Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
103
its intensity and the deep flows, especially in the NA, take different routes. Also,
the NA conveyor branch became shallower than at present. However, as show in
Seidov et al. (1996), the NA conveyor still existed at the LGM. Since there was a
weaker modern conveyor in the cited study, the relative changes are larger here
(see above). The present-day overturning rate (here we ended up with 23 Sv) is
perhaps an overestimate. We tried to obtain the convection in the northern NA
that would penetrate deep enough (deeper than 2 km) by cooling the sea-surface
by 2 °C (see above). The course resolution employed in this study led to an overestimate of the convective mixing and to overturning somewhat stronger than in
other coarse resolution studies. The mediating estimate of the glacial to presentday overturning ratio is perhaps within 50 to 70%.
The major changes took place at the MWE. Although the impact is tied to the
NGS and the northern NA only, the whole deep conveyor to the eastern Australian coast is affected. The NADW production was completely switched off, and
there is no deep southward flow in the western Atlantic. Moreover, the reversed
deep ocean flow is found in this simulation to the north of a substantially curtailed deep ACC in the Indian and Atlantic sectors of the Southern Ocean. Although our calculations do not unambiguously indicate the reversal of the whole
deep conveyor branch from the Atlantic to the northern Pacific, one may easily
recognize that the deep flow has indeed an opposite direction over rather a long
leg from the eastern Indian ocean to the northern NA. Additionally, a clearly
seen deep southward-flowing western boundary current developed in the northern Pacific (Fig. 8b). This flow, which is absent from the present-day deep current system, is a signature of the possibly reversed Pacific branch of the global
conveyor.
6.3
Trajectory Tracing
We have already emphasized above that the horizontal velocity vectors, even if
inspected at each level, may be misleading because they do not show vertical
motion. This is especially true in the areas of convection where water is mixed
vertically and particles may be transferred to the deep ocean. Hence, the true
3-D motion would be essentially different from what the vector maps might
have suggested. To illuminate the deep ocean ventilation and subsequent water
transport by the conveyor, the particles were deployed in the areas where they
can be transferred to the deep ocean within convection chimneys to trace the
deep ocean conveyor leg.
Some of the particles deployed in the NGS and northern NA penetrated into
the deep ocean and traveled southward in the western boundary current (Fig.
9a). Some of these particles pass further into the South Atlantic, although many
remained trapped in the NA subtropcal gyre. Some of those which manage to
travel to the South Atlantic turn backward. About 10 to 20% of all particles deployed in the shaded area in Fig. Sa reached the ACC and were transported further eastward in this current. Only 2 to 5% of these particles ever emerged in the
103
its intensity and the deep flows, especially in the NA, take different routes. Also,
the NA conveyor branch became shallower than at present. However, as show in
Seidov et al. (1996), the NA conveyor still existed at the LGM. Since there was a
weaker modern conveyor in the cited study, the relative changes are larger here
(see above). The present-day overturning rate (here we ended up with 23 Sv) is
perhaps an overestimate. We tried to obtain the convection in the northern NA
that would penetrate deep enough (deeper than 2 km) by cooling the sea-surface
by 2 °C (see above). The course resolution employed in this study led to an overestimate of the convective mixing and to overturning somewhat stronger than in
other coarse resolution studies. The mediating estimate of the glacial to presentday overturning ratio is perhaps within 50 to 70%.
The major changes took place at the MWE. Although the impact is tied to the
NGS and the northern NA only, the whole deep conveyor to the eastern Australian coast is affected. The NADW production was completely switched off, and
there is no deep southward flow in the western Atlantic. Moreover, the reversed
deep ocean flow is found in this simulation to the north of a substantially curtailed deep ACC in the Indian and Atlantic sectors of the Southern Ocean. Although our calculations do not unambiguously indicate the reversal of the whole
deep conveyor branch from the Atlantic to the northern Pacific, one may easily
recognize that the deep flow has indeed an opposite direction over rather a long
leg from the eastern Indian ocean to the northern NA. Additionally, a clearly
seen deep southward-flowing western boundary current developed in the northern Pacific (Fig. 8b). This flow, which is absent from the present-day deep current system, is a signature of the possibly reversed Pacific branch of the global
conveyor.
6.3
Trajectory Tracing
We have already emphasized above that the horizontal velocity vectors, even if
inspected at each level, may be misleading because they do not show vertical
motion. This is especially true in the areas of convection where water is mixed
vertically and particles may be transferred to the deep ocean. Hence, the true
3-D motion would be essentially different from what the vector maps might
have suggested. To illuminate the deep ocean ventilation and subsequent water
transport by the conveyor, the particles were deployed in the areas where they
can be transferred to the deep ocean within convection chimneys to trace the
deep ocean conveyor leg.
Some of the particles deployed in the NGS and northern NA penetrated into
the deep ocean and traveled southward in the western boundary current (Fig.
9a). Some of these particles pass further into the South Atlantic, although many
remained trapped in the NA subtropcal gyre. Some of those which manage to
travel to the South Atlantic turn backward. About 10 to 20% of all particles deployed in the shaded area in Fig. Sa reached the ACC and were transported further eastward in this current. Only 2 to 5% of these particles ever emerged in the