This manuscript presents an updated version of the Takahashi surface ocean CO2 climatology. Both this and the associated fluxes that are calculated are very useful for a large scientific community. The update is very welcome. I have a few relatively minor comments below.

Specific comments:

Please provide proper uncertainty assessments for the fluxes. You describe in section 4.2 what sources of uncertainty you include, but you appear to have taken numbers from Wanninhof et al (2013) rather than calculating your own numbers based on the %-uncertainty contributed by each term (from their Table 1). A map showing the uncertainties spatially would be highly useful.

Throughout the manuscript several averages are presented. Please provide also a standard deviation for all of them.

The SOCAT data product was first released in 2011.

It is very difficult to see the lowest numbers on Figure 1a. Could you make 0 white as in Figure 1b?

Did you test your assumption/hypothesis that there is no trend in \Delta_fCO_2 over the 40 year period? A figure in the supplement showing this would be nice I think.

How big is the area of the Arctic Ocean you make land?

Line 490: It is unclear which quantity you are referring to here.

Line 732-735: This needs revision for clarity. I assumed that the Southern and Northern hemispheres would add up to the global, but it does not (since the tropical area is missing).

I think including the section "LDEO flux" from the supplement in the main text would make sense.

I often find the manuscript a bit difficult to read. There are many very long and cumbersome sentences that I struggle to understand. There is also a rather excessive use of semicolons. I know many like semicolons, and I admit to having a particularly strong dislike of them, but they do not aid reading. A semicolon, most often, replaces a word (the word you would need if you used a comma instead). As a non-native reader my brain keeps stopping and trying to identify what the word is. I can't seem to fully grasp what the sentence says without mentally putting that word into it. This makes for slow and frustrating reading. I would therefore urge you to go through the text and simplify the language and ensure better readability. Many times shorter sentences would do the trick.

Review of the MS essd-2023-429 :

Title: Updated climatological mean delta fCO2 and net sea–air CO2 flux over the global open ocean regions , by Amanda R. Fay et al.

General comment:

Amanda Fay and CO2-authors revisit the Takahashi et al (2009) climatology using SOCAT version 2022. As there are much more fCO2 data available in SOCAT this is a very good idea to reconstruct such climatology extensively used to constraint atmospheric inversions (a-priori fluxes), methods that reconstruct ocean pCO2 fields (e.g. Rodenbeck et al, 2015) or to validate ocean carbon models (e.g. RECCAP 1 and RECCAP 2 stories). Compared to previous climatology (T-2009), authors found significant differences in the high latitudes (in winter in the SO; in summer in the North Atlantic) and in the south-east Pacific where data were sparse and are still missing in large regions (including in SOCAT-v2024 in progress). The paper is well structured, figures adapted. What is missing is a table (somehow like presented by Takahashi et al 2009, see their table 6) where authors would list: Regions (or biomes), their surface area, the mean DfCO2 and the Flux.

The paper is pleasant to read and suitable for publication in ESSD after few corrections. Below are listed specific comments.

Specific comments:

C-01: Line 22: In Key point: add error/uncertainty on the flux -1.79 PgC yr-1

C-02: Line 35: In Abstract: add error/uncertainty on the flux -1.79 PgC yr-1

C-03: Line 41: “now exceed 415 ppm”: As atmospheric CO2 increases rapidly, maybe specify the year for the value 415 ppm.

C-04: Line 55: Here maybe specify you list the approaches based on observations (i.e. not models):

“multiple approaches based on atmospheric and/or oceanic observations”.

C-05: Line 83: For Takahashi et al (2009), add also reference to Takahashi et al 2009b (corrigendum).

C-06: Line 83: I guess you can also refer to Tans et al (1990) who present the first global seasonal map of DpCO2 with the data they had in hand at that time. I think this was the first study that use a “DpCO2 climatology” to constraint a global carbon budget and clearly motivated the next steps (toward a full year ocean pCO2 and flux climatology, Takahashi et al 1997, 2002, 2009…). This also motivated the start of SOCAT first discussed in 2007 (Metzl et al, 2009) and released in 2011.

C-07: Line 196: You used fCO2, SST, SSS and sea level pressure from SOCAT; for SSS when there is no measurement for a cruise you can recall that you used the SSS from WOA also in SOCAT data files (see Pfeil et al, 2013). Should you indicate somewhere that you select only the Cruises with flags A-B-C-D and data for fCO2 with WOCE flag 2 ?

C-08: Line 275: add error/uncertainty on the flux for both the normalization method (-1.85 PgC yr-1) and the current method (-1.79 PgC yr-1).

C-09: Line 358: Low values of ΔfCO2 in the North Atlantic region also driven by biological activity in Spring-Summer ? (as you mentioned line 445)

C-10: Line 361: Is the near-global mean climatology seasonal curve (figure 2a) useful to discuss ? Would it be better to separate NH and SH (two curves) ?

