the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Aug 2024
Dual-tracer constraints on the Inverse-Gaussian Transit-time distribution improve the estimation of watermass ages and their temporal trends in the tropical thermocline
Abstract. Quantifying the mean state and temporal change of seawater age is crucial for understanding the role of ocean circulation and its change in the climate system. One commonly used technique to estimate the water age is the Inverse Gaussian Transit Time Distribution method (IG-TTD), which applies measurements of transient abiotic tracers like chlorofluorocarbon 12 (CFC-12). Here we use an Earth system model to evaluate how accurately the IG-TTD method infers the mean state and temporal change of true water age from 1981 to 2015 in the tropical thermocline (on isopycnal layer σ0=25.5 kg ⋅ m-3). To this end, we compared the mean age of IG-TTD (Γ) derived from simulated CFC-12 with the model "truth", the simulated ideal age. Results show that Γ underestimates the ideal age of 46.0 years by up to 50 %. We suggest that this discrepancy can be attributed to imperfect assumptions about the shapes of transit-time distribution of water parcels in the tropics and the short atmospheric history of CFC-12. As for the temporal change of seawater age, when only one transient tracer (CFC-12) is available, Γ might be an unreliable indicator and may even be of opposite sign to trends of it due to uncertainties of mixing ratio. The disparity between Γ and ideal age temporal trends can be significantly reduced by incorporating an additional abiotic tracer with a different temporal evolution, which we show by constraining Γ with sulfur hexafluoride (SF6) in addition to CFC-12.
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RC1: 'Comment on egusphere-2024-2552', Rolf Sonnerup, 07 Oct 2024
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This is a fine effort that provides guidance on how realistic changes in ventilation inferred from repeated CFC (and SF6) sections can be, and thus this can be an important contribution. It also focusses on the tropics, rather than the usual subtropical gyres. The work is careful and well focussed. I applaud the authors for their focus on saturation level uncertainties and trends. That said, I was disappointed that there was not much effort devoted to real-world scenarios. Researchers don't go out and measure a mean CFC-12 and a mean SF6 on a global isopycnal, they go out and measure discrete water samples. From those discrete samples, they attempt, perhaps optimistically, to infer basin-wide patterns of changes in ventilation. I suspect an 'upgrade' is easily within reach for these authors - choosing a location or two (or a section or two) and explicitly calculating the change in ventilation inferred from tracers as compared with the actual change. I would not insist on this happening - the work is fine as it is - but it may improve the impact of their paper.
Comments that the authors should consider and which (in my opinion) would improve the ms. markedly:
1). Line 153. Consider rewording this. The lessons from Waugh et al 2003 and from Stoeven et al 2015 are that CFCs and SF6 only provide meaningful constraints on Delta/Gamma when <= 1.8 or so. Those papers do not indicate that the real Delta/Gamma in the ocean is <= 1.8, merely that the tracer pair doesn’t provide useful information >= 1.8. Delta/Gamma could be 3, it could be 6, but this tracer combination wouldn’t tell us that, and wouldn't be able to distinguish 3 from 6.
2) Figs 2, 4 &5. It is very hard to see the patters described in the text. This paper focusses on the tropics. One way to improve visibility would be to remove latitudes > 40 (maybe even 35?)
Line 222-223: I cannot see this pattern in Figure 5. Perhaps re-scaling the figures will help?
3) Relatedly. Fig. 3b. Do these results vary by basin? The processes and pathways bringing tracers to the eastern tropical Pacific are quite different from, say, the Arabian Sea. This figure shows the mean.
I have the same comment about Figure 6b and 7b. Okay maybe the twin-tracer TTDs can do a good job representing changes in the global mean ventilation of this isopycnal. But people don’t go out and measure a global mean pSF6 and pCFC-12, they have section measurements from which they hope to derive larger-scale conclusions. How would these three figures look at some individual, spot locations or sections? Do these conclusions, which hold for the isopycnal mean, apply universally in individual regions?
4). Figure 4. It would be very useful and interesting to see a figure of the percent mismatch of at least one of these (your choice) from the reference, figure 2a.
