Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

Lachkar, Zouhair; Lévy, Marina; Smith, Shafer

doi:https://doi.org/10.5194/bg-15-159-2018

Articles | Volume 15, issue 1

https://doi.org/10.5194/bg-15-159-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-15-159-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 1

Research article

|

10 Jan 2018

Research article |

| 10 Jan 2018

Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

Zouhair Lachkar, Marina Lévy, and Shafer Smith

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Final revised paper (published on 10 Jan 2018)
Preprint (discussion started on 02 May 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Comments on Intensification and deepening of the Arabian Sea Oxygen Minimum Zone in response to increase in Indian monsoon wind intensity', Anonymous Referee #1, 01 Jun 2017
- RC2: 'Review', Anonymous Referee #2, 13 Jun 2017
  - AC2: 'Answer to Referee #2', Zouhair Lachkar, 25 Jul 2017
- AC1: 'Answer to Referee #1', Zouhair Lachkar, 25 Jul 2017

Peer-review completion

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

ED: Reconsider after major revisions (01 Aug 2017) by Tina Treude

AR by Zouhair Lachkar on behalf of the Authors (28 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Sep 2017) by Tina Treude

RR by Anonymous Referee #2 (18 Sep 2017)

Suggestions for revision or reasons for rejection

The authors improved the ms but the response of the OMZ to climate change is a crucial issue and involves a complex interaction of processes which are only partly addressed in the sensitivity experiments carried out by Lachkar et al.. As stated by the authors in chapter 4.4.2 first sentence, the study focuses on the sensitivity of OMZ to monsoon winds. That is interesting and I would suggest to adapt the abstract, introduction and conclusion to the focus of the ms.

Considering that Lachkar et al. are interested in the contemporary and even future ocean in addition to glacial also results obtained from Holocene records should be included in the discussion. First of all because of the similar boundary conditions and secondly also because results from the Holocene record contradict the presented interpretation of the model results.

For example, looking from today back into the past (the last 10000 years) decreasing d15N values within sedimentary records (especially from center of the OMZ in the north and east) correspond with decreasing burial rates of organic matter and an increasing summer monsoon strength. This lead to the conclusion that a strengthening of the summer monsoon weakens the OMZ due to associated changes in the ocean’s circulation and ventilation during the Holocene. Despite critics of the authors on global models recent results obtained from a global ocean circulation model support this conclusion by showing an increasing volume of oxygen depleted water (OMZ) associated with nearly constant productivity and weakening of the summer monsoon during the Holocene (from the past to the present).
See Gaye et al. 2017 https://www.biogeosciences-discuss.net/bg-2017-256/. and older references therein.

Furthermore the authors are correct figure 7b shows in line with observations the denitrification peak immediately below the mixed layer. Denitrification at depth between 400 and 1300 m with a small secondary peak at 500 m is unsupported by observations. This means there are two peaks: one shallow one supported by observations and another deep one which is unsupported by observations.
By increasing wind speeds the supported peak decreased and the unsupported peak increased. Since the latter overcompensated the first the authors concluded an intensifications of the summer monsoon winds deepens the zone of denitrification and increase denitrification rates. To base a conclusion on the intensification of an unsupported peak is to my understanding questionable. Ignoring the response of the unsupported peak would show that stronger summer monsoon winds lower denitrification, which is in line with the Holocene d15N records and the global ocean circulation model. These aspects should also be considered in the discussion and conclusion.

Some more minor comment refer to the discussion about sinking speeds and respirations rates, light limitation, classification of zones. If it does not matter whether one uses high sinking speeds and respirations rates or low sinking speed and low respiration rates why do not stick to observation in order to avoid such discussions. Please explain in more detail how light limitation was identified and translated into reduced carbon export rates and the why unused nutrient were not consumed somewhere else? According to fig 7a I would suggest to define upper OMZ from below the mixed layer to approximately 375 m (supported denitrifying zone) and the mesopelagic zone between 375 and 1800 m (unsupported denitrifying zone).

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ED: Reconsider after major revisions (27 Sep 2017) by Tina Treude

AR by Zouhair Lachkar on behalf of the Authors (10 Oct 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Oct 2017) by Tina Treude

RR by Anonymous Referee #2 (05 Nov 2017)

Suggestions for revision or reasons for rejection

Please find in following my response:

1) Link of the model result to future projections and the past evolution of the OMZ.

Yes, it is clearly stated in the ms that the simulations are highly idealized and are not intended to reproduce the past evolution of the OMZ or to predict its future trajectory and that they rather aim at exploring the sensitivity of the Arabian Sea OMZ to monsoon wind intensity changes but this is not reflected in the abstract and conclusion.

In the conclusion we can read : A set of coupled physical biogeochemical simulations of the Arabian Sea ecosystem reveals a tight coupling between the intensity of the summer monsoon wind and the size and intensity of the Arabian Sea OMZ. We find that the OMZ and ecosystem responses are largely determined by the perturbation of the summer SW monsoon ….

In the abstract the authors wrote that the model results show that the Arabian Sea productivity increases and its OMZ expands and deepens in response to monsoon wind intensification and that this lead to a strong intensification of denitrification at depth, resulting in a substantial amplification of fixed nitrogen depletion in the Arabian Sea.

This is misleading without mentioning the limitation of their simulation and that ventilation change in response to varying monsoon strength can cause the opposite.

The presented idea that large-scale ventilation changes act on longer and changes in wind intensity on shorter time scales is not supported by sediment tarp results. They indicate that increased winds speeds intensify upwelling but at a certain level decrease carbon export because of associated ventilation changes (Rixen et al 1996). This should be considered in the discussion.

That in the Holocene, large-scale ventilation changes may have played an important role together with fluctuations in monsoon intensity was not only suggested by Gaye et al and Das at 2017 but already in 2014 by Rixen et al 2014 (Biogeosciences) which should accordingly be cited.

However my main critics it that the authors present conclusion drawn form simulations (see abstract and conclusion) without mentioning the limitation of their simulation and that ventilation change in response to varying monsoon strength caused the opposite. This is misleading but can be solved by modifying the abstract and conclusion.

2) First of all I have suggested to discuss not to ignore the deep denitrification peak. That the response of low denitrification rates at depth could be more important as those of at shallower depth for the total denitrification is an interesting result. It suggests to pay more attention to denitrification at greater water-depth in future field studies.

The third point refers to the comment on sinking speed and respiration rates: That previous studies did it same is not an scientific argument.

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ED: Publish subject to minor revisions (review by editor) (15 Nov 2017) by Tina Treude

AR by Zouhair Lachkar on behalf of the Authors (16 Nov 2017) Author's response Manuscript

ED: Publish as is (21 Nov 2017) by Tina Treude

AR by Zouhair Lachkar on behalf of the Authors (21 Nov 2017) Author's response

Short summary

This study provides a new contribution to our understanding of the coupling between the oxygen minimum zones (OMZs) and climate. It explores how idealized changes in summer and winter Indian monsoon winds affect the productivity of the Arabian Sea and the size and intensity of its OMZ. We find that intensification of Indian monsoon winds can amplify climate warming on decadal to centennial timescales.