the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Dec 2025
Global Gridded Air-sea Oxygen Flux Inferred from a Machine Learning-based Dissolved Oxygen Product
Abstract. We present estimates of the monthly open ocean air-sea O2 flux from 2004 to 2024 on a 1°×1° grid spanning 64.5°S to 79.5°N. The flux is computed using 4-dimensional ocean dissolved oxygen (DO) fields derived from Argo float and shipboard observations via machine learning algorithms (GOBAI-O2, Sharp et al., 2023). Flux uncertainties are quantified by generating a large ensemble of flux estimates, first propagating errors from the dissolved oxygen product, then computing fluxes across all combinations of three gas exchange parameterizations that account for bubble-mediated flux and three wind products. We apply DO adjustments to align our resolved global annual mean fluxes with those derived from scaling global ocean heat uptake and regional annual mean fluxes with those derived from ocean inversions. Our results show larger seasonal flux variations at high latitudes than at low latitudes, with clear differences between major ocean basins. Both adjusted and unadjusted annual mean flux estimates exhibit strong ocean O2 sinks at high latitudes, weak sources in the low-to-mid latitude subtropics, and weak sinks near the Equator. The annual mean adjustment significantly enhances ocean O2 uptake in the northern high latitudes and tropical regions while reducing outgassing in the northern subtropics. We evaluate our flux seasonal cycles and annual mean values by comparing forward transport simulations with atmospheric O2 observations from global airborne surveys and surface sampling stations. The simulations reproduce the observed mean seasonal cycles well, but some differences remain in the annual mean latitudinal gradients. We analyze fractional variance contributions from DO, wind, and gas exchange scheme uncertainties and their interactions at regional and hemispheric scales for both climatological monthly and annual mean fluxes. This dataset marks a major improvement over existing air-sea O2 flux products, as it includes the resolution of interannual variability in the flux seasonal cycle, the use of advanced machine learning-based DO fields that better represent complex spatial and temporal patterns, and robust uncertainty quantification across various scales.
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CC1: 'Comment on essd-2025-738', Andrew Kowalski, 26 Dec 2025
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CC2: 'Reply on CC1', Andrew Kowalski, 20 Mar 2026
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During discussions in another forum, it has been argued that the oxygen community correctly applies solubility equations via an assumption that air directly at the water surface is essentially at 100% relative humidity, such that only the water temperature and not the atmospheric humidity is required to apply Henry's law. I would like to preclude such an argument here because it reflects a fundamental misunderstanding of air–sea gas exchange processes.By definition, 100% relative humidity occurs when the partial pressure of water vapor (e) equals the saturation vapor pressure (eₛ; Petty, 2008). Under such conditions, the system is in thermodynamic equilibrium with respect to phase changes of water, meaning that evaporation and condensation occur at equal rates, resulting in no net flux of water vapor. Such an equilibrium state is rarely realized at the sea surface and is typically limited to conditions such as fog. More commonly, net evaporation prevails, with evaporation exceeding condensation in proportion to the difference between e and eₛ (Brutsaert, 1982).Because it is e rather than eₛ that enters Dalton’s law and constrains the partial pressure of dry air—and thereby that of oxygen—it follows that ambient humidity necessarily influences oxygen dissolution. The assumption that it does not is untenable.ReferencesBrutsaert, W., Evaporation into the Atmosphere: Theory, History, and Applications, Springer Science+Business Media, Dordrecht, the Netherlands, ISBN 978-90-481-8365-4, 1982Petty, G. W.: A first course in atmospheric thermodynamics, Sundog Publishing, Madison, USA, ISBN-10 0-9729033-2-1, 2008Citation: https://doi.org/
10.5194/essd-2025-738-CC2
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CC2: 'Reply on CC1', Andrew Kowalski, 20 Mar 2026
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RC1: 'Comment on essd-2025-738', Anonymous Referee #1, 27 Jan 2026
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Yuming Jin and colleagues present a gridded data set of ocean-to-atmosphere oxygen exchange, based on an existing interpolation of dissolved oxygen measurements and gas exchange parameterizations. A specific adjustment is applied to the dissolved oxygen field to prevent otherwise unrealistic annual fluxes. The paper presents a detailed uncertainty analysis, as well as assessments based on independent data.
