Subtle influence of the Atlantic Meridional Overturning  Circulation (AMOC) on seasonal sea surface temperature  (SST) hindcast skill in the North Atlantic

Carvalho-Oliveira, Julianna; Borchert, Leonard Friedrich; Duchez, Aurélie; Dobrynin, Mikhail; Baehr, Johanna

doi:https://doi.org/10.5194/wcd-2-739-2021

Articles | Volume 2, issue 3

https://doi.org/10.5194/wcd-2-739-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wcd-2-739-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 2, issue 3

Research article

|

06 Aug 2021

Research article |

| 06 Aug 2021

Subtle influence of the Atlantic Meridional Overturning Circulation (AMOC) on seasonal sea surface temperature (SST) hindcast skill in the North Atlantic

Julianna Carvalho-Oliveira, Leonard Friedrich Borchert, Aurélie Duchez, Mikhail Dobrynin, and Johanna Baehr

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Final revised paper (published on 06 Aug 2021)
Preprint (discussion started on 16 Sep 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review Oliveira et al.', Anonymous Referee #1, 20 Oct 2020
- AC1: 'Response to Referee #1', Julianna Carvalho Oliveira, 01 Feb 2021
RC2: 'Review of 'AMOC fingerprints influence seasonal SST predictability in the North Atlantic' by Oliveira et al.', Anonymous Referee #2, 02 Nov 2020
- AC2: 'Response to Referee #2', Julianna Carvalho Oliveira, 01 Feb 2021

Peer-review completion

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

AR by Julianna Carvalho Oliveira on behalf of the Authors (17 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 Feb 2021) by Tim Woollings

RR by Anonymous Referee #1 (07 Mar 2021)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

The authors have responded to by comments and criticism, and the manuscript has been improved. The new analysis of ocean heat transport convergence is useful, and the new figure 9 summarizing prediction skill is most helpful. However, the new figure 9 also highlights what is still my major concern. Throughout the text the authors highlight the skill differences consistent with their proposed mechanism, but these differences are sometimes marginal and (based on the new figure 9) do not seem to be significant. Considering that this is the key result of the paper, this apparent lack of significant differences should be better discussed.

To conclude, although the manuscript has been improved, I still believe that it is in need of some substantial revisions before it is ready for publication. Detailed comments are found below.

Major comments:
1. Differences in prediction skill: As mentioned above, it is not clear from the text and figures to what extent there is a significant increase in prediction skill by considering the state of the AMOC. On l.310 it is stated that “significant skill difference (defined by non-overlapping confidence intervals) between strong and weak AMOC phases”, but in my opinion the differences in skill shown in figure 9 do not reflect how the authors present the results and conclude. Comparing with the similar figure 11 in Borchert et al. (2018), it looks like the skill differences in the present study are much smaller (less significant). Hence, I think the presentation and discussion of skill differences could still be improved.

2. Ocean heat transport convergence: The authors assess OHT convergence as the difference between two latitude bands (l.109-110, l.200-201). Why not calculate the actual convergence? From the location of Box 1 and 2 it is hard to believe that zonal transports are negligible. This should at least be checked and mentioned in the text.

3. Surface heat fluxes: The authors find negative correlations between SST and surface heat fluxes, i.e., the ocean forcing the atmosphere (l.243). However, as stated in the manuscript (l.228), it is known from observations that seasonal SST anomalies are strongly linked to atmospheric forcing. Why would the model be different, and would this in any way influence your interpretation of your results?

