Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
105
The Lagrangian calculations help to elaborate the true three-dimensional water motion and therefore are the only means by which conveyor modes can be
genuinely visualized. Moreover, the trajectory-tracing technique may indicate
whether specific parts of the global ocean are interconnected via the deep conveyor branches, or essentially isolated. For example, the northern NA during the
MWE is characterized by very old nonventilated intermediate-to-deep water
(Sarnthein et al. 1995). Our calculations may shed some additional light on this
problem. If the MWE water in the NA was a mixture of the AABW, originating in
the Weddell Sea and some water sinking in the eastern Indian ocean, it may explain extreme aging of the Atlantic water, stronger than it would be if only the
AABW ventilated the NA areas.
A combined circulation/sedimentation/particle tracing modeling approach
was employed to understand particular aspects of the glacial-interglacial change
of the North Atlantic and World Ocean circulation which are difficult to address
using single-component models. Primary among those are the ventilation regimes, including the characterization of vertical mixing in convective chimneys
and the advective ages of ventilated water. Circulation studies might focus on the
NADW outflow or deep ocean circulation routing. Most of these questions are
traditionally addressed using geochemical tracers such as 8 18 0, 8l3c, and ,:1 14 C,
which are extremely useful in ocean circulation studies and hence widely employed. However, the tracers alone cannot provide sufficient constraints over a
simulated past circulation, as has been shown recently by LeGrand and Wunsch
(1995). In their study they showed that there exist an infinite number of states
that would satisfy a tracer distribution aimed at constraining the circulation, at
least for the currently available proxy data sets.
Another problem arising in paleoceanographic investigations is the parallel
analysis of both surface and benthic habitats and/or sediment transport features. Commonly, assumed features of a water flow thought to be suitable for explaining a particular distribution of proxy data are largely based on speculation.
There is no guarantee that this hypothetical flow would satisfy hydrodynamic restrictions posed over the ocean by the wind stress, ocean geometry, bottom
morphology, and sea-surface heat and fresh waterfluxes.
Based on only partly known SSS, recently corrected SST, and simulated glacial
wind stress, our results largely conform to current ideas about ventilation and
overturn in the North Atlantic at the height of the last glaciation and during the
subsequent meltwater event near 13 500 14C years B.P. The sediment transport
model output contains features that agree well with interpretation of sediment
data in the northern North Atlantic. In Seidov and Haupt (1997) we discussed
the LGM sedimentation pattern resulting from our simulation in comparison
with the observed or inferred data of Cremer et al. (1993) and McCave and Tucholke (1986). It was pointed out that our simulations are in good qualitative
agreement with their findings. Moreover, in our experiments, the sediment load
in the vicinity of the North American coast is lower during the LGM than today,
in agreement with the interpretation given by Boyle and Keigwin (1987) on the
basis of nutrient concentration analysis. The Caribbean water at depths just
105
The Lagrangian calculations help to elaborate the true three-dimensional water motion and therefore are the only means by which conveyor modes can be
genuinely visualized. Moreover, the trajectory-tracing technique may indicate
whether specific parts of the global ocean are interconnected via the deep conveyor branches, or essentially isolated. For example, the northern NA during the
MWE is characterized by very old nonventilated intermediate-to-deep water
(Sarnthein et al. 1995). Our calculations may shed some additional light on this
problem. If the MWE water in the NA was a mixture of the AABW, originating in
the Weddell Sea and some water sinking in the eastern Indian ocean, it may explain extreme aging of the Atlantic water, stronger than it would be if only the
AABW ventilated the NA areas.
A combined circulation/sedimentation/particle tracing modeling approach
was employed to understand particular aspects of the glacial-interglacial change
of the North Atlantic and World Ocean circulation which are difficult to address
using single-component models. Primary among those are the ventilation regimes, including the characterization of vertical mixing in convective chimneys
and the advective ages of ventilated water. Circulation studies might focus on the
NADW outflow or deep ocean circulation routing. Most of these questions are
traditionally addressed using geochemical tracers such as 8 18 0, 8l3c, and ,:1 14 C,
which are extremely useful in ocean circulation studies and hence widely employed. However, the tracers alone cannot provide sufficient constraints over a
simulated past circulation, as has been shown recently by LeGrand and Wunsch
(1995). In their study they showed that there exist an infinite number of states
that would satisfy a tracer distribution aimed at constraining the circulation, at
least for the currently available proxy data sets.
Another problem arising in paleoceanographic investigations is the parallel
analysis of both surface and benthic habitats and/or sediment transport features. Commonly, assumed features of a water flow thought to be suitable for explaining a particular distribution of proxy data are largely based on speculation.
There is no guarantee that this hypothetical flow would satisfy hydrodynamic restrictions posed over the ocean by the wind stress, ocean geometry, bottom
morphology, and sea-surface heat and fresh waterfluxes.
Based on only partly known SSS, recently corrected SST, and simulated glacial
wind stress, our results largely conform to current ideas about ventilation and
overturn in the North Atlantic at the height of the last glaciation and during the
subsequent meltwater event near 13 500 14C years B.P. The sediment transport
model output contains features that agree well with interpretation of sediment
data in the northern North Atlantic. In Seidov and Haupt (1997) we discussed
the LGM sedimentation pattern resulting from our simulation in comparison
with the observed or inferred data of Cremer et al. (1993) and McCave and Tucholke (1986). It was pointed out that our simulations are in good qualitative
agreement with their findings. Moreover, in our experiments, the sediment load
in the vicinity of the North American coast is lower during the LGM than today,
in agreement with the interpretation given by Boyle and Keigwin (1987) on the
basis of nutrient concentration analysis. The Caribbean water at depths just