minimum of potential vorticity, potential density
or temperature (e.g. McCartney, 1982).
The low-salinity intermediate waters in the
North Atlantic and Southern Ocean are closely
associated with mode waters; this association is
described in more detail in the next section. The
low-salinity intermediate water of the North
Pacific on the other hand has not been shown to
be associated with mode waters. All three major
intermediate waters have some regions of low
potential vorticity resulting from convective formation at the sea surface, and in this sense there
are similarities to mode waters. The Labrador Sea
Water of the North Atlantic is formed from mode
waters of the subpolar gyre (Talley and McCartney, 1982). The Antarctic Intermediate Water of
the Pacific is a subducted mode water associated
with the Antarctic Circumpolar Current. The
Antarctic Intermediate Water of the Atlantic and
Indian Oceans also arises from this circumpolar
mode water, but through advection and modification of the Pacific mode water in Drake Passage
(McCartney, 1977).
5.4.3 Geographical distribution of
mixed-layer depth and mode waters
in the world’s oceans
Mode water formation areas are generally characterized by wintertime mixed layers that are relatively thick compared with other mixed layers
in the same geographical region. Talley (1999a)
mapped the global winter mixed-layer thicknesses,
using Reid’s (1982) approach employing the depth
of high oxygen saturation. This map is reproduced
here as Figure 5.4.2 (see Plate 5.4.2, p. 428).
Globally the thickest mixed layers are in the
northern North Atlantic and around the northern region of the Southern Ocean in the Pacific
and eastern Indian Oceans. These thick layers are
associated with the North Atlantic’s Subpolar
Mode Water and the Southern Ocean’s Subantarctic Mode Water, described below. Relatively thick
mixed layers are also found in the subtropical
mode water areas near the separated western
boundary currents.
Mode waters originate as thick winter mixed
layers, but are then subducted and advected away
from the formation areas. They are usually defined
as mode waters after they are capped by either a
seasonal pycnocline or the permanent pycnocline
under which they are subducted. The global distribution of mode waters as it was understood in the
late 1970s was mapped by McCartney (1982).
Talley (1999a) used the global hydrographic data
sets compiled by J. Reid and A. Mantyla (personal
communication) as well as numerous World Ocean
Circulation Experiment (WOCE) stations to produce a new schematic map of mode waters, including the same features as McCartney (1982) and
adding newly defined eastern subtropical mode
waters and central mode waters (see below).
Figure 5.4.3a (see Plate 5.4.3a, p. 428) is a slightly
updated version of Talley’s (1999a) map, including here more information regarding the density of
the mode waters.
Subtropical mode waters associated with western boundary current extensions of subtropical
gyres, Type I, are found in all basins (associated
with the Kuroshio, Gulf Stream, East Australian
Current, Brazil Current and Agulhas Current).
These arise from convection in the thickened layers on the south (north) side of the current axes in
the northern (southern) hemisphere, where a natural bowl in isopycnal surfaces occurs between the
separated current and its recirculation. STMWs,
especially in the northern hemisphere, are associated with large surface heat loss from the ocean as
a result of cold, dry air outbreaks in winter from
the nearby continents. Low potential vorticity
(weak stratification or thick mixed layer in winter)
associated with these mode waters arises as a
result of convection near the axis of the separated
western boundary currents, and possibly also from
the origin of some of the source water from the
equatorward, negative relative vorticity side of the
separated current. Equatorward Ekman transport
into the STMW formation area may also modify
STMW properties, particularly in making them
fresher to the east, as is observed in the North
Pacific (Suga and Hanawa, 1990). Eddy exchange
across the separated boundary current could also
accomplish a freshening (Talley, 1997).
A second type of subtropical mode water, of density not very different from STMWs associated with
the western boundary current, Type II, is found
in the eastern part of the subtropical gyres (light
pink in Figure 5.4.3a, see Plate 5.4.3a, p. 428). The
Madeira Mode Water (Käse et al., 1985; Siedler
et al., 1987) is the archetype of this mode water.
Hautala and Roemmich (1998) have described
this mode water for the North Pacific. A similar
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
376
or temperature (e.g. McCartney, 1982).
