102
D. Seidov . B. J. Haupt
indicates that the major change of the conveyor indeed took place at MWE, as
the consideration of the convection regime implies. Here we concentrate on tracing the conveyor using the Lagrangian particles. As was mentioned above, these
trajectories visualize the true three-dimensional water motion because they take
into account both vertical motion and mixing in the convection chimneys. Since
the vector maps suggest that the dramatic change of the conveyor took place at
MWE rather than at LGM, we present the trajectories for the present-day and
MWE time slices only. Figure 9a shows two pairs of trajectories calculated using
the modern velocity field and Fig. 9b depicts the trajectories calculated using the
MWE velocities. The particles to trace modern and MWE conveyor were deployed in different places. Two different sites were chosen because no deep convection has been found in the northern NA, whereas a site of deep convection
exists southwest of Australia at both HM and MWE. However, only at MWE did
particles deployed in the latter area progress westward and enter the central At1antic. First, we briefly overview the OGCM results represented by velocity vectors and then emphasize how the semi-Lagrangian calculation enhances water
transport analysis and understanding of the circulation change.
6.2
OGCM Results
The three-dimensional distribution of the horizontal currents is far more complex than Figs. 6-8 display. The deep inflow of AABW into the central and North
Atlantic is masked in these figures. However, Figs. 6 and 8 give a clear impression
of how different the deep and upper ocean flows are. The present-day deep flows
emerge as a truly global feature. This consolidated current system justifies, to a
certain extent, the term conveyor. The upper ocean circulation system, however,
does not give such an unambiguous impression of a continuous flow system. The
intensive subtropical gyres are indeed connected by their marginal extensions to
form a system that might have been recognized as connected rings of a chain
comprising the upper band of the conveyor. It should be noted that, because of
the course resolution, we cannot hope to model the correct operation of the upper band. For example, the Agulhas retroflection, which would extend the leg of
the Indian subtropical gyre into the South Atlantic, can only be resolved in an
eddy-resolving simulation. Nevertheless, one may say that no coherence comparable to the deep ocean conveyor structure can be found in the upper layers.
Moreover, the water traveling in the uppermost levels is strongly modified by
short-term air-sea interactions. The time scale of such interactions is about 2 to
3 months, which is an order of magnitude shorter than the time needed for water
to travel around the globe within this circulation system. Since the water characteristics would change on that short-term time scale, the question of their origin as the conveyors' water is therefore meaningless.
Because the structure of the glacial conveyor in the NA is not principally different from the modern one, the LGM conveyor is not illustrated here. Yet the
glacial conveyor is characterized by a noticeable (here almost 60%) decrease in
D. Seidov . B. J. Haupt
indicates that the major change of the conveyor indeed took place at MWE, as
the consideration of the convection regime implies. Here we concentrate on tracing the conveyor using the Lagrangian particles. As was mentioned above, these
trajectories visualize the true three-dimensional water motion because they take
into account both vertical motion and mixing in the convection chimneys. Since
the vector maps suggest that the dramatic change of the conveyor took place at
MWE rather than at LGM, we present the trajectories for the present-day and
MWE time slices only. Figure 9a shows two pairs of trajectories calculated using
the modern velocity field and Fig. 9b depicts the trajectories calculated using the
MWE velocities. The particles to trace modern and MWE conveyor were deployed in different places. Two different sites were chosen because no deep convection has been found in the northern NA, whereas a site of deep convection
exists southwest of Australia at both HM and MWE. However, only at MWE did
particles deployed in the latter area progress westward and enter the central At1antic. First, we briefly overview the OGCM results represented by velocity vectors and then emphasize how the semi-Lagrangian calculation enhances water
transport analysis and understanding of the circulation change.
6.2
OGCM Results
The three-dimensional distribution of the horizontal currents is far more complex than Figs. 6-8 display. The deep inflow of AABW into the central and North
Atlantic is masked in these figures. However, Figs. 6 and 8 give a clear impression
of how different the deep and upper ocean flows are. The present-day deep flows
emerge as a truly global feature. This consolidated current system justifies, to a
certain extent, the term conveyor. The upper ocean circulation system, however,
does not give such an unambiguous impression of a continuous flow system. The
intensive subtropical gyres are indeed connected by their marginal extensions to
form a system that might have been recognized as connected rings of a chain
comprising the upper band of the conveyor. It should be noted that, because of
the course resolution, we cannot hope to model the correct operation of the upper band. For example, the Agulhas retroflection, which would extend the leg of
the Indian subtropical gyre into the South Atlantic, can only be resolved in an
eddy-resolving simulation. Nevertheless, one may say that no coherence comparable to the deep ocean conveyor structure can be found in the upper layers.
Moreover, the water traveling in the uppermost levels is strongly modified by
short-term air-sea interactions. The time scale of such interactions is about 2 to
3 months, which is an order of magnitude shorter than the time needed for water
to travel around the globe within this circulation system. Since the water characteristics would change on that short-term time scale, the question of their origin as the conveyors' water is therefore meaningless.
Because the structure of the glacial conveyor in the NA is not principally different from the modern one, the LGM conveyor is not illustrated here. Yet the
glacial conveyor is characterized by a noticeable (here almost 60%) decrease in