Richardson-number-dependent parameterization of
vertical eddy diffusivity and viscosity; they remark
that ‘this parameterisation may give rise to enough
physically-based vertical mixing for equatorial
upwelling to be favored over more uniform
upwelling’. Dengler and Quadfasel (2001) provide
clear evidence of layers with low Richardson
number between reversing jets in the Indian Ocean
(see below).
Equatorial Intermediate Currents
The four Intermediate Currents seen in Fig. 4.3.3a
near 800 m (SEIC, SICC, LEIC, NICC; see caption
to Fig. 4.3.3a) appear to extend from 165°E to at
least 135°W (Firing et al., 1998). There is no obvious evidence of change in latitude at least from
165°E to 159°W, suggesting that they may basically be passive inertial jets. They occur at the
depth at which Antarctic Intermediate Water
crosses the equator, in the New Guinea Coastal
Undercurrent (Tsuchiya, 1991), and their existence
may be dynamically linked to the need for that
water to reverse the sign of its vorticity; but the
details have so far not been explained. The westward-flowing EIC (Delcroix and Henin, 1988)
near 1.5°N, 400 m in Fig. 4.3.3a has also received
little study so far. At 165°E the EIC lies directly on
the equator, with quite strong vertical shears
between it and the EUC above it (Fig. 4.3.3b), at
the level of the SCCs (see next subsection). Low
Richardson numbers have been observed in this
region, suggesting that internal mixing may occur
here (Toole et al., 1987). Wijffels (1993) ascribes
an eastern tropical Pacific origin to EIC water.
Bingham and Lukas (1995) name this water mass
‘South Pacific Tropical Intermediate Water’, and
note that a similar but slightly different water
mass (‘North Pacific Tropical Intermediate Water’)
lies just north of the eastward-flowing NECC.
Both water masses have oxygen minima; Bingham
and Lukas suggest they may be trapped in unventilated ‘shadow zones’ (Luyten et al., 1983).
Subsurface Countercurrents – ‘Tsuchiya Jets’
These features – the off-equatorial lobes of the
dumbbell-shaped region of eastward flow in Fig.
4.3.3b, near 250 m – have been extensively studied
recently (Johnson and Moore, 1997; Rowe et al.,
2000), following on from work by Tsuchiya
(1972, 1975, 1981), Tsuchiya et al. (1989) and
McPhaden (1984). Johnson and Moore obtained
temperature, salinity and pressure averages on
neutral density surfaces from CTD data available
in 1997, and objectively analysed the result to
form a sharper picture of the SCCs than had been
obtained earlier. Confirming the earlier work, they
found two jets that proceed from west to east with
rather constant transport (8.5<1.5 Sv for the
NSCC; 5.5<1.5 Sv for the SSCC). Peak velocities
shoaled and diverged from 240–250 m (<2.5°) at
165°E to 130–160 m (<5°) at 110°W. The northern jet was slightly shallower than the southern.
The jets become ‘thicker’ with distance downstream. The transport-weighted neutral density of
the SCCs also decreases from west to east. Both
SCCs are marked by minima in buoyancy frequency, N
2
. The NSCC marks a strong salinity
front (fresher water to the north), apparently
marking the northern limit of South Pacific water
carried north along the Australian and Papua New
Guinea coast and across the equator at this density
level. A weaker core of high salinity marks the
SSCC (see also Johnson and McPhaden, 1999).
Tsuchiya (1981) found a southern hemisphere
origin for the SSCC, while Bingham and Lukas
(1995) found that water from both hemispheres
contribute to the NSCC.
Johnson and McPhaden (1999) show maps of
depth, acceleration potential, salinity and planetary potential vorticity on the 26.5 neutral surface,
which cuts through the SCCs. The SSCC proceeds
directly to the South American coast near 10°S,
confirming Lukas’ (1986) conclusion that the
SSCC and EUC turn south to feed the Peru–Chile
Undercurrent and Countercurrent. The NSCC
fades out just before reaching central America,
apparently entrained into the northernmost edge
of the North Equatorial Current. It is likely to be
strongly freshened in the process, and to enter the
southernmost edge of the North Equatorial Current. It is thus a candidate to supply upper level
water in the Indonesian Throughflow (Godfrey
et al., 1993; Lu et al., 1998). The SCCs form the
poleward boundaries of a broad equatorial region
of low planetary potential vorticity.
Johnson and Moore (1997) idealized the SCCs
with an inertial model, in which the jets were
bounded above by a neutral surface that sloped uniformly from east to west (the base of the thermocline). They were bounded below by a surface that
was flat on either side of each jet, but with a greater
depth equatorward of each jet than poleward.
