The choice of the simple thermocline ventilation models is based on the following observations: The mean transports in the eastward branch of the intermediate subtropical gyre usually range from 6 to 26 Sv. In the westward branch the transports are typically between 10 and 30 Sv. The Lagrangian current measurements suggest a gradual increase of the westward transport from 18 Sv at 30oW to 29 Sv at 36oW before they fall back to 19 Sv at 40oW and 13 Sv at 42oW. This strong decrease is caused by the bifurcation of the current at the western boundary, in the Santos Bifurcation. About 3/4 of the 19 Sv at 40oW recirculate in the subtropical gyre. The remaining quarter flows north along the western boundary. This mean subtropical circulation pattern of Antarctic Intermediate Water and the similarity of the structures of the intermediate and the near-surface subtropical gyre strongly support the assumption of an anticyclonic basin-wide recirculation of Antarctic Intermediate Water in the subtropical South Atlantic. From the observations we conclude that the subtropical wind-driven gyre reaches down to more than 1200 m depth.
The hypothesis of the existence of a subtropical gyre at intermediate depth is supported by a Sverdrup model driven with observed wind fields. The transport distribution obtained with the Sverdrup model is largely consistent with the observed transport between the sea surface and the lower boundary of the Antarctic Intermediate Water layer. This encourages the use of two types of models of the ventilated thermocline. In the present study the models are adapted to the South Atlantic, and they are improved by more realistic boundary conditions than in earlier applications.
The parameters for the models of the ventilated thermocline are chosen
such that layer 3 includes the Antarctic Intermediate Water. All simulations
reproduce the observed structure of the subtropical gyre quite well. In
simulation 1 the prediction of the latitude of the eastward branch and
the latitude of the northern edge of the westward branch of the subtropical
gyre in layer 3 as well as the maximum total depth (more than 1200 m)
of the gyre coincide with observations. The width of the westward current,
however, deviates from observations in being about 5
too broad. As a consequence the center of the subtropical gyre appears
too far south. Introducing a mixed layer into the model (simulation 2)
does not have much effect on the location of the eastward and the westward
branches of the gyre but has a negative effect on the maximum total depth,
which is reduced by about 100 m. Opening the eastern boundary by
introducing an inflow/outflow condition (simulation 4) results in an even
broader westward current while the width and location of the eastward current
remains very similar to simulation 1. For this reason the agreement of
the location of the subtropical gyre center in simulation 4 with observations
is not as good as in simulation 1.
The main reasons for the difficulty of the models of the ventilated thermocline in reproducing the correct width of the westward branch of the subtropical gyre are the dynamics of the eastern boundary current (the Benguela Current) and the propagation of the Agulhas rings. In the present models the Benguela Current separation is too far south due to the lack of lateral friction and nonlinear processes. Thus the early separation is also found when an inflow through the eastern boundary is specified (simulation 4). Actually the Benguela Current separation is observed between 30oS and 25oS whereas it is located south of 30oS in the simulations. The Agulhas rings have been observed to propagate towards the northwest along the eastern boundary before they turn westward near 30oS. This behavior can also be seen in the Semtner/Chervin Parallel Ocean Climate Model. The Agulhas rings are considered responsible for a substantial part of the volume transport from the Indian to the Atlantic Ocean.
The comparison of the mean westward transport across about 30oW in layer 3 for the available seasonal ECMWF (European Center for Medium-range Weather Forecasting) wind fields suggests that about 90% of the observed transport might be caused by the wind field (simulation 1). In contrast to this the mean intermediate eastward transport derived from the simulation is only approximately half as large as the observed transport. There is an indication that this deviation can mainly be attributed to the missing interoceanic exchange across the eastern boundary. Opening the eastern boundary (simulation 4) with an inflow/outflow condition can reduce the discrepancy of the eastward transport from 50% to about 20%.
The seasonal variability is studied with simulations 1 and 5. Simulation 1 uses the seasonal ECMWF wind fields from austral autumn 1991 until austral spring 1993, while simulation 5 uses the seasonal ERS-1 wind fields from austral spring 1991 until austral summer 1995. The output of the Semtner/Chervin Parallel Ocean Climate Model (POCM). was also used to estimate the seasonal means. At 30oW in the third layer of simulation 1 transports ranging from 12 Sv to 24 Sv are estimated. In the POCM the transport varies between 8 Sv and 11 Sv in the 710 m - 1335 m layer. The larger amplitudes in the model of the ventilated thermocline are caused by the stationarity of the model. In the diagnostic model the applied wind field produces a corresponding equilibrium circulation, whereas in the real ocean and in prognostic models the influence of the wind field is considerably damped. Thus the equilibrium circulation for a synoptic wind field cannot be found in the ocean or in the POCM. The simulations 1 and 5 suggest that a minimum of the westward transport near 30oW occurs in the second half of the year, whereas the POCM indicates that a transport maximum occurs in the second half of the year.
The results show that the intermediate circulation is to a large extent part of the wind-driven subtropical gyre system which extends to more than 1200 m depth, including the Antarctic Intermediate Water.