As you will see from the reports, referee 1 takes the review that the revised paper is now suitable for publication, referee 2 takes the view that the revisions have not significantly improved the paper and that it should therefore be rejected.

Whilst I have some sympathy with the view of referee 1 that the paper contains sufficient interesting material to warrant publication, notwithstanding the significant remaining uncertainty, I am also sympathetic to referee 2's view that in several aspects the paper is not of the standard required for publication. Referee 2 is essentially saying that the paper should not be published until some of the uncertainty and speculation can be resolved by model experiments. I would be prepared to take a different view if the paper can be revised to address referee 2's particular concern about proper review of the literature on changes in the Hadley circulation, if speculative discussion can be minimised and, wherever uncertainty remains, that can be made absolutely clear in the text. I'm not particularly convinced that the current title is helpful -- it needs to be made clear that any role of lower stratospheric cooling is 'possible', not 'established'.

There are a number of places in the text where the information presented seems non-essential -- e.g. the 4th paragraph of Section 4 which provides a lengthy/detailed explanation of why it might not be possible to conduct model studies -- that could surely be said much more concisely, or the logic seems odd -- e.g. later in Section 4 the sudden decrease in stratospheric water vapour in 2000 is mentioned, that was a short-term event that does not seem to have any simple relation to the longer term cooling discussed by Abalos et al and others.

You are welcome to submit a further revised version of the paper, together with responses, that addresses in concrete ways the continuing concerns of Referee 2 and some of the concerns that I have expressed above.

I have looked carefully through your revised paper and your responses to Referee 2.

I was hoping to be able to accept the paper for publication at this stage without consulting Referee 2, but I don’t feel able to do this, since my own opinion is that Referee 2’s concerns have not been adequately addressed.

I have made some detailed comments on the paper below and I have set out three different options (A-C) for taking the paper further. Please can you specify which of those options you wish to follow and if you wish to follow B or C provide an appropriately revised version of the paper, plus responses.

I see a variety of problems with the current version of the paper:

(1) You have spent some time in your responses arguing that what the referee identifies as work on ‘change in ITCZ locations’ is only of tangential relevance to your work. (You have also very briefly mentioned this point in the revised paper at the end of Section 4 — and — minor point — your citation is incorrect.)

Your line is basically that the ICTZ is all to do with the ‘shallow branch’ in your Figure 1 and that what you are discussing is to do with the ‘deep branch’. But then looking through the paper there are many cases where you discuss quantities such as OLR, omega at 300hPa, SST — so I’m not convinced that you can separate the ICTZ from what you are discussing. You might argue that the ICTZ is a primarily oceanic feature whereas you are emphasising changes over land, i.e. that changes in convection over land in summer can somehow be considered to be independent of changes in convection over the ocean, but to me this seems highly speculative.

(2) The same applies to a lesser extent to ‘Hadley Cell widening’ or ‘tropical expansion’. You assert in Section 1 that this is, again, distinct from what you are discussing. But your argument is that the ‘expansion’ is ‘due to changes in the positions of jet streams and storm tracks’. The expansion might be manifested by those changes, but I don’t believe that anyone has clearly ruled out that the changes in say, tropical convection, are not part of the cause.

(3) You have included a new section 3.1 ‘Deep branch of the Hadley circulation’ primarily, it seems to me, to counter Referee 2’s recommendation that there is more discussion of the relation between your work and other previous work on shifts in the ITCZ/broadening of the tropics. But while this section puts on record that there is considerable variation in the properties of the tropospheric circulation with height, and that, for example, there is no single pattern of geographical variation for all physical quantities, it doesn’t convincing make the case, to me, that what you are describing can be regarded as completely independent of, say, the shift in the ITCZ and therefore that all previous discussion can be ignored. To be specific — you are essentially looking at Fig 1a and, pretty arbitrarily in my view, deciding that it can be decomposed into two parts, one composed of streamlines that reach above 100hPa and the other of streamlines that do not, that can be regarded as separate entities. This seems particularly difficult to justify for the narrow upwelling part of the circulation.

(4) You have also included a completely new section 3.5 on the ‘Stepwise seasonal transition’. This is an interesting topic, but this material has not been considered by either of the referees. So if you want to include this material then I think the only way forward is to treat this version of your paper as a new submission and start the refereeing process again.

(5) There are many detailed points in the paper which need clarification, some of them relating to Referee 2’s comments about ‘logical inconsistency and unsupported mechanisms’. I have listed these detailed points below.

OPTIONS:

(A) The current version of the paper is treated as a new submission.

(B) The current version of the paper is sent to referee 2. If referee 2 still has significant concerns then I reject and you have (A) as a possible future option. This will require you to submit a new revised version of the paper, but the changes to the existing version might, if you choose, be small.

