Review of the paper: egusphere-2024-1388: Ocean Acidification trends and Carbonate System dynamics in the North Atlantic Subpolar Gyre during 2009-2019, by David Curbelo-Hernández et al.

General Comment:

Quantifying and understanding the variations of anthropogenic CO2 (Cant) in the oceans is important not only to reduce the uncertainty in the present Cant inventories but also to better predict the climate/carbon coupling, i.e. how the ocean will capture atmospheric CO2 in the future. In addition the increase of Cant leads to ocean acidification and potential damage for marine species. The North Atlantic Ocean is an important CO2 sink (Takahashi et al. 2009) and this region contains high concentrations of anthropogenic CO2 (Cant) in the water column (Khatiwala et al 2013). Decadal variations of the Cant inventories were recently identified at basin scale probably linked to the change of the Atlantic Meridional Overturning Circulation (AMOC) (Gruber et al, 2019; Müller et al, 2023; Perez et al, 2024). It is worth noting that biases of AMOC in the GOBMs have been identified (Terhaar et al, 2024) that may explain differences of the Cant inventories between simulations and observations. Therefore, to better evaluate and correct current GOBMs comparisons with observed CT, AT and data-based Cant estimates are highly needed.

In this context David Curbelo-Hernández and co-authors present a detail analysis of the carbonate system (CS) changes based on 8 cruises conducted between 2009 and 2019 in the North Atlantic, here the NASPG region. After a nice introduction authors described in detail the dataset and the methods (measurements and calculations) used to investigate the temporal changes in the water column. I am convinced with all results that offer new views of the CS changes in the north Atlantic. The manuscript is clear, figures and tables adapted. The data presented here will be useful when revisiting the decadal changes of Cant inventories in the ocean (for RECCAP 3 ?), especially in the north Atlantic where decadal change of the Cant inventory could reached -1 PgC per decade (Müller et al, 2023). I support publication of this paper with minor revisions. Few comments and suggestions are listed below.

Specific comments:

C-00: Title: “Ocean Acidification trends and Carbonate System dynamics in the North Atlantic Subpolar Gyre during 2009-2019”. As you investigate acidification trends in the water column (other studies analyzed this in the surface only, e.g. Lauvset et al, 2015; Leseurre et al, 2020) you may change the title: “Ocean Acidification trends and Carbonate System dynamics in the North Atlantic Subpolar Gyre water masses during 2009-2019”.

C-01: In the Introduction, maybe recall that acidification rates were also investigated from surface data (Lauvset et al 2015; Chau et al, 2024). For example, in the Irminger Sea, Chau et al (2024) evaluated trend over 1985-2021 for pH (-0.016 ±0.001 per decade) and for War (-0.039 ±0.009 per decade).

C-02: Line 149: Authors write: “A detailed overview and metadata of the cruises is given in Table 1.”

I guess the metadata are not listed in Table 1: add a DOI or a reference for each cruise in Table 1?

C-03: Line 162: accuracy of ±1.5 μmol kg-1 for AT and ±1.0 μmol kg-1 for CT: this is very good. Impressive this is lower that the “climate goal” of ±2.0 μmol kg-1 (Newton et al, 2015).

C-04: Line 181: Are you sure that DO measurement is described in Dickson and Goyet (1994) ?

Maybe refer to Dickson (1995).

C-05: Line 382: Authors write: “The AT show a well-correlated direct relationship with salinity throughout the section (r2=0.89),…”. Curiosity: authors discussed significant changes in N-AT (Figure S4), is there a detectable difference of AT/S relationship over time ?

C-06: Line 392: Figure S2 presents the N-AT distribution for 2009 and 2016. Curiosity: Why in 2016 the N-AT distribution appeared noisy and concentrations higher in the eastern sector compared to 2009 ? Is the same was observed on other recent cruises 2014 or 2019 ?

C-07: Line 438: Authors write: “The interannual ratios are presented along with their respective standard error of estimate and correlation factors (r2 and p-value) in Table 3 and S4.”. Correct: Table 2 (not 3).

C-08: Line 451: “SEANOE [https://www.seanoe.org/], Pascale et al., 2022”. Correct name: Lherminier et al 2022.

