the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2024
Anomalous Summertime CO2 sink in the subpolar Southern Ocean promoted by early 2021 sea ice retreat
Abstract. The physical and biogeochemical processes governing the air-sea CO2 flux in the Southern Ocean are still widely debated. The "Southern Ocean Carbon and Heat Impact on Climate" cruise in summer 2022 aimed at studying these processes in the Weddell Sea and in its vicinity. A "CARbon Interface OCean Atmosphere" (CARIOCA) drifting buoy was deployed in January 2022 in the subpolar Southern Ocean, providing hourly surface ocean observations of fCO2 (fugacity of CO2), dissolved oxygen, salinity, temperature and chlorophyll-a fluorescence for 17 months. An underwater glider was piloted with the buoy for the first 6 weeks of the deployment to provide vertical ocean profiles of hydrography and biogeochemistry. These datasets reveal an anomalously strong ocean carbon sink for over 2 months occuring in the region of Bouvet Island and associated with large plumes of chlorophyll-a (Chl-a). Based on Lagrangian backward trajectories reconstructed using various surface currents fields, we identified that the water mass reaching the Bouvet Island region originated from the south-west, from the vicinity of sea ice edge in spring 2021. We suggest that a strong phytoplankton bloom developed there in November 2021 through dissolved iron supplied by early sea ice melt in 2021 in the Weddell Sea. These waters, depleted in carbon, then travelled to the position of the CARIOCA buoy. The very low values of ocean fCO2, measured by the buoy (down to 310 μatm), are consistent with net community production previously observed during blooms occurring near the sea ice edge, partly compensated by air-sea CO2 flux along the water mass trajectory. Early sea ice retreat might therefore have caused a large CO2 sink farther north than usual in summer 2022, in the Atlantic sector of the subpolar Southern Ocean. Such events might become more frequent in the future as a result of climate change.
- Preprint
(3477 KB) - Metadata XML
-
Supplement
(579 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-2668', Anonymous Referee #1, 29 Nov 2024
General
This is an interesting work with significance for research communities interested and working on similar topics. It shows the importance of in-situ observations and the synergies between different platforms to better understand the spatio-temporal dynamics of biogeochemical parameters coupled to physical phenomena. The hypothesis on what is driving the CO2 sink anomaly is sound (i.e. dissolved iron), however the results from this study only make an indirect connection and do not provide a way to quantify this. The arguments and references in the discussion section provide a justification but not strong enough to have an explicit statement as the one mentioned in the abstract (line 24).
The methodological approach and the subsequent analysis are robust to constrain air – sea CO2 fluxes, considering the inherent limitations that in situ- technology and the deployment area impose. The use of satellite data is coherent and complements well that in-situ observations.
One surprising point is that SOCAT, GLODAP, BGC ARGO/SOCCOM data (exception: the Briggs et al., 2018 paper) are not used or explored to identify whether they can increase the data density and/or comparisons with previous years. If such data are not relevant due to spatio-temporal differences, it will be useful to mention it. To an extent this might strengthen the relevance of the dataset and work as the dataset will be even more valuable.
On that note the relevance and impact of this work is evident, especially if one considers that it’s in a severely under-sampled areas and with a large contribution to earth’s climate.
Specific comments on sections
Introduction
This section is concise and comprehensive. The reference coverage is ok but some additional references like, Sutton, A. et al., 2021 (https://doi.org/10.1029/2020GL091748), Landschützer, P. et al., 2015 (https://doi.org/10.1126/science.aab2620), Sarmiento, J. et al., (https://doi.org/10.1016/j.pocean.2023.103130) seem relevant.
Line 52: For additional simplicity it might be useful. The references are adequate, but it will be helpful for the reader if values for interdecadal and interannual variability are mentioned.
Line 74 – 75: Please add some references
Line 77: The point is not visible in Fig 1.
Methods
Overall, the section is well presented. The lack of calibrations procedures for the optode is a short-fall. The authors do explain how this is addressed but there’s no assessment of whether this assumption is adequate for this work. The 8 uM correction is high and there is a question whether this is drifting with time. The fact that O2 is used as a diagnostic and showcasing trends, makes this shortfall less important, however there might be an impact on the O2-O2sat calculation.
Why is the oxygen flux (FO2) mentioned (see comment in results as well) since it’s not used later in the results or analysis?
Line 91-92: Please mention the model, type of sensors and how good they performed.
Line 93: Is the unit umol l-1 or umol Kg-1?
Line 127: typo
Line 133: How close is “nearest”? Not clear whether this is 20 km or less.
Results
Surprised that flux data for both CO2 and O2 are not presented at all (even in the appendix). The authors do make a valid point of the use of DIC and NCP fluxes, however it’s a bit confusing to mention sinks and fluxes in the title, explain how they are calculated and mention them throughout the manuscript and don’t include a single graph.
Line 223: “…probably entraining waters rich in DIC…”. Can GLODAP provide more quantitative information on this and enhance the XLD analysis?