(C) You make the changes I recommend below (or argue convincingly against them) and I then accept the paper on the basis that I am confident that Referee 2’s concerns have been addressed. The changes I have marked with * are those that I regard as essential.

DETAILED COMMENTS:

*p1 l13:

Large changes in tropical circulation, in particular those related to the summer monsoon and cooling of the sea surface in the equatorial eastern Pacific, were noted from the mid-to-late 1990s.’
— needs clarification — change to —
‘Large changes in tropical circulation from the mid-to-late 1990s to the present, in particular changes related to the summer monsoon and cooling of the sea surface in the equatorial eastern Pacific, are noted.’

[The above comment, and several other of my comments, are based on the my interpretation that the the ‘recent trends’ you are describing are represented by a change from the mid-late 1990s to the present. But this is never completely clear — and if I have misinterpreted what you mean that is by itself good reason for you to clarify what you mean by terms like ‘trend’ and ‘anomaly’ whenever you use them.]

*p1 l15: ‘deep ascending branch’ — I don’t believe this is a standard term, so you need to define it carefully. See comments above, particularly(C), and further detailed comments below. Amongst the various problems I see with this term a relatively minor one is that the terms ‘shallow’ and ‘deep’ are very likely to be confused with the shallow and deep branches of the Brewer-Dobson circulation. (These terms seem to have become widely used.)

*p1 l26: ‘Large changes in tropical circulation occurred from the mid-to-late 1990s.’ — needs clarification — change to — ‘Large changes in tropical circulation occurred from the mid-to-late 1990s to the present.’

p1 l27: Personally I find it a bit weird to describe a ‘slowdown, or hiatus, of global warming’ as a ‘large change in tropical circulation’.

p2 l3: My reading of Evan and Camargo (2011) is that they are very careful NOT to say that there is a long-term systematic change over the period that they consider. (They use terms like ‘interannual variation’.) The Wang et al (2012) paper is a brief Comment on another paper, and is disputed by the authors of that paper.

p2 l6: ‘Indeed, recent numerical model studies show that cooling of the tropopause impacts the intensity of tropical storms as
well as SSTs’ — change to (at least according to my interpretation) — ‘Indeed, recent numerical model studies show that cooling of the tropopause, as well as SSTs, impacts the intensity of tropical storms’

*p2 l16: ‘the anomalous tropical circulation of the 1990s’ — this comes back to the question of what you mean by ‘trends’ or ‘anomalous circulation’. The phrase you use here sounds as if you mean an anomaly that was present in the 1990s, but has since disappeared — but I don’t believe that you mean that. Please clarify.

*p2 l28: ‘In this paper, we show that the fundamental cause of the recent
decadal trend in the tropics from the mid-to-late 1990s is not the PDO, but rather a strengthening of the deep ascending branch of the summertime Hadley circulation extending into the stratosphere.’ — this statement is MUCH too strong. You surely don’t show that this is true — you suggest that it is true and provide some circumstantial evident to support your suggestion.

*p2 l31: See comment (2) above. This paragraph is at one level OK — you say that you are looking at different quantities to those being considered by those interested in tropical expansion — but I recommend that you an additional comment (which could be in your final section) along the lines of ‘For the purposes of this paper we assume that the changes that we discuss are independent from those associated with tropical expansion’ — i.e. an explicit statement that your arguments for a role for the stratosphere should not be interpreted as applying to tropical expansion (because if they did you would have to give more detail on the various mechanisms that have been proposed for that).

p4 l1: Include a very brief description (perhaps just a few words) of the criteria being used for an ‘overshooting cloud’.

**p4 l9-14: This is new material that has not been considered by the referees and should be removed if you want to follow routes (B) or (C) above.

*p4 l28: ‘Because the upwelling of the deep branch is driven by convective activity’ — unless you are defining the ‘deep branch’ in this way — in which I can’t see how you can identify the deep branch from any of the information in Fig 1 (since none of this information tells you what drives what) — this statement is pure speculation. I think that the only acceptable way to proceed is for you to define carefully what you mean by ‘deep branch’ — by certain features appearing in observational fields, rather than being caused by something — and then to say that your working hypothesis is that the deep branch can be considered independently of other aspects of the tropical tropospheric circulation (such as the ITCZ over oceans).

*p4 l31: Please refer to explicit Figure/sub-Figure numbers e.g. Fig 1b, rather than ‘left panel’ etc. One reason for that is that you are making the quite strong and at first sight surprising statement that theta_e at 925hPa is part of the ‘deep branch’ rather than the ‘shallow branch’. You should note this explicitly in the text as a further aspect of your working hypothesis.

