96
D. Seidov . B. J. Haupt
longer paths in the subtropical gyre take more than 10 to 15 years to return to
ventilation zone. It is obvious from the colors of the trajectories in Fig. 3 that the
glacial thermocline is far more deeply ventilated than its modern analogue.
There is evidence (Slowey and Curry 1992 1995) that the subtropical thermocline was indeed better ventilated during the last glacial period. As Fig. 4 reveals,
our Lagrangian calculations agree well with these findings.
Figs. 3 and 4 indicate that the speed of the particles in the upper and intermediate layers was not lower at the LGM than today. The deep and abyssal glacial
flows, though routed differently from today, were even stronger in the eastern
part of the basin. This confirms the concept that the LGM conveyor was at least
as intense as the present one (Yu et al.1996; Boyle 1996). However we stress that,
in contrast to a similar or even higher intensity meridional conveyor at intermediate depths, the forward conveyor was definitely weaker at the upper to intermediate depths.
The MWE trajectories indicate that at that time slice there was no ventilation
of the deep ocean in the NA (Fig. 3c), although there was still quite intensive ventilation of the subtropical thermocline. The spaghetti form of the MWE trajectories indicate also that the Nordic Seas were rather isolated at that time. The selected trajectories illustrate the collapse of the forward conveyor and delineate
the shallow and slow motion within the upper ocean layers.
6
Modeling of the Global Conveyor
6.1
Convection Regime
The global ocean conveyor is weaker than in the regional NA experiments. The
MWE data, in the amount suitable for setting the data up on a regular grid, exist
only for a small region in the northern North Atlantic and the Nordic Seas (Sarnthein et al. 1995). The LGM salinity data over the entire globe are rather speculative if compared to the North Atlantic, where they aresufficient to form a data
set on a regular grid (Duplessy et al. 1988). On the other hand, the global simulations are far more advanced and physically consistent than any regional modeling because they give a continuous hydrodynamics of interconnected oceans
without any need to introduce artificial sponge layers. Hence, it is a question of
tradeoff when one considers advantages and disadvantages of global versus regional modeling based on limited data. One definite advantage of global simulations is the presence of all major sources of deep water. The convection patterns
(Fig. 5 ) clearly delineate key differences in the dominant process of deep ocean
ventilation in the northern and southern hemisphere at present and in the past.
In Fig. 5 the convection depth (the depth to which the convection due to hydrostatic instability penetrates) is shown as vertical bars. The deeper the convection, the higher the bars (see legend in the diagrams). If multiplied by the area
over which the convection occurred (here the surface of the grid cells), the depth
D. Seidov . B. J. Haupt
longer paths in the subtropical gyre take more than 10 to 15 years to return to
ventilation zone. It is obvious from the colors of the trajectories in Fig. 3 that the
glacial thermocline is far more deeply ventilated than its modern analogue.
There is evidence (Slowey and Curry 1992 1995) that the subtropical thermocline was indeed better ventilated during the last glacial period. As Fig. 4 reveals,
our Lagrangian calculations agree well with these findings.
Figs. 3 and 4 indicate that the speed of the particles in the upper and intermediate layers was not lower at the LGM than today. The deep and abyssal glacial
flows, though routed differently from today, were even stronger in the eastern
part of the basin. This confirms the concept that the LGM conveyor was at least
as intense as the present one (Yu et al.1996; Boyle 1996). However we stress that,
in contrast to a similar or even higher intensity meridional conveyor at intermediate depths, the forward conveyor was definitely weaker at the upper to intermediate depths.
The MWE trajectories indicate that at that time slice there was no ventilation
of the deep ocean in the NA (Fig. 3c), although there was still quite intensive ventilation of the subtropical thermocline. The spaghetti form of the MWE trajectories indicate also that the Nordic Seas were rather isolated at that time. The selected trajectories illustrate the collapse of the forward conveyor and delineate
the shallow and slow motion within the upper ocean layers.
6
Modeling of the Global Conveyor
6.1
Convection Regime
The global ocean conveyor is weaker than in the regional NA experiments. The
MWE data, in the amount suitable for setting the data up on a regular grid, exist
only for a small region in the northern North Atlantic and the Nordic Seas (Sarnthein et al. 1995). The LGM salinity data over the entire globe are rather speculative if compared to the North Atlantic, where they aresufficient to form a data
set on a regular grid (Duplessy et al. 1988). On the other hand, the global simulations are far more advanced and physically consistent than any regional modeling because they give a continuous hydrodynamics of interconnected oceans
without any need to introduce artificial sponge layers. Hence, it is a question of
tradeoff when one considers advantages and disadvantages of global versus regional modeling based on limited data. One definite advantage of global simulations is the presence of all major sources of deep water. The convection patterns
(Fig. 5 ) clearly delineate key differences in the dominant process of deep ocean
ventilation in the northern and southern hemisphere at present and in the past.
In Fig. 5 the convection depth (the depth to which the convection due to hydrostatic instability penetrates) is shown as vertical bars. The deeper the convection, the higher the bars (see legend in the diagrams). If multiplied by the area
over which the convection occurred (here the surface of the grid cells), the depth