and, as pointed out by Woods (1985b), the absence
of a mixed layer in idealized models precludes lateral induction (by horizontal advection) across the
sloping base of the mixed layer. Williams (1989)
therefore included a spatially varying mixed layer
in the model of Luyten et al. (1983), and found
that the volume of ventilated fluid within the
subtropical gyre was indeed much increased due
to lateral induction. In fact, several studies have
now shown that the subduction rate in the North
Atlantic subtropical gyre due to lateral induction
is about two to four times greater than that resulting from Ekman pumping alone (Jenkins, 1982;
Sarmiento, 1983; Marshall et al., 1993a; New
et al., 1995). The readers may refer to Chapter 5.3
of this book by Jim Price for more detailed discussion of subduction.
Mode water studies are useful from various view
points. First, mode water reflects temporal variations of oceanic and hence climatic conditions.
Variations in mode water properties, distribution
and circulation are manifestations of variations in
wintertime air–sea interaction (surface cooling) in
the formation area, oceanic heat transport to the
formation area, eddy activity in the formation area
and spin-up/spin-down of the gyre. Second, mode
water simulation is a good target for numerical
models, particularly those with mixed layers. In
order to model mode waters and their variations
accurately various processes must be correctly simulated, including: plausible separation of western
boundary currents and their extensions, frontal systems, mixed-layer processes given proper surface
forcing, eddy activity in the formation area, advection/ventilation/subduction processes and isopycnal/diapycnal mixing. Numerical simulations of
NASTMW (Marsh and New, 1996; Hazeleger and
Drifhout, 1998, 1999) and SAMW (Ribbe and
Tomczak, 1997b) have been carried out. Reproducing mode water distribution, circulation and
variability in numerical models will assist mode
water studies as well as improvement of the numerical models themselves. Third, mode water in the
sense of its potential vorticity signal is a good
tracer of subtropical ventilation, as useful as chemical tracers of ventilation (Sarmiento et al., 1982;
Talley, 1988; Joyce et al., 1998; Schneider et al.,
1999b). This could be particularly useful in the
case of the eastern subtropical mode waters and
the SAMW, which are subducted in the eastern
parts of the gyres.
In this chapter, we describe the distribution and
water properties of mode waters in the world’s
oceans. Mode waters are defined and their general
characteristics presented in Section 5.4.2. The
global distributions and basin descriptions of mode
waters are presented in Section 5.4.3. Intermediate
waters are discussed in several chapters (see Davis
and Zenk, Chapter 3.2; Rintoul et al., Chapter 4.6;
Gordon, Chapter 4.7; and Schlosser et al., Chapter
5.8). Here in Section 5.4.3 a brief discussion is
presented of the low-salinity intermediate waters
as they relate to mode waters. In Section 5.4.4,
temporal variations of subtropical mode water
properties in the North Pacific and North Atlantic
are reviewed, and some information about variation of SPMW properties is presented.
5.4.2 Definition, detection and general
characteristics of mode waters
Mode waters are characterized by homogeneity
of water properties in the vertical as well as the
horizontal. Thickening of isopycnal layers occurs
in many places, so the following several characteristics are generally used to identify mode waters.
(1) In a volumetric sense, a mode water has a substantial volume in some region, in comparison
with surrounding water masses. (2) Water properties such as temperature, salinity and oxygen are
highly homogeneous in the horizontal and vertical.
(3) At a given station, that is, in a single vertical
profile, mode water appears as a pycnostad (low
vertical density gradient) between the seasonal and
main (or lower) pycnoclines (high vertical density
gradient). (4) Mode water is found well beyond
its outcropping area as a result of advection.
(5) Mode water formation or maintenance is usually associated with wintertime convective mixing
due to buoyancy loss from the ocean surface, in a
much more limited region than the total area occupied by the mode water. (6) Mode water formation areas occur in conjunction with permanent
fronts, on the low-density side of the front, where
the isopycnal slopes precondition the region for a
thicker layer than occurs on the high-density side
of the front.
