Technical note: Characterising and comparing different palaeoclimates with dynamical systems theory

Messori, Gabriele; Faranda, Davide

doi:https://doi.org/10.5194/cp-17-545-2021

Articles | Volume 17, issue 1

https://doi.org/10.5194/cp-17-545-2021

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

https://doi.org/10.5194/cp-17-545-2021

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

Articles | Volume 17, issue 1

Technical note

|

02 Mar 2021

Technical note |

| 02 Mar 2021

Technical note: Characterising and comparing different palaeoclimates with dynamical systems theory

Gabriele Messori and Davide Faranda

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Final revised paper (published on 02 Mar 2021)
Preprint (discussion started on 27 Aug 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review of “Technical Note: Characterising and comparing different palaeoclimates with dynamical systems theory” by Messori and Faranda', Anonymous Referee #1, 13 Sep 2020
- AC1: 'Reply to Reviewer #1', Gabriele Messori, 13 Nov 2020
RC2: 'review', Anonymous Referee #2, 14 Sep 2020
- AC2: 'Reply to Reviewer #2', Gabriele Messori, 13 Nov 2020

Peer-review completion

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

ED: Reconsider after major revisions (24 Nov 2020) by Martin Claussen

AR by Gabriele Messori on behalf of the Authors (08 Dec 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Dec 2020) by Martin Claussen

RR by Anonymous Referee #1 (13 Dec 2020)

RR by Anonymous Referee #2 (14 Dec 2020)

Suggestions for revision or reasons for rejection

The authors have made a commendable effort to answer every point raised by the reviewers in detail. Almost all criticism has been adequately addressed and the revised manuscript is substantially improved such that I now recommend its publication in CP. Upon re-reading the manuscript, I have however found a few minor issues.

Section 2.1 I overall approve of the "raindrop" analogy although it does become a little strained when the vegetation is added. Unfortunately I don't have a better idea either. Maybe it would have been more consistent to show a stylized image of a raindrop flowing down a rock instead of the Lorenz attractor in Figure 1. The zoomed in parts of the trajectories could easily represent paths of rain drops instead of the abstract trajectory along the "butterfly". On the other hand most readers with any kind of climate science background are probably familiar with L63.

Answer to reviewer #2 major comment 2: I would like to clarify my original comment because I don't think it came across properly. If the prp field contains only ones and zeros, the distance between such fields is effectively an integer. Regardless of the regular or fractal nature of the binary point set, the distance d is a fundamentally different kind of random variable than for slp and z500 because its density consists of a finite number of point masses at whole numbers. The members of the GEV family, on the other hand, are continuous functions. I appreciate that your methodology may nonetheless be applicable but I do not see the "robust theoretical arguments": In Lucarini et al. (2012), the attractor of the system is a fractal but the considered distance measure is continuous and not discrete. Faranda et al 2017a present an empirical study which may give evidence that this strategy for precipitation differences is appropriate in practice but they do not give theoretical arguments for its validity. I therefore believe that intermittent variables like precipitation are not ideal targets for your kind of analysis.

L.136, equation (3): What do you actually mean by the "U"-symbol? I thought this was the symbol for the union of two sets but the expressions left and right of it are not sets but logical statements in which case you probably mean "v" for a logical "or"? In either case I think what you actually want is a logical "and" which would be an inverted "v" (in Faranda et al 2020 it is formulated in terms of conditional probability using the "|"-symbol which also means that both thresholds must be exceeded): If you actually meant "or", the numenator would be the number of cases where either g(x) or g(y) exceed their thresholds which is at least as large as the number of cases where only g(x) is above s_x; alpha would then be bounded from below by 1.

L. 306f. "significantly increased precipitation across the southern portion of the domain [are] favored by negative SLP and Z500 anomalies to the North of the strongest precipitation anomalies [...]. These are likely the footprint of a strengthened heat low [...]"
Heat lows are generally shallow structures with negative pressure anomalies in the lower levels and divergence with associated positive pressure (or geopotential) anomalies above. In their discussion of West African heat lows, Lavaysse et al. (2009) explicitly state that "A heat low is an area of low atmospheric pressure near the surface resulting from heating of the lower troposphere and the subsequent lifting of isobaric surfaces and divergence of air aloft." If anything, I would expect a positive geopotential anomaly at 500hPa associated with this kind of pressure system. Your interpretation of the negative Z500 anomaly as an indicator of increased heat low activity is therefore questionable.

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ED: Publish subject to minor revisions (review by editor) (29 Dec 2020) by Martin Claussen

AR by Gabriele Messori on behalf of the Authors (07 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 Jan 2021) by Martin Claussen

AR by Gabriele Messori on behalf of the Authors (28 Jan 2021) Manuscript

Short summary

The palaeoclimate community must both analyse large amounts of model data and compare very different climates. Here, we present a seemingly very abstract analysis approach that may be fruitfully applied to palaeoclimate numerical simulations. This approach characterises the dynamics of a given climate through a small number of metrics and is thus suited to face the above challenges.