Investigating extreme marine summers in the Mediterranean Sea

Denaxa, Dimitra; Korres, Gerasimos; Flaounas, Emmanouil; Hatzaki, Maria

doi:https://doi.org/10.5194/egusphere-2023-1709

Preprints

https://doi.org/10.5194/egusphere-2023-1709

Preprints

03 Aug 2023

| 03 Aug 2023

Investigating extreme marine summers in the Mediterranean Sea

Dimitra Denaxa, Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki

Abstract. The Mediterranean Sea (MS) has been experiencing significant surface warming, particularly pronounced during summers and associated with devastating impacts. Τhis study proposes the concept of Extreme Marine Summers (EMSs) and investigates their characteristics in the MS, using Sea Surface Temperature (SST) reanalysis data spanning 1950–2020. A marine summer may evolve as extreme, in terms of mean summer SST, under different SST substructures. Results suggest that EMSs identified in most of the basin, are formed mainly due to the warmest part of the ranked daily SST distribution being warmer than normal. Areas where the warmest (coldest) part of the ranked daily SST distribution is more variable, experience EMSs primarily due to the contribution of the warmest (coldest) part of the distribution. Marine heatwaves (MHWs) within EMSs are more intense, longer lasting, and more frequent than usual, mainly in northern MS regions. However, the relative contribution of MHWs in EMSs is more pronounced in the central and eastern basin. Furthermore, a metric is proposed to quantify the driving role of air-sea heat fluxes in forming EMSs. Results suggest that surface fluxes primarily drive EMSs in the northern half of the MS, while oceanic processes play a major role in southern regions. Upper ocean preconditioning is also found to contribute to the EMS formation. Finally, a detrended dataset was produced to examine how the SST multi-decadal variability affects the studied EMS features. Despite leading to warmer EMSs basin-wide, the multi-decadal signal does not significantly affect the dominant SST substructures during EMSs. Results also highlight the fundamental role of latent heat flux in modulating the surface heat budget during EMSs, regardless of long-term trends.

Received: 26 Jul 2023 – Discussion started: 03 Aug 2023

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

26 Mar 2024

Investigating extreme marine summers in the Mediterranean Sea

Dimitra Denaxa, Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki

Ocean Sci., 20, 433–461, https://doi.org/10.5194/os-20-433-2024,https://doi.org/10.5194/os-20-433-2024, 2024

Short summary

Dimitra Denaxa, Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1709', Anonymous Referee #1, 31 Aug 2023

Please see comments in the attached document

Citation: https://doi.org/10.5194/egusphere-2023-1709-RC1
- AC1: 'Reply on RC1', Dimitra Denaxa, 01 Nov 2023
  
  We would like to thank you for your careful reading and valuable comments, which have significantly strengthened our manuscript. We have addressed all comments and have revised the manuscript accordingly. Our point-by-point response is provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC1
RC2:
'Comment on egusphere-2023-1709', Anonymous Referee #2, 01 Oct 2023
The paper untitled “Investigating extreme marine summers in the Mediterranean Sea” - by Dimitra Denaxa Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki – investigates Marine Heat Waves (MHW) during “extreme marine summers” in the Mediterranean Sea. The authors use a reanalysis to identify MHW characteristics and understand the main drivers explaining regional differences during particularly hot summers.

In general, I found the paper is well-written with a straightforward general structure. I really appreciated efforts of authors to clearly describe their results, even if some parts could be shortened to reduce the long length of the manuscript.

However, I have several concerns from the beginning of the paper regarding methodological choices:

I do not understand why the authors decide to only consider 4 events as “extreme marine summers”. Even if Figure 4 gives an idea about the distribution of summer SST, this is only at three locations. The authors need to give a more precise definition of what they call “extreme marine summer” regarding the distribution.

My second concern is a lack of justification about applying methodologies for each “grid point”. Results gives a very statistical point of view. Is there any criterion about spatial scales? This is confusing as they give physical interpretation associated with regional patterns. I would at least explain why they are not considering EMS composites.

If I understand Figure 3 correctly, it needs more explanation since patterns might results from one “very extreme” event for instance, and might not be representative of the 4 events. This also might be an issue in other figures.

It misses a discussion about the interest for the community of this new term “ESM”.

I think these points need to be addressed before introducing this new concept. A major revision is required to clarify these points.

Minor comments:

L128 – “derives from a combination of HadISST2 and OSTIA datasets” – ERA5 SST is based on a reanalysis which includes model and observations datasets. I would reformulate.

L133 – “In addition to ERA5 (reference SST dataset), we use the CMEMS L4 satellite SST product (EU Copernicus Marine Service Product, 2022c) for the period 1982-2019, at 0.05°x0.05° grid spacing, to cross check the quality of the reference dataset against a high-resolution observational SST dataset.” What did you compare? What did you conclude?

