Manuscript Number: egusphere-2025-1736

Title: The Atmospheric Potential Oxygen forward Model Intercomparison Project (APO-MIP1): Evaluating simulated atmospheric transport of air-sea gas exchange tracers and APO flux products

Authors: Y. Jin, B. B. Stephens, M.C. Long, N. Chandra, F. Chevalier, J.J.D. Hooghiem, I.T. Luijkx, S. Maksyutov, E.J. Morgan, Y. Niwa, P.K. Patra, C. Rödenbeck, J. Vance

The authors conducted an atmospheric transport model (ATM) intercomparison (TransCom) experiment by using APO as a transport tracer to evaluate the representations of the transport and fluxes. They used three air-sea APO fluxes and eight atmospheric transport models including the model variants to simulate spatiotemporal variations in APO. The annual means, seasonal amplitudes, and day of seasonal minimum of the simulated APO were compared to those of observed APO from the Scrips surface measurement network, several airborne meridional cross-section observation programs, and shipboard measurements made on transects crossing the Drake Passage. Intensive comparison revealed that there is large spread in the performance of the ATMs. Especially from the comparison of the global airborne measurements between observation and simulation, the authors succeeded in evaluating the validity of APO fluxes. Additionally, the authors, applying a previously developed framework to evaluate diabatic mixing rate to this study, investigated the simulation-based and observation based diabatic mixing rates and APO gradients. The result also clearly showed the ATM-related biases in the diabatic mixing rate and differences in the representation of APO fluxes.

Previous TransCom experiments, using atmospheric tracers like CO2 with strong land sources and/or sinks, did not necessarily evaluate the atmospheric mixing above the ocean. Since APO is a unique tracer to study air-sea gas exchange, this TransCom experiment can provide a new perspective on the evaluation of the mixing processes in the ATMs especially above the ocean.

I found that the paper is excellent, well organized, and contains material that should be published in EGUsphere. I highly recommend the manuscript to be published with the minor corrections as suggested below.

Specific comments:

Page 1, line 25: Although air-sea CO₂ exchange seldomly affects APO seasonal cycle, it may be oversimplified to say that APO is a tracer of air-sea O₂ exchange. I think it would be better to reword the relevant wordings to “a unique tracer to study air-sea gas exchange” or something like that.

Page 3, line 84: There are two “Nevison et al., 2008” in Reference. (“Nevison et al., 2008” is also cited in line 442.)

Page 6, line 165-166: Is it possible to quantify the extent of the “minor effect” for the ±0.05 change in the O₂:C exchange ratio for the terrestrial biota.

Page 9, line 256: It would be better to change “the ERA5 reanalysis” to “the ERA5 reanalysis wind fields (Hersbach et al., 2020)”. Additionally, the reference should be removed from line 270.

Page 13, line 339: “Schuldt et al., 2021” is not listed in Reference.

Page 16, line 382-385: I think that it’s not easy to imagine the meridional gradients in the annual mean APO from Fig. 4A. It would be better to refer Fig. 5A here.

Page 16, line 389: In this study, the authors mentioned that the days of seasonal minimum are latitudinally uniform in each hemisphere. Is it the same for the seasonal maximum? As far as I know, onboard observation in the western Pacific revealed that there was a difference in the meridional distribution pattern of the phases between seasonal minimum and maximum (Tohjima et al., 2012). If the APO observations adopted in this study show different meridional distribution patterns between seasonal minimum and maximum, it would be better to describe it in the manuscript to avoid misunderstanding of the readers.

Page 482-483: I’m not sure what the sentence means. Figure 6B seems to show that the simulated SCAs based on CESM agree with the observation while those based on DISS are underestimated.

Page 24, Figure 6: Please check the titles of figures: “( C ) Column-average APO SCA” and “( E ) Column-average APO Seasonal Minimum Day”. Additionally, it would be better to add black line indicating the observation in the upper figure legends.

Page 29, line 664: It seems that only two key aspects are described in the paragraph.

Page 31, line 694-695: “(a-b)”, “(c-d)”, and “(e-f)” should be “(A-B)”, “(C-D)”, and “(E-F)”.

Page 37: line 807: I’m not sure why the authors indicate Figs. 8 and 9 here.

Page 40, line 925: I think it would be better to explain the “rectifier effect” in the manuscript or to add appropriate reference here.

