Abstract
The circulation of the low-salinity Antarctic Intermediate Water in the
South Atlantic and the associated dynamical processes are studied, using
recent and historical hydrographic profiles, Lagrangian and Eulerian current
measurements as well as wind stress observations. The circulation pattern
inferred for the Antarctic Intermediate Water supports the hypothesis of
an anticyclonic basin-wide recirculation of the intermediate water in the
subtropics. The eastward current of the intermediate anticyclone is fed
mainly by water recirculated in the Brazil Current and by the Malvinas
Current. An additional source region is the Polar Frontal Zone of the South
Atlantic. The transport in the meandering eastward current ranges from
6 Svto 26 Sv (1 Sv = 106m3s-1).
The transport of the comparably uniform westward flow of the gyre varies
between 10 Sv and 30 Sv. Both transports vary with longitude.
At the western boundary near 28oS, in the Santos Bifurcation,
the westward current splits into two branches. About three quarters of
the 19 Sv at 40oW go south as an intermediate western
boundary current. The remaining quarter flows north along the western boundary.
Simulations with a simple model of the ventilated thermocline reveal that
the wind-driven subtropical gyre has a vertical extent of over 1200 m.
The transports derived from the simulations suggest that about 90% of the
transport in the westward branch of the intermediate gyre and about 50%
of the transport in the eastward branch can be attributed to the wind-driven
circulation. The structure of the simulated gyre deviates from observations
to some extent. The discrepancies between the simulations and the observations
are most likely caused by the interoceanic exchange south of Africa, the
dynamics of the boundary currents, the nonlinearity and the seasonal variability
of the wind field. A simulation with an inflow/outflow condition for the
eastern boundary reduces the transport deviations in the eastward current
to about 20%. The results support the hypothesis that the wind field is
of major importance for the subtropical circulation of Antarctic Intermediate
Water followed by the interoceanic exchange. The simulations suggest that
the westward transport in the subtropical gyre undergoes seasonal variations.
The transports and the structure of the intermediate subtropical gyre from
the Parallel Ocean Climate Model (Semtner/Chervin model) agree better with
observations.