L143 – “We then define EMSs, separately at each grid point, as the four summers with the highest average JAS SST, i.e., exceeding the 95th percentile of the 71 available summer periods from 1950 to 2020. “ As the authors want to introduce a new concept, it needs more explanation and justification.

L239 – “we provide examples of different patterns for three grid points of the domain ». Why these points? Are they representative a regional patterns?

Figure 4 – This figure is interesting, but this arises questions. I would imagine with such “Extreme Marine Summer” concept, that the spatial scales would be close to those of the basin. Maybe, the authors should show several plot with seasonal SST anomaly during two or three typical ESM.

L627 – “These values indicate that oceanic processes are primarily responsible for the observed EMS SSTs in these areas “ – You often refer to oceanic processes, but can you give references that support this result.
Citation: https://doi.org/10.5194/egusphere-2023-1709-RC2
- AC2: 'Reply on RC2', Dimitra Denaxa, 01 Nov 2023
  
  We would like to thank you for your comments and constructive suggestions, which have significantly improved our manuscript. Our point-by-point response is provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC2
RC3:
'Comment on egusphere-2023-1709', Anonymous Referee #3, 05 Oct 2023

Dear editor,

I apologise for taking so much time to review this paper.
The writers have investigated the concept of Marine Extreme Summers (EMS) in the Mediterranean Sea, their spatial distribution and potential drivers. In principle, the EMS idea sounds interesting, Despite the fact that the international literature on marine heatwaves (MHWs,a widely accepted framework for the identification of extreme temperatures in the ocean) has shown that a shift on the mean sea surface temperatures of the ocean is responsible for the development of extreme temperature in the oceans, the writers have tried to deal with the contribution and role of the SST variance in the development of extreme ocean temperatures in the Mediterranean. They have done a considerable amount of work in trying to prove the drivers of multiple EMS in the basin, concluding on the significant contributions from increased air-sea heat fluxes, reduced turbulent heat fluxes (mainly Latent) and winds, along with shallower than normal Mixed layer depth. Although this is a well known result for individual extreme ocean temperature events of the past in the Mediterranean Sea , it has not been shown before for multiple extreme events.
However, due to the length of the paper and the absence of coherence in its structure, writing and presentation of results, i could not a) get a clear message or outcome from it, b) understand exactly how the EMS are defined, or what is their usefulness/added value compared to the MHW definition. As i was approaching the description of the drivers behind EMS, i was striving to understand the methodologies used, i felt very often different methodologies were used to prove the same thing (e.g Sections, 3.3.1,3.3.2,3.3.3) without getting any new knowledge. In general, there was no structure in the paper which was discouraging, difficult and time-consuming to try to understand what the writers were doing most of the time, despite my expertise on marine extremes. I also find a lot of results are often repeated, some methodologies were only used in the beginning (e.g. the de-trending from the summer period) and then they were never mentioned for the reminder of the paper.
I find the paper badly written, and it should be re-written with the help from a native speaker. Since right now i could not get one clear message, rather many different results that very often were only inferred but not well-described or proven.For this reason I also provide a line-by-line review hoping that it will help the writers improve their manuscript.
At the moment I advise against the publication of this paper, unless major revisions are performed in which case the concept of EMS should be better described and once there is a clear message, the idea should be highlighted.

Regards

Citation: https://doi.org/10.5194/egusphere-2023-1709-RC3
- AC3: 'Reply on RC3', Dimitra Denaxa, 01 Nov 2023
  
  We greatly appreciate the time and effort you dedicated to reviewing our manuscript. We have carefully addressed all comments and we have made substantial revisions to the manuscript. Our point-by-point response is provided in the attached pdf.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1709', Anonymous Referee #1, 31 Aug 2023

Please see comments in the attached document

Citation: https://doi.org/10.5194/egusphere-2023-1709-RC1
- AC1: 'Reply on RC1', Dimitra Denaxa, 01 Nov 2023
  
  We would like to thank you for your careful reading and valuable comments, which have significantly strengthened our manuscript. We have addressed all comments and have revised the manuscript accordingly. Our point-by-point response is provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC1
RC2:
'Comment on egusphere-2023-1709', Anonymous Referee #2, 01 Oct 2023
The paper untitled “Investigating extreme marine summers in the Mediterranean Sea” - by Dimitra Denaxa Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki – investigates Marine Heat Waves (MHW) during “extreme marine summers” in the Mediterranean Sea. The authors use a reanalysis to identify MHW characteristics and understand the main drivers explaining regional differences during particularly hot summers.

