diapycnal mixing throughout much of the water
column is enhanced over rough topography
(Polzin et al., 1997; Ledwell et al., 2000). Diapycnal mixing may also be enhanced near the boundaries of the ocean (Wunsch, 1970; Armi, 1978).
To date there are few direct measurements of
diapycnal mixing, and so it is not yet clear if (nonuniform) interior mixing will prove to be sufficient
to balance the sinking at high latitude.
An alternative view is that the required conversion of dense to light water occurs primarily in the
Southern Ocean, where deep isopycnals outcrop
and are exposed to air–sea fluxes of heat and fresh
water (Döös and Coward, 1997; Toggweiler and
Samuels, 1998). Here we reconsider the meridional overturning in the Southern Ocean, this time
from the perspective of its connection to the rest of
the world ocean.
As explained in the introduction, an active overturning circulation in the Southern Ocean was
inferred by early investigators on the basis of the
prominent core layers (Figs 4.6.2 and 4.6.3) which
extend across the ACC (e.g. Sverdrup, 1933).
Deep water spreads south and upward across the
ACC, and is balanced by sinking and northward
flow of both lighter intermediate water and denser
bottom water. This picture is broadly consistent
with the requirements of closure of the NADW
overturning – import of NADW to the Southern
Ocean is balanced by export of IW and BW, as
observed in the South Atlantic (Rintoul, 1991;
Saunders and King, 1995b) – and suggests that the
water mass conversions taking place in the Southern Ocean are a key element of the overall cell.
The ‘intermediate’ cell (conversion of DW to IW)
is consistent with northward Ekman transport driven by the strong westerly winds: divergent Ekman
transport drives upwelling of deep water, which in
steady state must be converted to lighter water by
buoyancy input from the atmosphere as it is driven
north across mean density contours (Fig. 4.6.1;
Toole, 1981; Speer et al., 2000).
However, in the ensuing 60 years, few attempts
have been made to quantify the flow paths in Sverdrup’s diagram. One recent exception is Schmitz’s
(1995, 1996a,b) attempts to synthesize global view
from a large number of published estimates for
individual branches of the overturning circulation.
Summing his estimates for the three individual
basins at about 40°S, the overturning in the
Southern Ocean consists of 53 Sv of deep water
(NADW/UCDW) flowing south, balanced by 48 Sv
of bottom water (AABW/LCDW) and 5 Sv of intermediate water (SAMW/AAIW) flowing north.
Schmitz’s summary of ‘best guess’ values from the
literature suggests that in the zonal integral the deep
cell (DW to BW) is much stronger than the intermediate cell (DW to IW) of Sverdrup’s diagram.
Schmitz’s circulation scheme is derived from a
number of individual estimates, which may not be
internally consistent. His results are, however, very
similar to estimates from a recent Southern Ocean
inverse model, which provides an internally consistent solution that explicitly includes air–sea buoyancy forcing and diapycnal mixing (Sloyan and
Rintoul, 2001b). In the zonal integral across
roughly 30°S they find 52 Sv of deep water flowing
south, balanced by 46 Sv of lower deep and bottom
water and 6 Sv of intermediate water flowing north
(Fig. 4.6.14; see caption for layer definitions).
These results suggest the deep overturning cell,
in which dense AABW/LCDW exported to the
Indian and Pacific is balanced by import of slightly
less dense DW, dominates the overall Southern
Ocean overturning. Note that the contribution of
NADW to the zonally integrated overturning is
small by comparison: for example, the 10 Sv poleward flow of lower NADW (neutral density
between 28.0 and 28.2) is more than compensated
by strong equatorward flow in this density class in
the Indian and Pacific. The intermediate cell (conversion of DW to IW) is weak in the zonal integral
at 40°S (although the gross exchanges in this density class are large, as described in Section 4.6.4.1).
The observations of flow entering and leaving
the Southern Ocean across 30–40°S imply significant poleward transport in density layers shallower than the Drake Passage sill (neutral density
of 27.4–28.0 kg m
93 ). In addition, the net northward transport of light water is much smaller than
the Ekman transport, suggesting that much of the
Ekman transport returns poleward at similar density south of 40°S. Substantial poleward transport
in density layers not blocked by topography
implies divergence of the interfacial form stress, or
equivalently, the eddy buoyancy flux (Section
4.6.3). Speer et al. (2000) show that there are
strong gradients in isopycnal thickness (or potential vorticity) across the ACC in the UCDW layer;
eddy mixing will therefore tend to smooth out the
gradient, resulting in a volume flux to the south.