The data set is useful for oceanographic and biogeochemical research, particularly valuable due to the thorough evaluation presented here. Methods and results are presented in a clear and convincing way. I clearly recommend to publish this contribution. Below are a few minor suggestions the authors may consider.
Minor comments:
L69: “reduces both the upwelling of nutrients”: doesn’t that reduce photosynthesis and thus reduce O2 outgassing?
L105: “with high accuracy”: According to which metric? A brief information would be useful
L130: replace “data” by “fields” to avoid confusion with point measurement data
L138: “gridded values”: how are these interpolated in space and time? A brief information would be useful
L300: “bi-monthly”: every 2 months or twice per month? I think the term is used ambiguously
L357: replace “SCA” by “seasonal cycle” as you do not mean amplitude here
L542: “double-peak structure”: unclear because I do not actually see a double-peak structure in the 60N-80N panel of Fig 6
L548: “in the flux product”: would be good to announce the later discussion on model errors, otherwise the reader keeps wondering
L831-832: “We note that uncertainty due to DO may essentially stem from the annual mean
flux adjustment that corrects DO concentrations for each ensemble member.”: not sure I fully understand this sentenceL849-850: “resolving flux interannual variability”: but didn’t you say only the IAV of the seasonal cycle is resolved?
Typos:L167: “Mixed”
Citation: https://doi.org/10.5194/essd-2025-738-RC1
Data sets
GOBAI-O₂-Flux v1.0: Global Gridded Air-sea Oxygen Flux Inferred from a Machine Learning-based Dissolved Oxygen Product Yuming Jin https://doi.org/10.5281/zenodo.17765275
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As in many previous studies of atmospheric oxygen, this manuscript overlooks the fundamental and controlling influence of water vapor on oxygen availability (Kowalski et al., 2025). The repeated assumption of a constant atmospheric oxygen mole fraction of 0.2094 is inappropriate for many terrestrial surface environments and introduces a systematic bias into the air–sea flux calculations in equations (2) through (4). By neglecting humidity, the authors implicitly assume that the amount of oxygen in air is invariant, whereas it is directly constrained by the presence of water vapor.
The effect is readily illustrated by contrasting typical polar and tropical surface air compositions. In very dry polar air, the water vapor mole fraction may be as low as ~0.0004, leaving dry air to comprise ~99.96% of the total. Under such conditions, representative mole fractions for nitrogen, oxygen, and argon are approximately 0.7808, 0.2095, and 0.0093, respectively. In contrast, tropical surface air commonly exhibits water vapor mole fractions exceeding 0.03, reducing dry air to ≤97% of the total. The corresponding mole fractions of nitrogen, oxygen, and argon are then reduced to less than 0.7577, 0.2033, and 0.0090, respectively—representing a decrease of more than 6000 ppm for oxygen. Assuming a constant oxygen mole fraction therefore leads to a substantial overestimation of the oxygen available for dissolution in tropical surface waters.
This issue is more clearly framed in terms of oxygen’s true thermodynamic control: according to Henry’s law, oxygen solubility depends on its partial pressure. By Dalton’s law, total atmospheric pressure is the sum of the partial pressures of all constituents. Elevated water vapor partial pressure in tropical air does not generally coincide with elevated surface pressure; in fact, mean sea-level pressure near the equator is typically below the global average of 1013.25 mb. Consequently, increased humidity necessarily depresses the partial pressure of dry air and, proportionally, that of oxygen. Ignoring this effect systematically skews estimates of oxygen exchange at the air–sea interface.
The manuscript overlooks this humidity dependence in several key locations, including (i) the second paragraph of the Introduction, (ii) Section 2.3 as a whole, and (iii) Section 2.7, which notes that water vapor is indeed measured but its effect on oxygen availability is not taken into account. Given that humidity is already observed, the omission is both unnecessary and consequential.
The authors should therefore recompute oxygen mole fractions and partial pressures in a manner consistent with atmospheric humidity. Doing so is essential for physically correct estimates of air–sea oxygen fluxes.
Reference
Kowalski, A. S., Janssens, I. A., and Pérez-Priego, O., 2025, Water vapour dynamics as a key determinant of atmospheric composition and transport mechanisms, Biogeosciences, 22, 8005–8012, https://doi.org/10.5194/bg-22-8005-2025