Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
95
umn of high-density water (Fig. 3b). As particles deployed at the surface during
the LGM became trapped inside this column, they were propelled downward,
forming most of the 8 Sv of glacial NADW (see above).
Fig. 3b indicates that there was some cold intermediate water produced in the
NGS at the LGM. This water, flowing farther into the North Atlantic over the
Greenland-Iceland sill, was not dense enough to subduct under the intermediate
to deep water formed in the central part of the subpolar gyre. It mixed with the
subpolar water and stayed at an intermediate depth flowing along the S-shaped
route at that depth in the north central part of the basin (see in Plates 1d and 2b
in Seidov and Haupt 1997). Most of the deep return flow occurred along the eastern slope of the Mid-Atlantic Ridge, though some water still contoured the
American east coast. In contrast, water sinking in the NGS today is dense
enough to descend even deeper after spilling over the sills into the North Atlantic. Together with the portion of NADW formed east of Greenland and in the
Labrador Sea, this water travels southward in the western boundary current
comprising most of the 13 Sv of simulated modern NADW outflow. Most of the
present-day deep flow contours the American east coast, and a smaller portion
is routed along the west slope of the Mid-Atlantic Ridge. We point out that the
curtailment of the forward conveyor in the western part of the ocean was not
complete. This implies that the conclusions based on analysis of the glacial proxy
data assembled along the eastern meridional sections probably are not valid for
the western part of the basin.
Fig. 3a and 3b compares present and the glacial trajectories'''spaghetti'' in the
subtropical anticyclonic gyre. The modern and LGM trajectory maps conform
to the Luyten-Pedlosky-Stommel (LPS) ventilated thermocline theory (Luyten
et al. 1983) and the computer experiment of Cox and Bryan (1984). The thermocline in the subtropics is maintained by Ekman pumping. Therefore as water circulates in the gyre, it descends and ventilates the thermocline. The water is
brought up to subsurface layers in the western boundary along upward sloping
isopycnals (orange-colored segments of the trajectories in the vicinity of the
western boundary in Fig. 3). Flowing eastward, in the segment of western
boundary current outflow, the water convects because of heat loss to the atmosphere (because the specified SST is colder than the outflowing subsurface water
in this outflow zone). This shallow convection is shown in light gray in Fig. 3.
The subsurface water parcels in this ventilation zone come in contact with the
atmosphere. They then start to descend again to repeat the whole ventilation cycle. Those water parcels which flow northeastward below the convection depth
do not contact the atmosphere again and therefore do not ventilate. Yet, at any
given time, new water parcels descend from the surface in the ventilation zone.
However, ventilation is restricted to the central and western parts of the gyre. In
complete agreement with the LPS theory, there is a shadow zone attached to the
eastern boundary, practically unreachable for ventilation. Modern elapsed
times (Fig. 3a) for a single ventilation cycle in the gyre agree well with the estimates of Cox and Bryan (1984); short loops have an advection age of no more
than 3 to 5 years before the water returns to the ventilation zone, whereas the
95
umn of high-density water (Fig. 3b). As particles deployed at the surface during
the LGM became trapped inside this column, they were propelled downward,
forming most of the 8 Sv of glacial NADW (see above).
Fig. 3b indicates that there was some cold intermediate water produced in the
NGS at the LGM. This water, flowing farther into the North Atlantic over the
Greenland-Iceland sill, was not dense enough to subduct under the intermediate
to deep water formed in the central part of the subpolar gyre. It mixed with the
subpolar water and stayed at an intermediate depth flowing along the S-shaped
route at that depth in the north central part of the basin (see in Plates 1d and 2b
in Seidov and Haupt 1997). Most of the deep return flow occurred along the eastern slope of the Mid-Atlantic Ridge, though some water still contoured the
American east coast. In contrast, water sinking in the NGS today is dense
enough to descend even deeper after spilling over the sills into the North Atlantic. Together with the portion of NADW formed east of Greenland and in the
Labrador Sea, this water travels southward in the western boundary current
comprising most of the 13 Sv of simulated modern NADW outflow. Most of the
present-day deep flow contours the American east coast, and a smaller portion
is routed along the west slope of the Mid-Atlantic Ridge. We point out that the
curtailment of the forward conveyor in the western part of the ocean was not
complete. This implies that the conclusions based on analysis of the glacial proxy
data assembled along the eastern meridional sections probably are not valid for
the western part of the basin.
Fig. 3a and 3b compares present and the glacial trajectories'''spaghetti'' in the
subtropical anticyclonic gyre. The modern and LGM trajectory maps conform
to the Luyten-Pedlosky-Stommel (LPS) ventilated thermocline theory (Luyten
et al. 1983) and the computer experiment of Cox and Bryan (1984). The thermocline in the subtropics is maintained by Ekman pumping. Therefore as water circulates in the gyre, it descends and ventilates the thermocline. The water is
brought up to subsurface layers in the western boundary along upward sloping
isopycnals (orange-colored segments of the trajectories in the vicinity of the
western boundary in Fig. 3). Flowing eastward, in the segment of western
boundary current outflow, the water convects because of heat loss to the atmosphere (because the specified SST is colder than the outflowing subsurface water
in this outflow zone). This shallow convection is shown in light gray in Fig. 3.
The subsurface water parcels in this ventilation zone come in contact with the
atmosphere. They then start to descend again to repeat the whole ventilation cycle. Those water parcels which flow northeastward below the convection depth
do not contact the atmosphere again and therefore do not ventilate. Yet, at any
given time, new water parcels descend from the surface in the ventilation zone.
However, ventilation is restricted to the central and western parts of the gyre. In
complete agreement with the LPS theory, there is a shadow zone attached to the
eastern boundary, practically unreachable for ventilation. Modern elapsed
times (Fig. 3a) for a single ventilation cycle in the gyre agree well with the estimates of Cox and Bryan (1984); short loops have an advection age of no more
than 3 to 5 years before the water returns to the ventilation zone, whereas the