Inter-annual variability in the tropical Atlantic from the Last Glacial Maximum into future climate projections simulated by CMIP5/PMIP3

Brierley, Chris; Wainer, Ilana

doi:https://doi.org/10.5194/cp-14-1377-2018

Articles | Volume 14, issue 10

https://doi.org/10.5194/cp-14-1377-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-14-1377-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 10

Research article

|

01 Oct 2018

Research article |

| 01 Oct 2018

Inter-annual variability in the tropical Atlantic from the Last Glacial Maximum into future climate projections simulated by CMIP5/PMIP3

Chris Brierley and Ilana Wainer

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Final revised paper (published on 01 Oct 2018)
Supplement to the final revised paper
Preprint (discussion started on 28 Nov 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Reviewer n.1', Anonymous Referee #1, 15 Jan 2018
- SC1: 'Methodolgy clarification', Christopher Brierley, 07 Mar 2018
- AC1: 'Author Response to Editor and Referee', Christopher Brierley, 03 Apr 2018
RC2: 'Editor review of the manuscript', Pascale Braconnot, 28 Mar 2018
- AC1: 'Author Response to Editor and Referee', Christopher Brierley, 03 Apr 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (04 Apr 2018) by Pascale Braconnot

AR by Chris Brierley on behalf of the Authors (13 Jun 2018) Manuscript

ED: Referee Nomination & Report Request started (07 Jul 2018) by Pascale Braconnot

RR by Anonymous Referee #1 (23 Jul 2018)

Suggestions for revision or reasons for rejection

This work investigated the change in the leading modes of the Tropical Atlantic Variability the Atlantic Meridional Mode (AMM) and the Atlantico Niño (ATL3) in different climate scenarios: the historical, the last glacial maximum, the mid-holocene and future simulations in the multi-model ensemble of the PMIP3/CMIP5. Authors used this set of experiments in order to find robust signal of change in the Tropical Atlantic Variability. They found that all models across all experiments are able to represent main characteristics of dominant modes of variability in the Tropical Atlantic in spite of the mean state bias.

After the revision, the paper is improved and authors addressed carefully referee comments on the previous version of the manuscript. I am fine with that. More work is needed on some typos or considering to rephrase some sentence. I therefore I recommend a minor revision following comments below and I suggest to publish the paper in a close version to the current one.

Minor Comments:
There are some typos in the figures: the “i” somehow appears as an “l”. Please correct.
Pg. 2, Ln 29: Typo: “to follows” remove the s.
Pg. 2 Ln 30-31: The jump in the flow is to abrupt. Consider rephrasing. This sentence “Schneider et al. (2014) presents a comprehensive review of the dynamics of the ITCZ and associated meridional shifts” seems to be out of the contest.
Pg. 3 Ln 5: A blank is missing after the full stop.
Pg. 3 Ln 19: “positioning” -> position
Pg. 4 Ln 2: I would mention the last millennium here since your are not using it in the study.
Pg. 4 Ln 5: “The climatology … is used” -> is calculated.
Pg .6 Ln 27: “ The Atlantic is warmest on the Equator” is the warmest …. ? I do not understand… Consider to rephrase.
Pg. 7 Ln 8-9: …they have too much rain falling south of the ITCZ” -> … they show too strong positive precipitation anomaly on the southward flank of the ITCZ. Please be more accurate in the description of Figure and use proper English.
Pg. 7 Ln 9: “ whilst the models simulate too much convection over N.E. Brazil” -> rephrase. You are not showing specifically convective precipitation only, but instead the total precipitation. I fully understand that the monsoonal rainfall is mainly due to the convection, but it is inappropriate to use the term convection here.
Pg. 8 Ln 18: typo: onto not on to.
Pg. 8 Ln 29: “… regressions with from the reanalysis”. Remove with.
Pg. 8 Ln 30: “ PPT patterns” … what is PPT pattern?
Pg. 11 Ln 21: typo “timeseries” -> time series.

