What drives the latitudinal gradient in open-ocean surface dissolved inorganic carbon concentration?

Wu, Yingxu; Hain, Mathis P.; Humphreys, Matthew P.; Hartman, Sue; Tyrrell, Toby

doi:https://doi.org/10.5194/bg-16-2661-2019

Articles | Volume 16, issue 13

https://doi.org/10.5194/bg-16-2661-2019

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

https://doi.org/10.5194/bg-16-2661-2019

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

Articles | Volume 16, issue 13

Research article

|

11 Jul 2019

Research article |

| 11 Jul 2019

What drives the latitudinal gradient in open-ocean surface dissolved inorganic carbon concentration?

Yingxu Wu, Mathis P. Hain, Matthew P. Humphreys, Sue Hartman, and Toby Tyrrell

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Final revised paper (published on 11 Jul 2019)
Preprint (discussion started on 15 Aug 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review', Anonymous Referee #1, 14 Sep 2018
- AC1: 'Response to Referee#1', Yingxu Wu, 25 Sep 2018
RC2: 'Review of manuscript submitted by Wu et al. “What drives the latitudinal gradient in open ocean surface dissolved inorganic carbon concentration?”', Anonymous Referee #2, 09 Oct 2018
- AC2: 'Response to Referee#2', Yingxu Wu, 20 Oct 2018

Peer-review completion

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

ED: Reconsider after major revisions (23 Oct 2018) by Jack Middelburg

AR by Yingxu Wu on behalf of the Authors (23 Oct 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Oct 2018) by Jack Middelburg

RR by Anonymous Referee #3 (04 Dec 2018)

Suggestions for revision or reasons for rejection

Review of ”What drives the latitudinal gradient in open ocean surface dissolved
inorganic carbon concentration?” by Yingxu Wu et al.
The authors present an analysis of driving mechanisms behind the north-south
gradient of surface ocean DIC. The study is based on DIC and alkalinity data from
the database GLODAP v2.
Major issues:
The authors calculate pCO2 from DIC and TA data, which results in a nonequilibrium
pCO2. This is then converted to a standard temperature and
calculated back to TA and DIC assuming that pCO2 is back to its initial value. As a
major part of the pCO2 disequilibrium in the ocean is associated to temperature
changes wouldn’t it be more useful to assume equilibrium with the atmosphere
(at in-situ temperature as well as at standard temperature) when calculating the
temperature effect on DIC?
The two major weaknesses raised during the first review iteration are still
persisting, the normalization of DIC without a freshwater component and the
disregard of seasonality.
I am not convinced that excluding areas strongly influenced by riverine input is
enough to solve the problem with the DIC normalization. Fitting the alkalinity
against the salinity gives also in the open ocean a non-zero intercept that can be
vary from region to region. With the GLODAP v2 dataset there is an excellent
database to calculate this intercept and it’s regional variations. It has at least to
be discussed what influence this would have on the latitudinal gradients.
Regarding the seasonality I do understand that there is not enough data in the
Southern Ocean to resolve and discuss seasonalities. But nevertheless these
influences need to be discussed. The salinity normalization of phosphate, for
example, can produce relatively high residual phosphate and thus ‘unused DIC’
during summer in regions with low salinities although these might experience
phosphate limitation during summer. Also I am missing a notice that using
phosphate concentrations as tracer for the influence of primary production is a
simplification. C:P ratios are highly variable, not only, as mentioned, regionally,
but also seasonally. Under phosphate limitation phytoplankton is still able to fix
carbon, and release it as DOC.
As a last major point, the manuscript is partly very difficult to read. The
discussion of effects on DIC and nDIC is sometimes confusing and the reader
might get lost which of both is discussed and why.
Minor issues:
p. 10 l. 21: Here DICobs and nDICobs are discussed, right?
p. 11 l. 12: Rephrase this sentence. It reads as if you have data at 70˚S in the
northern Atlantic.
p. 12 l. 13: This part needs some changes. Most of the DIC variability is in fact not
caused by changes in alkalinity but by dilution and evaporation and therefore is
not existent anymore when discussing the nDIC. Alkalinity and DIC are just
influenced in a similar manner.
p. 12 l. 25: TA and salinity are only in the open ocean highly correlated
p. 13 l. 9: nutrients, nDIC and nTA?
p. 13 l. 10: delete ‘then’
p. 13 l. 30: change of nTA, or change of TA relative to the DIC changes
p. 14 l. 22: this should also be a function of the chemical composition: how low is
the iron concentration in the upwelled water in comparison to the other
nutrients.
p. 14 l. 23: delete ‘.’
Figure 1: increase the resolution of this picture. The description at the colour
axis only says DIC
Figure 2: Here a direct influence from evaporation & precipitation to DIC is
missing. Also, in the description of the figure it is written about both DIC and
nDIC. For which of these two is the diagram?
Figure 5: The resolution of the picture should be improved. The map should be
deleted from this picture. Show the location of the transects in Figure 3 instead. I
think all figures should follow a common design when it comes to setup and
fonts. Please change the design of this figure match the other figures. I don’t
understand the use of the black contour lines. Either, choose them in a way that
they cover the entire data range or delete them.
Figure 8: Are these the same transects as in figure 5? please specify.
Figure 9: Change the rotation of the longitudinal label to 0˚.

