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D. Seidov . B. J. Haupt
lation patterns, fundamental differences between glacial and interglacial ventilation and sedimentation regimes cannot be revealed using the velocity maps
solely. The transport in a transit area of intense ventilation can differ principally
depending on whether convection is taken into account or ignored. Let us hypothesize that a velocity field is not computed using a prognostic circulation
model, but is instead diagnostically calculated from the ocean climatology, e.g.,
using data of Levitus (1982). These climatological fields were formed by all processes including convection. However, the convection is not present in the data
directly. Moreover, the convection pattern cannot be reconstructed using these
data without running a prognostic model. Hence direct impact of convection on
the water motion is missed from diagnostic calculations based on these climatological data. In other words, diagnostic velocity would contain no information
about convection because hydrostatic instability was removed from the processed climatological data. This means that the Lagrangian particles whose trajectories were calculated on the basis of the observed ocean climatology instead
of a prognostic model would drift along the trajectories undisturbed by convection, and therefore these simulated trajectories would be wrong.
In our experiments, the particles illustrating the flow were deployed in different areas in the northern NA (Fig. 3 in Seidov and Haupt 1997). In each of the
areas, about 30 particles started to travel through the Eulerian velocity fields in
all three cases - the HM, MWE and LGM. In the following maps (Fig. 3 and Fig.
4) we employ two different techniques to show the trajectories. In Fig. 3 the trajectories are colored to show the depth of a particle. (convection sites are depicted by different shades of gray: the deeper the convection, the darker the shade).
We use the sunlight spectrum colors, from dark red in the uppermost layer
« 100 m) to violet and black in the two deepest layers, to visualize vertical
migration of the water parcels. In contrast, Fig. 4 shows the particles' pathways with both depth and elapsed time shown in small rectangles attached
to the trajectories (only two pairs of the trajectories are repeated in the black
and white Fig. 4). Although the model time in the trajectory-tracing calculations was over 500 years, only the tracks for the first 100-200 years of the
elapsed time are shown in the maps to avoid confusion.
A striking feature of the trajectory map is the change in the glacial deep ocean
circulation regime which is not so obvious from the velocity maps. This change
is far more complex than a simple increase in the zonality of the surface current,
a well-known feature of the LGM surface circulation (CLIMAP 1981; Ruddiman
and McIntyre 1981; Kellogg 1980).
The deep water production at the LGM is found in the model only in the central part of the north central North Atlantic (Fig. 3), which is in agreement with
the convection pattern in Fig. 4b. We note that water descends in the subpolar
gyre in spite of upward motion induced by Ekman divergence. Hence thermohaline currents would drive water along isopycnals in the subsurface layers regardless of ventilating convection; i.e., ventilation of subsurface and intermediate water would occur regardless of deep convection. The pronounced convective chimney forms an intensive cyclonic circulation around a homogenized col-
D. Seidov . B. J. Haupt
lation patterns, fundamental differences between glacial and interglacial ventilation and sedimentation regimes cannot be revealed using the velocity maps
solely. The transport in a transit area of intense ventilation can differ principally
depending on whether convection is taken into account or ignored. Let us hypothesize that a velocity field is not computed using a prognostic circulation
model, but is instead diagnostically calculated from the ocean climatology, e.g.,
using data of Levitus (1982). These climatological fields were formed by all processes including convection. However, the convection is not present in the data
directly. Moreover, the convection pattern cannot be reconstructed using these
data without running a prognostic model. Hence direct impact of convection on
the water motion is missed from diagnostic calculations based on these climatological data. In other words, diagnostic velocity would contain no information
about convection because hydrostatic instability was removed from the processed climatological data. This means that the Lagrangian particles whose trajectories were calculated on the basis of the observed ocean climatology instead
of a prognostic model would drift along the trajectories undisturbed by convection, and therefore these simulated trajectories would be wrong.
In our experiments, the particles illustrating the flow were deployed in different areas in the northern NA (Fig. 3 in Seidov and Haupt 1997). In each of the
areas, about 30 particles started to travel through the Eulerian velocity fields in
all three cases - the HM, MWE and LGM. In the following maps (Fig. 3 and Fig.
4) we employ two different techniques to show the trajectories. In Fig. 3 the trajectories are colored to show the depth of a particle. (convection sites are depicted by different shades of gray: the deeper the convection, the darker the shade).
We use the sunlight spectrum colors, from dark red in the uppermost layer
« 100 m) to violet and black in the two deepest layers, to visualize vertical
migration of the water parcels. In contrast, Fig. 4 shows the particles' pathways with both depth and elapsed time shown in small rectangles attached
to the trajectories (only two pairs of the trajectories are repeated in the black
and white Fig. 4). Although the model time in the trajectory-tracing calculations was over 500 years, only the tracks for the first 100-200 years of the
elapsed time are shown in the maps to avoid confusion.
A striking feature of the trajectory map is the change in the glacial deep ocean
circulation regime which is not so obvious from the velocity maps. This change
is far more complex than a simple increase in the zonality of the surface current,
a well-known feature of the LGM surface circulation (CLIMAP 1981; Ruddiman
and McIntyre 1981; Kellogg 1980).
The deep water production at the LGM is found in the model only in the central part of the north central North Atlantic (Fig. 3), which is in agreement with
the convection pattern in Fig. 4b. We note that water descends in the subpolar
gyre in spite of upward motion induced by Ekman divergence. Hence thermohaline currents would drive water along isopycnals in the subsurface layers regardless of ventilating convection; i.e., ventilation of subsurface and intermediate water would occur regardless of deep convection. The pronounced convective chimney forms an intensive cyclonic circulation around a homogenized col-