The low-salinity intermediate waters in the
North Atlantic and Southern Ocean are closely
associated with mode waters; this association is
described in more detail in the next section. The
low-salinity intermediate water of the North
Pacific on the other hand has not been shown to
be associated with mode waters. All three major
intermediate waters have some regions of low
potential vorticity resulting from convective formation at the sea surface, and in this sense there
are similarities to mode waters. The Labrador Sea
Water of the North Atlantic is formed from mode
waters of the subpolar gyre (Talley and McCartney, 1982). The Antarctic Intermediate Water of
the Pacific is a subducted mode water associated
with the Antarctic Circumpolar Current. The
Antarctic Intermediate Water of the Atlantic and
Indian Oceans also arises from this circumpolar
mode water, but through advection and modification of the Pacific mode water in Drake Passage
(McCartney, 1977).
5.4.3 Geographical distribution of
mixed-layer depth and mode waters
in the world’s oceans
Mode water formation areas are generally characterized by wintertime mixed layers that are relatively thick compared with other mixed layers
in the same geographical region. Talley (1999a)
mapped the global winter mixed-layer thicknesses,
using Reid’s (1982) approach employing the depth
of high oxygen saturation. This map is reproduced
here as Figure 5.4.2 (see Plate 5.4.2, p. 428).
Globally the thickest mixed layers are in the
northern North Atlantic and around the northern region of the Southern Ocean in the Pacific
and eastern Indian Oceans. These thick layers are
associated with the North Atlantic’s Subpolar
Mode Water and the Southern Ocean’s Subantarctic Mode Water, described below. Relatively thick
mixed layers are also found in the subtropical
mode water areas near the separated western
boundary currents.
Mode waters originate as thick winter mixed
layers, but are then subducted and advected away
from the formation areas. They are usually defined
as mode waters after they are capped by either a
seasonal pycnocline or the permanent pycnocline
under which they are subducted. The global distribution of mode waters as it was understood in the
late 1970s was mapped by McCartney (1982).
Talley (1999a) used the global hydrographic data
sets compiled by J. Reid and A. Mantyla (personal
communication) as well as numerous World Ocean
Circulation Experiment (WOCE) stations to produce a new schematic map of mode waters, including the same features as McCartney (1982) and
adding newly defined eastern subtropical mode
waters and central mode waters (see below).
Figure 5.4.3a (see Plate 5.4.3a, p. 428) is a slightly
updated version of Talley’s (1999a) map, including here more information regarding the density of
the mode waters.
Subtropical mode waters associated with western boundary current extensions of subtropical
gyres, Type I, are found in all basins (associated
with the Kuroshio, Gulf Stream, East Australian
Current, Brazil Current and Agulhas Current).
These arise from convection in the thickened layers on the south (north) side of the current axes in
the northern (southern) hemisphere, where a natural bowl in isopycnal surfaces occurs between the
separated current and its recirculation. STMWs,
especially in the northern hemisphere, are associated with large surface heat loss from the ocean as
a result of cold, dry air outbreaks in winter from
the nearby continents. Low potential vorticity
(weak stratification or thick mixed layer in winter)
associated with these mode waters arises as a
result of convection near the axis of the separated
western boundary currents, and possibly also from
the origin of some of the source water from the
equatorward, negative relative vorticity side of the
separated current. Equatorward Ekman transport
into the STMW formation area may also modify
STMW properties, particularly in making them
fresher to the east, as is observed in the North
Pacific (Suga and Hanawa, 1990). Eddy exchange
across the separated boundary current could also
accomplish a freshening (Talley, 1997).
A second type of subtropical mode water, of density not very different from STMWs associated with
the western boundary current, Type II, is found
in the eastern part of the subtropical gyres (light
pink in Figure 5.4.3a, see Plate 5.4.3a, p. 428). The
Madeira Mode Water (Käse et al., 1985; Siedler
et al., 1987) is the archetype of this mode water.
Hautala and Roemmich (1998) have described
this mode water for the North Pacific. A similar
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
376