the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2025
On the Response of the Equatorial Atmosphere and Ocean to Changes in Sea Surface Temperature along the Path of the North Equatorial Counter Current
Abstract. The CESM climate model is used to test the hypothesis that changes observed during El Niños are, at least in part, a response of the coupled ocean/atmosphere system to changes in sea surface temperature along the path of the North Equatorial Counter Current.
The results from the second month in a set of forced runs show that increased temperatures at the latitudes of the North Equatorial Counter Current produce a significant increase in deep atmospheric convection within the Intertropical Convergence Zone. This has a local effect on the ocean’s surface pressure field which reduces pressures on the Equator. The increased deep atmospheric convection also affects the longitude structure of the Hadley Circulation. In the south-east Pacific, an area associated with Hadley Cell sinking, surface pressure decreases. In the western Pacific, the pressure field increases with maxima north and south of the Equator.
Together the surface pressure changes have similarities with those associated with the Southern Oscillation. They reduce the zonal component of wind stress along the Equator and produce an El Niño type response in the ocean.
Competing interests: The author was, with Prof J. Johnson, one of the founding editors of Ocean Science
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 08 Oct 2025)
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RC1: 'Comment on egusphere-2025-3734', Mike Bell, 12 Sep 2025
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AC1: 'Response to Reviewer 1', David Webb, 20 Sep 2025
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I would like to thank the reviewer for taking on the m/s for a second time, and for his thoughtful comments. In response to the detailed comments:
1. Title
I agree a change of title is required. A possibility is "ENSO and the temperature of the North Equatorial Counter Current".
2. Abstract
Yes, I agree. I will revise the abstract taking into account the points raised.
3. "The longitude structure of the Hadley Circulation".
I use the term Hadley Circulation here because the NECC is in a region where the atmosphere is responding to the Coriolis term - so it is not part of the walker Circulation.
While I am on the subject : I think the Hadley and Walker Cells to be names which oversimplify what is going on. Deep convection occurs wherever the conditions are right and the surface air has enough energy to punch through the middle atmosphere to reach 300 hPa and less. Air then sinks whenever it can radiate to space or can slip sideways and downwards along constant potential density surfaces.
Some of the convection occurs where the Coriolis term is small - some sinking also occurs at such latitudes - but I know of nothing, except convention, to think that convection and sinking near the Equator are connected by anything but luck.
4. Typos
Umm. Perhaps this is AI pretending not to be AI.
5. Section 21. Heat Forcing
I hope that I understand correctly the point made by the reviewer, but my previous studies using high resolution NEMO and satellite data, indicated that most of the NECC temperature changes in the central and eastern Pacific were due to advection. See for example the SST figure in my 2017 paper. The comment shows that I need to say more about the previous study and the role of advection.
6. Lines 175-190 - Section on Analysis
The earlier section is more about setting up the initial model run and the subsequent runs. In the present form of the paper section 3.1.1 is self contained, with analysis theory and results. Moving the analysis theory to an earlier position in the paper would break the connection. It would also break the connection between the section describing setting up the tests and the description of the initial qualitative results. My preference is not to change the text.
7. Line 216.
Yes.
8. Lines 270-274. Definition of South-Eastern Pacific
I will be more specific and specific a line roughly between 230E, 15S and 280E, 35S.
9. Section 5 Addition to the M/S
My problem here is that, at this stage in research, further analysis of the set of runs inevitably results in some changes in emphasis. The effect of the forcing on the low surface pressure region in the SW Pacific may have been unusually large in the first run studied. I will say something to this effect in the revised m/s but plan to say more when I write up the results from the full set of runs that has now been completed.
10. Figure B1. Direct heating of the NEMO 3.1 region from NECC latitudes
An interesting point that needs further thought. Figure 8 shows heat being advected from the NECC to the Equator, but this may be an artifact of the forcing due to (a) the increased temperature of the NECC and (b) the tropical instability eddies/waves not having time to spin down. One of my previous studies found that they were less active during the development of strong El Ninos.
I'll look at the ERA5 SST and velocity fields and see if there is anything worth adding to the revised m/s.
Citation: https://doi.org/10.5194/egusphere-2025-3734-AC1
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AC1: 'Response to Reviewer 1', David Webb, 20 Sep 2025
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RC2: 'Comment on egusphere-2025-3734', Anonymous Referee #2, 14 Sep 2025
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The authors investigate the atmospheric and oceanic responses to prescribed SST anomalies along the path of the North Equatorial Counter Current (NECC) using the CESM climate model. They show that warming in this region can excite deep convection within the ITCZ, induce pressure changes resembling the Southern Oscillation, weaken equatorial zonal wind stress, and trigger an El Niño–like ocean response. The paper provides an interesting perspective on the role of the NECC region in modulating ENSO dynamics. The manuscript is generally well written and the scientific question is important. At present, I have several concerns and suggestions.
- At present, the experiments are based on a single model member. Atmospheric responses in particular can be heavily influenced by internal variability, and thus it is difficult to clearly separate the forced signal from noise. To convincingly isolate the response to prescribed SST anomalies, it is strongly recommended to perform multiple ensemble members and analyze the ensemble mean. At a minimum, the authors could briefly clarify the potential limitations of the single-member design.
