Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations with AWI-CM1.0

Rackow, Thomas; Sein, Dmitry V.; Semmler, Tido; Danilov, Sergey; Koldunov, Nikolay V.; Sidorenko, Dmitry; Wang, Qiang; Jung, Thomas

doi:https://doi.org/10.5194/gmd-12-2635-2019

Articles | Volume 12, issue 7

https://doi.org/10.5194/gmd-12-2635-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gmd-12-2635-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 7

Model evaluation paper

|

05 Jul 2019

Model evaluation paper |

| 05 Jul 2019

Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations with AWI-CM1.0

Thomas Rackow, Dmitry V. Sein, Tido Semmler, Sergey Danilov, Nikolay V. Koldunov, Dmitry Sidorenko, Qiang Wang, and Thomas Jung

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Final revised paper (published on 05 Jul 2019)
Preprint (discussion started on 10 Aug 2018)

Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 26 Sep 2018
RC2: 'Review of initial submission (referee 2)', Anonymous Referee #2, 28 Sep 2018
AC1: 'Response to reviewers', Thomas Rackow, 03 Jan 2019

Peer-review completion

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

AR by Thomas Rackow on behalf of the Authors (07 Jan 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Jan 2019) by Sophie Valcke

RR by Anonymous Referee #2 (08 Feb 2019)

RR by Anonymous Referee #1 (16 Feb 2019)

Suggestions for revision or reasons for rejection

Second review of “Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations with AWI-CM1.0” by Rackow and Co-authors

Although the authors addressed the majority of the comments and suggestions of both reviewers, I think there are still a few lingering items that should be addressed before publication. The first concerns consideration of other metrics to access the overall impacts of going to higher resolution. The authors indicate that they added performance indices (PIs) for some ocean fields. However, as far as I can tell from the explanation in Appendix B, this effort only concerns 3D temperature and salinity fields. In my request, I specifically asked for AMOC, and northward heat and salt transports from all the cases. Perhaps one should also add the Antarctic Circumpolar Circulation (ACC) transport at Drake Passage to this list. I would like to see spatial distributions of AMOC from all the cases as well as line plots of northward heat and salt transports. Plus a table of ACC transports. Based on Fig. 12, I am expecting that AMOC will be rather weak in MR. Second, it is clear from Table 3 that the improvements are not really monotonic with increasing resolution. Specifically, going to HR and MR0 do not improve the solutions as far as the metrics in PI are concerned. So, statements like “We show that increasing ocean resolution locally to resolve ocean eddies leads to a major reduction in deep ocean biases.” are not quite correct without a qualifier. A third item concerns the temperature and salinity biases depicted in Fig. 6 for the Strait of Gibraltar region. Specifically, I suspect that those biases are simply reflecting a vertical shift / change of the Mediterranean Outflow waters by a few hundred meters. Because these plots are at a constant depth of 1000 m, such shifts are not captured. I suggest adding vertical profiles of regionally-averaged potential temperature and salinity to capture such shifts. Although these items are not necessarily onerous, I would like to classify this revision as major because they can impact the conclusions.

Minor items:

Fig. 1 and related discussion: Indicate how many CMIP5 models are included. Explain why DJF was chosen. Both here and elsewhere, change depth axis values to be positive because depth is positive downwards and negative values indicate a vertical coordinate that is positive upwards.

p.3, l.2: Please explain what “mapping” refers to.

p.3, l.33: Clearly state that the simulations are 100 years long.

Table 1 caption: “…. at 1000 m depth.”

p.4, l.11-12: This sentence is not clear. Please rephrase.

Figure 6: Either use K or degree C throughout the manuscript. This figure uses degree C, elsewhere it is K.

p.14, l.11-12: This is true for a 30-year mean. What about seasonal and inter-annual excursions? If there are such excursions to deeper levels, then this general statement is not correct which can change your conclusions.

p.14, l.26-20: Can be deleted as already discussed earlier.

Fig. 9 caption: Is there a missing URL?

p.18, last line: “…. climatology as the circulation and eddy fields are ….”

Fig. 11 caption: These are not maps of along-isopycnal fields, but rather fields on a constant isopycnal.

p.20, l.4: “is systematically reduced when moving to successively higher resolution ….” Not justified.

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ED: Reconsider after major revisions (04 Mar 2019) by Sophie Valcke

AR by Thomas Rackow on behalf of the Authors (20 May 2019) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (05 Jun 2019) by Sophie Valcke

Dear Author,

Thank you for your careful review of your manuscript following the 2nd round of reviews. I think that your manuscript is now almost ready for publication but I would like you to consider the following comments from my side for its final version:

1- Regarding the first comment of referee #1, thank you for including the AMOC and ocean heat transport figures and analysis. Your arguments on why not to include the meridional salt transport are convincing to me. However, I think you should answer the referee’s demand to include ACC numbers and analysis, as you do in your reply (but not in the revised manuscript). Thank you for doing so in the final version of the manuscript.

2- Regarding your coupling procedure, I am not sure why you include reference to (Valcke, 2013; Valcke et al., 2013 ) when discussing the bicubic mapping. Also, it is just when reading the code availability section that one can understand that you use OASIS3-MCT. It would probably be more appropriate to remove references to (Valcke, 2013; Valcke et al., 2013 ) but mention in section 2 that you are using OASIS3-MCT and make a reference to
A. Craig, S. Valcke, L. Coquart, 2017: Development and performance of a new version of the OASIS coupler, OASIS3-MCT_3.0, Geosci. Model Dev., 10, 3297-3308, https://doi.org/10.5194/gmd-10-3297-2017, 2017.

3- Still regarding the coupling procedure, I don't understand what you mean by “direct coupling” and why this “.... effectively reduces the number of necessary interpolations...”; OASIS3-MCT still has to express the coupling fields of the source grid on the target grid, as the source and target grids do not match, right? Can you clarify (or remove this mention to the exchange grid and to this reduction of necessary interpolations, if you think it is complex to explain and not very relevant here)?

4- P.16, l.21 “the latter source”, maybe clarify that you are talking about the north-eastern North Atlantic source, as the mention is somewhat far in the preceding paragraph.

With best regards,
Sophie Valcke

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AR by Thomas Rackow on behalf of the Authors (15 Jun 2019) Author's response Manuscript

ED: Publish subject to technical corrections (18 Jun 2019) by Sophie Valcke

AR by Thomas Rackow on behalf of the Authors (18 Jun 2019) Manuscript

Short summary

Current climate models show errors in the deep ocean that are larger than the level of natural variability and the response to enhanced greenhouse gas concentrations. These errors are larger than the signals we aim to predict. With the AWI Climate Model, we show that increasing resolution to resolve eddies can lead to major reductions in deep ocean errors. AWI's next-generation (CMIP6) model configuration will thus use locally eddy-resolving computational grids for projecting climate change.