low-density mode water is evident in the southeastern South Pacific (mapped in Talley, 1999a,
but not fully described anywhere yet). The mechanism for the existence of eastern STMWs, however, has not been elucidated. They appear in the
subtropical gyre regions bounded by the equatorward side of permanent zonal fronts (such as the
Azores Front in the North Atlantic and the subtropical fronts in the North and South Pacific) and
the western side of eastern boundary current
fronts (such as the Canary, California and Peru
Currents). These eastern mode waters are much
weaker than the western STMWs in terms of
thickness and area of extent. The North Atlantic
and North Pacific eastern STMWs have been studied through the seasons and show major seasonal
variability associated with advection and seasonal
outcropping. These mode waters are in the primary subduction areas of the gyres and thus provide a useful tracer for ventilation studies within
the subtropical gyre.
A third, denser, type of subtropical mode water,
Type III, is associated with the subpolar fronts on
the poleward boundaries of the subtropical gyres
(dark in Fig. 5.4.3a, see Plate 5.4.3a, p. 428). The
densities of these mode waters are considerably
higher than for the first two types just described.
The archetype is the Subantarctic Mode Water,
found just north of the Subantarctic Front (McCartney, 1977). A similar mode water is found in the
North Pacific (North Pacific Central Mode Water;
Nakamura, 1996a; Suga et al., 1997), just south of
the Subarctic Front. In the North Atlantic, the Subpolar Mode Water of density near 26.9 to 27.0
on the south side of the North Atlantic Current
should be identified as this denser type of subtropical mode water, circulating southward into the subtropical gyre, as described by McCartney (1982).
Mode waters are also found around the outside
of the cyclonic subpolar circulations (Type IV). The
North Atlantic Subpolar Mode Water (SPMW;
McCartney and Talley, 1982) is the most obvious
of these mode waters, since it is associated with
very thick mixed layers (see Figure 5.4.2 (see Plate
5.4.2, p. 428) and description below). A weak version of sub-polar mode water is found around the
periphery of the North Pacific subpolar gyre,
although it has not been singled out and named.
Thick layers occur in the Nordic Seas but have not
been singled out as mode waters. Finally, although
mode waters have not been reported around the
periphery of the cyclonic gyres south of the
Antarctic Circumpolar Current, it is possible that
they exist as a result of the downward slope of
isopycnals towards the outer side of cyclonic
circulations. This could be explored further.
A weak mode water, Type V, has been described
in the high evaporation region of the South
Atlantic, in conjunction with the high-salinity water
observed in the subsurface layer of the subtropical
gyre (Tsuchiya et al., 1994), as described below. A
feature like this has also been found in the North
Pacific, assumed to be due to subduction of highsalinity surface water (Shuto, 1996). High-evaporation, high-salinity surface waters and subducted
saline subsurface layers occur in every subtropical
ocean. Thus a subsurface potential vorticity minimum (mode water) might be associated with the
subtropical underwater in every ocean. These shallow salinity extrema have been given various names
in different oceans. We follow the name ‘Subtropical Underwater’ used by Worthington (1976) for
the feature in the North Atlantic. These water
masses are described in numerous publications (for
the North Atlantic, e.g. Worthington, 1976; for the
South Pacific, e.g. Tsuchiya and Talley, 1996; and
for the Indian Ocean, e.g. Wyrtki, 1973a; Toole
and Warren, 1993).
The mode waters of each basin are described
now in more detail. The two northern hemisphere
oceans are described first, as they have many features in common, both with limited subtropical
and subpolar gyres. The subtropical mode waters
of the southern hemisphere subtropical oceans are
then treated together since they share similar features. The circumpolar Subantarctic Mode Water,
which as mentioned above is actually a type of
subtropical mode water, is then described in a section on the Southern Ocean. Table 5.4.1 is the
summary of mode waters in the world’s ocean, in
which acronym, full name, type of mode waters,
characteristic temperature (°C), salinity, potential
density ( ) and references are given.
Here note that in the following description, terminology is based on original references as much
as possible: Eighteen Degree Water and North
Atlantic Subtropical Mode Water (NASTMW) are
synonymous.
