convinced, on the basis of their own work and others, that El Ninos were connected with Madden Julian events and would accept no other hypothesis or theory. There is evidence that such oscillations were involved in early 1982 so I think I need to work up this aspect the manuscript.
A related reason is that I think the apparent link with the Madden Julian oscillation and the associated westerly wind bursts has helped the theoreticians believe they are on the right track by concentrating on the equatorial wave guide and equatorial Kelvin waves when developing theories of the El Nino. I accept that I may be biased but the result here helps show that other solutions are possible.
7. Figures
"Many of the model comparison figures, for example Figs. 3&4, 5&6 etc. can be combined to the benefit of the reader’s ability to make the intended visual comparison. Reducing the total number of figures may also improve the presentation; 24 is perhaps an over-abundance of figures for the scope of this paper."
The figures were designed to work with the 2-column version of the manuscript - where most of the figures do lie adjacent to their mate. Unfortunately Ocean Science requires the single column version for reviewers.
8. Progress
"There has been considerable progress made in understanding El Nino development since Wyrtki (1973, 1974) offered hypotheses about the role of enhanced NECC. This manuscript would benefit from taking into account what has been learned and described in many of the relevant publications since then."
You may guess that I have a problem with current theories of the El Nino and for that reason it is probably best that they are not raised in the manuscript (because the m/s is not concerned with the general problem). That said if you know of any work on thermocline movement or SSH changes between 140E and 170E and around 6N I would be pleased to include references.
After reading your comment I read through the review by Wang et al 2016. OK its not right up to date (I do not have a copy of the recent AGU book) and for some reason there is at least one theory involving thermocline displacement it does not include but otherwise it is reasonably detailed.
As I understand it, any physics that has been learnt is usually summarised in a decent theory. If you go to the section on ENSO mechanisms in Wang et al there is no proper discussion of the NECC. Instead it is full of the unphysical assumption that equatorial Kelvin waves can transport heat horizontally over significant portions of the Pacific. The associated Stokes drift can move water particles and heat but not over significant distances.
For the detailed theory, seven separate ENSO mechanisms are put forward dating from 1969 (Bjerknes), 1988 (Suarez and Schoph), 1987 (Jin), 1997 (Weisberg and Wang), 1997 (Picaut et al), 2001 (Wang), 1985 (Lau) as well as models like Zebiac and Cane (1987). They may be "conceptual models capable of producing ENSO-like oscillations" (I like the capable) but as far as I can tell over a period of 50 years none of these contain enough hard physics for them to have been tested and and if necessary discarded - using modern computers or the huge amount of data now available. They cannot all be right and if they cannot be rigorously tested or used for realistic prediction, what are they?
And if there is no decent theory, what really has been learnt? - except that the ECMWF model with data assimilation can predict El Ninos a few months in advance.
Anyway, sorry to go on but I think that mine is a valid point. At the same time I appreciate that many people have worked hard in the subject and I should include more references.
9. FinallyMany thanks for your specific comments which I'll make full use of.
Regards,
David Webb.
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AC3: 'Response to Reviewer #2', David Webb, 06 Apr 2021
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RC3: 'Comment on os-2021-5', Anonymous Referee #3, 16 Apr 2021
This manuscript seeks to understand the cause and the physical processes behind the anomalously low sea level in the western North equatorial Pacific in the lead-up of the 1982/83 El Nino event. The author therefore makes use of a set of short (1 year) ocean model simulations with varying wind forcings and initial conditions. It is found that local wind anomalies in the central Pacific north of the Equator after April 1982 trigger Ekman divergence and hence cause a deepening of the North Equatorial Trough. The accompanying sea level gradient leads to an increase in the eastward NECC transport of warm waters supporting the development of a strong El Nino. The author further argues that this mechanism is based on prolonged westerly wind anomalies over several months and therefore provides an alternative to the classic Kelvin wave mechanism which relies on short term westerly wind bursts.
The study is novel and interesting and the methods use are applicable. However, there are various shortcomings in the presentation (mistakes in the writing, organisation of figures) that need to be rectified. Also, some more explanation for the author’s reasoning is required at some instances. I encourage the author to revise the respective parts of the manuscript before I can recommend publication.