A minimum in the vertical gradient of potential
density ( ), or equivalently in the Brunt–Väisälä
frequency, is often used to identify mode water
(Fig. 5.4.1). Isopycnic potential vorticity is a
related quantity that is useful as well, since it is a
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
374
of a mixed layer in idealized models precludes lateral induction (by horizontal advection) across the
sloping base of the mixed layer. Williams (1989)
therefore included a spatially varying mixed layer
in the model of Luyten et al. (1983), and found
that the volume of ventilated fluid within the
subtropical gyre was indeed much increased due
to lateral induction. In fact, several studies have
now shown that the subduction rate in the North
Atlantic subtropical gyre due to lateral induction
is about two to four times greater than that resulting from Ekman pumping alone (Jenkins, 1982;
Sarmiento, 1983; Marshall et al., 1993a; New
et al., 1995). The readers may refer to Chapter 5.3
of this book by Jim Price for more detailed discussion of subduction.
Mode water studies are useful from various view
points. First, mode water reflects temporal variations of oceanic and hence climatic conditions.
Variations in mode water properties, distribution
and circulation are manifestations of variations in
wintertime air–sea interaction (surface cooling) in
the formation area, oceanic heat transport to the
formation area, eddy activity in the formation area
and spin-up/spin-down of the gyre. Second, mode
water simulation is a good target for numerical
models, particularly those with mixed layers. In
order to model mode waters and their variations
accurately various processes must be correctly simulated, including: plausible separation of western
boundary currents and their extensions, frontal systems, mixed-layer processes given proper surface
forcing, eddy activity in the formation area, advection/ventilation/subduction processes and isopycnal/diapycnal mixing. Numerical simulations of
NASTMW (Marsh and New, 1996; Hazeleger and
Drifhout, 1998, 1999) and SAMW (Ribbe and
Tomczak, 1997b) have been carried out. Reproducing mode water distribution, circulation and
variability in numerical models will assist mode
water studies as well as improvement of the numerical models themselves. Third, mode water in the
sense of its potential vorticity signal is a good
tracer of subtropical ventilation, as useful as chemical tracers of ventilation (Sarmiento et al., 1982;
Talley, 1988; Joyce et al., 1998; Schneider et al.,
1999b). This could be particularly useful in the
case of the eastern subtropical mode waters and
the SAMW, which are subducted in the eastern
parts of the gyres.
In this chapter, we describe the distribution and
water properties of mode waters in the world’s
oceans. Mode waters are defined and their general
characteristics presented in Section 5.4.2. The
global distributions and basin descriptions of mode
waters are presented in Section 5.4.3. Intermediate
waters are discussed in several chapters (see Davis
and Zenk, Chapter 3.2; Rintoul et al., Chapter 4.6;
Gordon, Chapter 4.7; and Schlosser et al., Chapter
5.8). Here in Section 5.4.3 a brief discussion is
presented of the low-salinity intermediate waters
as they relate to mode waters. In Section 5.4.4,
temporal variations of subtropical mode water
properties in the North Pacific and North Atlantic
are reviewed, and some information about variation of SPMW properties is presented.
5.4.2 Definition, detection and general
characteristics of mode waters
Mode waters are characterized by homogeneity
of water properties in the vertical as well as the
horizontal. Thickening of isopycnal layers occurs
in many places, so the following several characteristics are generally used to identify mode waters.
(1) In a volumetric sense, a mode water has a substantial volume in some region, in comparison
with surrounding water masses. (2) Water properties such as temperature, salinity and oxygen are
highly homogeneous in the horizontal and vertical.
(3) At a given station, that is, in a single vertical
profile, mode water appears as a pycnostad (low
vertical density gradient) between the seasonal and
main (or lower) pycnoclines (high vertical density
gradient). (4) Mode water is found well beyond
its outcropping area as a result of advection.
(5) Mode water formation or maintenance is usually associated with wintertime convective mixing
due to buoyancy loss from the ocean surface, in a
much more limited region than the total area occupied by the mode water. (6) Mode water formation areas occur in conjunction with permanent
fronts, on the low-density side of the front, where
the isopycnal slopes precondition the region for a
thicker layer than occurs on the high-density side
of the front.
A minimum in the vertical gradient of potential
density ( ), or equivalently in the Brunt–Väisälä
frequency, is often used to identify mode water
(Fig. 5.4.1). Isopycnic potential vorticity is a
related quantity that is useful as well, since it is a
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
374