| 23 May 2024 | Subsequently updated
Water masses in the Atlantic Ocean: water mass ages and ventilation
Abstract. The distribution of oceanic water masses and their properties, such as ventilation constitute fundamental parameters, for instance, the thermohaline circulation patterns and biogeochemical processes in the marine systems. The distributions of main water masses in the Atlantic Ocean have been comprehensively documented in a companion study (Liu and Tanhua, 2021), this study presents quantitative assessments of water mass age characteristics and ventilation time scales through multi-tracer analysis incorporating chlorofluorocarbon-12 (CFC-12), sulfur hexafluoride (SF6), and argon-39 (39Ar). Here we use two distinct age concepts: mean-age as an integrative metric of water mass chronology, and mode-age as a proxy for advective time scales. Empirical results demonstrate systematic age progression with increasing pressure and along water mass trajectories. Surface layer central waters exhibit mean ages up to ~100 years and mode ages reaching ~30 years. In the intermediate layer, meridional age gradients characterize the Antarctic Intermediate Water (AAIW) reaching maximum mean-age (~300 years) and mode-age (~80 years) at 30 °N, whereas zonal variations manifest in Mediterranean Water (MW) with peak values (~400 years in mean-age, ~100 years in mode-age) observed in equatorial regions. As the dominant deep water component, North Atlantic Deep Water (NADW) exhibits extreme ages in the Antarctic Circumpolar Current (ACC) region at 50°S, achieving mean age ~600 years and mode age ~100 years. Bottom layer water masses display their oldest signatures: Antarctic Bottom Water (AABW) from the Weddell Sea reaches ~600 years (mean) and ~100 years (mode) at equatorial latitudes, while its extension, Northeast Atlantic Bottom Water (NEABW), attains exceptional values of ~800 years (mean) and ~120 years (mode) at 50°N. The age analysis reveals significant basin-scale asymmetries, with eastern basins exhibiting younger ages compared to western counterparts. Ventilation efficiency modulates these age distributions, as evidenced by lower mode-ages and reduced apparent oxygen utilization (AOU) in better-ventilated western basins. The calculated oxygen utilization rate (OUR) demonstrates spatial concordance with dissolved oxygen (DO) concentrations, corroborating enhanced oxidative processes in high-oxygen regimes. This integrated age framework provides novel insights into water mass ventilation dynamics and their biogeochemical implications through quantitative characterization of temporal-spatial age distributions across multiple oceanographic provinces.
Key words: Water Mass, Atlantic Ocean, Transient Tracer, Mean- and Mode-age, Ventilation, GLODAPv2 data product
Comment on
“Water masses in the Atlantic Ocean: water mass ages and ventilation”
submitted by Mian Liu and Toste Tanhua to EGUsphere
Overview:
Mian Liu and Toste Tanhua submitted a manuscript about Atlantic water mass distribution and their (different types of) water mass ages, using transient tracers (the older anthropogenic CFC-12, the younger anthropogenic SF6, and the radioactive 39Ar) and the well-established TTD-method.
They introduce the paper with a short review of Atlantic water masses and their relevance in ocean circulation and for biogeochemical processes, followed by introducing the transient tracers and relevant methods. Moreover, they introduce the oxygen utilization and oxygen utilization rate and their importance in biogeochemical cycles.
Next, they introduce in more detail the application of the TTD method to derive water mass ages, they differentiate different types or definitions of ages and other technical aspects to compute them from the transient tracer data, which they obtained from the GLODAP data repository.
In the large results-section they show and discuss their computed water mass ages along two prominent WOCE/GO-SHIP sections and on maps in various density layers incorporating a large amount of GLODAP tracer data. They do that for several relevant Atlantic water masses, starting from the near surface layer to the deepest Atlantic water layers.
Additionally, they use their previously discussed water mass ages to derive the oxygen utilization rates for the most prominent Atlantic water masses. Finally, they compare the CFC and SF6 based water mass ages with ages derived from the radio nuclide 39Ar and discuss the deviations between the two.
Lastly, they summarize and discuss their findings and interpretation in a conclusion-section.
General comment:
The manuscript is very well written, easy to read and good to understand. The strength of the manuscript is the systematic application of the transient tracer-based TTD method on a Atlantic-wide large data set and the comprehensive discussion of the most relevant Atlantic water masses from surface to bottom. Moreover, the authors apply their findings by calculating and discussing oxygen utilization rates Atlantic-wide. Both might be a good data base for further investigations of biogeochemical cycles in the Atlantic Ocean.
I’d like to recommend publication after minor revision.
