88
D. Seidov . B. J. Haupt
and 65°N gave 22 Sv NADW production (see below), which is probably too high;
if one mediates the regional and global results there would be about 18 Sv, a
number most appropriate for a coarse resolution study. This would give an LGM
forward conveyor twice as weak but still comparable outflow which would fit the
idea of still intensive glacial thermohaline circulation (Yu et al. 1996; Webb et al.
1997).
As we have already noted, the 3-D conveyor is essentially more complex than
its 2-D image given by the total meridional overturning. The velocity fields are
discussed in great detail in Seidov et al. (1996) and Seidov and Haupt (1997).
Here we briefly overview how the glacial and MWE currents differ from the
modern ones. The present-day, LGM, and MWE simulated velocity fields may be
inspected in Seidov et al. (1996). The most noticeable feature is the deviation of
the paleo-North Atlantic Drift from its modern northeastern path. Strong zonality of the subpolar front indicates a reduced supply of water that can be downwelled as NADW in the northern North Atlantic and the NGS. Moreover, the
route of the return southward flow in the deep ocean changed radically, a robust
feature emerging in all our glacial experiments regardless of complexity. In the
eastern part of the basin, the incursion of AABW dominates the near-bed transport up to the Faeroe-Shetland Ridge, in agreement with the water mass contouring by Sarnthein et al. (1994). The most striking feature of the MWE currents in addition to the changes found at LGM is the reversal of the Norwegian
Current and the inflow-outflow regime in the Norwegian-Greenland Seas (see
Seidov et al. 1996 for details). Hence the redeposited sediment in the eastern mid
to high latitudes might be of different origin. Today, sediment drifts there transport material largely from northeast to west and southwest. In contrast, during
the LGM some sediment drifts could change direction of transport to redeposit
the grains from south to north and northwest.
Today's southward deep return current is a deep ocean western boundary
flow forming a strong countercurrent under the Gulf Stream. This western
boundary current is well recognized as the most prominent feature of the thermohaline circulation (Stommel and Arons 1960). Though a noticeably weaker
western boundary current still existed at the LGM and MWE, the descending
branch returned to the southwestern basin largely as a broad zonal westward
flow in the mid latitudes. The glacial countercurrent under the paleo-Gulf
Stream was weaker, deeper, and farther eastward. At the LGM a southward deep
ocean flow originated near the Rockall Plateau at depth of about 2 km, occurring
in the eastern part of the basin, rather than in the western part as today. This
particular feature of the computer model in the eastern North Atlantic is again
in good agreement with the contouring by Sarnthein et al. (1994). Using the trajectory-tracing model below, we demonstrate that the simulated glacial deep water indeed moved along the eastern flank of the Mid-Atlantic Ridge. Therefore
as water in the western deep Atlantic contained more AABW at the LGM, the
deep and abyssal water during the glacial time was older than at present.
To understand the mechanism of change of deep water formation, one should
consider changes in the convection regime. The modern deep convection sites
D. Seidov . B. J. Haupt
and 65°N gave 22 Sv NADW production (see below), which is probably too high;
if one mediates the regional and global results there would be about 18 Sv, a
number most appropriate for a coarse resolution study. This would give an LGM
forward conveyor twice as weak but still comparable outflow which would fit the
idea of still intensive glacial thermohaline circulation (Yu et al. 1996; Webb et al.
1997).
As we have already noted, the 3-D conveyor is essentially more complex than
its 2-D image given by the total meridional overturning. The velocity fields are
discussed in great detail in Seidov et al. (1996) and Seidov and Haupt (1997).
Here we briefly overview how the glacial and MWE currents differ from the
modern ones. The present-day, LGM, and MWE simulated velocity fields may be
inspected in Seidov et al. (1996). The most noticeable feature is the deviation of
the paleo-North Atlantic Drift from its modern northeastern path. Strong zonality of the subpolar front indicates a reduced supply of water that can be downwelled as NADW in the northern North Atlantic and the NGS. Moreover, the
route of the return southward flow in the deep ocean changed radically, a robust
feature emerging in all our glacial experiments regardless of complexity. In the
eastern part of the basin, the incursion of AABW dominates the near-bed transport up to the Faeroe-Shetland Ridge, in agreement with the water mass contouring by Sarnthein et al. (1994). The most striking feature of the MWE currents in addition to the changes found at LGM is the reversal of the Norwegian
Current and the inflow-outflow regime in the Norwegian-Greenland Seas (see
Seidov et al. 1996 for details). Hence the redeposited sediment in the eastern mid
to high latitudes might be of different origin. Today, sediment drifts there transport material largely from northeast to west and southwest. In contrast, during
the LGM some sediment drifts could change direction of transport to redeposit
the grains from south to north and northwest.
Today's southward deep return current is a deep ocean western boundary
flow forming a strong countercurrent under the Gulf Stream. This western
boundary current is well recognized as the most prominent feature of the thermohaline circulation (Stommel and Arons 1960). Though a noticeably weaker
western boundary current still existed at the LGM and MWE, the descending
branch returned to the southwestern basin largely as a broad zonal westward
flow in the mid latitudes. The glacial countercurrent under the paleo-Gulf
Stream was weaker, deeper, and farther eastward. At the LGM a southward deep
ocean flow originated near the Rockall Plateau at depth of about 2 km, occurring
in the eastern part of the basin, rather than in the western part as today. This
particular feature of the computer model in the eastern North Atlantic is again
in good agreement with the contouring by Sarnthein et al. (1994). Using the trajectory-tracing model below, we demonstrate that the simulated glacial deep water indeed moved along the eastern flank of the Mid-Atlantic Ridge. Therefore
as water in the western deep Atlantic contained more AABW at the LGM, the
deep and abyssal water during the glacial time was older than at present.
To understand the mechanism of change of deep water formation, one should
consider changes in the convection regime. The modern deep convection sites