Section 3.4: The comparison is valid, yet difficult to provide strong conclusions considering that it’s a comparison against only one season 2 years before. Maybe SOCAT data can provide a bit more coverage.
Discussion
Line 316: What’s the definition of “massive”? Also related to the general comment in the results section, is the fact that there’s no value with the term “unusually large CO2 sink”.
Line 322: Again what’s the definition of “more massive”?
4.2. Interannual variation: It would have been useful to elaborate more on air-sea co2 fluxes variability.
Wondering whether it will be informative to show and attempt connections with sea ice retreat anomalies. The paper from Morioka, Y., et al., 2024 https://doi.org/10.1038/s43247-024-01783-z is very recent and wouldn’t have been possible to include it but might be a source of inspiration(?)
Citation: https://doi.org/10.5194/egusphere-2024-2668-RC1 -
RC2: 'Comment on egusphere-2024-2668', Anonymous Referee #2, 02 Dec 2024
Review of “Anomalous Summertime CO2 sink in the subpolar Southern Ocean promoted by early 2021 sea ice retreat” by Kirtana Naêck et al.
This is an interesting study, that reveals the importance of sea ice melt or iron to support large phytoplanktonic blooms in the Southern Ocean, even in areas where enrichment or iron from land aeolian transport or hydrothermal vents could have been suspected at first sight. In addition, the authors put the observed bloom in perspective with earlier ice retreat, a feature that is predicted to repeat more frequently in the future. I am on the same line as the authors and think this study is valuable and deserves publication. I have no doubts about that.
Still, I have a few concerns.
The first one does not necessarily need to be addressed. SO-CHIC project proposed a very original combination of Carioca and glider surveys. I’m wondering if more emphasis could have been put on the physical processes. Gliders provide very valuable information about the physical processes underneath the buoy. The concepts of MLD vs XLD have been rapidly addressed in the results, but their impact was not discussed in depth elsewhere. I have the feeling that the authors are doing a bit of cherry-picking, presenting only a part of the data set till 27 June 2022 or some periods where primary production strongly affects the DIC signal (first two weeks of February and first two weeks of march). Personally, I am not encouraging a selective or partial interpretation of the data, but I can understand that you want to deliver one message (the impact on primary production, away from the SIZ) and then not discuss every single aspect. But then you should refrain from writing some sentences like “This suggests that biological activity was the dominant driver of the DIC seasonal variation “line 229, while you are looking into detail only some few weeks, and then during these few weeks, the DIC was actually increasing overall, not decreasing. At the end, the incredible opportunity and amount of energy needed to deploy the glider in conjunction with the buoy was not really necessary. Results from the glider are not addressed in the discussion or in the conclusion.
Second, there are more than 80 plots in the paper in three different places (in the text, in the 6 parts of the appendix and in the supplemental material). It’s really a massive amount of information, and while it’s a rigorous approach, to be honest, it confused me at some point. There is repeatedly the same information with different products ( e.g. Figure B2) or the same data appears in different graphs (DIC changes and O2-O2 sat appear in figure 2, 4 and A1). The figures are not appearing in the same order that in the text (e.g. line 251 reads “Fig C1 and fig E1in appendix” – then why there is appendix D between C1 and E1). There is a long discussion on Figure C1 that is a bit awkward for me and not needed. I mean, the presence of low salinity in the Southern Ocean due to sea ice melting is an information relatively straightforward and widely admitted. The back trajectories are very useful, one distribution of salinity from remote sensing of reanalysis validated by the onboard thermosalinograph is largely enough for me, but personally, I don’t need more proof of that. It is, of course, the authors' responsibility to add more figures, but keep in mind that at some point, the paper is becoming cumbersome with non-essential or repeated information. At some point, reading the results, I was starting to be confused, jumping from one figure to another one, in the main text, the appendix, or the supplemental, going back and forth. I would either remove some figures, like C1, or I would order them better, in only two places, in the order of appearance in the text, with careful references in the text to help the reader to follow your ideas.
Minor comment
Line 79: It's up to you, but personally, I found the sentences “In the next section, the instruments deployed, the different data sets, and the methodology used will be described. There will then be a description of the results followed by a discussion " unnecessary. In each paper, that is roughly what we expect to find in that sequence. I would remove it therefore.
Line 84: “It was anchored at 15m”. What does it mean? I presume that the buoy was NOT anchored, at least with a regular anchor, or it might have used a floating anchor, but then it should be precise.
Line 90: Carioca buoys are formidable instruments, able to withstand the rigours of the Southern Ocean and provide precious data at mesoscale or synoptic time scales. Still, there has been a lot of discussion about the accuracy of pCO2 derived from drifters in the Southern Ocean(Long et al., 2021; Williams et al., 2017; Wu et al., 2022; Zhang et al., 2024), especially SOCCOM drifters. I acknowledge that SOCCOM and Carioca sensors are different, but that is still the same principle, and some potential biases are similar (measurement of pH instead of pCO2, assumption on alkalinity, and so on). When the error of ICOS measurements, with direct measurements of pCO2 using shower head equilibrator, CO2 CRD analysers, and regular calibration with standard gas is 2 µatm, I think that the absolute precision of 3 µatm claimed for Carioca buoy should be carefully assessed. I’m sure it has been, but a reference will be very welcome here. And do you mean precision or accuracy? The concept of absolute and relative precision, both in µatm is not clear to me. Is the absolute precision the accuracy? Should the relative precision be in %? I’m sorry for my naïve questions.
