North Atlantic, upwelling elsewhere in the ocean,
and a return flow of lighter water. The ACC carries NADW from the Atlantic to the other ocean
basins and so forms an important link in this circulation path. But the significance of the Southern
Ocean to the global overturning goes beyond the
passive role of redistributing NADW. Upwelling
and buoyancy forcing in the Southern Ocean drive
the water mass transformations required to close
the NADW cell, as well as several additional overturning circulations, both shallow and deep. The
nature of the overturning circulation is also intimately related to the dynamics of the ACC, as
described above.
To balance the sinking of NADW in the
North Atlantic, somewhere deep water must be
converted to lighter water. Given that no zones of
concentrated upwelling from the abyss to the thermocline have been observed, the traditional assumption is that the upwelling is more or less broadly
distributed in the ocean interior. This assumption,
together with conservation of potential vorticity,
has profound implications for the deep flow, as
illustrated in the simple and elegant theory of the
abyssal circulation of Stommel and Arons (1960a).
Direct measurements of mixing in the ocean
pycnocline, however, typically find diffusivities an
order of magnitude too low to support the
upwelling required to balance the sources of deep
water (Ledwell et al., 1993; Toole et al., 1994).
Radiocarbon distributions also argue against widespread upwelling of deep water through the
pycnocline (e.g. Toggweiler and Samuels, 1993).
On the other hand, recent measurements suggest
4.6 The Antarctic Circumpolar Current System
297
Rintoul, Hughes and Olbers
0
o
0
o
180
o
90
o W
40
o S
50
o S
90
o E
0 .0
5
>0.1
0.1
0.15
0.15
>0.4
0.4
0.4
0.3
0. 2
0. 2
0.4
0.2
< 0 .1
5
0 .1 5
0 .1
>0.2
0. 2
0 .5
0 .3
0.3
>3.5
0 .1 5
0 .1
0.0
5
0.6
0 .4
0 .5
0 .3 5
2
0 .8
0 .3
0 .2
2
0.6
0
.6
0
.8
0 .4
0.3
<0.05
0.2
0.15
0.1
0 .0 5
0 .0 3
0. 02
3
2
<1
1
> 0 .5
1
>1
1
0.5
3
>2
>
2
> 0. 7
1
>0 .5
0 .1
0 .0
5
1
AABW
CFC-11 (pmol kg
–1
)
Fig. 4.6.13 Inventory of CFC-11 in the AABW layer (between neutral density 28.27 kg m
3 and the seafloor), from
Orsi et al. (1999).The layer-mean CFC-11 concentrations in the Atlantic (Weddell–Enderby), Indian (Australian
Antarctic), and Pacific basins are 0.17, 0.47 and 0.18 pmol kg
91 , respectively.
and a return flow of lighter water. The ACC carries NADW from the Atlantic to the other ocean
basins and so forms an important link in this circulation path. But the significance of the Southern
Ocean to the global overturning goes beyond the
passive role of redistributing NADW. Upwelling
and buoyancy forcing in the Southern Ocean drive
the water mass transformations required to close
the NADW cell, as well as several additional overturning circulations, both shallow and deep. The
nature of the overturning circulation is also intimately related to the dynamics of the ACC, as
described above.
To balance the sinking of NADW in the
North Atlantic, somewhere deep water must be
converted to lighter water. Given that no zones of
concentrated upwelling from the abyss to the thermocline have been observed, the traditional assumption is that the upwelling is more or less broadly
distributed in the ocean interior. This assumption,
together with conservation of potential vorticity,
has profound implications for the deep flow, as
illustrated in the simple and elegant theory of the
abyssal circulation of Stommel and Arons (1960a).
Direct measurements of mixing in the ocean
pycnocline, however, typically find diffusivities an
order of magnitude too low to support the
upwelling required to balance the sources of deep
water (Ledwell et al., 1993; Toole et al., 1994).
Radiocarbon distributions also argue against widespread upwelling of deep water through the
pycnocline (e.g. Toggweiler and Samuels, 1993).
On the other hand, recent measurements suggest
4.6 The Antarctic Circumpolar Current System
297
Rintoul, Hughes and Olbers
0
o
0
o
180
o
90
o W
40
o S
50
o S
90
o E
0 .0
5
>0.1
0.1
0.15
0.15
>0.4
0.4
0.4
0.3
0. 2
0. 2
0.4
0.2
< 0 .1
5
0 .1 5
0 .1
>0.2
0. 2
0 .5
0 .3
0.3
>3.5
0 .1 5
0 .1
0.0
5
0.6
0 .4
0 .5
0 .3 5
2
0 .8
0 .3
0 .2
2
0.6
0
.6
0
.8
0 .4
0.3
<0.05
0.2
0.15
0.1
0 .0 5
0 .0 3
0. 02
3
2
<1
1
> 0 .5
1
>1
1
0.5
3
>2
>
2
> 0. 7
1
>0 .5
0 .1
0 .0
5
1
AABW
CFC-11 (pmol kg
–1
)
Fig. 4.6.13 Inventory of CFC-11 in the AABW layer (between neutral density 28.27 kg m
3 and the seafloor), from
Orsi et al. (1999).The layer-mean CFC-11 concentrations in the Atlantic (Weddell–Enderby), Indian (Australian
Antarctic), and Pacific basins are 0.17, 0.47 and 0.18 pmol kg
91 , respectively.