the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
ISASO2 : Recent trends and regional patterns of Ocean Dissolved Oxygen change
Abstract. Recent estimates of the global inventory of dissolved oxygen (DO) have suggested a decrease of 2 % since the 1960s. However, due to the sparse historical oxygen data coverage, the DO inventory exhibits large regional uncertainties at interannual timescale. Using ISASO2, a new DO Argo-based optimally interpolated climatology https://doi.org/10.17882/52367 (Kolodziejczyk et al.,2021), we have estimated an updated regional oxygen inventory. Over the long term (~1980–2013), comparing the ISASO2 Argo fields with the first guess WOA18 built from the DO bottle samples fields extracted from WOD18, the broad tendency to global ocean deoxygenation remains robust in the upper 2000 m with -451±243 Tmol per decade. The oxygen decline is more pronounced in the key ventilation areas of the Southern Ocean and North Atlantic, except in the Nordic Seas, where oxygen has increased. Over the shorter timescale of the Argo period (2005–2019), the deoxygenation tendency seems globally amplified (-1211±218 Tmol per decade). However, DO changes exhibit stronger amplitude and contrasted regional patterns, likely driven by interannual modes of variability in regions as, for instance, the North Atlantic Subpolar-gyre. The recent changes in Apparent Oxygen Utilization mainly explain the interannual variability in the ventilation regions. However, Argo DO coverage is still incomplete at the global and calibration method development are still in progress. Continuing the monitoring of the seasonal to interannual and regional to global DO variability from ISASO2 will improve our ability to reduce uncertainties on global and regional DO inventory.
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RC1: 'Comment on essd-2024-106', Anonymous Referee #1, 13 May 2024
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Comment on “ISASO2 : Recent trends and regional patterns of Ocean Dissolved Oxygen change” by Nicolas Kolodziejczyk et al.
General comments:
This study focused on assessing changes in dissolved oxygen (DO) levels in the world's oceans over the last decades. Using a new dataset, ISASO2, derived from Argo-based optimally interpolated climatology, the study updates regional oxygen inventories and examines long-term and short-term deoxygenation trends. The paper is well written.
My primary concern is the need for a direct comparison with previously established gridded data, such as those by Schmidtko et al. (2017) and Sharp et al. (2023). It would be beneficial to emphasize how this new dataset might address some of the uncertainties previously noted in the field. Additionally, clarifying what distinguishes ISASO2 as potentially more reliable or different from earlier datasets could strengthen the impact and relevance of your findings.
Specific comments:
Line 17: "developpement" should be "development"
Line 65-66: Specify S-profiles and flag 1&2 to the general audience.
Citation: https://doi.org/10.5194/essd-2024-106-RC1 -
RC2: 'Comment on essd-2024-106', Anonymous Referee #2, 10 Jun 2024
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Review: Recent trends and regional patterns of Ocean Dissolved Oxygen change
It is very good to finally see some attention being paid to the promise of the ARGO based chemical climatology data set. This has been long promised, and is now much overdue. I commend the authors on their skill and diligence in ingesting all this information and on devising and implementing the techniques needed to extract information from this large and balky data set. I appreciate the hard work.
BUT publication in this form would embarrass the authors, the Editor, and the Journal for it is rare to see an analysis that so completely misses the point of the study. The manuscript calculates the changes in the quantity of dissolved oxygen in the ocean seen as trends over time and comments on the losses due to the lower solubility of oxygen in warmer water. This is of no significance, although the calculations serve to beef up the observations. It is not clear in reading through this if care is taken to avoid including data in the euphotic zone where highly variable active oxygen production occurs – the brief mention that oxygen levels are increasing in the Nordic Seas seems to hint at that?
The concerns are for the impacts of changes in oxygen on marine life for food security, approaching anoxia, etc. The warm surface waters of the Mediterranean are just as hospitable to marine life as are the cold surface waters of the Nordic seas even though they contain less dissolved O2. The critical value is pO2, and a careful account is given in:
Hofmann, A.F., E.T. Peltzer, P.M. Walz, and P.G. Brewer. 2011. Hypoxia by degrees: Establishing definitions for a changing ocean. Deep-Sea Research I, 58: 1212–1226. http://dx.doi.org/10.1016/j.dsr.2011.09.004
It is true that absolute values are important too at very low oxygen concentrations where competition with NO3 ions comes into play:
Brewer, P.G., A.F. Hofmann, E.T. Peltzer, and W. Ussler III. 2014. Evaluating microbial chemical choices: The ocean chemistry basis for the competition between use of O2 or NO3− as an electron acceptor. Deep Sea Research I, 87: 35–42. http://dx.doi.org/10.1016/j.dsr.2014.02.002
The original purpose of the ARGO program was strongly focused on observing the uptake of heat by the ocean and the penetration to depth of the present day warming signal. It would be of great value if the authors were to calculate the changes in pO2 – not just total oxygen – by combining the changing temperature and dissolved oxygen fields in a professional manner.
You may also wish to look at:
Brewer, P.G., and E.T. Peltzer. 2017. Depth perception: the need to report ocean biogeochemical rates as functions of temperature, not depth. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2102): 1–18. https://doi.org/10.1098/rsta.2016.0319
for understanding the critical role that temperature plays in controlling ocean oxygen consumption rates.
Based on the well-developed skills I see displayed here this should be well within the capabilities of the authors.
Citation: https://doi.org/10.5194/essd-2024-106-RC2
Data sets
ISAS temperature, salinity, dissolved oxygen gridded fields Nicolas Kolodziejczyk, Annaig Prigent-Mazella, and Fabienne Gaillard https://doi.org/10.17882/52367
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