Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
83
trophic, and therefore it belongs to the "intermediate" or "planetary geostrophic" class of models (Hasselmann 1982; Seidov 1986 1996; Colin de Verdi ere 1988;
Maier-Reimer et al.19911993; Zhang et al.I992). Except for the linear dynamics
inherent to planetary geostrophic models, such models retain all features of the
most advanced primitive equation models (PEM) that are the present-day standards in physical oceanography (e.g., Bryan 1969, Cox 1984, Semtner 1986). Planetary geostrophic formalism allows some simplifications of the momentum and
vorticity balance equations, and a longer time step, and consequently becomes
computationally more effective. Parallel runs have been made on a coarse-resolution grid using this model and the Geophysical Fluid Dynamics Laboratory
(GFDL) (Bryan 1969; Cox 1984; Pacanowski et al.1993},a PEM. Seidov and Prien
(1996) have shown that results of this planetary geostrophic model match well
with the performance of the GFDL model. However, since the planetary-geostrophic approach may lead to significant discrepancies in the equatorial region,
the global ocean circulation was simulated using the GFDL-Modular Ocean
Model (MOM, version 1.1) (Cox 1984; Pacanowski et al.I993).
Any OGCM contains a procedure to allow convective mixing that develops
due to hydrostatic instability if denser water is formed (or advected) over lighter
water. This is the key process for deep water production and transport. As done
commonly in an OGCM, this instability is accommodated by mixing water vertically until it regains complete hydrostatic stability (Cox 1984). This vertical adjustment takes place as a step process with several successive mixings of adjacent
layers until complete hydrostatic stability is restored. As the water convects, the
particles associated with the mixing volumes exchange their positions vertically,
and hence the convection facilitates deep ventilation.
3.2
Ocean Sedimentation Model
The sedimentation model employed here was designed by Haupt (1995) and
tested in Haupt et al. (1994 1995}.1t is a large-scale sedimentation model consisting of two components: (1) a 3-D sediment transport model in the ocean interior, and (2) a 2-D sediment transport model in a thin near-bottom layer following
smoothed bottom topography. The 3-D component models the advection-diffusion of sediment similar to the equations of advection-diffusion of heat and salt
in the OGCM, and includes an added term to compute the settling of sediment
in the water column.
The 3-D component of the sedimentation model simulates the lateral transport and the entry of sediment particles at the sea-surface, including the entry
of ice-rafted sedimentary material. The supply of large-grain sediment at the
sea-surface can be increased or decreased depending on the amount of icebergs
injected into the open ocean due to decay of major ice sheets. The icebergs distribution over the NA in the past can be inferred from analyses of proxy data
from foraminifera (e.g. Maslin et al. 1995). The mass of sediment covering the
sea floor depends only on the balance of sources and sinks, whereas the spatial
83
trophic, and therefore it belongs to the "intermediate" or "planetary geostrophic" class of models (Hasselmann 1982; Seidov 1986 1996; Colin de Verdi ere 1988;
Maier-Reimer et al.19911993; Zhang et al.I992). Except for the linear dynamics
inherent to planetary geostrophic models, such models retain all features of the
most advanced primitive equation models (PEM) that are the present-day standards in physical oceanography (e.g., Bryan 1969, Cox 1984, Semtner 1986). Planetary geostrophic formalism allows some simplifications of the momentum and
vorticity balance equations, and a longer time step, and consequently becomes
computationally more effective. Parallel runs have been made on a coarse-resolution grid using this model and the Geophysical Fluid Dynamics Laboratory
(GFDL) (Bryan 1969; Cox 1984; Pacanowski et al.1993},a PEM. Seidov and Prien
(1996) have shown that results of this planetary geostrophic model match well
with the performance of the GFDL model. However, since the planetary-geostrophic approach may lead to significant discrepancies in the equatorial region,
the global ocean circulation was simulated using the GFDL-Modular Ocean
Model (MOM, version 1.1) (Cox 1984; Pacanowski et al.I993).
Any OGCM contains a procedure to allow convective mixing that develops
due to hydrostatic instability if denser water is formed (or advected) over lighter
water. This is the key process for deep water production and transport. As done
commonly in an OGCM, this instability is accommodated by mixing water vertically until it regains complete hydrostatic stability (Cox 1984). This vertical adjustment takes place as a step process with several successive mixings of adjacent
layers until complete hydrostatic stability is restored. As the water convects, the
particles associated with the mixing volumes exchange their positions vertically,
and hence the convection facilitates deep ventilation.
3.2
Ocean Sedimentation Model
The sedimentation model employed here was designed by Haupt (1995) and
tested in Haupt et al. (1994 1995}.1t is a large-scale sedimentation model consisting of two components: (1) a 3-D sediment transport model in the ocean interior, and (2) a 2-D sediment transport model in a thin near-bottom layer following
smoothed bottom topography. The 3-D component models the advection-diffusion of sediment similar to the equations of advection-diffusion of heat and salt
in the OGCM, and includes an added term to compute the settling of sediment
in the water column.
The 3-D component of the sedimentation model simulates the lateral transport and the entry of sediment particles at the sea-surface, including the entry
of ice-rafted sedimentary material. The supply of large-grain sediment at the
sea-surface can be increased or decreased depending on the amount of icebergs
injected into the open ocean due to decay of major ice sheets. The icebergs distribution over the NA in the past can be inferred from analyses of proxy data
from foraminifera (e.g. Maslin et al. 1995). The mass of sediment covering the
sea floor depends only on the balance of sources and sinks, whereas the spatial