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Full text: Anthropogenic carbon storage and its decadal changes in the Atlantic between 1990-2020

3862 R. Steinfeldt et al.: Anthropogenic carbon in the Atlantic Appendix D: Variation in TTD parameters The effect of the choice of the TTD parameters on the shape of the TTD and the inferred Cant concentration is illustrated in Fig. D1 and Table C1. We assume a CFC-12 concentra- tion of 0.4 pmol kg?1 observed in 2010, a CFC saturation of 0.85, a potential temperature of ? = 3 °C and a salinity of S = 34.9. Cant is calculated for the reference year 2010 and an alkalinity of 2308 µmol kg?1 (the value derived from Lee et al., 2006, for the North Atlantic). The maximum of the TTD occurs at younger ages for 1/0 = 2 compared to the case with 1/0 = 1. This is a fairly common behavior; i.e., increasing the 1/0 ratio leads to a younger age of the mode of the TTD. Reducing the fraction of young water leads to an even younger mode, although in this case the 1/0 ratio is always chosen as 1. The younger the mode of the possi- ble TTDs derived from a given CFC-12 concentration, the smaller the inferred Cant concentration (see Table D1). The TTD Gyoung for the young water in Fig. D1 only rep- resents half of the water, as the fraction f in this example equals 0.5. To illustrate what the complete TTD might look like, we assume an old TTD Gold with 0 = 500 years and 1 = 250 years. The sum of Gyoung and Gold is shown as the dashed line in Fig. D1. This choice of the parameters for Gold is arbitrary, but it fulfills the condition that the inferred Cant concentration is small (< 0.5 µmol kg?1 in this case) and thus might be neglected. The sum of Gyoung and Gold only has one clear maximum. The mode of Gold is much less pronounced than that of Gyoung, so the complete TTD has a kind of saddle point around the mode of Gold (250 years in the example in Fig. D1). Table D1. TTD parameters derived for a CFC-12 concentration of 0.4 pmol kg?1 in 2010, assuming a CFC saturation of 0.85, a potential temperature of ? = 3 °C and a salinity of S = 34.9. Cant is calculated for the reference year 2010 and an alkalinity of 2308 µmol kg?1 (the value derived from Lee et al., 2006, for the North Atlantic). Also given is the fraction of water older than 200 years. 0 1 Cant f > 200 years [year] [year] [pmol kg?1] 1/0 = 1 f = 1 178.5 178.5 14.0 0.26 1/0 = 2 f = 1 564.5 1129 12.5 0.38 1/0 = 1 f = 0.5 104.5? 104.5? 10.4 0.13? ? Values are for the young TTD component only. Figure D1. TTDs derived from an observed CFC-12 concentration of 0.4 pmol kg?1 in 2010, a CFC saturation of 0.85, a potential temperature of ? = 3 °C and a salinity of S = 34.9. Shown are TTDs for f = 1, 1/0 = 1 and 1/0 = 2, as well as TTDs for f = 0.5 and 1/0 = 1. For the latter case, an assumed old TTD with 0 = 500 years and 1 = 250 years is also added (dashed cyan line). Appendix E: Relative Cant change (1tCant) between 1990 and 2020 Figure E1 shows the relative increase in the Cant column in- ventory for the periods of 1990–2000, 2000–2010 and 2010– 2020. In contrast, in Fig. 12 from the main text the abso- lute values of the Cant increase are shown. From the rising atmospheric CO2 concentration and a steady-state ocean, a decadal increase in the Cant inventory of about 19 % would be expected. Biogeosciences, 21, 3839–3867, 2024 https://doi.org/10.5194/bg-21-3839-2024
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