Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
85
priate boundary conditions. Integration in time was continued for several thousand years in the regional and extended over 10000 years in the global runs to
gain absolutely stationary solutions. Then for both modern and LGM conditions, the steady-state current velocities were used to calculate the trajectories of
the particles that move with the water-volumes. The MWE velocity field is the result of integration over only 500 years from the LGM steady-state with the LGM
sea-surface conditions replaced by the MWE ones (the duration of this meltwater event is estimated to be of several hundreds to a thousand years; Sarnthein et
al. 1995).
The regional NA study includes modeling of sediment transport, that is, these
simulations comprise a threefold approach. The global ocean circulation modeling at this stage employs only the OGCM and trajectory tracing models, and
therefore forms a twofold approach to water transport and ventilation problem.
For the regional NA studies we used a 2° x 2° grid with 12 vertical levels. In
view of uncertainties of the past sea-surface conditions, and in order to facilitate
multiple runs extended over 10 years with different parameter settings, we employed a coarse resolution of 6° x 4° in longitude and latitude respectively with
12 vertical levels. Although all major currents, except for the largely wind-driven
Antarctic Circumpolar Current (ACC), are two to three times weaker than the
observed ones, most of these currents are still clearly seen on the vector maps.
At the same time, the meridional thermohaline overturning, which largely depends on the deep convection and isopycnal outcrop, is modeled far better than
the horizontal flows. For the present-day overturning we have obtained a value
that agrees well with experiments of much finer resolution. The overturning in
the NA, which comprises the NADW production, amounts to 23 Sv (1 Sv=10 6
m 3 s- 1 ), for comparison Antarctic Bottom Water (AABW) inflow into the Atlantic
Ocean is about 9 Sv. Since we are mainly interested in the major changes in conveyor operation and rely only on the relative changes of the conveyor intensity,
the coarse resolution is legitimate for global paleocirculation studies such as
that presented here. We therefore consider it to be sufficient for the semi-Lagrangian simulations which are the core of our investigation.
In contrast to sensitivity studies which operate with so-called mixed boundary conditions (e.g., Manabe and Stouffer 1994; Rahmstorf 1995), we restore the
upper-layer thermohaline fields to the specified sea-surface temperature (SST)
and sea-surface salinity (SSS). The sea-surface boundary conditions are described the in previous section. In our study, the important reason for the chosen approach is the local nature of the freshwater driving. Indeed, the LGM seasurface conditions were disturbed by the meltwater invasion only in the small
area in the northern NA and Norwegian-Greenland Seas where reliable proxy
data are available (see above). This restoring technique implies that the freshwater fluxes were those that maintained the reconstructed SSS in these areas. In
fact, we merely diagnose the circulation regimes which would satisfy the observed sea-surface conditions.
85
priate boundary conditions. Integration in time was continued for several thousand years in the regional and extended over 10000 years in the global runs to
gain absolutely stationary solutions. Then for both modern and LGM conditions, the steady-state current velocities were used to calculate the trajectories of
the particles that move with the water-volumes. The MWE velocity field is the result of integration over only 500 years from the LGM steady-state with the LGM
sea-surface conditions replaced by the MWE ones (the duration of this meltwater event is estimated to be of several hundreds to a thousand years; Sarnthein et
al. 1995).
The regional NA study includes modeling of sediment transport, that is, these
simulations comprise a threefold approach. The global ocean circulation modeling at this stage employs only the OGCM and trajectory tracing models, and
therefore forms a twofold approach to water transport and ventilation problem.
For the regional NA studies we used a 2° x 2° grid with 12 vertical levels. In
view of uncertainties of the past sea-surface conditions, and in order to facilitate
multiple runs extended over 10 years with different parameter settings, we employed a coarse resolution of 6° x 4° in longitude and latitude respectively with
12 vertical levels. Although all major currents, except for the largely wind-driven
Antarctic Circumpolar Current (ACC), are two to three times weaker than the
observed ones, most of these currents are still clearly seen on the vector maps.
At the same time, the meridional thermohaline overturning, which largely depends on the deep convection and isopycnal outcrop, is modeled far better than
the horizontal flows. For the present-day overturning we have obtained a value
that agrees well with experiments of much finer resolution. The overturning in
the NA, which comprises the NADW production, amounts to 23 Sv (1 Sv=10 6
m 3 s- 1 ), for comparison Antarctic Bottom Water (AABW) inflow into the Atlantic
Ocean is about 9 Sv. Since we are mainly interested in the major changes in conveyor operation and rely only on the relative changes of the conveyor intensity,
the coarse resolution is legitimate for global paleocirculation studies such as
that presented here. We therefore consider it to be sufficient for the semi-Lagrangian simulations which are the core of our investigation.
In contrast to sensitivity studies which operate with so-called mixed boundary conditions (e.g., Manabe and Stouffer 1994; Rahmstorf 1995), we restore the
upper-layer thermohaline fields to the specified sea-surface temperature (SST)
and sea-surface salinity (SSS). The sea-surface boundary conditions are described the in previous section. In our study, the important reason for the chosen approach is the local nature of the freshwater driving. Indeed, the LGM seasurface conditions were disturbed by the meltwater invasion only in the small
area in the northern NA and Norwegian-Greenland Seas where reliable proxy
data are available (see above). This restoring technique implies that the freshwater fluxes were those that maintained the reconstructed SSS in these areas. In
fact, we merely diagnose the circulation regimes which would satisfy the observed sea-surface conditions.