Influence of ENSO on North American subseasonal surface air temperature variability

Martineau, Patrick; Nakamura, Hisashi; Kosaka, Yu

doi:https://doi.org/10.5194/wcd-2-395-2021

Articles | Volume 2, issue 2

https://doi.org/10.5194/wcd-2-395-2021

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

https://doi.org/10.5194/wcd-2-395-2021

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

Articles | Volume 2, issue 2

Research article

|

28 Apr 2021

Research article |

| 28 Apr 2021

Influence of ENSO on North American subseasonal surface air temperature variability

Patrick Martineau, Hisashi Nakamura, and Yu Kosaka

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Final revised paper (published on 28 Apr 2021)
Preprint (discussion started on 13 May 2020)

Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Comment on underlying mechanisms', Paul Pukite, 13 May 2020
- AC2: 'Reply to 'Comment on underlying mechanisms'', Patrick Martineau, 23 Sep 2020
RC1: 'comments', Anonymous Referee #1, 07 Jun 2020
RC2: 'reviewer comments', Anonymous Referee #2, 17 Jun 2020
AC1: 'Replies to all reviewer comments', Patrick Martineau, 23 Sep 2020

Peer-review completion

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

AR by Patrick Martineau on behalf of the Authors (01 Oct 2020) Manuscript

ED: Referee Nomination & Report Request started (02 Oct 2020) by Yang Zhang

RR by Hai Lin (21 Oct 2020)

RR by Anonymous Referee #2 (30 Nov 2020)

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

This study is interested in the influence of tropical Pacific SSTs on
subseasonal variability in North American SAT during the winter season. The
authors use SVD, regression/correlation and composite analyses to investigate
how ENSO affects subseasonal variability through modulation of subseasonal
eddies - specifically, via changes to the vertical structure of the eddies
which have bearing on the amount of baroclinic energy conversion that occurs.

The revised manuscript has gone a long way towards addressing a number of
concerns about the clarity, interpretation and statistical significance of the
study, and I very much appreciate the work the authors have put in. The subject
is interesting and relevant to improving our understanding of climate dynamics,
as improving near-term climate predictions, and better understanding the
large-scale conditions for extreme events. It would be good to see a few final
issues resolved before publication.

MAJOR COMMENTS

1. Thanks for the explanation of why this SVD technique was used over
traditional ENSO indices. I agree it is worthwhile exploring the "flavour" of
tropical Pacific variability associated with SSV of temperature over North
America. As I understand, it turns out that the SST pattern from the SVD is
ENSO-like, so it doesn't really give us any new information on "flavours" -
other than perhaps the discussion of the residual in section 3.6. If so, then
it seems okay to leave the title as is, but in other places, this result and
implications should be clarified (e.g., ENSO should be replaced by "tropical
Pacific variability" in places like the first line of the abstract and title of
section 3.2; the abstract and summary should convey that the important thing
for North American SSV turns out to be pretty much ENSO). Otherwise, the nice
addition/explanation on L105 is completely disconnected from the rest of the
manuscript.

2. The original review included a comment about the portion of extratropical SSV
related to ENSO. I noted that it seemed important to establish this up-front,
since later on in Fig. 10, you show an SSV signal unrelated to your ENSO index
(SVD1) that is both substantial in amplitude and very similar to the
ENSO-related signal. The addition of Fig. 1 showing the climatology is very
useful, and I see now about 10% of the local interannual variance is related to
ENSO, compared to 50% unrelated. Can you explain a bit more then why you
conclude that ENSO plays a prominent role in modulating SSV over North America
(L356)? Is the idea that the 10% we get from ENSO is at least predictable,
compared to the rest of the SSV that is just internal atmospheric variability?
It seems important to mention this in the abstract also.