In general, I found the paper is well-written with a straightforward general structure. I really appreciated efforts of authors to clearly describe their results, even if some parts could be shortened to reduce the long length of the manuscript.

However, I have several concerns from the beginning of the paper regarding methodological choices:

I do not understand why the authors decide to only consider 4 events as “extreme marine summers”. Even if Figure 4 gives an idea about the distribution of summer SST, this is only at three locations. The authors need to give a more precise definition of what they call “extreme marine summer” regarding the distribution.

My second concern is a lack of justification about applying methodologies for each “grid point”. Results gives a very statistical point of view. Is there any criterion about spatial scales? This is confusing as they give physical interpretation associated with regional patterns. I would at least explain why they are not considering EMS composites.

If I understand Figure 3 correctly, it needs more explanation since patterns might results from one “very extreme” event for instance, and might not be representative of the 4 events. This also might be an issue in other figures.

It misses a discussion about the interest for the community of this new term “ESM”.

I think these points need to be addressed before introducing this new concept. A major revision is required to clarify these points.

Minor comments:

L128 – “derives from a combination of HadISST2 and OSTIA datasets” – ERA5 SST is based on a reanalysis which includes model and observations datasets. I would reformulate.

L133 – “In addition to ERA5 (reference SST dataset), we use the CMEMS L4 satellite SST product (EU Copernicus Marine Service Product, 2022c) for the period 1982-2019, at 0.05°x0.05° grid spacing, to cross check the quality of the reference dataset against a high-resolution observational SST dataset.” What did you compare? What did you conclude?

L143 – “We then define EMSs, separately at each grid point, as the four summers with the highest average JAS SST, i.e., exceeding the 95th percentile of the 71 available summer periods from 1950 to 2020. “ As the authors want to introduce a new concept, it needs more explanation and justification.

L239 – “we provide examples of different patterns for three grid points of the domain ». Why these points? Are they representative a regional patterns?

Figure 4 – This figure is interesting, but this arises questions. I would imagine with such “Extreme Marine Summer” concept, that the spatial scales would be close to those of the basin. Maybe, the authors should show several plot with seasonal SST anomaly during two or three typical ESM.

L627 – “These values indicate that oceanic processes are primarily responsible for the observed EMS SSTs in these areas “ – You often refer to oceanic processes, but can you give references that support this result.
Citation: https://doi.org/10.5194/egusphere-2023-1709-RC2
- AC2: 'Reply on RC2', Dimitra Denaxa, 01 Nov 2023
  
  We would like to thank you for your comments and constructive suggestions, which have significantly improved our manuscript. Our point-by-point response is provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC2
RC3:
'Comment on egusphere-2023-1709', Anonymous Referee #3, 05 Oct 2023

Dear editor,

I apologise for taking so much time to review this paper.
The writers have investigated the concept of Marine Extreme Summers (EMS) in the Mediterranean Sea, their spatial distribution and potential drivers. In principle, the EMS idea sounds interesting, Despite the fact that the international literature on marine heatwaves (MHWs,a widely accepted framework for the identification of extreme temperatures in the ocean) has shown that a shift on the mean sea surface temperatures of the ocean is responsible for the development of extreme temperature in the oceans, the writers have tried to deal with the contribution and role of the SST variance in the development of extreme ocean temperatures in the Mediterranean. They have done a considerable amount of work in trying to prove the drivers of multiple EMS in the basin, concluding on the significant contributions from increased air-sea heat fluxes, reduced turbulent heat fluxes (mainly Latent) and winds, along with shallower than normal Mixed layer depth. Although this is a well known result for individual extreme ocean temperature events of the past in the Mediterranean Sea , it has not been shown before for multiple extreme events.
However, due to the length of the paper and the absence of coherence in its structure, writing and presentation of results, i could not a) get a clear message or outcome from it, b) understand exactly how the EMS are defined, or what is their usefulness/added value compared to the MHW definition. As i was approaching the description of the drivers behind EMS, i was striving to understand the methodologies used, i felt very often different methodologies were used to prove the same thing (e.g Sections, 3.3.1,3.3.2,3.3.3) without getting any new knowledge. In general, there was no structure in the paper which was discouraging, difficult and time-consuming to try to understand what the writers were doing most of the time, despite my expertise on marine extremes. I also find a lot of results are often repeated, some methodologies were only used in the beginning (e.g. the de-trending from the summer period) and then they were never mentioned for the reminder of the paper.
I find the paper badly written, and it should be re-written with the help from a native speaker. Since right now i could not get one clear message, rather many different results that very often were only inferred but not well-described or proven.For this reason I also provide a line-by-line review hoping that it will help the writers improve their manuscript.
At the moment I advise against the publication of this paper, unless major revisions are performed in which case the concept of EMS should be better described and once there is a clear message, the idea should be highlighted.