Minor comments:
l.3: It is not clear what “a seesaw-like mechanism” means.
l.24: You should consider explaining the SST anomalies associated with the tripole.
l.37-41: Suggest to move these sentences up to l.34 (before “[Here] We evaluate…”)
l.44: delete “as”
l.45-46: “by sub-selecting ensemble members that meet certain physical criteria, thus filtering atmospheric noise in the ensemble” -> I think this argument needs to better explained.
l.47-48: I don’t see the logic between the first and second part of this sentence (“…which is why it focuses”).
l.133-135: The use of smoothing is still confusing. In their reply, the authors state that “applying the low-pass filter only for plotting time series (e.g. Fig. 1, 4) , but not for any analysis of seasonal means”. This is not easy to understand from the text. Are the grey lines in Fig.1 used/necessary? You could rather show some form of error bars.
l.153: “up to 7 months” – ahead?
l.156-157: I don’t think you need a new paragraph here.
l.160-161: I’m still not convinced that any pronounced displacement of correlation is seen along the northern boundary of the STG. Could you help the reader somehow by e.g., adding the mean barotropic streamfunction to one of the panels?
l.177: “Hence,…” Not sure this sentence is consistent with the previous two, but rather agrees with what is stated on l. 173.
l.186: No need for a new paragraph here
l.203-205: The authors state that in the tropical lobe (Box 1) the correlation is significant (negative), whereas in the subtropical lobe (box 2) the correlation is weakly positive. However, in Fig.5 it is stated the mean correlation for Box 1 is -0.33 and +0.46 for Box 2. So I’m not sure I understand the authors interpretation here. Also, as the relation between AMOC and SST vis OHT convergence/divergence is central in the mechanism of D16, I think these correlations (or lack thereof) should be discussed in more detail (l.209-210 just states “other factors”, could be elaborated).
l.216: “an assessment of an attribution” – unclear
l.217 (and elsewhere): the authors refer to other drivers than AMOC as “non-oceanic”. However, this is not justified and there are also other sources of oceanic variability.
l.238-239: Some repetition, consider rewriting.
l.273-275: Some repetition in the description of skill. Consider rewriting.
l.288-289: How does this statement resonate with Fig.9? Except for MAM in Box 1, it seems like the confidence intervals are overlapping for “strong AMOC” and “all years”.
l.299: “skill differences we find agree to some extent with D16’s physical mechanism” – the authors should better explain what skill differences they refer to and more specifically how this relates to D16’s mechanism.
l.308: Again, how do relate this to what is shown in Fig.9? In Box 1 there is a skill decrease for “strong AMOC” versus “all years” (although overlapping).
l.310: “only during summer a significant skill difference” – Confidence intervals seem to be overlapping for JJA. Ans what about MAM in Box 1?
l.353: Better skill for MAM in box 1?
l.399-400: Again, from Fig.9 skill improvement is only seen in JJA (but not significant?).
l.402-403: This I assume refers to only the small patch also referred to earlier in the text. The area-averaged skill is still only 0.4 (Fig. 9).
l.407-408: I know I’m repeating myself, but the skill improvement the authors refer to is marginal for DJF and not significant for JJA.
Figure 4: It’s very hard to see the significance lines.
Figure 9: As in Borchert et al (2018), you should also add the persistence skill here as a comparison.

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RR by Anonymous Referee #2 (08 Mar 2021)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

The manuscript has greatly improved since the first version. My main concern with the paper is section 3.3.1. This section contains some of the main conclusions of this paper regarding the improvements in skill, however this mainly relies on visual comparison of the maps and it is not very robust. Perhaps computing maps of correlation differences including the statistical significance, as done in seasonal and decadal forecast studies, would help the clarity and would be a more robust way of determining skill improvements, providing stronger conclusions.

Comments by line:

Line 144: AMC-EKM -> AMOC-EKM

Line 153: Second ‘subtropical’ should be ‘tropical’?

Lines 189-196: Check this paragraph as I believe that several sentences reference wrong panels of figure 4.

Lines 203-205: In figure 4e the correlation is -0.33, while in figure 4f the correlation stated is 0.46 (although the latitudinal bands are inconsistent with the figure caption). Are these values correct? If so, how come the latter is referred to as 'weak' even though it's magnitude is greater than the former?

Figure 4 caption: The latitude bands in the caption for f) are different to what is indicated in figure.

Line 258: ‘less’ is repeated.

Lines 288-289: Concluding that there is an improvement in ACC skill over the subtropics in the strong AMOC cases with respect to the entire time period by visually comparing the maps does not seem very evident to me, perhaps only in JJA. As indicated in my previous review comments, the magnitude of the skill greater but the regions of positive skill are smaller. Furthermore, in Fig. 9, you have shown that JJA has greater skill in strong AMOC event, but this is not the case for DJF nor MAM.

Lines 309-311: Figure 9 is a nice addition to the paper. It would help the discussion if this figure would include if the correlations and the differences in correlations are statistically significant. It would provide more robust results.

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ED: Publish subject to revisions (further review by editor and referees) (16 Mar 2021) by Tim Woollings

AR by Julianna Carvalho Oliveira on behalf of the Authors (16 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 Jun 2021) by Tim Woollings

Short summary

This work questions the influence of the Atlantic Meridional Overturning Circulation, an important component of the climate system, on the variability in North Atlantic sea surface temperature (SST) a season ahead, particularly how this influence affects SST prediction credibility 2–4 months into the future. While we find this relationship is relevant for assessing SST predictions, it strongly depends on the time period and season we analyse and is more subtle than what is found in observations.