C-09: Line 517: Authors write: “while the Cnat experienced a slightly decrease throughout the region (Figure 5 and Table 2).” Any chance to specify the origin of this decrease ? (e.g. less export over time ?)

C-10: Line 519: Authors write: “The increase in the ventilation rates during this decade…”. Which decade ? Maybe specify the period: “The increase in the ventilation rates over 2009-2019…”

C-11: Line 521: Authors write: “explained the higher growth in Cant than expected due to the atmospheric CO2 increase.” Could you recall the expected Cant trend due to the atmospheric CO2 increase? +0.5, +1, +1.5 µmol/kg/yr ?

C-12: Line 561: Authors write: “keeping approximately constant the CT (Table 3)”. Correct to Table 2. Maybe recall in the text the value of the CT trend = +0.05 µmol/kg/yr in ENACW.

C-13: Line 583: Authors write: “…where strong slowdowns in ventilation were observed from 2009 to 2010 and from 2013 to 2014, resulted in a relatively increase in Cnat and decrease in Cant observed in SPMW”.  Is this signal could be associated with the low NAO index in 2010 ?

C-14: Line 635: Authors write: “The year-to-year variability in the biogeochemical patterns after 2012 may be attributed to the fluctuations in the spreading into the Rockall Trough of several water masses occupying different depths coming from the south and east”. As there is also variability in AT and N-AT, would that be also associated to local or regional biological processes (e.g. Cocco Blooms) and linked to the NAC transports (as discussed lines 556-559).

C-15: Line 672: Authors write: “The substantial variability introduced by these processes made it difficult to discern the pattern of acidification and its drivers on an interannual scale in the shallow Rockall Trough. Therefore, long-term monitoring and the development of multidecadal-scale studies are required in this area to derive significant conclusions.” I agree that maintaining long-term monitoring is important and it is often difficult to detect and explain the pH variations: however it seems that, and opposed to other regions, the Cant trend in the ENACW is relatively well estimated (quasi-linear) in the Rockall Trough (Figure 5) with value of 0.85 ±0.11 µmol/kg/yr and that Cnat decrease is also well evaluated (-0.84 ± 0.50 µmol/kg/yr). For this water mass would it be possible to suggest some process that explain the Cnat decreasing trend since 2009 (physics and/or biology ?).

C-16: Line 697: Authors write: “the positive NAT trends encountered in the upper layers lead a rise in pHT, while the diminished NAT contributed to decrease the pHT toward the interior ocean.” Is the N-AT trend could be related to change in Cocco blooms distributions or the signal is too low to interpret the link with the various biological processes in the north atlantic (Ostle et al, 2022) ?

C-17: Line 742: Figure 7, Table 2:  The trend of War in the Irminger Sea of around -0.07 per decade seems high compared to that deduced from reconstructed products (e.g. -0.039 ±0.009 per decade, Chau et al, 2024). Could that be discussed ? Would that be explained by seasonal difference of the trends (e.g. Leseurre et al, 2020) ?

C-18: Line 811: Authors write: “The driver analysis exhibited the strongest interannual decrease in Ώ in the upper layers governed by the uptake of Cant weakly compensated by the increase in NAT and favoured by the cooling and freshening.” Here, again, you identify that the N-AT increase explains part of the W changes: what is the process associated to this variations (Cocco bloom ?) ?

C-19: Line 816: Authors write: “The progressive reduction in ΏArag is driving a long-term decrease in the depth of the aragonite saturation horizon (ΏArag=1) by 80-400 m since the preindustrial era”. Curiosity: could you show a section (or profiles) of preindustrial War calculated with Cnat to compare with modern values (to add in Supp Mat ?).

C-20: Line 834: Authors write “In fact, several studies reported that CWC ecosystems are anticipated to be among the first deep-sea ecosystems to experience acidification threats”. Maybe refer to Gehlen et al (2014) (and also in the introduction).

C-21: Line 856: Authors write “The observational period is relatively short to quantify long-term trends and to formulate significant future projections.” Maybe delete here, as this was also written on line 840:  “Despite the observational period is relatively short to quantify long-term trends and to formulate significant future projections,…”.