4.3 The Tropical Ocean Circulation
225
Godfrey, Johnson, McPhaden, Reverdin and Wijffels
vertical eddy diffusivity and viscosity; they remark
that ‘this parameterisation may give rise to enough
physically-based vertical mixing for equatorial
upwelling to be favored over more uniform
upwelling’. Dengler and Quadfasel (2001) provide
clear evidence of layers with low Richardson
number between reversing jets in the Indian Ocean
(see below).
Equatorial Intermediate Currents
The four Intermediate Currents seen in Fig. 4.3.3a
near 800 m (SEIC, SICC, LEIC, NICC; see caption
to Fig. 4.3.3a) appear to extend from 165°E to at
least 135°W (Firing et al., 1998). There is no obvious evidence of change in latitude at least from
165°E to 159°W, suggesting that they may basically be passive inertial jets. They occur at the
depth at which Antarctic Intermediate Water
crosses the equator, in the New Guinea Coastal
Undercurrent (Tsuchiya, 1991), and their existence
may be dynamically linked to the need for that
water to reverse the sign of its vorticity; but the
details have so far not been explained. The westward-flowing EIC (Delcroix and Henin, 1988)
near 1.5°N, 400 m in Fig. 4.3.3a has also received
little study so far. At 165°E the EIC lies directly on
the equator, with quite strong vertical shears
between it and the EUC above it (Fig. 4.3.3b), at
the level of the SCCs (see next subsection). Low
Richardson numbers have been observed in this
region, suggesting that internal mixing may occur
here (Toole et al., 1987). Wijffels (1993) ascribes
an eastern tropical Pacific origin to EIC water.
Bingham and Lukas (1995) name this water mass
‘South Pacific Tropical Intermediate Water’, and
note that a similar but slightly different water
mass (‘North Pacific Tropical Intermediate Water’)
lies just north of the eastward-flowing NECC.
Both water masses have oxygen minima; Bingham
and Lukas suggest they may be trapped in unventilated ‘shadow zones’ (Luyten et al., 1983).
Subsurface Countercurrents – ‘Tsuchiya Jets’
These features – the off-equatorial lobes of the
dumbbell-shaped region of eastward flow in Fig.
4.3.3b, near 250 m – have been extensively studied
recently (Johnson and Moore, 1997; Rowe et al.,
2000), following on from work by Tsuchiya
(1972, 1975, 1981), Tsuchiya et al. (1989) and
McPhaden (1984). Johnson and Moore obtained
temperature, salinity and pressure averages on
neutral density surfaces from CTD data available
in 1997, and objectively analysed the result to
form a sharper picture of the SCCs than had been
obtained earlier. Confirming the earlier work, they
found two jets that proceed from west to east with
rather constant transport (8.5<1.5 Sv for the
NSCC; 5.5<1.5 Sv for the SSCC). Peak velocities
shoaled and diverged from 240–250 m (<2.5°) at
165°E to 130–160 m (<5°) at 110°W. The northern jet was slightly shallower than the southern.
The jets become ‘thicker’ with distance downstream. The transport-weighted neutral density of
the SCCs also decreases from west to east. Both
SCCs are marked by minima in buoyancy frequency, N
2
. The NSCC marks a strong salinity
front (fresher water to the north), apparently
marking the northern limit of South Pacific water
carried north along the Australian and Papua New
Guinea coast and across the equator at this density
level. A weaker core of high salinity marks the
SSCC (see also Johnson and McPhaden, 1999).
Tsuchiya (1981) found a southern hemisphere
origin for the SSCC, while Bingham and Lukas
(1995) found that water from both hemispheres
contribute to the NSCC.
Johnson and McPhaden (1999) show maps of
depth, acceleration potential, salinity and planetary potential vorticity on the 26.5 neutral surface,
which cuts through the SCCs. The SSCC proceeds
directly to the South American coast near 10°S,
confirming Lukas’ (1986) conclusion that the
SSCC and EUC turn south to feed the Peru–Chile
Undercurrent and Countercurrent. The NSCC
fades out just before reaching central America,
apparently entrained into the northernmost edge
of the North Equatorial Current. It is likely to be
strongly freshened in the process, and to enter the
southernmost edge of the North Equatorial Current. It is thus a candidate to supply upper level
water in the Indonesian Throughflow (Godfrey
et al., 1993; Lu et al., 1998). The SCCs form the
poleward boundaries of a broad equatorial region
of low planetary potential vorticity.
Johnson and Moore (1997) idealized the SCCs
with an inertial model, in which the jets were
bounded above by a neutral surface that sloped uniformly from east to west (the base of the thermocline). They were bounded below by a surface that
was flat on either side of each jet, but with a greater
depth equatorward of each jet than poleward.
4.3 The Tropical Ocean Circulation
225
Godfrey, Johnson, McPhaden, Reverdin and Wijffels