The fresh intermediate water of the North
Atlantic, the Labrador Sea Water (LSW), arises
from convection in the western Labrador Sea
(Clarke and Gascard, 1983; Pickart et al., 1997;
Lazier et al., Chapter 5.5) and is advected
throughout the subpolar Atlantic and into the subtropical Atlantic (Figure 5.4.3b, see Plate 5.4.3b,
p. 428; from Talley, 1999a). The source waters of
LSW are the Subpolar Mode Waters – LSW can be
considered the densest of the SPMWs. It is characterized in the subpolar North Atlantic as a pycnostad, a salinity minimum and an oxygen maximum
(Talley and McCartney, 1982). It is a salinity minimum in the subpolar region because of the large
inflow of saline subtropical surface waters (that
become the SPMW). LSWs signature in the subtropical North Atlantic is not as clear because it is
opposed by the saline Mediterranean Outflow
Water, which occupies a similar density range and
which mixes with LSW. In the tropical and South
Atlantic, LSW and Mediterranean Outflow Water
are considered to be part of the southward-flowing
North Atlantic Deep Water. Here LSW contributes
the high-oxygen core known as Middle North
Atlantic Deep Water (Wüst, 1935).
5.4.3.2 North Pacific Ocean
North Pacific Subtropical Mode Water (NPSTMW)
is found throughout the northwestern part of the
subtropical gyre (Hanawa, 1987; Bingham, 1992).
Hanawa and Suga (1995) provided a thorough
review of NPSTMW studies.
The core of NPSTMW is 16.5°C, 34.85 psu and
25.2 . The approximate outcrop area is the zonal
band from the Izu Ridge to the international dateline in longitude and from 28°N to the Kuroshio
Extension (Hanawa and Hoshino, 1988; Yasuda
and Hanawa, 1997, 1999; Hanawa and Yoritaka,
2000). In late winter, a well-developed mixed layer
thicker than 300 m is formed in this area. In the
formation area, temperature and salinity decrease
from west to east (Suga and Hanawa, 1990;
Bingham, 1992). This decrease of salinity to the
east reflects the input of northern surface water
with lower salinity through Ekman transport.
The source water of NPSTMW is apparently
the Kuroshio Extension (Bingham, 1992). The surface temperature of the Kuroshio Extension
decreases eastward, corresponding to a local heat
loss of more than 800 W m
92 in winter. Maps of
acceleration potential, which yield the geostrophic
flow on isopycnals (Montgomery, 1938), suggest
that the Kuroshio water detrains southward into
the thick layers in the formation area of NPSTMW.
Thus the eastward decrease in NPSTMW temperature is more a function of the Kuroshio Extension
temperature than an indication of eastward advection of the NPSTMW itself. The NPSTMW formation area lies dynamically between the Kuroshio
Extension and a westward recirculation south of
the Kuroshio Extension. Winter convection in the
narrow region just south of the Kuroshio, perhaps
accentuated in anticyclonic meander regions,
thickens the NPSTMW layer further. Using maps
of potential vorticity on isopycnals, Suga and
Hanawa (1995b) showed the seasonal movement
of NPSTMW formed in the western part of the
formation region. Based on synoptic surveys in
1987 and 1888, Suga and Hanawa (1995a)
described the substantial advection of NPSTMW
by anticyclonic eddies propagating westward in
the region south of Japan. During Kuroshio large
meander periods, the intrusion of NPSTMW into
the region just south of Japan is blocked and the
NPSTMW path is shifted further south (Suga and
Hanawa, 1995b).
A denser type of subtropical mode water was
described by Nakamura (1996a) and Suga et al.
(1997), who named it North Pacific Central Mode
Water (NPCMW). This mode water is distributed
in the central North Pacific (approximately 170°E
to 150°W) between the Kuroshio Extension Front
(approx. 33°N) and the Kuroshio Bifurcation Front
(approx. 40°N). NPCMW has a temperature of
9–12°C and potential density around 26.2 . This
water is synonymous with the ‘stability gap’
described by Roden (1970), by which was meant
the lowered stability region south of the Subarctic
Front. Southward Ekman transport of cool, fresh
water may play a role in decreasing the stability of
this region. Reid (1982) showed that winter mixed
layers in this region are thicker than anywhere else
in the North Pacific, including the NPSTMW
region. Very thick mixed layers (9500 m) are
found locally in Kuroshio warm core rings just
east of Honshu, which may precondition the central region to have particularly thick winter mixed
layers, and hence be a precursor to the NPCMW.
The third, low-density, mode water of the subtropical gyre is found in the eastern North Pacific.