Regards

Citation: https://doi.org/10.5194/egusphere-2023-1709-RC3
- AC3: 'Reply on RC3', Dimitra Denaxa, 01 Nov 2023
  
  We greatly appreciate the time and effort you dedicated to reviewing our manuscript. We have carefully addressed all comments and we have made substantial revisions to the manuscript. Our point-by-point response is provided in the attached pdf.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1709-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Dimitra Denaxa on behalf of the Authors (28 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Dec 2023) by Anne Marie Treguier

RR by Anonymous Referee #1 (05 Dec 2023)

Suggestions for revision or reasons for rejection

Having read the authors' response to my previous review I still have doubts about the authors' definition of EMS. Although in general they have satisfactorily answered or clarified most of the issues I raised in the review I cannot yet recommend the publication of this paper.
Thanks to the authors' detailed explanations I have clearly understood their definition of EMS but I must say that, even understanding their intention, I think their concept of EMS is incomplete to my understanding and I do not see the usefulness of its application in research on SST and MHWs in the Mediterranean. First of all, reading the title, the reader would expect a definition that includes both temporal and spatial aspects, since it speaks of "Mediterranean Sea". The fact that the definition is based on a grid point approach without any spatial consideration, even if it is posterior, I believe detracts from the interest or usefulness of the EMS concept.
The definition, as it stands, may lead to a theoretical situation, although unlikely but not impossible by the way it is defined, where the 4 years qualified as EMS may be different at each neighbour point in the grid, which makes the EMS concept less consistent. Knowing about the spatio-temporal coherence of the SST, it is possible that in a large part of the points of the grid the years qualified as EMS coincide, reducing the theoretically possible variability, but this is something that the authors should check/show in their results. In fact, in the figures representing three points in the Mediterranean there are few coincidences in the EMS years. I do not feel comfortable looking at a map of, for example, mean SST anomaly without knowing if the averages at nearby, or distant, points have been made over the same years (or a short series of years). The authors should report whether the years considered EMS are a short distribution or whether there is a large set of EMS years with large variability. A suggestion would be to evaluate the number of grid points having an EMS each year to look for years concentrating the highest number of grid points.
Moreover, the definition would only be consistent for the period under study. In the years 2022 and 2023, very intense and extensive SST anomalies and MHWs have been recorded in the Mediterranean that would most likely fall into the group of 4 detectable EMSs in the analysis. Only a slight extension of the series could change the nature of 50% of the EMS over large areas of the basin. This is a major weakness of the definition even though the authors have performed analyses that indicate that the spatial distribution or substructure of SSTs does not change, which is interesting. But the EMS years could change. From such a definition one would expect that subsequent years could be analysed and determined to be extreme or non-extreme without affecting the qualification of previous years, at least not 50% of them.
At this point, I cannot recommend the publication of this article unless the authors consider introducing a requirement for spatial coherence. In this regard I recommend the publication of one of the co-authors "Flaounas, E., et al (2021). Extreme wet seasons – their definition and relationship with synoptic-scale weather systems. Weather and Climate Dynamics", specifically section 2.2 "Spatial and temporal coherence of extreme wet seasons" for a similar analysis.
I encourage you to run the analysis suggested as an improved definition of EMS could be an interesting/important metric for SST analysis both for scientists and stakeholders.

Hide

ED: Reconsider after major revisions (07 Dec 2023) by Anne Marie Treguier

AR by Dimitra Denaxa on behalf of the Authors (18 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 Jan 2024) by Anne Marie Treguier

RR by Anonymous Referee #1 (21 Jan 2024)

ED: Publish as is (23 Jan 2024) by Anne Marie Treguier

AR by Dimitra Denaxa on behalf of the Authors (26 Jan 2024)

Journal article(s) based on this preprint

26 Mar 2024

Investigating extreme marine summers in the Mediterranean Sea

Dimitra Denaxa, Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki

Ocean Sci., 20, 433–461, https://doi.org/10.5194/os-20-433-2024,https://doi.org/10.5194/os-20-433-2024, 2024

Short summary

Dimitra Denaxa, Gerasimos Korres, Emmanouil Flaounas, and Maria Hatzaki

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This study explores Extreme Marine Summers (EMSs) in the Mediterranean Sea using Sea Surface Temperature (SST) data. EMSs arise mainly due to the warmest summer days being unusually warm. Air-sea heat fluxes drive EMSs in northern regions, where also marine heatwaves (MHWs) highly contribute. Long-term SST changes lead to warmer EMSs and more intense MHWs, while not affecting the way daily SST values are organized during EMSs. Findings help understanding anomalously warm conditions in the basin.


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