Major Comments:
Pg. 2 Ln 1: “…the latitudinal displacement of the rain producing Inter-Tropical Convergence Zone”. I think that the rain belt is produced by the convergence of the surface winds into the ITCZ where air masses converge and uplift inside the Hadley Cell. This is where the rain comes. Consider to rephrase.
Pg. 2 Ln 15: “most well-known mode of TAV is” you did not specified TAV acronym before in the Introduction. Correct please.
Pg. 3 Ln 22: “The leading mode is a meridional mode…”. The leading mode of what? It is not clear in the text.
Pg. 3 Ln 24: “The inter hemispheric: SST-gradient is accompanied by a cross-equatorial atmospheric flow in the same direction”. I do not understand what you mean. I think that this sentence is incorrect. Just to clarify the concept: if the northern hemisphere is warmer than the southern hemisphere (e.g. strong inter-hemispheric thermal contrast), then the cross-equatorial (winter - SH) Hadley Circulation is stronger than the summer counterpart. Therefore the Atmospheric Heat transport (AHT) is southward, following the cross-equatorial flow. From McGee et al., 2014: “In the solsticial seasons, the (cross-equatorial) Hadley circulation transports ∼2.5 PW of heat into the winter hemisphere, and the magnitude of this seasonal AHTEQ is linearly related to how far the ITCZ – located near the boundary between the Hadley cells – migrates into the summer hemisphere”. Therefore, the more the ITCZ migrates into the summer hemisphere, stronger the AHT toward the winter hemisphere is.
Continuing from McGee et al., 2014: “…the ITCZ’s position in the Northern Hemisphere and the associated southward AHTEQ is driven by the cross-equatorial ocean heat transport (OHTEQ), which has a magnitude of ∼0.4 PW and is principally driven by the Atlantic Ocean’s meridional overturning circulation (AMOC). Changes in ITCZ position, and thus in AHTEQ, may therefore provide insight into past changes in heat transport by the AMOC; alternatively, they may reflect changes in the hemispheric balance of TOA energy fluxes (e.g., asymmetries in hemispheric albedo). [..] a southward shift of the ITCZ during Heinrich Stadial S1 would increase AHT into the NH, compensating for a reduction or shutdown of the AMOC.” Consider to rephrase at Ln 24 in order to make it clear what you mean.
Pg. 3 Ln 1-18: Maybe it is matter of personal taste, but I suggest to work a bit on this paragraph. It seems you are listing only facts. Try to rephrase.
Pg. 4 Ln 14 “ although only in the case of GFDL’s last glacial maximum run was this for scientific rather than resourcing reasons.” It is not clear to me this sentence. Consider rephrasing.
Pg. 8 Ln 18-19: This sentence seems out of contest. Why are you talking about ENSO now?
Figure 8: Indeed, there are big differences between ATL3 and AMM with respect to Fig.1 in the response to reviewers recomputed using the common models for the PI ensemble mean. Can you comment on that? Furthermore, correct the colorbar: symmetric labels with respect to the zero.
Figure 10 and 12: Correct the colorbar: symmetric labels with respect to the zero.
Figure 13 and 14: Add a legend with colored dots associated to each experiment. Also Numbers associated to each model would be interesting to show, as well as showing the multimodel ensemble mean for each experiment.

Suggested literature:
McGee, D., Donohoe, A., Marshall, J., & Ferreira, D. (2014). Changes in ITCZ location and cross-equatorial heat transport at the Last Glacial Maximum, Heinrich Stadial 1, and the mid-Holocene. Earth and Planetary Science Letters, 390, 69-79.

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RR by Anonymous Referee #3 (26 Jul 2018)

Suggestions for revision or reasons for rejection

This is a useful paper that examines the response of Atlantic climate variability (the Atlantic Nino mode and the Atlantic Meridional Mode) to climate change in the past (Holocene, LGM) and future. Although there is no significant scientific insight gained from the work, the paper presents the first systematic examination of Atlantic variability across the three sets of experiments and therefore provide a useful reference that can be used for future studies.

Scientifically, perhaps, the most interesting point is that they have no conclusion what caused the change of these modes. In particular, there is no relation between E-W SST gradient and the amplitude of ALT3, and there is no relation between the N-S SST gradient and the AMM, although both relationships have been suggested as a mechanism in previous works. Although this is a negative conclusion, it is interesting in that sense that the result here questions our current understanding of the mechanism of these Atlantic modes. Indeed, the mechanism of Atlantic variability has been studied much less than their Pacific, and perhaps the Indian Ocean, counterparts. This study is a confirmation of this.

I recommend the publication of the paper after addressing one major question below.

It will be also interesting to show ENSO variability here for two purposes.
First of all, it gives a contrast example in the Pacific. Second, and perhaps more important, ENSO exerts significant impact on tropical Atlantic and may therefore have some impact on the response of the variability too. If, for example, there is no relation between the change of ENSO strength and the Atlantic modes, it is also an interesting negative conclusion that the response of ENSO is not causing the change of the Atlantic modes.

In this regard, it will be interesting to show the correlation among: ALT3, AMM and ENSO (pehrpas NINO3.4), may be as a table, in the observation and the models.

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ED: Publish subject to minor revisions (review by editor) (02 Aug 2018) by Pascale Braconnot

AR by Chris Brierley on behalf of the Authors (06 Aug 2018) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (23 Aug 2018) by Pascale Braconnot

AR by Chris Brierley on behalf of the Authors (29 Aug 2018) Author's response Manuscript

ED: Publish as is (17 Sep 2018) by Pascale Braconnot

AR by Chris Brierley on behalf of the Authors (17 Sep 2018)

Short summary

Year-to-year changes in rainfall over Africa and South America are influenced by variations in the temperatures of tropical Atlantic variability. Here we investigate how these variations behave under climate change using a series of multi-model experiments. We look at how cold and warm climates of the past relate to future shifts in variability.