*p5 l9: I see now that this is where you define what you mean by terms such as trend and anomaly — i.e. you are looking specifically at the difference between the 1999-2016 mean and the 1981-2010 mean. You need to make this absolutely clear much earlier in the paper — e.g. even in the first line of the paper when you talk about changes ‘from the mid to late 1990s’. Then here you should confirm that you will be using the term ‘anomaly’ or ‘anomalous …’ to mean precisely this difference.

*p5 l19: ‘Thus, the primary cause …’ — you can only use ‘Thus’ if the following statement follows logically from the preceding statement. I don’t see how your statement about the cause of cold tongues etc can possibly follow logically from the previous sentence — I guess that it is something that has been proposed/demonstrated by Xie and others. So ‘Thus’ should be removed.

*p5 l21: ‘Therefore, changes in the zonal-mean meridional circulation … can affect eastern Pacific SSTs.’ — again I don’t follow the logic at all — and I think that it is this sort of statement that troubled referee 2. The simplest change would be to remove this sentence, or you could say ‘We suggest in this paper that …’. Anything else needs much clearer supporting argument.

p5 l30: I’m not very clear about the logic here either, but neither of the referees seemed particularly worried about this.

*p6 l1: ‘The impact of the recent decadal variation …’ — this makes it sound as though what you are showing is caused by decadal variation — but, at the level of the discussion in this paper, this is simply an aspect of decadal variation. So please modify appropriately.

*p6 l25: ‘corresponding to the deep ascending branch of …’ — change to ‘corresponding to what we have identified as the deep ascending branch of’ — to make it absolutely clear that this statement depends on your working hypothesis. The same applies to p6 l16 — ‘which according to our hypothesis correspond to the locations …’

*p7 l13: ‘In addition to changes in the troposphere, Abalos et al (2015) identified …’ — this makes it sound as if Abalos et al identified changes in the troposphere. Change to ‘Alongside the interdecadal changes in the troposphere just discussed, an increasing trend … was identified by Abalos et al (2015).’

*p7 l14: ‘Because the upwelling in the deep ascending …, it is expected … may exist.’ — make it clear that it is you that expect this — there are almost certainly many other people who do NOT expect it — i.e. it would be better if this sentence was re-written along the lines of ‘we suggest that’.

p7 l31: ’These results suggest a stronger connection …’ — why ‘stronger’ — stronger than what? More importantly you should add an explicit qualifier that causality cannot be deduced from this analysis alone — you have used ‘connection between’ which is good, but the explicit qualifier is needed too.

*p7 l32: ’This is consistent with results from …’ — this is too strong a statement. Both of these papers (which are certainly interesting) were about variations on SSW-type timescales, not on interannual variability. Also the Eguchi et al study is very preliminary in the sense that only one simulation is carried out when really an ensemble is needed and the Kodera et al study is largely (but not exclusively) about changes in the TTL rather than through the depth of the troposphere. So the sentence needs to be moderated appropriately.

*p8 l8-20: There seems to be confusion between Fig 7 and Fig 8 in this paragraph.

*p9 l19: ‘Results of the above analyses are summarised in Fig 10(left).’ — change to make it very clear that you are advancing a hypothesis. It certainly isn’t the case that the results you present lead inevitably to the mechanism depicted in Fig 10. E.g. you could say ‘On the basis of the results presented in the previous sections we suggest the mechanism depicted in Fig 10.’ Then you need to include a sentence or two explaining the mechanism that Fig 10 is supposed to depict — the Fig alone is not enough.

p9 l32: ‘This time evolution tentatively suggests a causality among the variables’ — that’s fine, but I recommend including a sentence spelling out what causality you are inferring (to make sure the reader is clear about that).

*p10 l4-32: As noted previously, this section (and Figs 11-13) should to be removed unless you want to go straight to option A. (But note that on l6 you have referred to Fig 10, but surely you mean Fig 11.)

p11 l4: I still find this use of ‘changes from the middle to the end of the 1990s’ unnecessarily mysterious. It sounds as though you mean change from 1995 to 2000, but surely you don’t. In practice you must, given the quantities that you have been showing, mean the difference between 1999-2016 relative to 1981-2010 (or surely equivalently — difference between 1999-2016 on the one hand and 1981-1998 on the other). It would be much better if you could find a clear simple term for this — and use it consistently.

*p11 l11: ’Nevertheless, time lags introduced in selection of variables from summer to autumn demonstrate that the processes are related, …’ — you say ‘are related’, but earlier you have said that these time correlations suggested a causal relationship and you should use the term ‘causal’ again here (if you mean it). But ‘demonstrate’ is too strong — you should be clear that you (i.e. the authors) are suggesting a causal relationship on the basis of these correlations, but also that you accept that (much) more evidence is needed to demonstrate this conclusively.

