84
D. Seidov . B. J. Haupt
variation of sedimentation rates depends on the circulation pattern and particle
grain size. The 2-D model is initialized at every time step by the exchange of
sediment between the ocean body and the ocean floor. The sediment in the bottom layer is transported by a corrected benthic flow which is largely a projection
of the OGCM velocity field onto the smoothed bottom layer (1 cm thick). Additionally, the near-bottom velocities are reduced to take bottom friction into account (Miller et al. 1977; Zanke 1978; Siindermann 1983).
The impact of convection was not included in the previous version of the sedimentation model (Haupt et al. 1994). In the updated version the convection is
incorporated in the form of convection depths from the OGCM (Seidov and
Haupt 1997). Vertical mixing, similar to that employed in the OGCM, is applied
to concentration. The sediment transport model 'knows' when to mix water because we have encoded the convection depths in the velocity field as an additional parameter. When used in the sedimentation model, this information is decoded to enable vertical mixing in the grid points where convection occurs (see below in the discussion of convection patterns).
3.3
Particle-Tracing Model
The water-volume trajectory-tracing model was developed by Haupt (1995) and
was employed to trace particle drifts in the northern North Atlantic (Haupt et al.
19941995). This model exercises a hybrid Eulerian-Lagrangian (or semi-Lagrangian) approach; the velocity components are interpolated to the current positions of
the Lagrangian particles from the nearby grid points of a Eulerian numerical grid.
As in the sediment transport model described above, the Eulerian velocity field is
provided by the OGCM, whereas the coordinates of Lagrangian particles are calcu1ated straightforwardly, using the Lagrangian velocity along the trajectory.
As we have emphasized above, ventilating convection induced by hydrostatic
instability is included in all three components of our simulations. The semi-Lagrangian trajectory-tracing model of Haupt et al. (1994) was upgraded in Seidov
and Haupt (1997) to facilitate vertical ventilation in convective "chimneys"
(Send and Marshall (1995) show that ventilating convection occurs as water mixing in water columns or "chimneys"). Here the velocity field from the OGCM is
supplemented by the convection depths showing where and to what depth the
vertically mixing volume should be propelled in the turbulent chimney. A detailed discussion of the parameterization of chimney mixing in the particletracing model is given in Seidov and Haupt (1997).
More details of the sediment transport model and the particle-tracing model
can be also found in Haupt et al. (1997; see this Vol.).
4
The Setup of Numerical Experiments
The numerical experiments in both the regional NA and the global circulation
cases were carried out in two steps. First, the OGCM was run using the appro-
D. Seidov . B. J. Haupt
variation of sedimentation rates depends on the circulation pattern and particle
grain size. The 2-D model is initialized at every time step by the exchange of
sediment between the ocean body and the ocean floor. The sediment in the bottom layer is transported by a corrected benthic flow which is largely a projection
of the OGCM velocity field onto the smoothed bottom layer (1 cm thick). Additionally, the near-bottom velocities are reduced to take bottom friction into account (Miller et al. 1977; Zanke 1978; Siindermann 1983).
The impact of convection was not included in the previous version of the sedimentation model (Haupt et al. 1994). In the updated version the convection is
incorporated in the form of convection depths from the OGCM (Seidov and
Haupt 1997). Vertical mixing, similar to that employed in the OGCM, is applied
to concentration. The sediment transport model 'knows' when to mix water because we have encoded the convection depths in the velocity field as an additional parameter. When used in the sedimentation model, this information is decoded to enable vertical mixing in the grid points where convection occurs (see below in the discussion of convection patterns).
3.3
Particle-Tracing Model
The water-volume trajectory-tracing model was developed by Haupt (1995) and
was employed to trace particle drifts in the northern North Atlantic (Haupt et al.
19941995). This model exercises a hybrid Eulerian-Lagrangian (or semi-Lagrangian) approach; the velocity components are interpolated to the current positions of
the Lagrangian particles from the nearby grid points of a Eulerian numerical grid.
As in the sediment transport model described above, the Eulerian velocity field is
provided by the OGCM, whereas the coordinates of Lagrangian particles are calcu1ated straightforwardly, using the Lagrangian velocity along the trajectory.
As we have emphasized above, ventilating convection induced by hydrostatic
instability is included in all three components of our simulations. The semi-Lagrangian trajectory-tracing model of Haupt et al. (1994) was upgraded in Seidov
and Haupt (1997) to facilitate vertical ventilation in convective "chimneys"
(Send and Marshall (1995) show that ventilating convection occurs as water mixing in water columns or "chimneys"). Here the velocity field from the OGCM is
supplemented by the convection depths showing where and to what depth the
vertically mixing volume should be propelled in the turbulent chimney. A detailed discussion of the parameterization of chimney mixing in the particletracing model is given in Seidov and Haupt (1997).
More details of the sediment transport model and the particle-tracing model
can be also found in Haupt et al. (1997; see this Vol.).
4
The Setup of Numerical Experiments
The numerical experiments in both the regional NA and the global circulation
cases were carried out in two steps. First, the OGCM was run using the appro-