4.1. Large-Scale Consequences
 We showed that about a third of the MOC water upwells in the tropics. A substantial amount of that water recirculates in the STCs. Upwelling of MOC water masses in the tropical Atlantic implies that its properties can change which may affect the stability of the MOC. The transformation of MOC water masses is clear from the temperature and salinity of MOC parcels when they enter the tropical Atlantic at 10°S compared to when they leave the region at 10°N (Figure 13). At 10°S, South Atlantic Central Water is found that is characterized by a linear temperature-salinity relationship [e.g., Poole and Tomczak, 1999]. The freshening of surface waters induced by the runoff and the Intertropical Convergence Zone is eminent. Also, the MOC water masses warm substantially. These results are consistent with the progressive warming and freshening toward the north found by Stramma and England  in observations. The warming stands out when the volume transport of the MOC is binned in temperature and salinity classes (Figure 14). The redistribution of MOC waters is very clear as indicated by the large peak of volume transport at a temperature of about 26 degrees C at 10°N at the expense of volume transport in a range between 6 and 22 degrees C (Figure 14a). The slight cooling at low temperatures is also visible in the temperature salinity diagram and reflects the drift of intermediate water masses toward North Atlantic Deep Water that should be considered as unrealistic. Both freshening and salinifying of MOC water is found (Figure 14b). As can be seen from the Figure 13, the water masses at higher densities become more salty while the warmer surface waters freshen due to runoff and precipitation.
Figure 13. Temperature-salinity diagrams of MOC water that passes 10°S for the first time (black, shown by crosses) and that passes 10°N for the last time (grey, shown by pluses).
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Figure 14. MOC volume transport binned in (a) temperature classes (bins of 0.1 C, smoothed with a 10-point box filter) and (b) salinity classes (bins of 0.1 psu, smoothed with a 10-point box filter) at sections at 10°S and 10°N.
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 To put these changes into perspective of the global ocean circulation, the heat uptake of MOC water masses in the tropical Atlantic is compared to estimates of divergences of meridional heat transport from observations in the Atlantic basin. We determine the amount of heat carried by the upper branch of the MOC from the thermodynamic properties of the trajectories at the sections at 10°N and 10°S according to
Here Hmoc is the heat transport by the upper branch of the MOC, Vi is the volume transport carried by each particle in the upper branch of the MOC, Ti is the temperature of each particle, ρ is density of sea water, cp is the heat capacity, and N is the number of particles that crosses a section. The heat transport by particles that make up the upper branch of the MOC is 0.86 PW (= 1015W) at 10°N. The change of heat that is carried northward by these particles between 10°S and 10°N, that is, the heat transport divergence is 0.22 PW. The observed total heat transport divergence in the tropical Atlantic Ocean between 10°N and 10°S is 0.43 PW based on Da Silva et al. . Using estimates from NCEP reanalysis data [Trenberth and Caron, 2001], we obtain a heat transport divergence of 0.49 PW. From ECMWF data we obtain a divergence of 0.42 PW.
 Similar to the heat transport through a section, the fresh water transport can be estimated from
Here, Fmoc is the fresh water transport by the upper branch of the MOC, S0 is a reference salinity that has been set to 35 psu, and Si is the salinity of each particle that crosses a section. The fresh water transport divergence of MOC water masses between 10°N and 10°S is 0.16 Sv. The fresh water transports obtained from indirect and direct measurements vary strongly. Especially over regions of large precipitation, large variations are found. Wijffels  summarizes different estimates and the total fresh water divergence is found between 0.2 and 0.4 Sv.
 The results from the trajectory analysis indicate that the tropics is indeed active in modifying MOC properties substantially. We conclude that in the tropical Atlantic at least half of the heat received from the atmosphere is used to heat up MOC water masses which transfer the heat northward in the ocean. The freshening of MOC water masses is also about half of the total fresh water transport divergence in the tropical Atlantic region.
4.2. Comparison to Observations
 The results shown in this paper are obtained with a numerical ocean model. Most ocean currents that are important for the MOC and STC circulations in the tropical Atlantic are well represented in this model. From the south the tropical Atlantic is fed by the South Equatorial Current and the North Brazil Undercurrent in accordance with, for instance, Schott et al.  and Stramma and England  (see Figures 1, 2, 3, and 11). In the equatorial region, the Equatorial Undercurrent is well represented [see also Hazeleger et al., 2003] as well as the equatorial branches of the South Equatorial Current and the North Equatorial Counter Current. However, compared to observed transports presented by Schott et al.  and Stramma et al. , the model has too weak off-equatorial undercurrents. Also, Schott et al.  report a permanently present Equatorial Intermediate Current beneath the Equatorial Undercurrent which we only find in autumn (Figure 11).
 The water mass analysis shown in Figures 13 and 14 shows that the main water masses are well represented by the model. In particular, South Atlantic Central Water is well represented as indicated by the linear temperature-salinity relationship [Poole and Tomczak, 1999]. These water mass characteristics compare well to the origin of Equatorial Undercurrent water presented by Snowden and Molinari  and the observations presented by Stramma and England . In the following we will discuss in more detail comparisons between observed and modeled transports that are relevant to the MOC and STC circulations.
