circulation. Our present theoretical understanding
of the deep ocean circulation is based on the work
of Stommel and Arons (1960a,b). They assumed
that the deep water left the abyssal layer through a
spatially uniform upwelling (this implies spatially
uniform vertical mixing). Vorticity balance would
then require that the abyssal circulation would
consist of a series of narrow western boundary
currents (Fig. 4.5.1, Hogg, Chapter 4.5) feeding
interior gyres.
Based on this understanding and the observational evidence, Fig. 1.2.7 (see Plate 1.2.7, p. 44)
from Schmitz (1996b) shows a schematic representation of the three-dimensional circulation of deep
water masses. In the Atlantic, the southward-flowing western boundary current transports NADW
to the Southern Ocean. This flow is reflected in the
tongue of high-salinity NADW (centred at depths
of 2–3 km in the South Atlantic) in a meridional
section through the Atlantic (Fig. 1.2.4, see Plate
1.2.4, p. 44). Below this there is a northward flow
of low-salinity Antarctic Bottom Water (AABW)
originating from the Weddell Sea. The NADW
joins lower Circumpolar Deep Water (CDW) in
the Southern Ocean where it is transported eastward as part of the Antarctic Circumpolar Current, and then northward along with AABW into
the Pacific Ocean (Fig. 1.2.5, see Plate 1.2.5, p. 44)
and Indian Ocean. Part of the CDW is upwelled
south of the ACC where it undergoes transformation to lighter Antarctic Intermediate Water
(AAIW) or denser Antarctic Bottom Water. In the
North Pacific, no deep water is formed at the surface. Rather, part of the northward flow of deep
and bottom water is returned southward as North
Pacific Deep Water, with the high-silicate (and
low-oxygen – not shown) tongue centred at a
depth of about 3 km (Fig. 1.2.5, see Plate 1.2.5,
p. 44). The high silicate and low oxygen concentrations of this water indicate it has not been in
contact with the atmosphere for centuries.
The flow of NADW from the Atlantic to the rest
of the world ocean must be replaced by waters
entering the Atlantic through its southern boundary.
Antarctic Intermediate Water enters through Drake
Passage (Rintoul et al., Chapter 4.6). This is referred
to as the ‘cold water route’ in contrast to the ‘warm
water route’ by which Agulhas Current water enters
around southern Africa (Gordon, Chapter 4.7).
Water also passes from the North Pacific to the
Atlantic through the Arctic Ocean. The balance
between these three exchanges is thought to
determine the salinity of the North Atlantic. The
Indonesian Throughflow carrying warm water from
the Pacific to the Indian Ocean (Fig. 1.2.7, see Plate
1.2.7, p. 44, Gordon, Chapter 4.7) is another
important part of this global circulation.
The deep circulation is strongly influenced by
bathymetry. The deep basins are separated from
one another by oceanic ridges whose sill depth will
determine the maximum density of the waters that
move from one basin to another. The deepest
passages between ocean basins are often relatively
narrow (ϳ10 km) and long (ϳ100 km) and the
transport through such passages is governed by
stratified hydraulic conditions. At the time WOCE
was being planned, many of the connections
between basins had not been surveyed. Rather,
their location and sill depths had been inferred
by the differences between water properties
between the basins. WOCE has made a significant
contribution by quantifying the transport of these
deep currents by the deployment of current meter
moorings and repeat hydrographic sections and
bathymetric surveys (Figs 4.5.3 and 4.5.4, Hogg,
Chapter 4.5).
The main source regions of North Atlantic
Deep Water (NADW) are separated from the rest
of the North Atlantic and hence from the global
ocean by the Shetland–Faroe–Iceland–Greenland
ridge system. The water mass transformations that
take place poleward of this ridge create water that,
at the sill depths of 500 to 800 m, is denser than
any other water found equatorward of the ridges.
The water passes across the sill, cascades down
the slope while undergoing intense mixing and
entrainment (Saunders, Chapter 5.6). Through this
descent, the volume transport of the current
increases while the density of the water decreases.
These same intense flows and entrainment
processes take place at the exits of the Mediterranean (Candela, Chapter 5.7) and Red Seas. In
both cases, evaporation within the nearly enclosed
basins produces salty and dense deep water. When
this water exits the sea and begins its descent, the
mixing and entrainment are so great that the
descending water’s density is reduced to intermediate values and the newly produced water
mass spreads into the ocean interior at depths of
approximately 1000–1500 m.
