Since then other ideas have been presented on the possible pathways of the AAIW circulation. Earlier studies supported the hypothesis of a continuous western boundary current (Defant, 1941; Sverdrup et al., 1942). Later it was suggested that a northward western boundary current of AAIW exists only south and north of an intermediate anticyclonic subtropical gyre (Riley, 1951; Martineau, 1953; Buscaglia, 1971). Taft (1963) also concluded that the AAIW flows around the subtropical anticyclonic gyre, but he could not exclude the existence of a persistent equatorward western boundary current between the gyre and the shelf break. Although more recent hydrographic observations do not show a northward western boundary current between 40oS and 28oS, the spatial resolution is still not sufficiently high to exclude the possibility of such a current (Reid, 1989; Talley, 1996; Larqué et al., 1997 and Schmid, 1998). Lagrangian current measurements, however, do not indicate a northward western boundary current between 40oS and 28oS (Boebel et al., 1997b; Schmid, 1998). These measurements clearly support the subtropical gyre hypothesis instead. The shape of the intermediate water subtropical gyre differs from the subtropical near-surface gyre (compare Figs. 1; Reid, 1989). The intermediate gyre does not extend as far north and the northern branch of the gyre is more zonally oriented than the near-surface gyre (Reid, 1989; Boebel et al., 1997b).
A summary of meridional AAIW transports from the literature is given in Table 1. The estimates of net northward transports (Mn) of AAIW range from 5 Sv to 8 Sv (1 Sv = 106m3s-1). At latitudes south of 30oS northward transports are observed along the eastern boundary (Me) and in the interior of the western South Atlantic, whereas the western boundary current transport (Mw) is directed to the south. It has been suggested that the northward transports of AAIW between 30oS and 36oS adjacent to the southward western boundary current can be linked to an offshore return current (Rintoul, 1991). It remains an open question whether these transport estimates indicate the existence of a permanent return current analogous to the Brazil Return Current at the surface (Stramma, 1989; Rintoul, 1991) or whether they are caused by eddies or meanders.
The locations of the South Atlantic fronts (Table 2) are relevant for the AAIW generation and circulation. This applies particularly to the Subantarctic Front (SAF) which coincides with the southern boundary of the AAIW core layer. Between the SAF and the Polar Front (PF), in the Polar Frontal Zone (PFZ), the low salinity can be seen from the surface down to more than 500 m. In this region the AAIW cannot be discerned from the overlying low-salinity water. The Subtropical Front (STF) is an indication for the southern boundary of the near-surface subtropical gyre, with the South Atlantic Current to the north of this front (Stramma_Peterson, 1990).
Several possible sources of AAIW have been discussed. Deacon (1933,
1937) and Wüst (1935) believed that a significant amount of Antarctic
Surface Water (AASW) subducts underneath the Subantarctic Surface Water
in the PFZ. They thought that these two water masses mix in the process
to form AAIW. Another hypothesis is that the Subantarctic Mode Water (SAMW)
with temperatures below 4.5oC, formed in the southeast Pacific
Ocean, is a major source of the AAIW (McCartney, 1977; Molinelli, 1981;
Keffer, 1985). Molinelli (1981) estimated a transport of 6 Sv in
the density range 
= (27.1 - 27.2) kg m-3 for this source. Another 4 Sv
originate from the Indian Ocean but part of it is retroflected south of
Africa and returns into the Indian Ocean. Molinelli (1981) believed that
an isopycnal transport of AASW across the Polar Front in the Pacific Ocean
feeds the water in the density range
= (27.2 - 27.3) kg m-3. He stated that this process
could explain the existence of isohaline thermoclines which contribute
about 3 Sv to the AAIW transport of the Atlantic Ocean. Another
2 Sv originate from the Indian Ocean. Molinelli's error estimate
for these transports was 10%.
The hypothesis that the SAMW input from the Pacific Ocean represents a major contribution to the AAIW seems to be the most likely explanation for the observations even though the coldest variety of the SAMW is warmer than the freshest AAIW in the Atlantic Ocean. This discrepancy might be explained by the change of characteristics of the SAMW on its way through the Drake Passage due to surface fluxes, cross-frontal mixing and/or the existence of a secondary AAIW source. Piola and Georgi (1982) assumed that a strong source of AAIW near the PF of the South Atlantic is necessary to explain the observed water mass characteristics. This is supported by the results from hydrographic surveys in the Drake Passage and the larger Malvinas Current region (Piola and Gordon, 1989).
The question of the AAIW sources and pathways is closely linked to the question of the dynamics governing the AAIW circulation. Evans and Signorini (1985) assumed that a northward western boundary current driven by thermohaline forcing exists all along the coast of South America. In contrast Buscaglia (1971) argued that the subtropical AAIW circulation is governed by the wind field, i. e. that the anticyclonic Sverdrup gyre circulation reaches down to more than 1000 m.
In the present study we address the question of the wind forcing. The discussion will be based on the analysis of hydrographic and Lagrangian observations as well as simulations with models of the ventilated thermocline. In section 2 we will present our observations and discuss the inferences. In section 3 we will check whether Sverdrup dynamics can be considered an important factor for the AAIW circulation. Two models of the ventilated thermocline will be applied to the South Atlantic and the results will be discussed in conjunction with in situ observations and output fields of a primitive equations model. Our conclusions are summarized in section 4.