Numerical Study of Glacial and Meltwater Global Ocean Thermohaline Conveyor
107
ter. Deeper glacial ventilation, evident in Fig. 3b, implies that the base of thermocline had to rise upward because water pumped into the thermocline was
more than 4 °C colder at the glacial ventilation points (westward boundary outflow region; compare Fig. 3a and b). To quantify this effect, the difference between to day's and the LGM simulated temperature is shown in a meridional section of the temperature anomaly field in Fig. 8 in Seidov and Haupt (1997). In essence, Fig. 8 displays a penetration of cold anomalies deep into the thermocline.
It is clear, however, that this penetration is limited to the upper 1 km and that the
lower 500 m of this layer is more strongly cooled than the upper 500 m. This implies that the thermocline base has been raised. A comparison of the temperature profiles in the subtropical gyres indicates that on average the thermocline
depth was about 200 m shallower than today, which differs from the estimates of
Slowey and Curry (1995). There may be several reasons for this discrepancy.
Firstly, the subtropical gyre is dangerously close to the sponge layer at the southern wall, which might have distorted the behavior of the thermocline base. Second, we use the annual mean surface forcing and, therefore, we are unable to
simulate an extreme winter southward migration of the density outcrop intersecting the Ekman pumping within the subtropical gyre. Nevertheless, the general tendencies sketched in Fig. 1 of Slowey and Curry (1995, p. 717) are evident
in our trajectory maps (Fig. 3b as compared to Fig. 3a). The particles dive deeper
into the thermocline and stay there longer. The northern limit of the subtropical
gyre, marked by the shallow convection along the North Atlantic Drift, is shifted
noticeably southward.
As some findings have indicated (e.g., Sarnthein et al. 1995), the Nordic Seas
were more isolated from the northern North Atlantic at the LGM than today.
This is strongly supported by our Lagrangian calculations. As Fig. 7 displays, the
water from the subtropics probably had a very limited chance to enter the Nordic
Seas, largely because the outflow over the Iceland-Greenland Ridge was reduced
during the LGM.
Based on a threefold numerical simulation of the North Atlantic circulation
and sedimentation and twofold simulation of the World Ocean water transport
we draw the following conclusions:
1. present-day and LGM meridional thermohaline circulation is characterized by
a forward global deep ocean conveyor. At MWE the freshwater fluxes that maintain the "observed" high latitudinal sea-surface salinity appear to be sufficient
to suppress this forward conveyor. A reversed deep ocean conveyor replaced the
forward one on the whole distance from NA to the southeast Indian Ocean.
2. The convection regime is the most crucial process for the conveyor dynamics.
At MWE the bimodal convection regime, with both NADW and AADW sources, was replaced by the regime with only southern deep water source driving
the conveyor. As a result, a reversed conveyor emerged in the Indian-Atlantic
sector of the Southern Ocean. The meltwater North Atlantic was essentially
isolated from the other parts of the World Ocean.
3. Sedimentation in the North Atlantic is non-linearly coupled to the circulation
modes associated with different surface climatology. The LGM and MWE sed-
107
ter. Deeper glacial ventilation, evident in Fig. 3b, implies that the base of thermocline had to rise upward because water pumped into the thermocline was
more than 4 °C colder at the glacial ventilation points (westward boundary outflow region; compare Fig. 3a and b). To quantify this effect, the difference between to day's and the LGM simulated temperature is shown in a meridional section of the temperature anomaly field in Fig. 8 in Seidov and Haupt (1997). In essence, Fig. 8 displays a penetration of cold anomalies deep into the thermocline.
It is clear, however, that this penetration is limited to the upper 1 km and that the
lower 500 m of this layer is more strongly cooled than the upper 500 m. This implies that the thermocline base has been raised. A comparison of the temperature profiles in the subtropical gyres indicates that on average the thermocline
depth was about 200 m shallower than today, which differs from the estimates of
Slowey and Curry (1995). There may be several reasons for this discrepancy.
Firstly, the subtropical gyre is dangerously close to the sponge layer at the southern wall, which might have distorted the behavior of the thermocline base. Second, we use the annual mean surface forcing and, therefore, we are unable to
simulate an extreme winter southward migration of the density outcrop intersecting the Ekman pumping within the subtropical gyre. Nevertheless, the general tendencies sketched in Fig. 1 of Slowey and Curry (1995, p. 717) are evident
in our trajectory maps (Fig. 3b as compared to Fig. 3a). The particles dive deeper
into the thermocline and stay there longer. The northern limit of the subtropical
gyre, marked by the shallow convection along the North Atlantic Drift, is shifted
noticeably southward.
As some findings have indicated (e.g., Sarnthein et al. 1995), the Nordic Seas
were more isolated from the northern North Atlantic at the LGM than today.
This is strongly supported by our Lagrangian calculations. As Fig. 7 displays, the
water from the subtropics probably had a very limited chance to enter the Nordic
Seas, largely because the outflow over the Iceland-Greenland Ridge was reduced
during the LGM.
Based on a threefold numerical simulation of the North Atlantic circulation
and sedimentation and twofold simulation of the World Ocean water transport
we draw the following conclusions:
1. present-day and LGM meridional thermohaline circulation is characterized by
a forward global deep ocean conveyor. At MWE the freshwater fluxes that maintain the "observed" high latitudinal sea-surface salinity appear to be sufficient
to suppress this forward conveyor. A reversed deep ocean conveyor replaced the
forward one on the whole distance from NA to the southeast Indian Ocean.
2. The convection regime is the most crucial process for the conveyor dynamics.
At MWE the bimodal convection regime, with both NADW and AADW sources, was replaced by the regime with only southern deep water source driving
the conveyor. As a result, a reversed conveyor emerged in the Indian-Atlantic
sector of the Southern Ocean. The meltwater North Atlantic was essentially
isolated from the other parts of the World Ocean.
3. Sedimentation in the North Atlantic is non-linearly coupled to the circulation
modes associated with different surface climatology. The LGM and MWE sed-