XBT data in this well-observed region, Hautala
and Roemmich (1998) described this mode water
(NPESTMW), whose low potential vorticity influence was documented by Talley (1988). This water
is analogous to the Madeira Mode Water of the
North Atlantic (Siedler et al., 1987). NPESTMW
has temperatures from 12 to 22°C and potential
density 24–25.4 . As with the Madeira Mode
Water, NPESTMW is subducted and advected
southward, mixing along its path. Hautala and
Roemmich showed the formation of NPESTMW
in each winter. The temperature of the potential
vorticity minimum was similar in the periods
1970–79 and 1991–97 (with inadequate data for
the intervening period). This eastern region of the
North Pacific has received some attention as a
possible source for decadal changes that might
propagate to the tropics and influence El Niño.
Schneider et al. (1999b) used depths of isopycnal
surfaces and its variations as a primary tracer of
subduction/advection changes in the eastern subtropical Pacific, finding that the tropical influence
of the subtropical waters was minimal.
Unlike the North Atlantic and Southern Ocean,
the North Pacific does not have a distinct subpolar
mode water, in the sense of thick layers that are
maintained by winter convection. However, relatively thick layers, marked by lower potential vorticity, are found around the rim of the subpolar
gyre, particularly in the Gulf of Alaska (Talley,
1988). These layers lie above the strong and relatively shallow pycnocline, which is maintained in
part by freshwater input at the sea surface. It
might, however, be misleading to call the thick
layers mode waters, except insofar as they are
dynamically similar to the much more dramatic
Subpolar Mode Water of the North Atlantic.
The low-salinity intermediate water of the
North Pacific, North Pacific Intermediate Water
(NPIW), is not closely associated with mode
waters, unlike the LSW of the North Atlantic. The
ventilation source of NPIW is the Okhotsk Sea
and Oyashio region (Reid, 1965; Talley, 1993). In
the Okhotsk Sea, brine rejection during sea-ice
formation on the broad shelves ventilates the intermediate densities (Kitani, 1973; Talley, 1991).
Convection in the southern Okhotsk Sea creates a
low potential vorticity signature in the upper portion of the ventilated layer (Yasuda, 1997), and
could be considered an indicator of mode water.
The newly ventilated waters enter the North
Pacific and are advected into both the subpolar
and subtropical gyres. NPIW is not apparent as a
core layer in the subpolar gyre because the overlying surface waters are even fresher than NPIW
(unlike the situation in the North Atlantic), and
because the potential vorticity signature of NPIW
is weak. NPIW is characterized by a salinity minimum in the evaporative subtropical gyre because
there are no other sources of water at this density,
unlike the North Atlantic, which has saline subsurface input from the Mediterranean.
5.4.3.3 Subtropical South Atlantic Ocean
The South Atlantic subtropical gyre has several
prominent fronts, each with its own subtropical
mode water (Tsuchiya et al., 1994; Provost et al.,
1999). Tsuchiya et al. documented three mode
waters along 25°W, plus a weak pycnostad at the
Antarctic Intermediate Water density. Provost et al.
showed the geographical distribution of these mode
waters and provided details regarding their formation regions. These include two types of Subantarctic Mode Water (SAMW; McCartney, 1977, 1982)
and a type of subtropical mode water that is associated with the surface salinity maximum of the
central subtropical gyre. The SAMWs are clearly
part of the subtropical gyre rather than of a highlatitude cyclonic gyre, and should perhaps be best
identified as subtropical mode waters, which is
how Provost et al. identify them. The southernmost
SAMW lies between the Subtropical Front and the
separated Brazil Current Front, with approximate
properties of 12°C, 35.1 psu and 26.7 . North of
the Brazil Current Front is found a less dense
SAMW, at about 13.5°C, 35.3 psu and 26.6 .
McCartney (1982) mapped these two modes as a
single mode water at about 26.5 , showing the
limited geographical area and weakness of the
mode compared with those of other oceans.
