of the coldest bottom waters influence the heat balance of the ocean and the Earth’s
climate as a whole.
The major part of the Antarctic waters from the Weddell Sea spreads to the north
in the western part of the Atlantic Ocean. The further propagation to the East
Atlantic occurs through numerous fracture zones of the Mid-Atlantic Ridge. Hence,
almost all Antarctic waters propagate through the Argentine and Brazil basins in the
southwestern part of the Atlantic. The zonally aligned Rio Grande Rise and Santos
Plateau separate these deep basins from each other and prevent the free propagation
of bottom water to the north. The key passages for AABW here are the Vema and
Hunter channels and also the small channels on the Santos Plateau. The Vema
Channel was formed due to the long-term erosion of the bottom by the abyssal
currents [7]. Now it is the deepest pathway for Antarctic waters. The Hunter
Channel and Santos Plateau are much shallower. Therefore, the coldest part of
AABW propagates through the Vema Channel; however, the flow over the wide
Santos Plateau makes a significant contribution to the total AABW transport [8, 18].
The most well studied part of the Vema Channel is the Vema Sill at 31° 12′–31°
14′ S. A bathymetric survey of the southern part of the channel was performed in
1993 with a multi-beam echo-sounder [22]. More than 20 CTD sections across the
channel have been occupied in this region since the 1970s. Several hydrographic
sections were occupied at the Vema Extension at 26° 40′ S [14, 16, 19]. The
measurements of velocities in the channel were performed using the Lowered
Acoustic Doppler Current Profiler (LADCP) and on a few moorings deployed in
1991–1992 and 2001–2003. The transport of Antarctic waters through the channel
is estimated at 1.3–5.3 Sv [15]. In contrast to the Vema Channel, the bottom
currents over the Santos Plateau are less studied. Velocities in this region were
measured on several moorings deployed across the southern boundary of the Brazil
Basin in 1991 [8] and in the northern part of the plateau in 2003 [15]. These
measurements show strong variability of the bottom currents. Existing estimates of
AABW transport over the Santos Plateau range from −1.5 to 2 Sv [13, 18] indicating that water transport here is possible in both directions.
As for numerical modeling, only one work to our knowledge was dedicated to
the simulation of the bottom currents in the Vema Channel [9]. The authors used a
two-dimensional version of the sigma-coordinate Princeton Ocean Model for
simulating the cross-channel distribution of the thermohaline properties. It was
shown that the bottom friction causes the cross-channel Ekman flux and modifies
the lateral structure of the flow. That work does not describe the flow in the
longitudinal direction. Existing three-dimensional global models of the ocean do
not have sufficient lateral resolution for the simulation of the currents in narrow
abyssal channels. In addition, the vertical resolution of these models in the bottom
layers is not satisfactory for studying the lower part of the ocean circulation.
The goal of this paper is to study the variability of the bottom currents along
abyssal channels in the Southwest Atlantic using regional three-dimensional model
of the oceanic circulation with high resolution near the bottom. The data of direct
measurements collected using the LADCP instrument were used for verification of
the simulated velocity fields. These measurements were made at a few locations
476
D. I. Frey et al.
climate as a whole.
The major part of the Antarctic waters from the Weddell Sea spreads to the north
in the western part of the Atlantic Ocean. The further propagation to the East
Atlantic occurs through numerous fracture zones of the Mid-Atlantic Ridge. Hence,
almost all Antarctic waters propagate through the Argentine and Brazil basins in the
southwestern part of the Atlantic. The zonally aligned Rio Grande Rise and Santos
Plateau separate these deep basins from each other and prevent the free propagation
of bottom water to the north. The key passages for AABW here are the Vema and
Hunter channels and also the small channels on the Santos Plateau. The Vema
Channel was formed due to the long-term erosion of the bottom by the abyssal
currents [7]. Now it is the deepest pathway for Antarctic waters. The Hunter
Channel and Santos Plateau are much shallower. Therefore, the coldest part of
AABW propagates through the Vema Channel; however, the flow over the wide
Santos Plateau makes a significant contribution to the total AABW transport [8, 18].
The most well studied part of the Vema Channel is the Vema Sill at 31° 12′–31°
14′ S. A bathymetric survey of the southern part of the channel was performed in
1993 with a multi-beam echo-sounder [22]. More than 20 CTD sections across the
channel have been occupied in this region since the 1970s. Several hydrographic
sections were occupied at the Vema Extension at 26° 40′ S [14, 16, 19]. The
measurements of velocities in the channel were performed using the Lowered
Acoustic Doppler Current Profiler (LADCP) and on a few moorings deployed in
1991–1992 and 2001–2003. The transport of Antarctic waters through the channel
is estimated at 1.3–5.3 Sv [15]. In contrast to the Vema Channel, the bottom
currents over the Santos Plateau are less studied. Velocities in this region were
measured on several moorings deployed across the southern boundary of the Brazil
Basin in 1991 [8] and in the northern part of the plateau in 2003 [15]. These
measurements show strong variability of the bottom currents. Existing estimates of
AABW transport over the Santos Plateau range from −1.5 to 2 Sv [13, 18] indicating that water transport here is possible in both directions.
As for numerical modeling, only one work to our knowledge was dedicated to
the simulation of the bottom currents in the Vema Channel [9]. The authors used a
two-dimensional version of the sigma-coordinate Princeton Ocean Model for
simulating the cross-channel distribution of the thermohaline properties. It was
shown that the bottom friction causes the cross-channel Ekman flux and modifies
the lateral structure of the flow. That work does not describe the flow in the
longitudinal direction. Existing three-dimensional global models of the ocean do
not have sufficient lateral resolution for the simulation of the currents in narrow
abyssal channels. In addition, the vertical resolution of these models in the bottom
layers is not satisfactory for studying the lower part of the ocean circulation.
The goal of this paper is to study the variability of the bottom currents along
abyssal channels in the Southwest Atlantic using regional three-dimensional model
of the oceanic circulation with high resolution near the bottom. The data of direct
measurements collected using the LADCP instrument were used for verification of
the simulated velocity fields. These measurements were made at a few locations
476
D. I. Frey et al.