5). Line 233 – This is very similar to the findings of Mecking et al 2004 based on simple pCFC ages in the early 1990s– and to the findings of Thiele and Sarmiento (1990) based on pCFC ratio ages in the 1980s. Some attribution to these pioneers, somewhere in this paper, would seem appropriate.
And some
Suggested, minor modifications, by line number
9. uncertainties in the mixing ratio
26-28. One citation will suffice – I’d recommend Jenkins 1980 and Weiss et al 1985
30. Gruber 1998
34. Doney and Bullister 1992 rather than Fine 2011
36 … individual transit times is that which results from one-dimensional transport and mixing, the Inverse Gaussian…
51 most of the studies focus on the subtropical gyres, and very few focus on the tropics.
60 … represent ideal age and temporal changes in ideal age…. (cumbersome but clearer)
147 …from measurement of a single transient tracer
159 - Tricky – the saturation level mis-assumption causes the derived age to be older than it should, however it is still younger than the ideal age (or, water age)
Maybe try wording like
…causes the tracer-derived age to be older than with a more realistic, undersaturated, boundary condition.
160 Shao et al 2013 should be cited here as well.
162 …rise in saturation levels, TTD-derived water ages range from being…
192 …calculation, so fixed spatially homogeneous…
205 More precisely, the spatial variance of Gamma increases with higher Delta/Gamma, with 1.2 being most representative of the spatial variation in ideal age.
(If I’m interpreting the Fig. 3a correctly, ignore this remark if I’m not)
251-252. …along outcrops and in the Atlantic, Pacific and Indian Ocean ranges from 80% to 100% (Fig. S3).
262 This is an important result. I would start the sentence ‘Notably, the assumption…’
278 An even more important result. Possibly clarify a bit …our results suggest that Gamma constrained by CFC-12 alone is not able…
325 Does this also mean that ‘ages’ from those concentrations would be about half the ideal age? That result is very similar to yours, and similar to those of Mecking et al 2004.
333. Rather than Shao et al I’d choose Waugh et al 2003 as the citation
333. ‘only began to be measurable in the atmosphere after 1936 and 1953’
These weren’t really measured back then, they were invented and presumably released into the atmosphere around those dates. Bullister’s reconstruction is a model using CFC and SF6 production and assuming some release function that is tuned to CFC SF6 measurements when those started to be readily available – 1970s?
Rather than muddy the waters, you could just write ‘These gases were released into the atmosphere after 1936 and 1953, respectively…’
337 – A set of 39-Ar simulations would be nice, but what would be nicer still is a set of 39-Ar measurements. For that study, when you do it, you may need to focus your efforts on waters that are a bit older. Good luck!
Fig. 8 – I can’t help but notice that all of these ‘data’ treatments capture the brief slowdown events of 1990 and 2006. Whether real-world sampling would capture this is another question, but you might note in the text that these events are reflected in the tracer concentrations (and in their derived ages).
Citations provided by reviewer
Doney, S. C., and J. L. Bullister (1992) A chlorofluorocarbon section in the eastern North Atlantic, Deep Sea Research 39, 11-12, 1857-1883.
Gruber, N., 1998 Anthropogenic CO2 in the Atlantic Ocean, Glob. Biogeochem. Cycles 12, 1, doi.org/10.1029/97GB03658
Jenkins, William J.. 1980. "Tritium and 3He in the Sargasso Sea." Journal of Marine Research 38, (3). https://elischolar.library.yale.edu/journal_of_marine_research/1518
Mecking, S., M. J. Warner, C. E. Greene, S. L. Hautala, and R. E. Sonnerup 2004 Influence of mixing on CFC uptake and CFC ages in the North Pacific thermocline, J. Geophys. Res. Oceans 109, C7, https://doi.org/10.1029/2003JC001988
Weiss, R., Bullister, J., Gammon, R. et al. Atmospheric chlorofluoromethanes in the deep equatorial Atlantic. Nature 314, 608–610 (1985). https://doi.org/10.1038/314608a0
Citation: https://doi.org/10.5194/egusphere-2024-2552-RC1
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