Using the WOCE high-density XBT data between
Honolulu and San Francisco as well as historical
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
380
Atlantic, the Labrador Sea Water (LSW), arises
from convection in the western Labrador Sea
(Clarke and Gascard, 1983; Pickart et al., 1997;
Lazier et al., Chapter 5.5) and is advected
throughout the subpolar Atlantic and into the subtropical Atlantic (Figure 5.4.3b, see Plate 5.4.3b,
p. 428; from Talley, 1999a). The source waters of
LSW are the Subpolar Mode Waters – LSW can be
considered the densest of the SPMWs. It is characterized in the subpolar North Atlantic as a pycnostad, a salinity minimum and an oxygen maximum
(Talley and McCartney, 1982). It is a salinity minimum in the subpolar region because of the large
inflow of saline subtropical surface waters (that
become the SPMW). LSWs signature in the subtropical North Atlantic is not as clear because it is
opposed by the saline Mediterranean Outflow
Water, which occupies a similar density range and
which mixes with LSW. In the tropical and South
Atlantic, LSW and Mediterranean Outflow Water
are considered to be part of the southward-flowing
North Atlantic Deep Water. Here LSW contributes
the high-oxygen core known as Middle North
Atlantic Deep Water (Wüst, 1935).
5.4.3.2 North Pacific Ocean
North Pacific Subtropical Mode Water (NPSTMW)
is found throughout the northwestern part of the
subtropical gyre (Hanawa, 1987; Bingham, 1992).
Hanawa and Suga (1995) provided a thorough
review of NPSTMW studies.
The core of NPSTMW is 16.5°C, 34.85 psu and
25.2 . The approximate outcrop area is the zonal
band from the Izu Ridge to the international dateline in longitude and from 28°N to the Kuroshio
Extension (Hanawa and Hoshino, 1988; Yasuda
and Hanawa, 1997, 1999; Hanawa and Yoritaka,
2000). In late winter, a well-developed mixed layer
thicker than 300 m is formed in this area. In the
formation area, temperature and salinity decrease
from west to east (Suga and Hanawa, 1990;
Bingham, 1992). This decrease of salinity to the
east reflects the input of northern surface water
with lower salinity through Ekman transport.
The source water of NPSTMW is apparently
the Kuroshio Extension (Bingham, 1992). The surface temperature of the Kuroshio Extension
decreases eastward, corresponding to a local heat
loss of more than 800 W m
92 in winter. Maps of
acceleration potential, which yield the geostrophic
flow on isopycnals (Montgomery, 1938), suggest
that the Kuroshio water detrains southward into
the thick layers in the formation area of NPSTMW.
Thus the eastward decrease in NPSTMW temperature is more a function of the Kuroshio Extension
temperature than an indication of eastward advection of the NPSTMW itself. The NPSTMW formation area lies dynamically between the Kuroshio
Extension and a westward recirculation south of
the Kuroshio Extension. Winter convection in the
narrow region just south of the Kuroshio, perhaps
accentuated in anticyclonic meander regions,
thickens the NPSTMW layer further. Using maps
of potential vorticity on isopycnals, Suga and
Hanawa (1995b) showed the seasonal movement
of NPSTMW formed in the western part of the
formation region. Based on synoptic surveys in
1987 and 1888, Suga and Hanawa (1995a)
described the substantial advection of NPSTMW
by anticyclonic eddies propagating westward in
the region south of Japan. During Kuroshio large
meander periods, the intrusion of NPSTMW into
the region just south of Japan is blocked and the
NPSTMW path is shifted further south (Suga and
Hanawa, 1995b).
A denser type of subtropical mode water was
described by Nakamura (1996a) and Suga et al.
(1997), who named it North Pacific Central Mode
Water (NPCMW). This mode water is distributed
in the central North Pacific (approximately 170°E
to 150°W) between the Kuroshio Extension Front
(approx. 33°N) and the Kuroshio Bifurcation Front
(approx. 40°N). NPCMW has a temperature of
9–12°C and potential density around 26.2 . This
water is synonymous with the ‘stability gap’
described by Roden (1970), by which was meant
the lowered stability region south of the Subarctic
Front. Southward Ekman transport of cool, fresh
water may play a role in decreasing the stability of
this region. Reid (1982) showed that winter mixed
layers in this region are thicker than anywhere else
in the North Pacific, including the NPSTMW
region. Very thick mixed layers (9500 m) are
found locally in Kuroshio warm core rings just
east of Honshu, which may precondition the central region to have particularly thick winter mixed
layers, and hence be a precursor to the NPCMW.
The third, low-density, mode water of the subtropical gyre is found in the eastern North Pacific.
Using the WOCE high-density XBT data between
Honolulu and San Francisco as well as historical
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
380