Tsuchiya and Talley (1998) in the eastern equatorial Pacific and McPhaden (1985) in the central
equatorial Pacific note the possibility that salt
fingering is an important mixing mechanism.
With regard to lateral mixing, tropical instability waves are the primary contributors to lateral
diffusivities in the tropical Pacific (Hansen and
Paul, 1984; Bryden and Brady, 1989; Flament
et al., 1996). The energetics of these waves are
complicated, and there appears to be more than
one form of instability of the major zonal currents
that can give rise to the observed variability
(Luther and Johnson, 1990; McCreary and Yu,
1992). Effective lateral heat and momentum diffusities due to these waves as inferred from observational studies are generally around 10
3 to
10
4 m
2 s
91 (Bryden and Brady, 1989; Swenson and
Hansen, 1999; Wang and McPhaden, 1999).
These diffusivities are generally positive, though
Lukas (1987) and Bryden and Brady (1989) found
instances (from mid-Pacific mooring data) when
eddy diffusivities were negative, both for momentum and heat. The implications of this result have
not to the authors’ knowledge been taken up yet
by the modelling community. Richards and Pollard (1991) and Richards (1998) have used finescale hydrographic observations across the western
Pacific to show that double-diffusion, combined
with observed interleaving flows with vertical
scales of a few tens of metres, can generate quite
large ‘horizontal diffusivities’ in the equatorial
western Pacific.
Dynamics of the Equatorial Undercurrent’s width,
depth and strength
Empirically, the simple balance dP/dx:
(x) holds
quite well along the equatorial Pacific and Atlantic
Oceans, where P is the depth-integrated pressure
above an assumed depth of no motion of (say)
400 db. For reasonable choices of ‘reduced gravity’, this implies that the thermocline depth must
be substantially greater than observed mixed-layer
depths in the western Pacific, or Atlantic. Below
the mixed layer, one might expect friction to be
small compared to the zonal pressure gradient;
since Coriolis force is zero, the balance should be
inertial – water simply accelerates down the pressure gradient. Pedlosky (1987a, 1991) developed
an inertial model of the EUC, matching it to the
Luyten et al. (1983) ‘ventilated thermocline’ away
from the equator; the model simulates Pacific EUC
width and strength quite well. Wacongne (1990)
notes that an integration of dP/dx:
(x) across the
Pacific and Atlantic implies greater thermocline
depth in the west Pacific compared with the
Atlantic, and suggested that this is the reason for
the substantially greater depth of the Pacific EUC
compared with the Atlantic. She compared OGCM
simulations of the Atlantic and Pacific EUCs; she
found an inertial acceleration zone at the western
end of the model EUCs. The maximum speed of
the EUC was found at the eastern end of this inertial region. Vertical friction was strongly dependent on Richardson number in this model, and
therefore only became important to the east of this
region, where shears became large and Richardson
numbers small. Qualitatively similar behaviour
occurred (in the OGCM) in the equatorial Pacific;
but here the inertial region extended about 72° of
longitude from the western boundary, compared
with only 14° in the Atlantic. Thus the Pacific
EUC reaches its maximum through the action of
gentle accelerations over a very long fetch; it is this
long fetch that allows the Pacific EUC to exceed
the Atlantic in strength. However, McPhaden
(1993) finds that similar conclusions also hold in a
linear, frictional model of the EUC, and he notes
that ‘the exact mix of linear and nonlinear
processes that leads to the observed differences
between the Atlantic and Pacific Oceans remains
to be determined’. The strongly non-linear nature
of both lateral and vertical friction, noted in
the preceding subsection, further complicates the
matter.
Temporal variability:Tropical Instability Waves,
Madden–Julian Oscillation, mean seasonal cycle, ENSO
As noted earlier, the task of working out the heat
and fresh water budget is still further complicated
by variability associated with the El Niño phenomenon, seasonal cycle, Madden–Julian Oscillations
of 40- to 60-day period, and Tropical Instability
Waves of 20- to 30-day period. To elucidate these,
it will be necessary to supplement WOCE observations with observations from the TOGA Observing System (McPhaden et al., 1998; referred to
below as TOGAObs).
Currents estimated from thermal data via geostrophy match more direct observations well. For
example, mean transport values for the EUC, SEC,
NEC and NECC along 150–158°W between March
1979 and June 1980 (Wyrtki et al., 1981) are
4.3 The Tropical Ocean Circulation
221
Godfrey, Johnson, McPhaden, Reverdin and Wijffels
equatorial Pacific note the possibility that salt
fingering is an important mixing mechanism.
