The dynamics of these exchange processes can be
explained in terms of simple thermocline models
(Liu, 1994; McCreary and Lu, 1994; Liu and
Philander, 1995). Liu (1994) describes how the
boundaries of these different subduction windows
depend on the structure of the windstress patterns.
The tropical–subtropical water exchange in the
Pacific is strongly affected by the complex spatial
pattern of the trade wind. The asymmetry of that
field relative to the equator, the Intertropical Convergence Zone, where the southeast and northeast
trades meet, is mostly in the northern hemisphere
and causes the pathways of water parcels to be
different in the two hemispheres. Routes to the
equator in the southern hemisphere can be direct
but the presence of the eastward North Equatorial
Countercurrent in the northern hemisphere complicates matters. That current is associated with a
ridge in the thermocline near 10°N (see Fig. 4.4.2).
A parcel conserving potential vorticity could have
difficulty negotiating that ridge, an impression that
one may also have from the salinity distribution of
Fig. 4.4.2c (Lu and McCreary, 1995). Nevertheless, the thermocline deepens westward and therefore it remains possible for the flow to conserve
potential vorticity while crossing the thermocline
ridge. Dynamically, this thermocline ridge is
forced by the dominant Ekman upwelling in this
region. The upwelling produces a northward
geostrophic current that tends to block the interior
equatorward penetration. However, in the central
Pacific, the Ekman pumping is downward in the
latitudes of North Equatorial Countercurrent. This
generates a southward geostrophic flow and therefore enables the interior pathway to penetrate
across the latitude of North Equatorial Countercurrent (McPhaden and Fine, 1988; Liu and
Huang, 1998). That is why, in Fig. 4.4.5 (see Plate
4.4.5, p. 300), the northern hemisphere interior
trajectory moves southwestward up to 10°N, then
continues southeastward in the Countercurrent.
These results are confirmed by recent modelling
(Huang, 1996; Blanke and Raynaud, 1997; Gu
and Philander, 1997; Lu et al., 1998; Rothstein
et al., 1998b; Huang and Liu, 1999; Harper, 2000)
and hydrographic (Fig. 4.4.6, see Plate 4.4.6,
p. 300; Johnson and McPhaden, 1999) studies.
Because of the different pathways for subducted
waters in the two hemispheres, the contributions
to equatorial thermocline waters (lighter than
26.2 ) by interior pathways is on the order of
14 Sv for the southern hemisphere, but only 4 Sv for
the northern hemisphere. Contributions via western
boundary currents amount to approximately 14 Sv
for each hemisphere (see Butt and Lindstrom, 1994;
Wijffels, 1993; Huang and Liu, 1999; Johnson and
McPhaden, 1999).
The details of the low-latitude western boundary flows are complex and involve several coastal
currents that include the Mindanao Current
(Lukas et al., 1991; Fine et al., 1994; Bingham and
Lukas, 1995), the New Guinea Coastal Undercurrent (Lindstrom et al., 1987), and the New Ireland
Coastal Undercurrent (Butt and Lindstrom, 1994).
These currents are highly variable so that information about them is tentative. It appears that waters
from the southern hemisphere penetrate into the
northern hemisphere. Some of it joins the eastward
North Equatorial Countercurrent, and some leaks
into the Celebes Sea and eventually into the Indian
Ocean because of the Indonesian Throughflow.
The salinity distribution in the Equatorial
Undercurrent (Figs 4.4.1b and 4.4.2c) suggests that
much of the water in that current comes from the
saline South Pacific. However, that inference is
tentative until we explore the role of entrainment
of subthermocline waters into the Equatorial
Undercurrent, the topic of the next section.
4.4.3 Tropical–subpolar exchange of
Intermediate Waters
Immediately beneath the equatorial thermocline is
a layer of low vertical stability, with temperatures
between 11°C and 14°C, known as the thermostad
(Montgomery and Stroup, 1962). The cause of this
low stability has received surprisingly little attention. It probably is a consequence of vertical
mixing induced by the large vertical shear of the
Equatorial Undercurrent. This mixing is a means
for Intermediate Waters to be entrained into shallower layers and hence is an important aspect of
the maintenance of the equatorial thermocline.
Clues to the origin of these Intermediate Waters
come from its properties that are evident in
Fig. 4.4.2: high salinity, high oxygen concentration
and low nutrient concentration. On the basis of
the salinity distribution along isopycnals, Tsuchiya
(1981) suggested that these waters have their
provenance in the surface layers of the ocean
northeast of New Zealand, and in the Tasman Sea,
which, according to Toggweiler et al. (1991), may
SECTION 4 THE GLOBAL FLOW FIELD
252
explained in terms of simple thermocline models
(Liu, 1994; McCreary and Lu, 1994; Liu and
Philander, 1995). Liu (1994) describes how the
boundaries of these different subduction windows
depend on the structure of the windstress patterns.