Referee Report: PDF

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ED: Reconsider after major revisions (05 Dec 2018) by Jack Middelburg

AR by Yingxu Wu on behalf of the Authors (29 Jan 2019)

ED: Referee Nomination & Report Request started (31 Jan 2019) by Jack Middelburg

RR by Anonymous Referee #2 (11 Mar 2019)

Suggestions for revision or reasons for rejection

Review of manuscript submitted by Wu et al. “What drives the latitudinal gradient in open ocean surface dissolved inorganic carbon concentration?”
Wu et al. present a detailed analysis of factors driving the surface ocean concentration of dissolved inorganic carbon (DIC). Their study is based on the recently released GLODAP v2 dataset. In order to compare DIC in a global perspective they use salinity normalized DIC (nDIC). The major conclusion of their study is that sea surface temperature (SST) is the major driver of DIC variability, followed by changes in alkalinity and Southern Ocean upwelling. Their conclusion points out that the upwelling effect in is ignored in global models but can change the DIC fields substantially.
This is my second review and I want to acknowledge that the manuscript improved and the authors took the comments of all reviewers serious. I don’t add specific comments in this reviews as I don’t think that this will improve the text as it might be even to the taste of the reader/author how to formulate certain sentences.
My major issues with the manuscript are:
(1) The normalization of DIC by salinity. I understand the authors motivation to do that, but I’m still wondering if the normalization of a parameter that is influenced by non-salinity related factors weakens there finding s too much. What is the introduced error of this simple normalization?
(2) Readability: In parts of the manuscript the authors jump between DIC, nDIC, DICobs. Is DDICFe the same as unused DIC? I’m not sure if the reader has always the right DIC in mind.
(3) The calculation of the effects is based on conversions to and from surface ocean pCO2 and atmospheric pCO2. Neither seasonality (air and water pCO2) nor latitudinal gradients (air pCO2) are considered. The latitudinal gradient in the atmospheric xCO2 can be easily more than 10 ppm. Thus I expect the introduced error in DpCO2 calculations and further DIC calculations to be higher than estimated.

I suggest to check the paper for possible sections that can be shortened. I think the overall finding that upwelling plays a considerable role in surface ocean DIC distribution stays even with the detailed calculations.

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ED: Publish subject to minor revisions (review by editor) (04 Jun 2019) by Jack Middelburg

AR by Yingxu Wu on behalf of the Authors (14 Jun 2019) Author's response Manuscript

ED: Publish as is (20 Jun 2019) by Jack Middelburg

AR by Yingxu Wu on behalf of the Authors (21 Jun 2019) Manuscript

Short summary

This study takes advantage of the GLODAPv2 database to investigate the processes driving the surface ocean dissolved inorganic carbon distribution, with the focus on its latitudinal gradient between the polar oceans and the low-latitude oceans. Based on our quantitative study, we find that temperature-driven CO₂ gas exchange and high-latitude upwelling of DIC- and TA-rich deep waters are the two major drivers, with the importance of the latter not having been previously realized.