Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
81
sity of such freshwater fluxes may become the most important paleoceanographic information in computerized studies of ancient ocean conveyors. In essence, the high-latitude SSS data become the major unknown and their absence
would jeopardize any systematic use of an OGCM in ocean paleoclimate studies.
The situation is not so hopeless, however. The conveyor operation is thought
to be hampered by suppression of the deep convection in the northern NA and
in the Nordic Seas because of major meltwater discharges. Fortunately, these areas are relatively well covered by proxy data and corresponding paleoreconstructions which provide both SST and SSS for LGM and a subsequent meltwater
event near 13 000 l4C years here abbreviated as MWE. Based on these proxies for
the LGM and MWE time slices, SST and SSS to the north of 400N were reconstructed (Duplessy et al. 1988; Sarnthein et al.1995). These reconstructions provided a basis for setting up thermohaline sea-surface conditions on a lOx 1 ° regular grid in the entire NA north of lOON (Seidov et al.1996). Such boundary conditions were compiled for both LGM and MWE. Seidov et al. (l996) used these
data in their simulations of the NA circulation during these two time slices. To
compare the emerging circulation patterns to the present-day circulation, a control (modern) run was also carried out. In the control run modern sea-surface
climatology from Levitus (l982) was used. The control run is henceforth referred to as the Holocene/Modern (HM) experiment.
To simulate the ocean circulation one needs not only the thermohaline seasurface forcing, but the wind stress data as well. The wind stresses for LGM and
for the control run were extracted from the output of the Hamburg atmosphere
general circulation model (Lautenschlager and Herterich 1991; Lorenz et al.
1996). At LGM this model is driven by the CLIMAP (l981) SST, whereas the
modern run was driven by present-day sea-surface climatology. The MWE
wind was assumed to be the same as at LGM.
Thermohaline sea-surface conditions and the wind-stress distributions comprise the necessary boundary conditions allowing simulation of the past ocean
annual mean circulation. Table 1 gives the sources of the data used to compile
the regional NA regular arrays of boundary conditions.
Based on these regional NA data, global distributions of SST from CLIMAP
(l981) were updated in the northern NA and the Nordic Seas to form LGM and
MWE SST distributions. Present-day sea-surface salinity was increased everywhere by 0.8 psu (to compromise between the upper limit estimate of 1 psu and
smooth connection of the low and high latitudes in the NA; see Seidov et al.
(l996) and the regional NA LGM SSS replaced the modified SSS to the north of
400N to form the global LGM and MWE SSS distribution. Modern mean annual
SST and SSS are specified from new ocean climatological data sets (Levitus and
Boyer 1994; Levitus et al. 1994), except for the area south of Greenland where
some additional cooling of up to 2°C mimicking cold spells from Greenland was
needed to obtain present -day annual NADW production.
81
sity of such freshwater fluxes may become the most important paleoceanographic information in computerized studies of ancient ocean conveyors. In essence, the high-latitude SSS data become the major unknown and their absence
would jeopardize any systematic use of an OGCM in ocean paleoclimate studies.
The situation is not so hopeless, however. The conveyor operation is thought
to be hampered by suppression of the deep convection in the northern NA and
in the Nordic Seas because of major meltwater discharges. Fortunately, these areas are relatively well covered by proxy data and corresponding paleoreconstructions which provide both SST and SSS for LGM and a subsequent meltwater
event near 13 000 l4C years here abbreviated as MWE. Based on these proxies for
the LGM and MWE time slices, SST and SSS to the north of 400N were reconstructed (Duplessy et al. 1988; Sarnthein et al.1995). These reconstructions provided a basis for setting up thermohaline sea-surface conditions on a lOx 1 ° regular grid in the entire NA north of lOON (Seidov et al.1996). Such boundary conditions were compiled for both LGM and MWE. Seidov et al. (l996) used these
data in their simulations of the NA circulation during these two time slices. To
compare the emerging circulation patterns to the present-day circulation, a control (modern) run was also carried out. In the control run modern sea-surface
climatology from Levitus (l982) was used. The control run is henceforth referred to as the Holocene/Modern (HM) experiment.
To simulate the ocean circulation one needs not only the thermohaline seasurface forcing, but the wind stress data as well. The wind stresses for LGM and
for the control run were extracted from the output of the Hamburg atmosphere
general circulation model (Lautenschlager and Herterich 1991; Lorenz et al.
1996). At LGM this model is driven by the CLIMAP (l981) SST, whereas the
modern run was driven by present-day sea-surface climatology. The MWE
wind was assumed to be the same as at LGM.
Thermohaline sea-surface conditions and the wind-stress distributions comprise the necessary boundary conditions allowing simulation of the past ocean
annual mean circulation. Table 1 gives the sources of the data used to compile
the regional NA regular arrays of boundary conditions.
Based on these regional NA data, global distributions of SST from CLIMAP
(l981) were updated in the northern NA and the Nordic Seas to form LGM and
MWE SST distributions. Present-day sea-surface salinity was increased everywhere by 0.8 psu (to compromise between the upper limit estimate of 1 psu and
smooth connection of the low and high latitudes in the NA; see Seidov et al.
(l996) and the regional NA LGM SSS replaced the modified SSS to the north of
400N to form the global LGM and MWE SSS distribution. Modern mean annual
SST and SSS are specified from new ocean climatological data sets (Levitus and
Boyer 1994; Levitus et al. 1994), except for the area south of Greenland where
some additional cooling of up to 2°C mimicking cold spells from Greenland was
needed to obtain present -day annual NADW production.