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A Model‐Based Evaluation of the Inverse Gaussian Transit‐Time Distribution Method for Inferring Anthropogenic Carbon Storage in the Ocean
Author(s) -
He YanChun,
Tjiputra Jerry,
Langehaug Helene R.,
Jeansson Emil,
Gao Yongqi,
Schwinger Jörg,
Olsen Are
Publication year - 2018
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2017jc013504
Subject(s) - tracer , alkalinity , saturation (graph theory) , environmental science , salinity , disequilibrium , inverse gaussian distribution , atmospheric sciences , climatology , oceanography , distribution (mathematics) , chemistry , geology , mathematics , physics , medicine , mathematical analysis , organic chemistry , combinatorics , nuclear physics , ophthalmology
The Inverse Gaussian approximation of transit time distribution method (IG‐TTD) is widely used to infer the anthropogenic carbon ( C ant ) concentration in the ocean from measurements of transient tracers such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF 6 ). Its accuracy relies on the validity of several assumptions, notably (i) a steady state ocean circulation, (ii) a prescribed age tracer saturation history, e.g., a constant 100% saturation, (iii) a prescribed constant degree of mixing in the ocean, (iv) a constant surface ocean air‐sea CO 2 disequilibrium with time, and (v) that preformed alkalinity can be sufficiently estimated by salinity or salinity and temperature. Here, these assumptions are evaluated using simulated “model‐truth” of Cant. The results give the IG‐TTD method a range of uncertainty from 7.8% to 13.6% (11.4 Pg C to 19.8 Pg C) due to above assumptions, which is about half of the uncertainty derived in previous model studies. Assumptions (ii), (iv) and (iii) are the three largest sources of uncertainties, accounting for 5.5%, 3.8% and 3.0%, respectively, while assumptions (i) and (v) only contribute about 0.6% and 0.7%. Regionally, the Southern Ocean contributes the largest uncertainty, of 7.8%, while the North Atlantic contributes about 1.3%. Our findings demonstrate that spatial‐dependency of Δ / Γ , and temporal changes in tracer saturation and air‐sea CO 2 disequilibrium have strong compensating effect on the estimated C ant . The values of these parameters should be quantified to reduce the uncertainty of IG‐TTD; this is increasingly important under a changing ocean climate.