Provost et al. also describe a lighter STMW, at
26.2 , in the Brazil Current recirculation region
and restricted to a smaller western region than the
mode waters they identified at 26.5 and 26.6
(corresponding to the mode waters identified by
Tsuchiya et al., 1994). They mapped the winter
outcrop windows and extent of influence of all
three of these mode waters, showing that all originate primarily in the western South Atlantic, in the
overshoot and recirculation area of the Brazil Current, i.e. the southwestern corner of the Brazil
Current. The dominant mode water has properties
5.4 Mode Waters
381
Hanawa and Talley
and Roemmich (1998) described this mode water
(NPESTMW), whose low potential vorticity influence was documented by Talley (1988). This water
is analogous to the Madeira Mode Water of the
North Atlantic (Siedler et al., 1987). NPESTMW
has temperatures from 12 to 22°C and potential
density 24–25.4 . As with the Madeira Mode
Water, NPESTMW is subducted and advected
southward, mixing along its path. Hautala and
Roemmich showed the formation of NPESTMW
in each winter. The temperature of the potential
vorticity minimum was similar in the periods
1970–79 and 1991–97 (with inadequate data for
the intervening period). This eastern region of the
North Pacific has received some attention as a
possible source for decadal changes that might
propagate to the tropics and influence El Niño.
Schneider et al. (1999b) used depths of isopycnal
surfaces and its variations as a primary tracer of
subduction/advection changes in the eastern subtropical Pacific, finding that the tropical influence
of the subtropical waters was minimal.
Unlike the North Atlantic and Southern Ocean,
the North Pacific does not have a distinct subpolar
mode water, in the sense of thick layers that are
maintained by winter convection. However, relatively thick layers, marked by lower potential vorticity, are found around the rim of the subpolar
gyre, particularly in the Gulf of Alaska (Talley,
1988). These layers lie above the strong and relatively shallow pycnocline, which is maintained in
part by freshwater input at the sea surface. It
might, however, be misleading to call the thick
layers mode waters, except insofar as they are
dynamically similar to the much more dramatic
Subpolar Mode Water of the North Atlantic.
The low-salinity intermediate water of the
North Pacific, North Pacific Intermediate Water
(NPIW), is not closely associated with mode
waters, unlike the LSW of the North Atlantic. The
ventilation source of NPIW is the Okhotsk Sea
and Oyashio region (Reid, 1965; Talley, 1993). In
the Okhotsk Sea, brine rejection during sea-ice
formation on the broad shelves ventilates the intermediate densities (Kitani, 1973; Talley, 1991).
Convection in the southern Okhotsk Sea creates a
low potential vorticity signature in the upper portion of the ventilated layer (Yasuda, 1997), and
could be considered an indicator of mode water.
The newly ventilated waters enter the North
Pacific and are advected into both the subpolar
and subtropical gyres. NPIW is not apparent as a
core layer in the subpolar gyre because the overlying surface waters are even fresher than NPIW
(unlike the situation in the North Atlantic), and
because the potential vorticity signature of NPIW
is weak. NPIW is characterized by a salinity minimum in the evaporative subtropical gyre because
there are no other sources of water at this density,
unlike the North Atlantic, which has saline subsurface input from the Mediterranean.
5.4.3.3 Subtropical South Atlantic Ocean
The South Atlantic subtropical gyre has several
prominent fronts, each with its own subtropical
mode water (Tsuchiya et al., 1994; Provost et al.,
1999). Tsuchiya et al. documented three mode
waters along 25°W, plus a weak pycnostad at the
Antarctic Intermediate Water density. Provost et al.
showed the geographical distribution of these mode
waters and provided details regarding their formation regions. These include two types of Subantarctic Mode Water (SAMW; McCartney, 1977, 1982)
and a type of subtropical mode water that is associated with the surface salinity maximum of the
central subtropical gyre. The SAMWs are clearly
part of the subtropical gyre rather than of a highlatitude cyclonic gyre, and should perhaps be best
identified as subtropical mode waters, which is
how Provost et al. identify them. The southernmost
SAMW lies between the Subtropical Front and the
separated Brazil Current Front, with approximate
properties of 12°C, 35.1 psu and 26.7 . North of
the Brazil Current Front is found a less dense
SAMW, at about 13.5°C, 35.3 psu and 26.6 .
McCartney (1982) mapped these two modes as a
single mode water at about 26.5 , showing the
limited geographical area and weakness of the
mode compared with those of other oceans.
Provost et al. also describe a lighter STMW, at
26.2 , in the Brazil Current recirculation region
and restricted to a smaller western region than the
mode waters they identified at 26.5 and 26.6
(corresponding to the mode waters identified by
Tsuchiya et al., 1994). They mapped the winter
outcrop windows and extent of influence of all
three of these mode waters, showing that all originate primarily in the western South Atlantic, in the
overshoot and recirculation area of the Brazil Current, i.e. the southwestern corner of the Brazil
Current. The dominant mode water has properties
5.4 Mode Waters
381
Hanawa and Talley