Dear Editor,
We thank the reviewers for their thoughtful comments on the paper entitled "Asymmetry in Zonal Phase Propagation of ENSO Sea Surface Temperature Anomalies" (009GL038774) and have revised it accordingly. Our responses to the reviewer comments are attached. We have uploaded two version of the revised paper, one with modifications tracked and one clean version.
Thank you for your consideration.
Sincerely,
Michael McPhaden
Reviewers’ comments in italics and responses in plain text
Reviewer #1:
I had one major problem…I suggest they add a few heavy arrows to the 2 panels to show more clearly the region and times where the zonal propagation is important. This should only be done for roughly 2 strong El Niño and 2 strong La Niña events.
Done.
I noticed a couple of minor points.
1) The Reynolds et al. (2002) data set starts in November 1981. So where did the SST data for 1980 and the rest of 1981 shown in Figure 1 come from?
Thanks for catching this. We used the Smith et al (2008) in situ SST analysis prior to November 1981. This is now noted in the text.
2) Page 5, line 7, the reference is Smith et al. (2008) not Reynolds et al.
Fixed. Thank you.
3) There is a double period on page 7 line 15.
Fixed.
Reviewer #2:
GENERAL COMMENTS
I like the fact that this article indicates that we should investigate this asymmetry further. However, while reading it I get the feeling that some explanation is missing. I understand that the authors do not have the intention to solve this problem in this article. However, if they have some ideas about it I would appreciate a slightly more extended discussion.
We struggled with this issue in the original and since we don’t have an explanation, we limited discussion of the possibilities. In view of the reviewers’ comment though we have added a little more discussion in the final section. It is speculative and therefore intentionally short.
MAJOR COMMENT
To what extend are the differences between 1950-1976 and 1980-2008 a result of the different datasets used? After 1980 SST has been observed by satellites, whereas before 1980 it was reconstructed mainly from scattered ship observations. This could in principle influence details of the evolution of El Niño and La Niña, such as phase propagation, substantially.
The addition of satellite measurements for SST beginning in the early 1980s does not affect the sense of phase propagation. We used two completely independent in situ only data sets for the post-1980 period, one based on moored measurements from the TAO/TRITON array and one based on MBTs/XBTs. They both show unambiguous eastward propagation of El Niño events and westward propagation of La Niña events.
MINOR COMMENTS AND SPELLING ERRORS
p2 line 3: add the abbreviation '(SST)'
p2 line 4: 'after mid- to late-' -> 'after a mid- to late-'
p2 line 14: remove 'cycle'
All the above changed as suggested.
p3 line 11: '1-3 seasons' Is it clear to readers that a season is 3 months in this context?
ModifedModified to “…up to 3 seasons (9 months)…”
p3 line 15: and further throughout the text: 'El Niños' -> 'El Niño events' (I believe in Spanish you can't use the plural in this way)
Although the reviewer is correct from a linguistic point of view, the terms El Niño and La Niña are now part of the technical oceanographic lexicon (like “tsunami”) and therefore not subject to the strict grammatical rules of Spanish. Our preference is not to add “events” after each plural reference to El Niño and La Niña.
p3 line 17: Can you explain very briefly why phase propagation can occur at all?
Some elaboration is now included.
p5 line 19: probably you can refer to Table 1 here.
Good suggestion. Done. Thanks.
p6 line 16 and further: Can you somehow quantify the direction of phase propagation? For instance using the Trans Nino Index (TNI: the lag of the zonal SST gradient between Nino12 and NINO4 with the Nino3 index, Trenberth & Stepaniak (2001)).
Introducing the TNI here will require more elaboration than space will allow. We have quantified the magnitude and direction of propagation though through regression analysis. We also add a short description of phase speed as estimated from correlation analysis.
p7 line 11: 'emphasizing the importance of remote forcing and ocean wave dynamics in the development of ENSO SST anomalies.' I think it emphasizes the role of advection over the climatological SST gradient just as well. Given that the heating in the composites 2c and 2d is concentrated strongly in the central Pacific, where the role of upwelling is small, the composites show clearly that the low-frequency features common to all ENSO events except the two very large ones are dominated by the surface mode.
Good point. We have not gotten into discussion of “surface” vs. “thermocline” modes here, but refer to the proximate cause of SST anomalies near the date line which is zonal advection.
p7 line 13: The sentence doesn't run properly.
p7 line 15: delete one dot
p8 line 3 and 21: For a non-native speaker: what does 'q.v.' mean?
Changes made to all the above to correct or clarify.
p8 line 20: You might want to list these double cold peaks in Table 1 as well.
We have modified the discussion since for some La Niñas it is not so much a double peak as it is simply a persistence of cold anomalies over a second year. We prefer not to modify the table with additional qualifiers to parse the cold events into long vs double peaked events and so have chosen to leave it as it is.
p9 line 5 and 6: The sentence doesn't run properly.
Modified for clarity.
p9 line 14: The sentence doesn't run properly. I suggest: '...eastern Pacific and then develop concurrently in the central Pacific while after the regime shift they show ...'
Thank you. We modified as suggested.
p11 line 8: I believe the correct spelling is 'Alexey'
Corrected. Thank you.