Meridional gradients of isopycnal thickness are
SECTION 4 THE GLOBAL FLOW FIELD
298
column is enhanced over rough topography
(Polzin et al., 1997; Ledwell et al., 2000). Diapycnal mixing may also be enhanced near the boundaries of the ocean (Wunsch, 1970; Armi, 1978).
To date there are few direct measurements of
diapycnal mixing, and so it is not yet clear if (nonuniform) interior mixing will prove to be sufficient
to balance the sinking at high latitude.
An alternative view is that the required conversion of dense to light water occurs primarily in the
Southern Ocean, where deep isopycnals outcrop
and are exposed to air–sea fluxes of heat and fresh
water (Döös and Coward, 1997; Toggweiler and
Samuels, 1998). Here we reconsider the meridional overturning in the Southern Ocean, this time
from the perspective of its connection to the rest of
the world ocean.
As explained in the introduction, an active overturning circulation in the Southern Ocean was
inferred by early investigators on the basis of the
prominent core layers (Figs 4.6.2 and 4.6.3) which
extend across the ACC (e.g. Sverdrup, 1933).
Deep water spreads south and upward across the
ACC, and is balanced by sinking and northward
flow of both lighter intermediate water and denser
bottom water. This picture is broadly consistent
with the requirements of closure of the NADW
overturning – import of NADW to the Southern
Ocean is balanced by export of IW and BW, as
observed in the South Atlantic (Rintoul, 1991;
Saunders and King, 1995b) – and suggests that the
water mass conversions taking place in the Southern Ocean are a key element of the overall cell.
The ‘intermediate’ cell (conversion of DW to IW)
is consistent with northward Ekman transport driven by the strong westerly winds: divergent Ekman
transport drives upwelling of deep water, which in
steady state must be converted to lighter water by
buoyancy input from the atmosphere as it is driven
north across mean density contours (Fig. 4.6.1;
Toole, 1981; Speer et al., 2000).
However, in the ensuing 60 years, few attempts
have been made to quantify the flow paths in Sverdrup’s diagram. One recent exception is Schmitz’s
(1995, 1996a,b) attempts to synthesize global view
from a large number of published estimates for
individual branches of the overturning circulation.
Summing his estimates for the three individual
basins at about 40°S, the overturning in the
Southern Ocean consists of 53 Sv of deep water
(NADW/UCDW) flowing south, balanced by 48 Sv
of bottom water (AABW/LCDW) and 5 Sv of intermediate water (SAMW/AAIW) flowing north.
Schmitz’s summary of ‘best guess’ values from the
literature suggests that in the zonal integral the deep
cell (DW to BW) is much stronger than the intermediate cell (DW to IW) of Sverdrup’s diagram.
Schmitz’s circulation scheme is derived from a
number of individual estimates, which may not be
internally consistent. His results are, however, very
similar to estimates from a recent Southern Ocean
inverse model, which provides an internally consistent solution that explicitly includes air–sea buoyancy forcing and diapycnal mixing (Sloyan and
Rintoul, 2001b). In the zonal integral across
roughly 30°S they find 52 Sv of deep water flowing
south, balanced by 46 Sv of lower deep and bottom
water and 6 Sv of intermediate water flowing north
(Fig. 4.6.14; see caption for layer definitions).
These results suggest the deep overturning cell,
in which dense AABW/LCDW exported to the
Indian and Pacific is balanced by import of slightly
less dense DW, dominates the overall Southern
Ocean overturning. Note that the contribution of
NADW to the zonally integrated overturning is
small by comparison: for example, the 10 Sv poleward flow of lower NADW (neutral density
between 28.0 and 28.2) is more than compensated
by strong equatorward flow in this density class in
the Indian and Pacific. The intermediate cell (conversion of DW to IW) is weak in the zonal integral
at 40°S (although the gross exchanges in this density class are large, as described in Section 4.6.4.1).
The observations of flow entering and leaving
the Southern Ocean across 30–40°S imply significant poleward transport in density layers shallower than the Drake Passage sill (neutral density
of 27.4–28.0 kg m
93 ). In addition, the net northward transport of light water is much smaller than
the Ekman transport, suggesting that much of the
Ekman transport returns poleward at similar density south of 40°S. Substantial poleward transport
in density layers not blocked by topography
implies divergence of the interfacial form stress, or
equivalently, the eddy buoyancy flux (Section
4.6.3). Speer et al. (2000) show that there are
strong gradients in isopycnal thickness (or potential vorticity) across the ACC in the UCDW layer;
eddy mixing will therefore tend to smooth out the
gradient, resulting in a volume flux to the south.
Meridional gradients of isopycnal thickness are
SECTION 4 THE GLOBAL FLOW FIELD
298