Line 102. Why stop on the 27 June 2022? It’s somehow uncommon to present only a part of the data and not the full data set. Is there a scientific rationale for that? Was the rest of the data not interesting enough?
Line 128. Could you provide details on that instrument, brand, capability? I’ve tried to find some information on it, but it was not that easy to find some.
Line 135. This approach is interesting. Could you provide some details about the Savitzky-Golay filter that would be useful for others, like width and order?
Line141. You’re providing details on the calibration of the fluorimeter, but actually, it’s calibrated against another fluorimeter (the one from the CTD). But then, how the fluorescence units are converted in chlorophyll (using built-in algorithms)? And why provide details for the sea glider and not from the Carioca buoy? It seems important for me to provide details on converting fluorescence to biomass.
Line 144. Remove one parenthesis after “de Boyer Montégut et al., 2004”
Line 144. “mixing” Should it be written “extreme layer depth” instead?
Figure 4. Why did you split the figure and remove the second half of February? It looks like you’re cherry-picking and presenting only part of the data set. This approach should be discouraged.
Line 229. “This suggests that biological activity was the 230 dominant driver of the DIC seasonal variation”. There is something I don’t quite understand. You put a lot of emphasis on the impact on biological production. I am not contesting that there was primary production, but, when looking at the DIC, comparing the 01/02/2022 to 13/02/2022 or the 01/03/2022 to 17/03/2022, in both cases, the DIC is increasing overall, NOT decreasing. So there are other processes at work, mixing, air-sea exchange, and water mass change, that outweigh the decrease in DIC. I would not write, therefore, “the dominant driver”. It is not for me.
Line 241 to line 245. Could you indicate the relevant figure? I am a bit lost among the 80 plots of the paper.
Line 247. “The decrease in salinity started near the South Sandwich trench, around 25° W and 60° S, near the sea ice edge in September 2021”. How can you see that? I mean, the data are blanked in the figure. I don’t understand the discussion actually. The ice retreats, but in the west, there is low salinity, and in the East, there is high salinity.
248 and 60° S, near the sea ice edge in September 2021, when the sea ice started to retreat
Part 4.3. I’m not convinced by the rigour of the approach (assuming no mixing), but more importantly, by the interest of such computation subjected to caution, while it’s not clear to me what this computation is bringing to the overall conclusion. It’s not my decision, but I would remove that part.
Supplemental.
There is a poor definition of the figures when zooming in. It’s difficult to see the details.
References
Long, M.C., Stephens, B.B., McKain, K., Sweeney, C., Keeling, R.F., Kort, E.A., Morgan, E.J., Bent, J.D., Chandra, N., Chevallier, F., Commane, R., Daube, B.C., Krummel, P.B., Loh, Z., Luijkx, I.T., Munro, D., Patra, P., Peters, W., Ramonet, M., Rödenbeck, C., Stavert, A., Tans, P., Wofsy, S.C., 2021. Strong Southern Ocean carbon uptake evident in airborne observations. Science 374, 1275–1280. https://doi.org/10.1126/science.abi4355
Williams, N.L., Juranek, L.W., Feely, R.A., Johnson, K.S., Sarmiento, J.L., Talley, L.D., Dickson, A.G., Gray, A.R., Wanninkhof, R., Russell, J.L., Riser, S.C., Takeshita, Y., 2017. Calculating surface ocean pCO2 from biogeochemical Argo floats equipped with pH: An uncertainty analysis. Global Biogeochemical Cycles 31, 591–604. https://doi.org/10.1002/2016GB005541
Wu, Y., Bakker, D.C.E., Achterberg, E.P., Silva, A.N., Pickup, D.D., Li, X., Hartman, S., Stappard, D., Qi, D., Tyrrell, T., 2022. Integrated analysis of carbon dioxide and oxygen concentrations as a quality control of ocean float data. Commun Earth Environ 3, 1–11. https://doi.org/10.1038/s43247-022-00421-w
Zhang, C., Wu, Y., Brown, P.J., Stappard, D., Silva, A.N., Tyrrell, T., 2024. Comparing float pCO2 profiles in the Southern Ocean to ship data reveals discrepancies (No. EGU24-3332). Presented at the EGU24, Copernicus Meetings. https://doi.org/10.5194/egusphere-egu24-3332
Citation: https://doi.org/10.5194/egusphere-2024-2668-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 205 | 32 | 101 | 338 | 13 | 2 | 3 |
- HTML: 205
- PDF: 32
- XML: 101
- Total: 338
- Supplement: 13
- BibTeX: 2
- EndNote: 3
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1