For minor revision I have listed a few specific comments (or better suggestions):
General comments:
Since I am a non-native speaker; I do not comment on wording, grammar, spelling, etc. I’d like to leave these issues to the editorial board.
In my pdf all figures are of minor, very blurred quality. I guess, that happened during crating the pdf for the reviewers from the submitted manuscript and submitted figures. So, this comment goes more to the editorial board than to the authors.
Specific comments:
Line 22-23: “eastern basin exhibiting younger ages compared to…” If this is meant in general, it should be rather opposite. Or is this specific for NEABW? Please, clarify.
Line 110-112: “The solubility (F)…” Find a better place in the methods’ section.
Line 113ff: You use the term “diffusive”, as widely common in this context. But wouldn’t it be more logic to use “disperse”. Many people automatically think on “molecular diffusion”, but here something different is meant, which might be better named “dispersion”.
Line 129-130: “Based on the TTD determination…” This sentence stays a little bit alone at this place. Skip or find better location.
Line 140: “the TTD is only a spike.” Moreover, in such a case the Δ becomes infinite small and the IG (Eq. in line 122) formally becomes a δ-function and can be computed the same way, so it is just a special case of the same formalism and not something completely different. Eventually worth to note here.
Line 141: “…underestimated by the tracer-age.” Eventually worth to refer to Sonnerup, 2001, who diels with different “tracer-ages” and the systematic deviation between tracer-ages from old and young or low and high concentrations.
Line 151-152: “The atmospheric concentration…” The sentence is important, bit at this place it stands a bit out of context. Please, find better location in section 3.
Line 152-175: The entire Δ/Γ-section could be shortened or focused. 1. The trouble is not enough tracers to compute Δ and Γ and there is almost no way out. Many authors previously discussed this issue and using Δ/Γ=1 is widely common and accepted. 2. The entire discussion here, if Δ/Γ= 0.6, 0.8, 1.0, 1.2 etc. does not lead to a final conclusion. Moreover, why not 0.1, 0.5, 1, 2, 5? 3. In principle and in theory the authors HAVE TWO tracers (or even three with the 39Ar) and COULD compute both Δ and Γ. But I guess, that might not help, since at least CFC-12 and SF6 do not provide enough independent information. So, keep it short. All tracer people know, why, and all others don’t care.
After line 175:
One would like to know, HOW the Γ is computed from the tracers. I guess a least square fit or something similar. But here is my most urgent question, if the authors do that by using CFC-12 and SF6 individually or at once. It is stated somewhere further below in the manuscript, but it should already be mentioned in the methods.
Which atmospheric data do the authors use? Pleas add a reference.
Line 177-178: In this study … vertical layers…” Could find a better place or, since independent from the following, mage an own paragraph of this sentence.
Line 188ff: As mentioned above, move this to methods-section?
Line 197-200: Move this paragraph to a better location, e.g. together with the sentence in lines 177-178.
Figure 6: Could you add “mean age” and “mode age” into the figure? (Holds for all such figures.)
Figure 7: Limit from 0 to 1000 dbar?
Figure 9: Limits from 0 to 3000 dbar?
Line 290: “over THE ridge”?
Line 359-366: The discussion why to favor the mode age instead the mean age does not convince me. In my understanding, the mode age and the mean age are almost one order of magnitude apart, hence, the OUR would also be almost 1 order of magnitude apart. Thus, an argument is needed, why the one and not the other. As I understand the authors, the mean age is subject to mixing with adjacent water masses. True. But the mode-age is, too. So, this argument does not count. I have also no clue, what is the right argument for favoring the mode-age. I guess it is related to either the “right” order of magnitude OR it has something to do with the integrative effect of the mean-age (the mean age sums all the damn history) and the more advective character of the mode-age.
Line 428-429: If the authors decide to put the technical aspect of how to compute Γ into the method-section, this could also go there. As I understand right, it is just the equation from line 122 with an additional decay factor e-λ/t ? Might be worth to note clearly.
Line 442: “(Fig. 18)” does not exist.
Line 455-465: What I would really enjoy would be a Figure 18 as a property-property plot as CFC/SF6 mean-age versus 39Ar mean-age and CFC/SF6 mode-age versus 39Ar mode-age and a short related discussion of such a figure. I’m sure one could learn something from it.
Line 467: I would put this sentence to the end of this paragraph.
Line 468: Add 39Ar to the bracket.
Line 470: Replace “investigated” by “discussed”?
Line 472: Replace “different parts” by “different fractions” or “different pathways”?
Line 498-505: Here I was puzzled. I can’t put this into the context of the manuscript. I can’t see climate change in the manuscript, since no repeat sections or stations are compared. Numerical simulation methods were not applied here. Please, either skip or clarify in the manuscript’s context.