- The current paper feels somewhat lengthy, which reduces readability. For instance, the introduction could be streamlined by briefly summarizing ENSO mechanisms and then quickly moving to the central scientific question. Similarly, the response sections are divided into many subsections, each describing detailed features. While such comprehensiveness is appreciated, the paper would benefit from focusing on the most important and robust aspects of the atmospheric and oceanic responses, rather than giving equal weight to every detail.
- In the sensitivity experiments, the imposed ocean temperature anomalies within a rectangular box may themselves generate dynamical responses outside the box via advection or wave processes, rather than being solely driven by wind stress anomalies. This point should be clarified, as it affects the interpretation of the mechanisms at play.
- The abstract emphasizes that the imposed SST anomalies produce reduced zonal wind stress and an El Niño–type ocean response. However, the conclusion (L530) states that the anomalies did not lead to any marked El Niño–like changes. These statements appear contradictory and should be reconciled to give readers a clear take-home message. Moreover, the abstract currently underrepresents the ocean response section, which is a key part of the study.
- The authors’ experimental region can influence ENSO evolution and diversity (including coastal El Niño) by altering Hadley circulation and exciting meridional coupled modes. The potential relevance of NECC forcing to these processes should be briefly discussed in light of previous studies (e.g., Xie et al. 2018; Peng et al. 2020; Peng et al. 2024).
Xie, S.P., Peng, Q., Kamae, Y., Zheng, X.T., Tokinaga, H. and Wang, D., 2018. Eastern Pacific ITCZ dipole and ENSO diversity. Journal of Climate, 31(11), pp.4449-4462.
Peng, Q., Xie, S.P., Wang, D., Kamae, Y., Zhang, H., Hu, S., Zheng, X.T. and Wang, W., 2020. Eastern Pacific wind effect on the evolution of El Niño: Implications for ENSO diversity. Journal of Climate, 33(8), pp.3197-3212.
Peng, Q., Xie, S.P., Passalacqua, G.A., Miyamoto, A. and Deser, C., 2024. The 2023 extreme coastal El Niño: Atmospheric and air-sea coupling mechanisms. Science Advances, 10(12), p.eadk8646.
- The paper currently uses expressions such as “240°E, 240°E (120°W)”. It is recommended to adopt the more common notation (e.g., 120°W) throughout the text for clarity.
- In Section 3.1, when discussing the convection response, a figure showing the difference between the two experiments would provide a clearer and more direct illustration of the atmospheric anomalies.
Citation: https://doi.org/10.5194/egusphere-2025-3734-RC2 -
AC2: 'Response to Reviewer 2', David Webb, 22 Sep 2025
reply
I would like to thank the reviewer for taking on the m/s and for his thoughtful comments. In response to the detailed comments:
1. Single Member Study
I agree with the comments on a single member study. However some of the results were so striking that I thought an initial paper was justified. I have now completed a set of ensemble runs and am planning how to report the results.
2. Length of Paper
I am sorry if the paper seems lengthy. The introduction maybe includes more history than is normal, but I think the supporting arguments may be needed before many people can accept the paper. There is also the question "Given the evidence, why has it taken 50 years or more for the potential importance of the NECC to be realised?".
The analysis section may also appear long but, because of the contrast with other models of the ENSO process, I thought it was important to go through each stage of the apparent mechanism logically and carefully.
3. Direct Dynamical responses
Yes I agree and there is evidence of a possible direct advective effect in the figure showing the SST response. It is for this reason that I have not claimed the surface warming at the Equator to be part of an ENSO response.
I will make this clearer in the revised m/s.
4. El Nino like changes
I need to be clearer on this issue. The forcing produced El Nino (ENSO) like changes in the Pacific east of 200E (160W) - i.e. the reduction in the strength of the easterlies and the resulting reduced upwelling and warming at depth. However the forcing did not produce the classic El Nino signals seen further west, i.e. the region of deep convection that develops around 180E and a region of westerly winds that develops on the Equator west of the deep convection.
There is also a question over the cold pool surface temperatures. The forcing does increase the Nino 3.4 temperature by a significant amount, but I judged that this was partly due to the advection discussed above, so did not think the result could be used.
Thus for many people the results will not explain the 'El Nino'.
5. Hadley Circulation
Yes, the increased deep convection above the ITCZ will change the longitudinal structure of the Hadley/Walker circulation. Such changes are almost certainly responsible for the global impacts of ENSO. This is quite a claim, so I thought it better to let readers make their own judgment.
I will think about what I can do to be more specific.
6. Use of 240E, 120 etc.
I realise that the use of 'degrees east' or 'degrees west' can raise the same amount of passion as 'data is' or 'data are'. I accept that seagoers have a convention of using degrees west in the western hemisphere and I am happy to use that convention for navigation and for the general public. However when running and analysing models and data, I always have to work in degrees east, whatever the longitude. So for a professional paper that involves such models and data, I believe that the use of degrees east everywhere is valid.
7. Figure 1.
I'll add the difference figure.
Citation: https://doi.org/10.5194/egusphere-2025-3734-AC2
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