86
D. Seidov . B. J. Haupt
5
North Atlantic Sediment Transport and Water Motion
5.1
OGCM Results
Here, we briefly review our regional NA modeling. More details can be found in
Seidovet al. (l996) and Seidov and Haupt (l997). The meridional overturning
stream function (Fig. 1) which gives the general impression of the overall thermohaline meridional circulation, conforms to a simple scheme that has emerged
from numerous computer simulations of differing complexity (e.g., England
1993; Toggweiler et al. 1989; Maier-Reimer et al. 1991; Wright and Stocker 1991;
Fichefet et al. 1994; Rahmstorf 1994; Sakai and Peltier 1995; Manabe and Stouffer
1995). Today's forward or clockwise (as seen from the eastern boundary looking
west through a vertical plane) gyre of the salinity conveyor occupies most of the
ocean from the surface-subsurface layers to a depth of 3 km. This gyre conveys
NADW and is thought of as the main wheel of the modern ocean climate. A
much weaker abyssal reverse (counterclockwise) gyre conveys AABW (largely in
the deepest layer, i.e., below 3 km).lts return (southward) flow joins the southward flow of NADW between 3 and 4 km. In addition, there is a weak, shallow,
wind-driven reverse (counterclockwise) gyre in the mid to high latitudes, induced by Ekman convergence in the subtropics.
Note that although glacial NADW production is 8 Sv, much lower than today's
value of 13 Sv, the intensity of the glacial conveyor is comparable to that of modern times. If we quantify the southward transport at 30 0 N, the Upper NADW outflow, or forward conveyor branch, does indeed rise to only 8 Sv (also see the inventory of the glacial water masses given by, e.g., Oppo and Lehman 1993 and
Oppo et al.1995). However, if we take into account the deep reverse branch comprising AABW and its mixture with lower NADW still produced in the central
northern Atlantic (see below in the discussion of convection patterns), the total
transport amounts to 12 Sv, which gives almost the same intensity of the deep
water outflow as the present-day North Atlantic combined deep outflow. The intensity of this NADW-depleted conveyor, similar to present-day conveyors' intensity, seems to fit well the recent finding based on proxy data analysis (Yu et al.
1996). However, the modern overturning strength is too low. A slight cooling of
the sea-surface to the southeast of Greenland and in the Nordic Seas may easily
double the NADW production (see Sec. 5.2). This is because the sea-surface density taken as averages of summer and winter values is somewhat biased toward
summer values, and therefore the convection is not deep enough to form the vigorous observed present-day overturning. Yet we may consider the numbers as a
first approximation, keeping in mind that the glacial conveyor was probably relatively even weaker than our regional modeling indicates. Hence, we believe that
the glacial conveyor was probably still noticeably weaker than the modern one,
in contrast to what is suggested byYu et al. (l996). The global circulation modeling with sea-surface cooled down by 2°C to the east of Greenland between 60°
D. Seidov . B. J. Haupt
5
North Atlantic Sediment Transport and Water Motion
5.1
OGCM Results
Here, we briefly review our regional NA modeling. More details can be found in
Seidovet al. (l996) and Seidov and Haupt (l997). The meridional overturning
stream function (Fig. 1) which gives the general impression of the overall thermohaline meridional circulation, conforms to a simple scheme that has emerged
from numerous computer simulations of differing complexity (e.g., England
1993; Toggweiler et al. 1989; Maier-Reimer et al. 1991; Wright and Stocker 1991;
Fichefet et al. 1994; Rahmstorf 1994; Sakai and Peltier 1995; Manabe and Stouffer
1995). Today's forward or clockwise (as seen from the eastern boundary looking
west through a vertical plane) gyre of the salinity conveyor occupies most of the
ocean from the surface-subsurface layers to a depth of 3 km. This gyre conveys
NADW and is thought of as the main wheel of the modern ocean climate. A
much weaker abyssal reverse (counterclockwise) gyre conveys AABW (largely in
the deepest layer, i.e., below 3 km).lts return (southward) flow joins the southward flow of NADW between 3 and 4 km. In addition, there is a weak, shallow,
wind-driven reverse (counterclockwise) gyre in the mid to high latitudes, induced by Ekman convergence in the subtropics.
Note that although glacial NADW production is 8 Sv, much lower than today's
value of 13 Sv, the intensity of the glacial conveyor is comparable to that of modern times. If we quantify the southward transport at 30 0 N, the Upper NADW outflow, or forward conveyor branch, does indeed rise to only 8 Sv (also see the inventory of the glacial water masses given by, e.g., Oppo and Lehman 1993 and
Oppo et al.1995). However, if we take into account the deep reverse branch comprising AABW and its mixture with lower NADW still produced in the central
northern Atlantic (see below in the discussion of convection patterns), the total
transport amounts to 12 Sv, which gives almost the same intensity of the deep
water outflow as the present-day North Atlantic combined deep outflow. The intensity of this NADW-depleted conveyor, similar to present-day conveyors' intensity, seems to fit well the recent finding based on proxy data analysis (Yu et al.
1996). However, the modern overturning strength is too low. A slight cooling of
the sea-surface to the southeast of Greenland and in the Nordic Seas may easily
double the NADW production (see Sec. 5.2). This is because the sea-surface density taken as averages of summer and winter values is somewhat biased toward
summer values, and therefore the convection is not deep enough to form the vigorous observed present-day overturning. Yet we may consider the numbers as a
first approximation, keeping in mind that the glacial conveyor was probably relatively even weaker than our regional modeling indicates. Hence, we believe that
the glacial conveyor was probably still noticeably weaker than the modern one,
in contrast to what is suggested byYu et al. (l996). The global circulation modeling with sea-surface cooled down by 2°C to the east of Greenland between 60°