 The observed cross-equatorial exchange associated with the basin-wide meridional overturning circulation is on the order of 13 to 17 Sv [Schmitz and Richardson, 1991; Schmitz 1995; Smethie and Fine, 2001]. In the upper tropical Atlantic this consists of northward transport of warm surface water in primarily the North Brazil Current as part of the thermohaline overturning cell [Gordon, 1986]. The model analyzed here shows a realistic amount of 16 Sv of warm water inflow into the Atlantic originating from the Indian Ocean (Figure 2). This warm water is brought northward by the southern branch of the South Equatorial current and from 15°S on along the western boundary in the North Brazil Undercurrent. In addition, the tropical Atlantic is fed with 8 Sv of light surface water (σθ < 24.5) from the central South Equatorial Current that entrains into the western boundary current at about 4°S as shown by Schott et al. . We see consistent amounts of transport when we add the STC and gyre flows at 4°S in the model (Figures 1 and 3).
 It is clear from our cross sections that the MOC waters stay along the western boundary on the Southern Hemisphere in the North Brazil Undercurrent. Although Figure 10 and 12 show that the MOC recirculates into the Equatorial Undercurrent, the Southern Equatorial Undercurrent, and Northern Equatorial Undercurrent, most of it remains at the western boundary while crossing the equator. Compared to an earlier model study by Blanke et al. , we find more recirculation, with separate recirculation cells north and south of the equator.
 On the basis of water mass analysis and the number of North Brazil current rings shed per year, Johns et al.  estimated that 9 Sv of South Atlantic water is transferred northward in rings along the western boundary. We find lower values (about 5 Sv, see Figure 2 and 10) and get a more dominant pathway in the interior. This also implies a more prominent role of the cyclonic circulation centered near 12°N and 25°W (the Guinea Dome) in which MOC water can upwell and then expel northward in the mixed layer and transfer westward in the North Equatorial Current. However, it is possible that the model underestimates the ring transport because the horizontal resolution is eddy-permitting. Alternatively, the pathways can be sensitive to the atmospheric forcing that is used to force the ocean [Inui et al., 2002]
 One of the purposes of this study is to disentangle the MOC transports from the STC transports and to study how much the STC and MOC interact. Zhang et al.  use western boundary current transport, interior transport, and upwelling transport to infer STC and MOC transports from observations. The values they find are upper limits of STC-MOC interaction.
 Zhang et al.  find equatorward flow of 2 Sv in the Northern Hemisphere in the interior, just as we do. At 6°S they find 4 Sv of equatorward transport in the interior which they interpret as STC transport. We find only 2 Sv equatorward transport in the STC at this latitude. However, Zhang et al. determine this equatorward transport using hydrographic data and this does not imply that the equatorward flow actually upwells. We find a similar number if we add the STC transport and the interior gyre flow.
 At the western boundary a 3 Sv equatorward flow on the Northern Hemisphere is assumed based on a sparse data set by Bourles et al. . We do not find equatorward STC transport along the western boundary, but the return flow between 65°W and 25°W seen at 6°N (Figure 7) may be interpreted as the southeastward Guiana Undercurrent and a southward branch of the North Equatorial Current of the model. In the Southern Hemisphere the western boundary current transports used by Zhang et al.  and those that are found here are similar: 12 Sv.
 The total upwelling found by Zhang et al.  is 21 Sv based on divergences from drifter data. We find a similar upwelling from Eulerian mean transports, but the Lagrangian mean upwelling is less, as some upwelling occurs within the mixed layer and is compensated by eddies [see Hazeleger et al., 2003; Hazeleger and de Vries, 2003]. The net obduction is only 7.5 Sv (5.5 from the MOC and 2 Sv from the southern STC). The 5.5 Sv upwelling of MOC waters is consistent with Roemmich , who found 6 Sv of MOC upwelling from observations. We find that 2.7 Sv of the 5.5 Sv recirculates in the STCs. This recirculation has not been taken into account by Zhang et al. The much higher upwelling rates and the different Northern Hemisphere western boundary current transport used by Zhang et al. imply much stronger STCs in their study. We argue that a substantial amount of the upwelling is associated with tropical cells which are largely compensated by eddy transports.
 It is hard to judge whether the observational data was too sparse or whether the model contains errors. The advantage of the model is that the upwelled water and the gyre flow could be separated unambiguously, as well as the interaction between the MOC and the STC. Zhang et al.  did not take this recirculation into account explicitly and had to close their budget with the relatively uncertain western boundary transports and the surface layer divergence obtained from drifter data. It is clear that this is a daunting task using the observations. However, model uncertainties can be the cause of these discrepancies as well. As stated before, the ventilation of the tropical thermocline depends on the wind product that is used. Also, the strength of the vertical mixing relates to the strength of STCs [Boccaletti et al., 2004] and the Eulerian mean transport through the base of the mixed layer differs from the upwelling with eddy-induced transports included. Here, we did include the eddy-induced transports that are hard to obtain from observations.