The tropical–subtropical water exchange in the
Pacific is strongly affected by the complex spatial
pattern of the trade wind. The asymmetry of that
field relative to the equator, the Intertropical Convergence Zone, where the southeast and northeast
trades meet, is mostly in the northern hemisphere
and causes the pathways of water parcels to be
different in the two hemispheres. Routes to the
equator in the southern hemisphere can be direct
but the presence of the eastward North Equatorial
Countercurrent in the northern hemisphere complicates matters. That current is associated with a
ridge in the thermocline near 10°N (see Fig. 4.4.2).
A parcel conserving potential vorticity could have
difficulty negotiating that ridge, an impression that
one may also have from the salinity distribution of
Fig. 4.4.2c (Lu and McCreary, 1995). Nevertheless, the thermocline deepens westward and therefore it remains possible for the flow to conserve
potential vorticity while crossing the thermocline
ridge. Dynamically, this thermocline ridge is
forced by the dominant Ekman upwelling in this
region. The upwelling produces a northward
geostrophic current that tends to block the interior
equatorward penetration. However, in the central
Pacific, the Ekman pumping is downward in the
latitudes of North Equatorial Countercurrent. This
generates a southward geostrophic flow and therefore enables the interior pathway to penetrate
across the latitude of North Equatorial Countercurrent (McPhaden and Fine, 1988; Liu and
Huang, 1998). That is why, in Fig. 4.4.5 (see Plate
4.4.5, p. 300), the northern hemisphere interior
trajectory moves southwestward up to 10°N, then
continues southeastward in the Countercurrent.
These results are confirmed by recent modelling
(Huang, 1996; Blanke and Raynaud, 1997; Gu
and Philander, 1997; Lu et al., 1998; Rothstein
et al., 1998b; Huang and Liu, 1999; Harper, 2000)
and hydrographic (Fig. 4.4.6, see Plate 4.4.6,
p. 300; Johnson and McPhaden, 1999) studies.
Because of the different pathways for subducted
waters in the two hemispheres, the contributions
to equatorial thermocline waters (lighter than
26.2 ) by interior pathways is on the order of
14 Sv for the southern hemisphere, but only 4 Sv for
the northern hemisphere. Contributions via western
boundary currents amount to approximately 14 Sv
for each hemisphere (see Butt and Lindstrom, 1994;
Wijffels, 1993; Huang and Liu, 1999; Johnson and
McPhaden, 1999).
The details of the low-latitude western boundary flows are complex and involve several coastal
currents that include the Mindanao Current
(Lukas et al., 1991; Fine et al., 1994; Bingham and
Lukas, 1995), the New Guinea Coastal Undercurrent (Lindstrom et al., 1987), and the New Ireland
Coastal Undercurrent (Butt and Lindstrom, 1994).
These currents are highly variable so that information about them is tentative. It appears that waters
from the southern hemisphere penetrate into the
northern hemisphere. Some of it joins the eastward
North Equatorial Countercurrent, and some leaks
into the Celebes Sea and eventually into the Indian
Ocean because of the Indonesian Throughflow.
The salinity distribution in the Equatorial
Undercurrent (Figs 4.4.1b and 4.4.2c) suggests that
much of the water in that current comes from the
saline South Pacific. However, that inference is
tentative until we explore the role of entrainment
of subthermocline waters into the Equatorial
Undercurrent, the topic of the next section.
4.4.3 Tropical–subpolar exchange of
Intermediate Waters
Immediately beneath the equatorial thermocline is
a layer of low vertical stability, with temperatures
between 11°C and 14°C, known as the thermostad
(Montgomery and Stroup, 1962). The cause of this
low stability has received surprisingly little attention. It probably is a consequence of vertical
mixing induced by the large vertical shear of the
Equatorial Undercurrent. This mixing is a means
for Intermediate Waters to be entrained into shallower layers and hence is an important aspect of
the maintenance of the equatorial thermocline.
Clues to the origin of these Intermediate Waters
come from its properties that are evident in
Fig. 4.4.2: high salinity, high oxygen concentration
and low nutrient concentration. On the basis of
the salinity distribution along isopycnals, Tsuchiya
(1981) suggested that these waters have their
provenance in the surface layers of the ocean
northeast of New Zealand, and in the Tasman Sea,
which, according to Toggweiler et al. (1991), may
SECTION 4 THE GLOBAL FLOW FIELD
252