|The paper “Effect of gas-transfer-velocity parameterization choice on CO2 air-sea fluxes in the North Atlantic and the European Arctic” by Wrobel and Piskozub study the uncertainty of air-sea CO2 flux estimates in the North Atlantic due to the air-sea gas parameterization, particularly with respect to five different commonly used gas transfer rate formulas. They indicate that up to 5% uncertainty can be attributed to this. They suggest that relatively stronger wind speed and lack of seasonal changes in the flux direction are the reason for this uncertainty.|
Although I found the study interesting, I feel that the analysis performed is too brief, which resulted in relatively limited new and significant advancement in our current knowledge understanding. There are also a number of English errors, which make the manuscript unpublishable at this stage. Below I have provided the authors with some ideas and suggestions to extent the current work and to further improve the overall presentation of the study as well as some minor corrections.
Although it is useful to quantify uncertainty attributed to the gas transfer formulation, there is very limited new information for the community. Given that the authors have done the computation for the whole global ocean, a more detailed investigation on regional features would be very useful, i.e., in addition to the North Atlantic. The authors state that the uncertainty could be substantial in regions of high wind speed. Is this only valid for the North Atlantic or also for other strong wind regions such as the Equatorial Pacific and the Southern Ocean?
To compute uncertainty highlighted in this study, one would only need to provide surface pCO2, SST, SSS, wind speed, and atmospheric CO2 fields. In addition to the above point, I think the authors should also consider studying how this uncertainty would evolve in the future, regionally and temporally. To do this, one can use the CMIP5 model outputs, which are publicly available. Questions such as will uncertainty due to atmospheric CO2 concentration dominates the uncertainty due to wind speed formulation, etc. has not been studied previously and can be addressed here.
In which regions, and when in the future, the uncertainty become significant thus provides some recommendation for improving the monitoring network to the observational community.
L210-214: I am not entirely convinced by this analysis and I think that this is not entirely true (e.g.based on Fig 4d and 3d). From my visual inspection, the spatial variations in Fig.4 actually resemble very much Fig. 2, and therefore, I would assume the difference shown in Fig. 4 to a large extent stems from the amplitude of delta pCO2 values, rather than the wind speed. Assuming that atmospheric CO2 has very little spatial variability, the delta pCO2 patterns will be very similar with Fig. 2.
Fig. 5 shows that the differences in wind speed parameterization is largest between McGills and Nightingale parameterizations. Based on this, it would be more informative to show the difference between fluxes computed based on these formulations in Fig. 4, as an indicator for upper uncertainty range.
Some of the existing analysis can be illustrated in a simpler and compacted way, such as put values from Figs. 6 and 7 into one single table (Line 234-237, the values from the global estimates, which are missing from Fig. 7 should be added to this table as well). In addition, to better illustrate regions where different wind parameterization could be important for air-sea fluxes estimation, I recommend showing a map of standard deviation/variance of gridded fluxes computed using the different equations.
L30-32: I suggest revising this sentence into something like: “The seasonal flux in the Arctic computed from the two climatology data sets are opposite to one another, possibly due to insufficient spatial and temporal data coverage, especially in the winter.”
L55: Studying the rate of the ocean CO2 sink…
L55: … especially its long-term trends, one needs …
L83 … European Space Agency funded OceanFlux …
L85: Maybe better to replace “opening sourced” with “publicly available”
L98 users can choose
L99: how is ice age affect air-sea flux? clarify. Do you mean sea-ice area?
L100: clarify what is meant by “whitecapping”?
L101: configure them in ..
L111: Bakker et al.
L117. clarify what is meant here by “preprocessed”?
L133: What is meant by “sea state”?
L137: the solubility unit should be g m-3 microatm-1
L175: in water])
L184: do you mean “Ocean Flux GHG project”?
L195: NIghtingale et al. (2000)
L195: Takahashi et al., (2009)
L197: replace “all-season” with “annual”
L205: are shown
L219 “… by up to 30% …”
L269:271, again not clear to me where are 33% and 50% quantities derived from?
L257-258, where does the 9% value come from? Fig. 7 only shows values for North Atlantic and Arctic.
Double check the units throughout the text. I believe most of the [Tg] should be [Pg].
Figs caption: would be useful to state that positive values represent outgassing and negative otherwise, when showing the air-sea flux of CO2 values.
Fig1. the color bar can be improved, e.g., use ranges from -20 to 20 since there seems to be no grid points showing values of -40.
Fig. 1. Add zero contour lines to distinguish uptake and outgassing areas.
Fig. 6a and 6b are identical, and the units in x-axes are wrong as well.
Fig. 7. Clarify what is meant by normalized here. Shouldn’t it be unit-less instead of Tg/year?
Fig. 8: units should be in Pg instead of Tg.
Last paragraph of introduction: In addition to formula in the gas transfer velocity, the selection of wind product also contribute to the uncertainty, therefore would be good to mention this, citing the study by Gregg et al., 2014 (Ocean Modelling: Sensitivity of simulated global ocean carbon flux estimates to forcing by reanalysis products.)
There is also a study by Landshutzer et al. (2011, in Biogeosciences Discussion, A model study on the sensitivity of surface ocean CO2 pressure with respect to the CO2 gas exchange rate), which can be cited as well.
I would also recommend a native speaker to review the English prior to resubmission.