Page 44, line 1025: Does the scaling factor of 0.82 partially explain the underestimation of the simulated SCA based on DISS flux? If it’s true, should it be mentioned in the manuscript?

Page 44, line 1025: “Naegler et al. (2006)” should read “Naegler et al. (2007)”.

Page 60, line 1410-1413: “Kenneth et al., 2021” is not cited in the manuscript.

This manuscript presents results from a forward model intercomparison project using Atmospheric Potential Oxygen (APO), comparing eight models/model variants and three APO flux products. The focus of the study is on evaluating uncertainty in atmospheric transport models (ATMs) by comparing to surface, shipboard and aircraft observations with a focus on latitudinal and seasonal variability, annual mean APO, seasonal cycle amplitude and seasonal minimum timing, as well as diabatic mixing rates. Unlike previous studies, which have tended to focus on tracers with land sources/sinks that are more abundant in the northern hemisphere, this study aims to add value to the field by simulating a tracer of surface ocean exchange with significant seasonal atmospheric variability in both hemisphere. Although a previous APO intercomparison study was already conducted in the mid-2000s, there have been substantial advances since in terms of improvements in ATM performance, in ocean flux simulations, and in methods to evaluate diabatic and adiabatic mixing processes, as well as additional observations; hence, the authors were motivated to conduct a more extensive study.

Overall I find this manuscript to be excellently written and executed, with detailed analyses that are well-explained and presented in high-quality figures. I consider this manuscript an excellent fit to the journal and recommend it be published with only minor corrections as follows:

Line 25: This definition of APO is not strictly true, but I think if the wording is changed to “APO… can be used as a tracer of air-sea O2 exchange…” or similar then this fixes the issue, which presumably only arises in the abstract for reasons of brevity as the full explanation in the introduction is correct.

Line 27: very minor point, but perhaps there should be a “the” before “Atmospheric Potential Oxygen forward Model Intercomparison Project” as per the title?

Line 52: Strictly speaking, I think the APO equation should have “≈” instead of “=” as this version excludes the oxidation of CH4 and CO molecules as per the original Stephens et al. 1998 publication. This also applies to Equation 1 around line 155.

Lines 71-72: Perhaps the citations here should be preceded by “e.g.” as this is probably not the complete list?

Line 140: This paragraph begins with an explanation of section 3, but perhaps 1-2 sentences of explanation on section 2 would also be helpful?

Line 168: Does the first mention of the unit “per meg” need a citation?

Line 339: Schuldt et al., 2021 is missing from the reference list.

Line 382: I think it is the difference between the models and the multi-station global mean shown by the colours in figure 4, not the model errors?

Line 383: Meridional gradients in the observations are clearer in Figure 5. Also, perhaps for clarity the figure 4 caption should mention that the sites are organised by latitude from south (left) to north (right)?

Line 384: I think it would help the reader to identify more clearly where this southern tropical bulge is being observed (presumably it is in the values for SMO?)

Lines 482-483: I disagree with this description of the figure, as it seems to me that Jena is over-estimating SCA in the northern mid-latitudes, while CESM agrees quite well and DISS is under-estimating at all latitudes. Perhaps instead the authors are referring to the annual mean panels, where CESM and DISS seem to over-estimate at those latitudes?

Figure 6: I think the plot labels here should read (A), (B), (C) as per the caption? Also it might be better to place the legend at the bottom of the plot rather than below the (A) panel label.

Line 1188: Should be “References”

Line 1410: The Kenneth et al 2021 reference is not mentioned in the text anywhere.

Lines 1448-1453 and1495-1503: There are two different Long et al (2021) references and two different Nevison et al. (2008) references that both need “a” and “b” suffixes.

General comments on the figures: Overall these are excellent, however, I’m not sure the colour schemes used are fully accessible? So the authors might want to consider alternatives, and perhaps also the use of additional symbols/line types. Resolution could also perhaps be improved for Figures 1, 6, and 8 in particular. The axes labels in Figures 6, 8 and 10 are also quite small and it might be beneficial to make these larger and easier to read. Finally, is it possible to either add error bars to the observations in Figures 3, 5, and 6, or if these are not easily visible in the plots, is it possible to give an indication of the typical observational error in the figure captions. This would help to place some of the model differences into context when these are small, especially for readers who are not overly familiar with APO and its typical measurement errors.