For global analysis and comparison with the study of Mikaloff Fletcher et al. , section data from the GLODAP database [Key et al., 2004] are used. based on the ΔC* [Gruber et al., 1996], the CFC [Thomas and Ittekkot, 2001] and the TTD [Waugh et al., 2004] methods are provided on the GLODAP site. In addition, we estimated with the TrOCA method [Touratier et al., 2007] from the GLODAP section data using carbon, oxygen, alkalinity (Alk) and temperature. Additionally, data from four different sections and from six different reconstruction methods [Vázquez-Rodríguez et al., 2008] are applied to infer fluxes in the Atlantic and to further explore and quantify the impact of systematic uncertainties in input data. Estimates from the ϕ − (M. Vázquez-Rodríguez et al., manuscript under review, submitted 2008) and the IPSL [Lo Monaco et al., 2005] method are available for these sections in addition to the four methods used in the global analysis. The sections are WOCE A14 (1995), WOCE I06-Sb (1996), CLIVAR Repeat Section A16N (Legs 1, 2) (2003) and WOCE AR01 (1998) (Figure 1). These four sections include the Atlantic water masses from 60°S to 60°N and are located in or close to (WOCE I06-Sb) the Atlantic basin.
Figure 1. (a) Map of the 22 regions used in the EnKF inversion and the cruises with estimates from the six different methods which were used to constrain fluxes in the Atlantic. The column inventories shown in Figures 1b, 1c, and 1d are taken from three cruises (I06-Sb, A14, A16N). There are no available values for the ΔC* method used in the GLODAP database for the A16 cruise in 2003. In the inversion, we have used ΔC* values from the 1993 cruise instead.
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 The ΔC* method [Gruber et al., 1996] is based on the quasi conservative tracer C* and the assumption that water masses exchange along isopycnal surfaces. C* accounts for the contributions of calcium carbonate dissolution and remineralization of organic matter to the measured dissolved inorganic carbon concentration (C) by assuming that these fluxes are linked to oxygen (O2) and Alk through constant and uniform Redfield ratios (r) (C* = C − rC:O2O2 − (Alk + rN:O2O2)). The ΔC* term is defined as the difference between present C* and preformed C*,0 and represents the sum of and a contribution from pCO2 disequilibrium between air and sea (ΔC*(t) = C*(t) − C*0 = + ΔCdis). Preformed C*,0 is assumed to be time-invariant. Preformed oxygen, O20, required to compute preformed C*, is estimated by assuming that O20 equals the concentration in equilibrium with the atmosphere at in situ temperature and salinity. Preformed alkalinity, Alk0, is computed from in situ tracer data by applying a multiple linear regression parameterization. The parameterization has been derived from surface water data. Finally, the ΔC* method considers a constant CO2 air-sea disequilibrium, ΔCdis that is subtracted from the ΔC* tracer to get an estimate of . ΔCdis is estimated for each isopycnal layer either from old free waters for deep-reaching isopycnals or alternatively by assigning a single water mass age to each water parcel using CFC or other tracers with a time information.
 The CFC-shortcut method [Thomas and Ittekkot, 2001], only uses CFC concentration measurements. A single time, t, denoting when a water parcel (sample) has lost contact with the atmosphere, is calculated from the measured CFC concentration. Carbonate chemistry equations are then used to compute the concentration in equilibrium with the known atmospheric CO2 partial pressure at time t and for the preindustrial time period. is then the difference between these two equilibrium concentrations. In contrast to the approaches above, no assumptions about the Redfield ratios between the different biogeochemical elements are required. It is implicitly assumed that a water parcel is characterized by a single discrete age, that the air-sea disequilibrium has not changed since preindustrial time, and that ocean circulation has remained in steady state. The Transit Time Distribution (TTD) method [Waugh et al., 2006, 2004] can be viewed as a sophistication of the CFC-shortcut method. A distribution for the transit time is assigned to a sample rather than a discrete age as in the CFC-shortcut method, thereby accounting for the mixing of water masses with different histories. The time distribution is based on the measurements of different transient tracers such as CFCs or tritium and helium.
 The TrOCA method [Touratier and Goyet, 2004; Touratier et al., 2007], uses a quasi-conservative tracer, similar to those of NO and PO [Broecker, 1974], called TrOCA which is based on the Redfield equation. In order to determine the anthropogenic carbon concentration in seawater, the conservative tracer TrOCA0 (TrOCA prior to anthropogenic contamination), is subtracted from the quasi-conservative tracer TrOCA. Since TrOCA0 can simply be calculated using the measured temperature and total alkalinity, anthropogenic carbon concentrations can be directly determined with measurements of oxygen, total alkalinity, total inorganic carbon and temperature. The ϕ − method (M. Vázquez-Rodríguez et al., manuscript under review, submitted 2008) constitutes a revised version of the C* method. The main modifications are (1) ΔCdis and preformed total alkalinity AT0 are parameterized taking subsurface layer (100–200 meter depth) data as the only reference and assuming a multi-end-member mixing model instead of an isopycnal mixing model; (2) the air-sea CO2 disequilibrium ΔCdis is not taken as constant over time, but its spatial variability and temporal evolution since preindustrial time is modeled; (3) no age estimates from CFCs or other age tracers are necessary for calculating anthropogenic carbon, except for the case of subsurface reference samples.
 In the IPSL method [Lo Monaco et al., 2005], anthropogenic carbon is estimated using the preformed method, a back-calculation technique developed by Brewer  and Chen and Millero . The method has been tested many times, e.g., in the North Atlantic [Körtzinger et al., 1998] and in the Southern Ocean [Lo Monaco et al., 2005]. In the latter study, the method was modified to allow oxygen disequilibria with the atmosphere on the basis of the ΔC* method by Gruber et al. , and an optimum multiparameter analysis was used to determine the relative contributions of northern and southern waters to the observed properties. Substantial differences in the reconstructed column inventories are found among methods [Vázquez-Rodríguez et al., 2008] (Figure 1). For example, the reconstructed is only 14 mol m−2 at 60°S for the ΔC* method, but around 50 mol m−2 for the ϕ − the TrOCA and TTD method, and around 75 mol m−2 for the IPSL method, and even higher for the CFC-shortcut method. Such large differences of a factor of three are quite astonishing, as computational differences between the reconstruction methods are often quite subtle.
 In general, the CFC-shortcut method yields much higher column inventories than all of the other methods (Figure 1). We attribute this to the application of a single time to characterize the age of a water parcel. This assumption does not seem very realistic and results from this method should be viewed with some caution [Matear et al., 2003; Waugh et al., 2006]. The IPSL approach yields somewhat higher column inventories along the four sections than the remaining four methods, except in high northern latitudes of section A16N. Consequently, we expect that the assimilation of the IPSL data will yield a high estimate for the air-sea flux of anthropogenic carbon in the Atlantic. The TrOCA and ϕ − methods yield very similar column inventories for all four sections, while inferred inventories from the TTD method are smallest along A16N (11°S to 66°N), but usually larger than the TrOCA and ϕ − values for I06-Sb (66°S to 28°S).