5.4.3.1 North Atlantic Ocean
The North Atlantic subtropical mode water
(Eighteen Degree Water: Worthington, 1959) has
5.4 Mode Waters
377
Hanawa and Talley
but not fully described anywhere yet). The mechanism for the existence of eastern STMWs, however, has not been elucidated. They appear in the
subtropical gyre regions bounded by the equatorward side of permanent zonal fronts (such as the
Azores Front in the North Atlantic and the subtropical fronts in the North and South Pacific) and
the western side of eastern boundary current
fronts (such as the Canary, California and Peru
Currents). These eastern mode waters are much
weaker than the western STMWs in terms of
thickness and area of extent. The North Atlantic
and North Pacific eastern STMWs have been studied through the seasons and show major seasonal
variability associated with advection and seasonal
outcropping. These mode waters are in the primary subduction areas of the gyres and thus provide a useful tracer for ventilation studies within
the subtropical gyre.
A third, denser, type of subtropical mode water,
Type III, is associated with the subpolar fronts on
the poleward boundaries of the subtropical gyres
(dark in Fig. 5.4.3a, see Plate 5.4.3a, p. 428). The
densities of these mode waters are considerably
higher than for the first two types just described.
The archetype is the Subantarctic Mode Water,
found just north of the Subantarctic Front (McCartney, 1977). A similar mode water is found in the
North Pacific (North Pacific Central Mode Water;
Nakamura, 1996a; Suga et al., 1997), just south of
the Subarctic Front. In the North Atlantic, the Subpolar Mode Water of density near 26.9 to 27.0
on the south side of the North Atlantic Current
should be identified as this denser type of subtropical mode water, circulating southward into the subtropical gyre, as described by McCartney (1982).
Mode waters are also found around the outside
of the cyclonic subpolar circulations (Type IV). The
North Atlantic Subpolar Mode Water (SPMW;
McCartney and Talley, 1982) is the most obvious
of these mode waters, since it is associated with
very thick mixed layers (see Figure 5.4.2 (see Plate
5.4.2, p. 428) and description below). A weak version of sub-polar mode water is found around the
periphery of the North Pacific subpolar gyre,
although it has not been singled out and named.
Thick layers occur in the Nordic Seas but have not
been singled out as mode waters. Finally, although
mode waters have not been reported around the
periphery of the cyclonic gyres south of the
Antarctic Circumpolar Current, it is possible that
they exist as a result of the downward slope of
isopycnals towards the outer side of cyclonic
circulations. This could be explored further.
A weak mode water, Type V, has been described
in the high evaporation region of the South
Atlantic, in conjunction with the high-salinity water
observed in the subsurface layer of the subtropical
gyre (Tsuchiya et al., 1994), as described below. A
feature like this has also been found in the North
Pacific, assumed to be due to subduction of highsalinity surface water (Shuto, 1996). High-evaporation, high-salinity surface waters and subducted
saline subsurface layers occur in every subtropical
ocean. Thus a subsurface potential vorticity minimum (mode water) might be associated with the
subtropical underwater in every ocean. These shallow salinity extrema have been given various names
in different oceans. We follow the name ‘Subtropical Underwater’ used by Worthington (1976) for
the feature in the North Atlantic. These water
masses are described in numerous publications (for
the North Atlantic, e.g. Worthington, 1976; for the
South Pacific, e.g. Tsuchiya and Talley, 1996; and
for the Indian Ocean, e.g. Wyrtki, 1973a; Toole
and Warren, 1993).
The mode waters of each basin are described
now in more detail. The two northern hemisphere
oceans are described first, as they have many features in common, both with limited subtropical
and subpolar gyres. The subtropical mode waters
of the southern hemisphere subtropical oceans are
then treated together since they share similar features. The circumpolar Subantarctic Mode Water,
which as mentioned above is actually a type of
subtropical mode water, is then described in a section on the Southern Ocean. Table 5.4.1 is the
summary of mode waters in the world’s ocean, in
which acronym, full name, type of mode waters,
characteristic temperature (°C), salinity, potential
density ( ) and references are given.
Here note that in the following description, terminology is based on original references as much
as possible: Eighteen Degree Water and North
Atlantic Subtropical Mode Water (NASTMW) are
synonymous.
5.4.3.1 North Atlantic Ocean
The North Atlantic subtropical mode water
(Eighteen Degree Water: Worthington, 1959) has
5.4 Mode Waters
377
Hanawa and Talley