the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2024
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 the 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 model results show that increased temperatures at the latitudes of the NECC produce more deep atmospheric convection within the ITCZ. This has a local effect on the ocean's surface pressure field. The increased deep atmospheric convection also appears to affect the longitude structure of the Hadley Circulation. This results in less sinking air in the south-east Pacific and produces changes in surface pressure similar to those of a Southern Oscillation. Both mechanisms reduce the zonal component of the surface pressure gradient and wind stress along the Equator, and produce an El Niño type response in the ocean.
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Status: open (until 20 Jan 2025)
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RC1: 'Comment on egusphere-2024-3560', Anonymous Referee #1, 17 Dec 2024
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This study uses the CESM coupled ocean-atmosphere climate model to test the hypothesis that strong El Niños are due, at least in part, to warmer than normal water along the path of the North Equatorial Countercurrent (NECC). The model setup involves comparing the results of a control run with no anomalous SST warming in the vicinity of the NECC to one with a prescribed warm anomaly of ~1C along the path of the NECC between 120W and the dateline. The differences in coupled ocean-atmosphere anomalies between these two runs provide a measure of NECC warming impact on the tropical Pacific. The NECC roughly coincides with the location of the ITCZ, a region of ascending air masses that significantly affects the oceanic and atmospheric circulation of the Pacific basin. So the imposition of anomalous SSTs in this region has significant impacts on atmospheric and oceanic fields that look like they could contribute to the development of an El Niño.
I have no particular issues with the results or their interpretation. My main criticism is that the prescribed SST anomalies are unlikely to occur in the real world in isolation of a broader ENSO warming across the Pacific basin. The author finds support for his hypothesis (lines 65-66) that "strong El Niños are due, at least in part, to warmer than normal water along the path of the North Equatorial Countercurrent." What is unstated and crucially important though, is that warmer than normal water along the path of the North Equatorial Countercurrent is due to strong El Niños. The processes that generate ENSO events involve, first and foremost, ocean-atmosphere interactions and ocean dynamics in the equatorial waveguide. A broader range of latitudes is involved but is it highly unlikely that the SST anomalies prescribed in this study would be present without the presence of a developing El Niño. As as long as these points are made clearly in the paper, I am comfortable with its acceptance.
Minor points:
1) Line 55. "extracted" is not a conventional dynamical term. Explain what is meant.
2) Using east longitudes > 180 degrees for west longitudes requires the reader to stop and do math to figure out what the real longitude is. Use west longitudes for west longitudes.
Citation: https://doi.org/10.5194/egusphere-2024-3560-RC1 -
AC1: 'Response to Reviewer 1', David Webb, 17 Jan 2025
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I would like to thank reviewer 1 for taking on this manuscript and for the large amounts of time and effort needed to do this. I also appreciate that the focus of the paper is not typical of mainstream ENSO research.
1. The first point made by the referee is that "the prescribed SST anomalies are unlikely to occur in the real world in isolation of a broader ENSO warming across the Pacific basin".
I am not sure that I completely agree. The NECC is not a constant current and the temperature of the source region in the western Pacific does change from year to year, so an extra one degree increase (or decrease) along the path of the current may not be unreasonable.
In terms of the manuscript, one degree was chosen so that if there was a response, it was likely to be large enough to be noticed. I could have tried a tenth of a degree or less, but the response would most likely be lost in the background noise.
I also stayed away from a background ENSO event, because of concern that fluxes may get saturated at high temperatures. This could have happened if the base state had close to 100% of the ITCZ convection reaching the tropopause. In this case the perturbation may have had little or no effect.
In the revision I will say more about the choice of a one degree perturbation and emphasise the point that although the perturbation generates changes that correlate with those observed during an El Niño, this is not an attempt to generate an El Niño.
2. A second point made by the reviewer is that "warmer than normal water along the path of the North Equatorial Countercurrent is due to strong El Niños".I am sorry but I disagree for two reasons. The first is that in my study of the forcing of strong El Niños (Webb, 2021 in the m/s) I found that the warm water started moving eastwards early in the year, well before any other aspect of a strong El Niño had developed.
In addition I an unhappy saying that anything is due to an El Niño. As far as I can tell, and ignoring some of the early fisheries papers, the term El Niño is used primarily to describe a large set of correlated events and measurements.
In particular cases I am happy to say that an influx of nutrient poor water, associated with an El Niño, resulted in fewer fish, or a westerly wind burst, occurring near the start of an El Niño, generated a Kelvin wave. I accept that a warm NECC needs a source of warm water in the western Pacific, but to say that a warm NECC is due to, i.e. caused by, a developing El Niño goes too far.
To conclude, I hope that we can agree to differ on some points and possibly sometime discuss our differences. In the mean time thank you again for reviewing the manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3560-AC1
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AC1: 'Response to Reviewer 1', David Webb, 17 Jan 2025
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RC2: 'Comment on egusphere-2024-3560', Mike Bell, 17 Jan 2025
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see attached file
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AC2: 'Resolution and Reproducibility', David Webb, 19 Jan 2025
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I would like to thank Dr Bell for both his detailed comments on the manuscript science and for his suggestions for improving the presentation.