With regard to lateral mixing, tropical instability waves are the primary contributors to lateral
diffusivities in the tropical Pacific (Hansen and
Paul, 1984; Bryden and Brady, 1989; Flament
et al., 1996). The energetics of these waves are
complicated, and there appears to be more than
one form of instability of the major zonal currents
that can give rise to the observed variability
(Luther and Johnson, 1990; McCreary and Yu,
1992). Effective lateral heat and momentum diffusities due to these waves as inferred from observational studies are generally around 10
3 to
10
4 m
2 s
91 (Bryden and Brady, 1989; Swenson and
Hansen, 1999; Wang and McPhaden, 1999).
These diffusivities are generally positive, though
Lukas (1987) and Bryden and Brady (1989) found
instances (from mid-Pacific mooring data) when
eddy diffusivities were negative, both for momentum and heat. The implications of this result have
not to the authors’ knowledge been taken up yet
by the modelling community. Richards and Pollard (1991) and Richards (1998) have used finescale hydrographic observations across the western
Pacific to show that double-diffusion, combined
with observed interleaving flows with vertical
scales of a few tens of metres, can generate quite
large ‘horizontal diffusivities’ in the equatorial
western Pacific.
Dynamics of the Equatorial Undercurrent’s width,
depth and strength
Empirically, the simple balance dP/dx:
(x) holds
quite well along the equatorial Pacific and Atlantic
Oceans, where P is the depth-integrated pressure
above an assumed depth of no motion of (say)
400 db. For reasonable choices of ‘reduced gravity’, this implies that the thermocline depth must
be substantially greater than observed mixed-layer
depths in the western Pacific, or Atlantic. Below
the mixed layer, one might expect friction to be
small compared to the zonal pressure gradient;
since Coriolis force is zero, the balance should be
inertial – water simply accelerates down the pressure gradient. Pedlosky (1987a, 1991) developed
an inertial model of the EUC, matching it to the
Luyten et al. (1983) ‘ventilated thermocline’ away
from the equator; the model simulates Pacific EUC
width and strength quite well. Wacongne (1990)
notes that an integration of dP/dx:
(x) across the
Pacific and Atlantic implies greater thermocline
depth in the west Pacific compared with the
Atlantic, and suggested that this is the reason for
the substantially greater depth of the Pacific EUC
compared with the Atlantic. She compared OGCM
simulations of the Atlantic and Pacific EUCs; she
found an inertial acceleration zone at the western
end of the model EUCs. The maximum speed of
the EUC was found at the eastern end of this inertial region. Vertical friction was strongly dependent on Richardson number in this model, and
therefore only became important to the east of this
region, where shears became large and Richardson
numbers small. Qualitatively similar behaviour
occurred (in the OGCM) in the equatorial Pacific;
but here the inertial region extended about 72° of
longitude from the western boundary, compared
with only 14° in the Atlantic. Thus the Pacific
EUC reaches its maximum through the action of
gentle accelerations over a very long fetch; it is this
long fetch that allows the Pacific EUC to exceed
the Atlantic in strength. However, McPhaden
(1993) finds that similar conclusions also hold in a
linear, frictional model of the EUC, and he notes
that ‘the exact mix of linear and nonlinear
processes that leads to the observed differences
between the Atlantic and Pacific Oceans remains
to be determined’. The strongly non-linear nature
of both lateral and vertical friction, noted in
the preceding subsection, further complicates the
matter.
Temporal variability:Tropical Instability Waves,
Madden–Julian Oscillation, mean seasonal cycle, ENSO
As noted earlier, the task of working out the heat
and fresh water budget is still further complicated
by variability associated with the El Niño phenomenon, seasonal cycle, Madden–Julian Oscillations
of 40- to 60-day period, and Tropical Instability
Waves of 20- to 30-day period. To elucidate these,
it will be necessary to supplement WOCE observations with observations from the TOGA Observing System (McPhaden et al., 1998; referred to
below as TOGAObs).
Currents estimated from thermal data via geostrophy match more direct observations well. For
example, mean transport values for the EUC, SEC,
NEC and NECC along 150–158°W between March
1979 and June 1980 (Wyrtki et al., 1981) are
4.3 The Tropical Ocean Circulation
221
Godfrey, Johnson, McPhaden, Reverdin and Wijffels