C-22: 900: “to favour the entrance of Cant in intermediate and deep-layers and this its acidification,”

Delete “this”

C-23: Conclusion: Maybe reduce the numbers of values listed in the conclusion (already listed in the MS).

Figures:

C-24: Figures: check the color code for blind (see accepted colors for the journal Biogeoscience)…

C-25: For Figures 4, 5, 6, 7, a suggestion: change the color code for ENACW (i.e. different than for uLSW)

C-26: In figure 5 the CT and Cant concentrations in ENACW were low in 2014. I think this is not discussed. Any idea to explain this anomaly ? A strong bloom in 2014 in the eastern NASPG or a shift in the data for this cruise ?

References:

C-27: Friedlingstein et al, 2022: add full reference, journal, doi

C-28: Pascale, L., 2022: change to

Lherminier P., Perez, F. F., Branellec, P., Mercier, H., Velo, A., Messias, M. J., Castrillejo, M., Reverdin, G., Fontela, M., Baurand, F. (2022). GO-SHIP A25 - OVIDE 2018 Cruise data. SEANOE. https://doi.org/10.17882/87394

;;;;;;;;;;;;; References added in this review not cited in the MS

Chau, T.-T.-T., Gehlen, M., Metzl, N., and Chevallier, F.: CMEMS-LSCE: a global, 0.25°, monthly reconstruction of the surface ocean carbonate system, Earth Syst. Sci. Data, 16, 121–160, https://doi.org/10.5194/essd-16-121-2024, 2024.

Dickson, A. D. 1995. Determination of dissolved oxygen in sea water by Winkler titration. WOCE Operations Manual, Part 3.1.3 Operations & Methods, WHP Office Report WHPO 91-1.

Gehlen, M., Séférian, R., Jones, D. O. B., Roy, T., Roth, R., Barry, J., Bopp, L., Doney, S. C., Dunne, J. P., Heinze, C., Joos, F., Orr, J. C., Resplandy, L., Segschneider, J., and Tjiputra, J.: Projected pH reductions by 2100 might put deep North Atlantic biodiversity at risk, Biogeosciences, 11, 6955-6967, 10.5194/bg-11-6955-2014, 2014.

Lauvset, S. K., Gruber, N., Landschützer, P., Olsen, A., and Tjiputra, J.: Trends and drivers in global surface ocean pH over the past 3 decades. Biogeosciences, 12, 1285-1298, doi:10.5194/bg-12-1285-2015, 2015

Lherminier P., Perez, F. F., Branellec, P., Mercier, H., Velo, A., Messias, M. J., Castrillejo, M.,Reverdin, G., Fontela, M., Baurand, F. (2022). GO-SHIP A25 - OVIDE 2018 Cruise data.SEANOE. https://doi.org/10.17882/87394

Müller, J. D., Gruber, N., Carter, B., Feely, R., Ishii, M., Lange, N., et al.: Decadal trends in the oceanic storage of anthropogenic carbon from 1994 to 2014. AGU Advances, 4, e2023AV000875. https://doi.org/10.1029/2023AV000875, 2023

Newton, J.A., Feely, R. A., Jewett, E. B., Williamson, P. and Mathis, J.: Global Ocean Acidification Observing Network: Requirements and Governance Plan. Second Edition, GOA-ON, https://www.iaea.org/sites/default/files/18/06/goa-on-second-edition-2015.pdf, 2015.

Ostle C., P. Landschützer, M. Edwards, M. Johnson, S. Schmidtko, U. Schuster, A. J. Watson and C. Robinson, 2022. Multidecadal changes in biology influence the variability of the North Atlantic carbon sink.  Environ. Res. Lett. 17, 114056, DOI : 10.1088/1748-9326/ac9ecf

Terhaar, J., Goris, N., Müller, J. D., DeVries, T., Gruber, N., Hauck, J., et al. (2024). Assessment of global ocean biogeochemistry models for ocean carbon sink estimates in RECCAP2 and recommendations for future studies. Journal of Advances in Modeling Earth Systems, 16, e2023MS003840. https://doi.org/10.1029/2023MS003840

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Review of the manuscript egusphere-2024-1388 « Ocean Acidification trends and Carbonate System dynamics in the North Atlantic Subpolar Gyre during 2009-2019» by Curbelo-Hernández et al. submitted for discussion in Biogeosciences