I think that most of the comments and suggestions are straightforward and, assuming that a revision is requested, will be dealt with in the revised paper and associated "Author's response".
However, in the short time remaining before the end of the discussion period, there are two points I would like to cover which may also be of interest to others considering similar tests in future.
1. The first is the problem of ocean model resolution and tropical instability waves/eddies. In the base CESM model used here, the ocean model resolution varies with position. Near the Equator in the Pacific the resolution is around 1 degree in longitude and 0.26 degrees in latitude.
The reviewer refers to Graham's 2014 paper on the effect of ocean model resolution on the simulation of the waves/eddies. He compared 1 degree and 0.25 degree resolution ocean models and found the higher resolution gave much better agreement with observations. My experience with the 0.25 degree OCCAM model is also that the agreement with the observations is good.
However in the CESM model results, the instability waves/eddies appear to be distorted, with predominantly a north-south axis. They are also much too energetic in the western equatorial Pacific.
This has consequences. First it means that some of the apparent heating of the cold pool in the forced run is likely a result of advection out of the NECC by over-energetic waves/eddies. Secondly it means that in discussing any El Niño like features, reduction in the strength of the equatorial easterlies and the resulting ocean upwelling is much more significant than any heating of the cold pool.
I would therefore recommend any further studies use ocean models at least 0.25 degree resolution in both longitude and latitude
2. The reviewer also asks about checking other months and other years.
In response I should say that since submitting the manuscript I have carried out a set of runs starting in August in each of ten years. Analysis so far is both preliminary and qualitative. I give some figures below which show there is some scatter in the results but I do not think that they are enough to affect the main conclusion of the manuscript.
The data from 7 of the 10 runs show pressure changes along the Equator, comparable to those of the manuscript. I the three other cases the lack of a significant drop in pressure along the Equator appears to be due to a reduced pressure drop in the south-east Pacific (around 240E, 20S) and pressure increases in the south central Pacific (around 210E, 20S). I take this to imply that sinking in the Hadley Cell is not just a simple local response to convection.
In most of the cases, the agreement continues into the following month (October). In each of the poor cases it improves slightly in the following month. In one of the good cases it is markedly worse.
Changes in the temperature of the cold pool are well correlated with zonal wind stress on the Equator. However in two of the cases, pressure reduction on the Equator is associated with slightly stronger easterlies. In two other cases, pressure increase on the Equator is associated with slightly weaker easterlies. Again I take this to imply that other processes are also involved.
Finally, many thanks for the review. I hope we have a chance to discuss the subject further.Citation: https://doi.org/10.5194/egusphere-2024-3560-AC2 -
RC3: 'Reply on AC2', Anonymous Referee #1, 21 Jan 2025
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The author in his reply makes two points in response to my review: 1) “The NECC is not a constant current and the temperature of the source region in the western Pacific does change from year to year, so an extra one degree increase...along the path of the current may not be unreasonable.” I agree that 1°C warming is not unreasonable in the vicinity of the ITCZ. However, my main point is that it is unlikely that a 1°C warming would occur along the path the ITCZ between the dateline and 120°W independent of El Niño development. I would also contend that how much of that anomaly is due to zonal advection by the NECC vs local air-sea interaction is a matter of debate; however, this is a secondary issue.
To emphasize the point above, in my review I inverted the author's statement in the paper that "strong El Niños are due, at least in part, to warmer than normal water along the path of the North Equatorial Countercurrent" to read "warmer than normal water along the path of the North Equatorial Countercurrent is due to strong El Niños" . (One can remove the word "strong" in my inversion and the statement is still valid). The second point of the author's reply notes that in the NECC prior to the development of strong El Niños "the warm water started moving eastwards early in the year, well before any other aspect of a strong El Niño had developed." However, the author is not testing whether slight eastward movement of warm water in the NECC early in the year can influence the development of an El Niño, but rather that anomalously warm water stretching over 6000 km in an narrow band of longitudes north of the equator in the vicinity of the ITCZ beginning in August has an effect on El Niño development. August is already well into the season of typical El Niño development and we have never seen an SST anomaly structure like this independent of El Niño. As a result, cause and effect are confounded in the experimental set up as far as I can tell and the author's response has not convinced me otherwise.
Citation: https://doi.org/10.5194/egusphere-2024-3560-RC3 -
RC4: 'Reply on RC3', Anonymous Referee #1, 21 Jan 2025
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Correction: "narrow band of longitudes" should be "narrow band of latitudes"
Citation: https://doi.org/10.5194/egusphere-2024-3560-RC4
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RC4: 'Reply on RC3', Anonymous Referee #1, 21 Jan 2025
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RC3: 'Reply on AC2', Anonymous Referee #1, 21 Jan 2025
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AC2: 'Resolution and Reproducibility', David Webb, 19 Jan 2025
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