104
D. Seidov . B. J. Haupt
northern Pacific. Hence, this study indicates that only a tiny fraction of the
NADW can physically travel along the whole leg of the deep conveyor. However,
it does not mean that the NADW water cannot reach the Pacific in significant
amounts. One must consider the duration of the process which can be of several
thousand years of a stable modern-like conveyor. Some additional calculations
that combine the Lagrangian technique with incorporation of geochemical tracers are required to quantify the total amount of NADW reaching the northern
Pacific at the slow rate indicated by our experiments (no more than 1-2 Sv in the
model).
In spite of uncertainties in our present -day calculations due to a rather coarse
horizontal resolution and a somewhat incomplete glacial and meltwater sea-surface climatology, the MWE curtailment of the conveyor and emergence of the reversed deep conveyor is a very robust feature. In many additional experiments
with additional disturbances of MWE sea-surface conditions, a switched-off or
even reversed NA conveyor was a permanent feature. All attempts to find a trajectory originating somewhere in the NA and continuing into at least the Indian
Ocean have failed. On the contrary, the particles deployed at the surface to the
southwest of Australia travel far into the South Atlantic. A tiny fraction of the
deployed ensemble was found north of the equator. However, no particles
managed to pass into the northern NA. Hence, although there was indeed a
strong incursion of the AABW into the NA, it upwelled mostly to the south of
SOoN, which agrees well with the results of Seidov et al. (1996), who show isopycnal outcrop to the south of SOON. Such a southward-shifted density outcrop
isolated the northern NA from the rest of the World Ocean. Hence, the main
driving mechanism of the deep ocean circulation during MWE was restricted
to the Southern Ocean.
7
Discussion and Conclusions
The primary task of our chapter is to demonstrate that the major meltwater
events in the NA might have affected the deep ocean branch of the global conveyor very substantially and that these changes can be clearly and unambiguously traced in Lagrangian calculations. The characteristics of ocean circulation, such as the ventilation of the deep ocean, sedimentation transport, water
parcel motion, meridional overturning and potential vorticity analysis, all indicate a weakening and some shallowing of the main conveyor at the LGM.
These characteristics indicate a complete collapse after the very localized
freshwater discharge at the MWE occurred. This may be instrumental for understanding and predicting global climate change on the basis of the top-analogue examples given by the major deglaciations. Our results also demonstrate, not only in an idealized sensitivity simulation, but based on numerous
proxy data, that the deep ocean circulation is indeed sensitive to localized
high-latitudinal forcing that might be able to destroy water convection in the
World Ocean.
D. Seidov . B. J. Haupt
northern Pacific. Hence, this study indicates that only a tiny fraction of the
NADW can physically travel along the whole leg of the deep conveyor. However,
it does not mean that the NADW water cannot reach the Pacific in significant
amounts. One must consider the duration of the process which can be of several
thousand years of a stable modern-like conveyor. Some additional calculations
that combine the Lagrangian technique with incorporation of geochemical tracers are required to quantify the total amount of NADW reaching the northern
Pacific at the slow rate indicated by our experiments (no more than 1-2 Sv in the
model).
In spite of uncertainties in our present -day calculations due to a rather coarse
horizontal resolution and a somewhat incomplete glacial and meltwater sea-surface climatology, the MWE curtailment of the conveyor and emergence of the reversed deep conveyor is a very robust feature. In many additional experiments
with additional disturbances of MWE sea-surface conditions, a switched-off or
even reversed NA conveyor was a permanent feature. All attempts to find a trajectory originating somewhere in the NA and continuing into at least the Indian
Ocean have failed. On the contrary, the particles deployed at the surface to the
southwest of Australia travel far into the South Atlantic. A tiny fraction of the
deployed ensemble was found north of the equator. However, no particles
managed to pass into the northern NA. Hence, although there was indeed a
strong incursion of the AABW into the NA, it upwelled mostly to the south of
SOoN, which agrees well with the results of Seidov et al. (1996), who show isopycnal outcrop to the south of SOON. Such a southward-shifted density outcrop
isolated the northern NA from the rest of the World Ocean. Hence, the main
driving mechanism of the deep ocean circulation during MWE was restricted
to the Southern Ocean.
7
Discussion and Conclusions
The primary task of our chapter is to demonstrate that the major meltwater
events in the NA might have affected the deep ocean branch of the global conveyor very substantially and that these changes can be clearly and unambiguously traced in Lagrangian calculations. The characteristics of ocean circulation, such as the ventilation of the deep ocean, sedimentation transport, water
parcel motion, meridional overturning and potential vorticity analysis, all indicate a weakening and some shallowing of the main conveyor at the LGM.
These characteristics indicate a complete collapse after the very localized
freshwater discharge at the MWE occurred. This may be instrumental for understanding and predicting global climate change on the basis of the top-analogue examples given by the major deglaciations. Our results also demonstrate, not only in an idealized sensitivity simulation, but based on numerous
proxy data, that the deep ocean circulation is indeed sensitive to localized
high-latitudinal forcing that might be able to destroy water convection in the
World Ocean.