C-11: Line 552: Chen et al: correct: Chen and Tsunogai (1998). For the large fluxes in the Indian sector and Arabian sea maybe refer to Sabine et al (2000) and/or Sarma et al (2023) ?

C-12: Line 560-564: The carbon source in the Southern Ocean during austral winter (Atlantic and Indian sector south of 45S) is likely linked to deep mixing and/or upwelling (import of high DIC in surface layers).

C-13: Line 567: “Uncertainties are higher in the Southern Ocean region due to the limited number of observations, particularly in winter”. Could you specify “uncertainties”: fCO2 reconstruction, fluxes, both ? Compared to T-2009, is there are more data in winter in SOCAT in the SO that would reduce these uncertainties ?

C-14: Line 611: For the comparison of data used in T-2009 and this study, could you show a map where data are in SOCAT-v2022 but not in T-2009. The supp mat (figure S1) shows the LDEOv2019, but might be useful to show the original LDEO when comparing fluxes with T-2009. I guess this is from Takahashi, Sutherland, and Kozyr (2009)

C-15: Line 741: “less data in socat since 2017”.  Maybe refer to the Bakker et al (2023)  ?

;;;;;;;;;;;; In Figures:

C-16: Figure 7: maybe change the range -50 to 50 µatm to better highlight the differences ?

C-17: Figure 8: maybe indicate in the caption that white area in the SO in JJA is because of  ICE extend ?

;;;;;;;;;;;; In references:

C-18: Line 771: Antonov et al : not cited in the MS

C-19: Line 834: De Vries et al (2023) now published

C-20: Line 1026: Takahashi et al 2009 (check list of authors)

;;;;;;;;;;; In Supplementary Material:

C-21: Table 1: add unit for number listed in this table (µatm/yr)

;;;;;;;;; Reference added in this review not listed in the paper

Bakker, D., R. Sanders, A. Collins, M. DeGrandpre, T. Gkritzalis, S. Ibánhez, S. Jones, S. Lauvset, N. Metzl, K. O’Brien,  A. Olsen, U. Schuster, T. Steinhoff, M. Telszewski, B. Tilbrook,  D. Wallace, 2023. Case for SOCAT as an integral part of the value chain advising UNFCCC on ocean CO2 uptake http://www.ioccp.org/images/Gnews/2023_A_Case_for_SOCAT.pdf

Metzl, N., Tilbrook, B., Doney, Scott C., Le Quere C., Feely, R. A., Bakker, D.C., Roy S., 2009. Dedication to Dr Taro Takahashi. Deep Sea Res., II, 56, 8-10. https://doi.org/10.1016/j.dsr2.2008.12.039

Sabine, C. L., Wanninkhof, R., Key, R. M., Goyet, C., & Millero, F. J. (2000). Seasonal CO2 fluxes in the tropical and subtropical Indian Ocean. Marine Chemistry, 72(1), 33–53. https://doi.org/10.1016/s0304-4203(00)00064-5

Sarma, V. V. S. S., Sridevi, B., Metzl, N., Patra, P. K., Lachkar, Z., Chakraborty, K., et al. (2023). Air-sea fluxes of CO2 in the Indian Ocean between 1985 and 2018: A synthesis based on observation-based surface CO2, hindcast and atmospheric inversion models. Global Biogeochemical Cycles, 37, e2023GB007694. https://doi.org/10.1029/2023GB007694

Takahashi, T., S.C. Sutherland, and A. Kozyr. 2009. Global Ocean Surface Water Partial Pressure of CO2 Database: Measurements Performed During 1968–2008 (Version 2008). ORNL/CDIAC-152, NDP-088r. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, doi: 10.3334/CDIAC/otg.ndp088r.

Takahashi, T., S C. Sutherland, R.Wanninkhof, C. Sweeney, R.A. Feely, D. Chipman, B. Hales, G. Friederich, F. Chavez, A. Watson, D. Bakker, U. Schuster,  N.Metzl, H.Y. Inoue, M. Ishii, T. Midorikawa, C.Sabine, M. Hoppema, J.Olafsson, T. Amarson, B.Tilbrook, T. Johannessen, A. Olsen, R. Bellerby, Y. Nojiri, C.S. Wong, B. Delille, N. Bates and H. De Baar, 2009. Corrigendum to Climatological Mean and Decadal Change in Surface Ocean pCO2, and Net Sea-air CO2 Flux over the Global Oceans (DSRII). Deep-Sea Res I, doi:10.1016/j.dsr.2009.07.007

Tans et al.  Observational Contrains on the Global Atmospheric Co2 Budget.Science247,1431-1438(1990).DOI:10.1126/science.247.4949.1431

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