General comments:
26 figures for a paper with a relatively short story seem way too many. E.g., there is no need for 16 figures only for model validation. Why not validate the model with one or two timeseries averaged over selected regions and only show one map for one date and one year as an example?
In general, throughout the manuscript it would be very helpful for the reader to have all related panels closer together (e.g., in one figure) so they can be seen and compared at once without flipping pages. Also, difference plots would be very helpful for the model validation as well as in the results section. A lot of the figures can be merged together to one figure with more panels.Another comment regarding the appearance of the figures. It should be made sure that all figures/panels are consistent among each other (e.g., same axis labels). Also, many (all?) axis labels and axis tick labels are very tiny and could be made larger.
Why is the author only focusing on the 1982/83 event and not also the 1997/98 event? It would be very interesting to know if the presented mechanism also applies to other strong El Nino events.
Section 1.2 seems unnecessarily long as it already goes into the details of the methods and even results. It could be condensed by leaving out all the details (shifting them to the Methods section) that are partly mentioned in the following sections and merged to the end of section 1.1.
There are multiple small spelling and grammatical mistakes (mostly missing words) throughout the manuscript. The manuscript should be carefully checked against such mistakes as it makes it harder for the reader to follow. I have started with a few in the specific comments below but they became too many to all list them here.
Specific comments along the manuscript:
line 25: Typo. missing “in” at the end of the line.
line 52: Typo. “started”.
line 53: Typo. Missing “on”.
line 74: word missing after “topography”.
Figure 1: This figure can be made clearer. The two panels should be closer to each other and also be at the same height. Moreover, the y-axis and axis labels should be the same for both panels. It might also be helpful to add a difference plot as panel c. There are also several typos/missing spaces in the figure caption.
Figure 2: My comment to Figure 1 applies here, too.
line 113: Wrong figure reference. It should be “Figs. 3-6”.
line 113: Why specifically these dates, 4th June and 2nd September? Probably related to the build-up and major phase of NECC transport. The author should explain/justify this.
Figures 3-6: Again, difference plots between Occam and Nemo would make it much easier to compare the variables.
Figures 3-6: In general, Occam seems to overestimate sea level variations as compared to Nemo. This should be mentioned in the discussion of the Figures. What are possible reasons for this? What impact on the results does this have?
line 135: “usefully used”?
lines 133-136: It should also be concluded that due to the lack of heat and freshwater fluxes Occam does not well capture the SST features (amplitude) of Nemo? How does this affect the usefulness of the model for this study?
lines 184-187: I am not sure I understand the reasoning here. Does the author really refer to the western Pacific rather than the eastern Pacific? To my ENSO understanding, the SST increases in the east, not the west, as a result of weakened easterlies/EUC. And the increase/decrease in sea level in the east/west is rather a result of changes in the water volume than of the density, although the temperature effect of course will also play some role.
lines 196-197: There are three grammatical mistakes in these two lines.
lines 198-203: Is this experiment necessary, after all, as the author has already shown in Figure 13 that the 1981 winds cannot produce a sea level low in the western Pacific?
line 207: This is not a very informative heading.
213-214: It is unclear to me how this statement contributes to the overall story presented in the manuscript. Please clarify.
Figure 22: Is panel a (left) really necessary as it was already shown before that the winds before 30th April do not play a role here.
Figure 23: Figure has neither a legend nor units.
Figures 25-26: Why is the Nemo output shown here instead of the Occam output?
line 256: I would say the Ekman divergence causes sea level changes, rather than Ekman pumping which causes isopycnal changes.
lines 262-264: What is the reason for this? Is it because of an increase in temperature as the El Nino develops?
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AC4: 'Reply on RC3', David Webb, 16 Apr 2021
Thanks for your comments - I've only a couple of hours until the end of the review period so briefly:
1. Re the figures - your comments are similar to those of the other referees. Originally many of them were designed to be side by side in the two column version of the manuscript. I'll take your points into account but hope that any revised version you see will have two columns.
2. Re validation - I check that.
3. Re all the 'english' - thanks for the corrections.
Regards,
David Webb.
p.s. As a result of other reviewers comments I've been rereading some old papers and found there is a bit more support for Wyrtki's ideas than I had remembered - including the original Myers and Donguy paper.
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AC4: 'Reply on RC3', David Webb, 16 Apr 2021
David J. Webb
David J. Webb
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