*p12 l1-3: ‘A poleward shift in the convective zone occurred because of enhanced convective activity in the deep ascending branch of
the Hadley circulation over the continental sector.’

The first sentence is unclear — are you suggesting that the shift occurred as a result of enhanced convective activity in the deep ascending branch …’, i.e. there are two separate phenomena here and one causes the other. To me it seems to be very difficult to separate the shift on the one hand and the enhanced convective activity on the other — aren’t they different aspects of a single phenomenon. If you want to claim that there is cause and effect then you need to be much clearer on how to separate the two things. Once again, yo u need to be very clear about what exactly you mean by the ‘deep ascending branch of the Hadley circulation over the continental sector’ — Fig 1 is helpful here but it is not enough — it needs to be reinforced by clear text. Also is your separation of the tropical circulation into (i) shallow branch of HC and (ii) deep branch of HC, or is the second part ‘the deep branch of the HC over the continental sector’? If one uses Fig 1a as a basis of the separation then it is not at all clear what part can be assigned to the continental sector and what to the oceanic sector.

*p12 l2: ‘This phenomenon studied here is different from the shift of the ITCZ over the oceanic sector due to perturbations in the atmospheric energy balance such as discussed by Schneider et al. (2017).’ — again this is not at all obvious and I see the only way forward without you doing more detailed work on this is to be clear that the study reported in this paper is based on the hypothesis that changes in the ‘deep branch of the HC over the continental sector’ or whatever can be considered as independent of the shift in the ITCZ studied by many other authors — Schneider (2017) being an example and Byrne et al (2018) being another — which you see as primarily relevant to the oceanic sector. You must remind the reader explicitly of this hypothesis here, at the end of the paper.

Other comments:

I can’t find any reference in the text to Fig 5a. If it is not mentioned in the text (i.e. it is not part of the arguments made in the paper then it doesn’t seem necessary to include it).

Many of the Figures are very small. Of course if reading a paper online the Figures can be magnified, but it should be easy to read text and Figures together. I recommend that most of the Figures are enlarged so that the entire Figure (i.e. all the panels as displayed) fill most of the width of the page.

Reference:

Byrne et al (2018) Current Climate Change Reports
https://doi.org/10.1007/s40641-018-0110-5.

The paper is now very close to a form that I will feel comfortable in accepting for ACP. Please can you consider the further detailed comments below, which are primarily concerned with wording, and make appropriate changes. I will then accept the paper.

p1 l16: 'resulted from' > 'were associated with' [There is no clear evidence that the Hadley Circulation is dynamically separate from the 'cross-equatorial southerlies' and therefore that a change in the Hadley Circulation is something that can be regarded as a CAUSE of a change in the cross-equatorial southerlies.]

p1 l27: 'are noted' doesn't seem necessary -- is it a relic from a previous edit?

p2 l8: '(Emanuel et al 2013)'

p2 l32: 'fundamental cause is not ... but rather a strengthening of the deep ascending branch of the summertime Hadley circulation extending into the stratosphere' -- again in what sense can the strengthening of the Hadley circulation be regarded as a cause -- i.e. in what sense is it dynamically separate from the aspects of the tropical tropospheric circulation that you are regarding as changing? Elsewhere you have identified increase in land surface and changes in lower stratospheric temperatures as possible causes -- that potentially makes more sense to me.

p5 l1: second quotation mark missing on 'deep ascending branch'

p10 l11: '(a) Cooling in the lower stratosphere adds to the global warming in the troposphere.' -- this sounds as if cooling in the lower stratosphere REINFORCES warming in the troposphere. Do you really mean that? Are you simply saying that the result of increase in greenhouse gases is warming in the troposphere and cooling in the stratosphere?

p11 l19: Bearing in mind the referee comments, personally I think it would be much better to add a sentence here along the lines of: 'However, suitably designed numerical experiments will be needed to add further support to these ideas.'


p12 l1: 'It should also be noted that the expansion rates of the tropics should be much smaller than those reported in past studies (Staten et al., 2018).' -- 'should' isn't the correct word to use here. A possible modification would be 'are much smaller' -- but that isn't really consistent with what Staten et al (2018) are saying. In their abstract they say 'However, although theory and modelling suggest increasing GHG concentrations should widen the tropics, previous observational-based studies depict disparate rates of expansion, including many that are far higher than those simulated by climate models.' -- i.e. some estimates seem much too large, but not that all estimates seem much too large. The simplest thing would be to omit this sentence -- but you could simply say something like 'The broad range of estimates for tropical widening has recently been discussed by Staten et al (2018).'

References: The Evan and Camargo (2011) is still in the reference list, though you have removed the citation from the text. Please check general consistency between citations in text and reference list.

You have made the further revisions that I was requesting and I have therefore accepted the paper for publication in ACP.