3. The results would be more compelling if the statistical tests were a bit
more systematic, to allow the reader (and the interpretation/discussion) to
focus on the robust signals.
- some figures use a 95% significance level, others use a 90% level
- also, I believe in some cases where "confidence level" is used, it should be
"significance level"
- some composites are defined with +/- 1 values of SVD1, others with +/- 0.5
- the correlations in Fig. 6 are quite noisy (spatially) and use a rather
generous significance level - it seems like field significance should be tested
- no significance indicated on Fig. 9, which explores the mechanism

4. Section 3.5 is clearer than it was previously. However, I'm not sure the
reasoning hangs together with the results as they're presented - it seems some
of the arguments would need to be backed up by more rigorous analysis of the
temperature distributions and how much they overlap. For example, I don't think
we can conclude whether the winter-mean sets the frequency of extremes by
shifting the distributions (L322), or whether the presence of a few extreme
days determines the winter-mean. Perhaps it would be better to make the
discussion more general overall - in some regions it seems the change in SSV
broadens the distribution towards one side or the other (cold or warm), and in
other regions, while in other regions, we see what may be more a shift. And
then show some histograms to bolster the discussion?

OTHER POINTS

- L173: "North America" might be more straightforward than "conterminous..."
- L173-174: lower-case "northwest-southeast-tilted"
- L143 and L179: slightly different filter details for high-pass
- L231: seems Fig. 6 is mentioned in the text before Fig. 5
- L239: How was 1500 chosen as the optimal number of resamples?
- L292: "Differences in the Z500..." - where do we see this?
- Captions should include more details so that the reader need not go back to
the text to look up information, abbreviations, etc.
- Fig. 3: colour bar for energy conversion should probably be adjusted
- Fig. 8: nice with the crosses, but they don't show up well in this colour

Hide

ED: Publish subject to minor revisions (review by editor) (09 Dec 2020) by Yang Zhang

Thank you for your detailed revision and reply to the reviewers’ comments. Based on the second round of reviews and my own evaluation, the manuscript may be acceptable for publication after some minor revision. Please address all remaining concerns raised by the referees, especially referee #2’s major comments. For the revision, I also have some suggestions for consideration.

1. In section 3.2, the authors need to make a clearer statement on the relation between ENSO and SVD1_SST, which also refers to referee #2’s major comment 1. Based on the results in Tab1, correlations between the time series of SVD1_SST and Nino 3 (Nino 3.4) index already reach -0.96 (-0.94), suggesting that SVD1_SST mostly reflects the variability of ENSO. I think this is the main rationale that SVD1_SSV (and the authors’ following analysis) can greatly represents ENSO’s influence. However, compared to this evident result, the authors’ argument on the relation between ENSO and SVD1_SST in the text seems a bit weak and less straightforward.

2. In section 3.5, I suggest the authors revise their argument on the relation between winter-mean state and extreme temperature events (i.e. line 321-322), which also refers to referee #2’s major comment 4. The winter-mean state is the average of the day-by-day temperature in winter season, in which the extreme events always make considerable contribution. Thus, the mean state and extreme events are intrinsically coupled. Merely from Figure 10, it is not convincing enough to conclude that the mean state sets the frequency of the extremes.

3. Line 161-162: the domain integrated energy flux term might be small over some region but not zero. For the west coast of North America, as it is in the downstream region of the jet stream, the contribution of this term may be not negligible. It is fine that the authors focus on energy transfer/conversion terms, but the description of the energy flux term need to be more accurate.

4. Acknowledgements: I think the reviewers deserve a mentioning here.

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AR by Patrick Martineau on behalf of the Authors (16 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (04 Mar 2021) by Yang Zhang

AR by Patrick Martineau on behalf of the Authors (11 Mar 2021) Author's response Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Patrick Martineau on behalf of the Authors (21 Apr 2021) Author's adjustment Manuscript

EA: Adjustments approved (22 Apr 2021) by Yang Zhang

Short summary

To better understand the factors that impact the weather in North America, this study explores the influence of the El Niño–Southern Oscillation on wintertime surface air temperature variability using reanalysis data. Results show that La Niña enhances subseasonal variability over western North America by amplifying the baroclinic conversion of energy from the winter-mean circulation to subseasonal eddies. Changes in the structural properties of eddies are crucial for this amplification.