General comment :

This manuscript reports a detailed analysis of carbonate chemistry parameters measured along the entire water column of a transect in the North Atlantic South Polar Gyre during 8 oceanographic cruises conducted between 2009 and 2019. The manuscript gives a nice description of this remarkable dataset. The dataset is used to study the trends of ocean acidification in this oceanic area with a particular focus on the spatial variability between three defined oceanic provinces (The Irminger Basin, the Iceland Basin and the Rockall Trough) and the different water layers encountered in the water column. Due to its importance as an oceanic carbon sink, the north Atlantic basin has been very intensively studied in the last decades. However this study is a significant contribution for (at least) three reasons : (1) it presents a new dataset, (2) it focuses on the last decade (2009-2019) and it (3) shows some original results in the eastern part of the basin. The authors have made a detailed literature review in order to put this study in context. The measurements and calculation methods are carefully described. The figures and tables are of good quality. The manuscript is well written and the results are convincing. However the manuscript is very long. This is certainly due to the fact that it is at the same time a “data paper” and a “scientific paper” . My point is not to say that it should be split in two (I appreciate having all the information in one manuscript) but I believe that it could be more synthetic in some parts. I would be glad to support the publication of this manuscript after some revisions (which I believe to be minor). My major concerns (detailed in the following sections) are related to the structure of the manuscript and some methodological points.

Specific comments :

Section 2.2.5 and 2.2.6: It’s the same method that is used to deconvolute the drivers of the pH and omega trends. This could go into one section called “deconvolution of the trends”. The beginning of section 2.2.6 on the calculation of the omega values could be added to section 2.2.2 “computational methods”.

On section 2.2 « Data Processing » : I am surprised about the fact that a huge amount of work has been done to compare the measured variable to variable estimated with CANYON-B but I haven’t found a simple “internal consistency test” of the three measured variables of the carbonate system. Following the table S2, a table could give some basic statistics about the difference “pHT measured – pHT calculated from AT/CT “, “AT measured – AT calculated from pHT/CT” or “CT measured – CT calculated from AT/pHT” in the cases where all three variable were measured.

On section 2.2.4 « Water mass characterization » : This section presents mostly results about the different water masses encountered during the cruises. Most of this section should be added (maybe in a more synthetic way ) into section 3.1 « Physicochemical characterization of the water column ».

On section 3.2. “ Temporal evolution of the physicochemical properties”. There are no real results in this section, most of the information here is more relevant to the method “section”.

On section 4.2 and Appendix B: I don’t really understand the reasons to give the information of the CT and AT trends in appendix. I would suggest putting this information directly in the main text.

Sections 4.3 “Acidification trends”, section 4.4 “Drivers pH” and section 4.5 “ Interannual changes in …” are very long. I really believe that these three sections could be synthesized into one single section. This is not an easy task but the drivers of the pH trends and the omega trends are mostly the same (I am of course aware that some differences exist) and could be treated in a more synthetic manner.

Conclusion : This is a really long conclusion ! I would suggest just giving the main messages in conclusion and It could certainly stand in 20 to 30 lines.

Technical corrections

L54 : What is meant by « conservative scenario » ?

L55 : Please define what is meant by « Anthropogenic carbon » before using it for the first time.

L84 : Missing reference “Gonzalez-Davilla and Santana-Casiano 2023”

L133 : The concept of « hydrographic CLIVAR 59.5°N Section » is not clear. Is this an historical WOCE section ?

L160 : What is meant by « in-situ calibrated » ?

L170-173 : This point is confusing. An uncertainty of 0.0047 pH Units does not necessarily correspond to a systematic bias. Please clarify this point ?

L199 : CANYON-B is trained and validated using GLODAPv2 data. Argo profiles are just used for comparison but not for formal validation.

L372-373 : This sentence is confusing. This section is not devoted to evaluating the temporal changes between 2009 and 2016.

L550 – 553 : This sentence is not completely clear to me. Why is the limitation of the ventilation to the subsurface water related to an enhanced entrance of CANT through the air-sea interface ?

L651 : The name of this section « Drivers pH » should be rephrased

L674-676 : This sentence is repeated in the conclusion. It could be removed here.