Producing models that adequately represent the
processes governing the thermohaline circulation is
SECTION 1 THE OCEAN AND CLIMATE
22
of the deep ocean circulation is based on the work
of Stommel and Arons (1960a,b). They assumed
that the deep water left the abyssal layer through a
spatially uniform upwelling (this implies spatially
uniform vertical mixing). Vorticity balance would
then require that the abyssal circulation would
consist of a series of narrow western boundary
currents (Fig. 4.5.1, Hogg, Chapter 4.5) feeding
interior gyres.
Based on this understanding and the observational evidence, Fig. 1.2.7 (see Plate 1.2.7, p. 44)
from Schmitz (1996b) shows a schematic representation of the three-dimensional circulation of deep
water masses. In the Atlantic, the southward-flowing western boundary current transports NADW
to the Southern Ocean. This flow is reflected in the
tongue of high-salinity NADW (centred at depths
of 2–3 km in the South Atlantic) in a meridional
section through the Atlantic (Fig. 1.2.4, see Plate
1.2.4, p. 44). Below this there is a northward flow
of low-salinity Antarctic Bottom Water (AABW)
originating from the Weddell Sea. The NADW
joins lower Circumpolar Deep Water (CDW) in
the Southern Ocean where it is transported eastward as part of the Antarctic Circumpolar Current, and then northward along with AABW into
the Pacific Ocean (Fig. 1.2.5, see Plate 1.2.5, p. 44)
and Indian Ocean. Part of the CDW is upwelled
south of the ACC where it undergoes transformation to lighter Antarctic Intermediate Water
(AAIW) or denser Antarctic Bottom Water. In the
North Pacific, no deep water is formed at the surface. Rather, part of the northward flow of deep
and bottom water is returned southward as North
Pacific Deep Water, with the high-silicate (and
low-oxygen – not shown) tongue centred at a
depth of about 3 km (Fig. 1.2.5, see Plate 1.2.5,
p. 44). The high silicate and low oxygen concentrations of this water indicate it has not been in
contact with the atmosphere for centuries.
The flow of NADW from the Atlantic to the rest
of the world ocean must be replaced by waters
entering the Atlantic through its southern boundary.
Antarctic Intermediate Water enters through Drake
Passage (Rintoul et al., Chapter 4.6). This is referred
to as the ‘cold water route’ in contrast to the ‘warm
water route’ by which Agulhas Current water enters
around southern Africa (Gordon, Chapter 4.7).
Water also passes from the North Pacific to the
Atlantic through the Arctic Ocean. The balance
between these three exchanges is thought to
determine the salinity of the North Atlantic. The
Indonesian Throughflow carrying warm water from
the Pacific to the Indian Ocean (Fig. 1.2.7, see Plate
1.2.7, p. 44, Gordon, Chapter 4.7) is another
important part of this global circulation.
The deep circulation is strongly influenced by
bathymetry. The deep basins are separated from
one another by oceanic ridges whose sill depth will
determine the maximum density of the waters that
move from one basin to another. The deepest
passages between ocean basins are often relatively
narrow (ϳ10 km) and long (ϳ100 km) and the
transport through such passages is governed by
stratified hydraulic conditions. At the time WOCE
was being planned, many of the connections
between basins had not been surveyed. Rather,
their location and sill depths had been inferred
by the differences between water properties
between the basins. WOCE has made a significant
contribution by quantifying the transport of these
deep currents by the deployment of current meter
moorings and repeat hydrographic sections and
bathymetric surveys (Figs 4.5.3 and 4.5.4, Hogg,
Chapter 4.5).
The main source regions of North Atlantic
Deep Water (NADW) are separated from the rest
of the North Atlantic and hence from the global
ocean by the Shetland–Faroe–Iceland–Greenland
ridge system. The water mass transformations that
take place poleward of this ridge create water that,
at the sill depths of 500 to 800 m, is denser than
any other water found equatorward of the ridges.
The water passes across the sill, cascades down
the slope while undergoing intense mixing and
entrainment (Saunders, Chapter 5.6). Through this
descent, the volume transport of the current
increases while the density of the water decreases.
These same intense flows and entrainment
processes take place at the exits of the Mediterranean (Candela, Chapter 5.7) and Red Seas. In
both cases, evaporation within the nearly enclosed
basins produces salty and dense deep water. When
this water exits the sea and begins its descent, the
mixing and entrainment are so great that the
descending water’s density is reduced to intermediate values and the newly produced water
mass spreads into the ocean interior at depths of
approximately 1000–1500 m.
Producing models that adequately represent the
processes governing the thermohaline circulation is
SECTION 1 THE OCEAN AND CLIMATE
22