LDEO Software (version IX.10), developed by M. Visbeck. This package is an
implementation of the velocity inversion method [20]. The results of processing
were corrected by subtracting the tidal velocities. The barotropic tide was calculated
from the data of the satellite observations using the method described in [5]. Usually,
the velocities of the bottom currents exceed the tidal velocities by one order of
magnitude. However, this correction is not negligible for some slow currents over
the Santos Plateau. The accuracy of velocity measurements is 2 cm/s, CTD temperature measurements is 0.003 °C, CTD salinity measurements is 0.005 PSU.
The simulation of the near-bottom dynamics was performed using the Institute of
Numerical Mathematics Ocean Model (INMOM). The INMOM is a σ-coordinate
ocean circulation model based on the primitive equations of ocean hydrothermodynamics with the Boussinesq and hydrostatic approximations [4]. Dimensionless
variable σ is used in the model as the vertical coordinate:
σ =
z − ζðx, y, tÞ
ðHðx, yÞ − ζðx, y, tÞ
,
where z is the ordinary vertical coordinate in meters measured from the unperturbed
sea surface towards the center of the Earth, H(x,y) is the depth at the given point,
ζðx, y, tÞ is the sea surface height (x,y) are the longitude and latitude, respectively.
The use of a σ-coordinate model for studying bottom currents is important because
the lower layers in the σ-coordinates exactly follow the bottom topography [6].
The INMOM has been well tested in the different regions of the World Ocean
including the Atlantic in the frame of the Coordinated Ocean-ice Reference
Experiments (CORE) international program [3] and in the eddy resolving regime [1].
In this work we used a regional version of the INMOM developed for the
Southwest Atlantic. We selected a rectangular domain between 37.0° and 22.8° S
and between 45.2° and 29.0° W, which completely covers the Santos Plateau, the
Rio Grande Rise, and parts of the Argentine and Brazil basins. The grid domain
contains 814 by 764 nodes in the horizontal plane, the spatial resolution is 0.02°.
The vertical direction is divided into 33 σ-levels not uniformly distributed over the
depth. The best resolution was at the bottom; at the ocean point with a depth of
4850 m, the 10 deepest levels were specified in the bottom layers from 4400 to
4850 m with a step of 50 m. The model time step was 3 min.
The bottom topography was constructed from the data of Smith and Sandwell
digital atlas with an initial resolution of 30′. The original data were corrected using
the multi-beam echo sounder survey [22], interpolated to the model domain and
smoothed using the Tukey filter. The data from the climatological Levitus Atlas [2,
12] were used to construct the initial conditions for temperature and salinity. These
monthly data include the mean climatic fields of temperature and salinity in the
World Ocean with a spatial resolution of 0.25°. The Levitus data were also specified
in a buffer zone with a width of 10 boundary points at the liquid boundaries [10].
The atmospheric forcing was taken into account by specifying the fluxes of heat,
freshwater, and momentum at the boundary. The surface turbulent fluxes were
calculated using the bulk formulas. The atmospheric surface characteristics
478
D. I. Frey et al.
implementation of the velocity inversion method [20]. The results of processing
were corrected by subtracting the tidal velocities. The barotropic tide was calculated
from the data of the satellite observations using the method described in [5]. Usually,
the velocities of the bottom currents exceed the tidal velocities by one order of
magnitude. However, this correction is not negligible for some slow currents over
the Santos Plateau. The accuracy of velocity measurements is 2 cm/s, CTD temperature measurements is 0.003 °C, CTD salinity measurements is 0.005 PSU.
The simulation of the near-bottom dynamics was performed using the Institute of
Numerical Mathematics Ocean Model (INMOM). The INMOM is a σ-coordinate
ocean circulation model based on the primitive equations of ocean hydrothermodynamics with the Boussinesq and hydrostatic approximations [4]. Dimensionless
variable σ is used in the model as the vertical coordinate:
σ =
z − ζðx, y, tÞ
ðHðx, yÞ − ζðx, y, tÞ
,
where z is the ordinary vertical coordinate in meters measured from the unperturbed
sea surface towards the center of the Earth, H(x,y) is the depth at the given point,
ζðx, y, tÞ is the sea surface height (x,y) are the longitude and latitude, respectively.
The use of a σ-coordinate model for studying bottom currents is important because
the lower layers in the σ-coordinates exactly follow the bottom topography [6].
The INMOM has been well tested in the different regions of the World Ocean
including the Atlantic in the frame of the Coordinated Ocean-ice Reference
Experiments (CORE) international program [3] and in the eddy resolving regime [1].
In this work we used a regional version of the INMOM developed for the
Southwest Atlantic. We selected a rectangular domain between 37.0° and 22.8° S
and between 45.2° and 29.0° W, which completely covers the Santos Plateau, the
Rio Grande Rise, and parts of the Argentine and Brazil basins. The grid domain
contains 814 by 764 nodes in the horizontal plane, the spatial resolution is 0.02°.
The vertical direction is divided into 33 σ-levels not uniformly distributed over the
depth. The best resolution was at the bottom; at the ocean point with a depth of
4850 m, the 10 deepest levels were specified in the bottom layers from 4400 to
4850 m with a step of 50 m. The model time step was 3 min.
The bottom topography was constructed from the data of Smith and Sandwell
digital atlas with an initial resolution of 30′. The original data were corrected using
the multi-beam echo sounder survey [22], interpolated to the model domain and
smoothed using the Tukey filter. The data from the climatological Levitus Atlas [2,
12] were used to construct the initial conditions for temperature and salinity. These
monthly data include the mean climatic fields of temperature and salinity in the
World Ocean with a spatial resolution of 0.25°. The Levitus data were also specified
in a buffer zone with a width of 10 boundary points at the liquid boundaries [10].
The atmospheric forcing was taken into account by specifying the fluxes of heat,
freshwater, and momentum at the boundary. The surface turbulent fluxes were
calculated using the bulk formulas. The atmospheric surface characteristics
478
D. I. Frey et al.