reproduced by estimates from the TOGA XBT network, within the standard deviations of both measurements. The mean EUC and SEC from the
Tahiti-to-Hawaii Shuttle data of Fig. 4.3.2 are also
well reproduced in XBT data at similar longitudes
(Picaut and Tournier, 1991). Drifter data confirm
the universally poleward mean nature of surface
flow in the Pacific (TOGAObs, Reverdin et al.,
1994). TAO mooring data provide mean seasonal
cycles of zonal flow and temperature along the
equator at four longitudes, with at least 7 years of
data in each average (TOGAObs). The boreal
spring decrease in the SEC and associated increase
in the upper parts of the EUC is particularly noteworthy at all longitudes; these are earliest and of
greatest magnitude in the east Pacific. There are
indications that the observed annual-cycle variations in current, SST and wind stresses may be
linked through an ENSO-like feed-back mechanism, and that the annual cycle is non-stationary –
e.g. annual thermocline variations were much
greater in the 1990s than the 1980s. TOGAObs
also reviews work on mean seasonal cycles elsewhere in the equatorial Pacific, using XBT, drifter
and tide gauge data.
For a review of variability on ENSO, Madden–
Julian Oscillations and Tropical Instability Wave
time scales, we refer the reader to TOGAObs and
the accompanying articles in the TOGA Special
Issue of the Journal of Geophysical Research, of
June 1998. However, one item of particular interest in the context of WOCE is the relation of the
Indonesian Throughflow (IT) to ENSO. Clarke and
Liu (1993, 1994) and Meyers (1996) showed that
the strength of the IT was closely related to ENSO,
using sea level and XBT data, respectively. According to Clarke and Liu, the ENSO variations of
thermocline depth in the western Pacific are transmitted by Kelvin and Rossby waves along the western coast of Australasia. Thermocline behaviour at
the Indonesian coast is determined by Indian
Ocean wind. Both have an ENSO component of
the same sign, but the latter is much smaller. As a
result, the peak-to-trough variation of the IT is
about <5 Sv – comparable to its mean value (7 Sv,
from steric heights relative to 400 db).
4.3.2.2 Mean subthermocline zonal currents in the
equatorial Pacific
Current structure below the thermocline has obviously received less attention than upper ocean flow.
However, Firing (1987, 1989) obtained 41 meridional sections of currents from top to bottom across
the equatorial Pacific, at 159°W, over 16 months in
1982–83. The resulting long-term mean section of
zonal flow (Fig. 4.3.3a) shows a remarkably complex pattern of interleaving jets. Even the nomenclature is difficult, with some significant features (e.g.
the eastward current at 3000 m near 2°S and the
westward flow near 4000 m) having no name as yet.
The observations spanned the 1982–83 ENSO event
but are nevertheless similar to earlier data sets, so
Firing (1987) suggested that these flows may be permanent. This has been largely borne out by the
results of Firing et al. (1998). They examined 12
top-to-bottom velocity sections, mostly measured on
WOCE cruises, and a geostrophic current estimate
based on the average of a large number of hydrographic sections at 165°E between 1984 and 1991
(Gouriou and Toole, 1993; Wijffels, 1993). The latter average (Fig. 4.3.3b) shows a remarkable similarity to Fig. 4.3.3a; the South and North Intermediate
CounterCurrents (SICC, NICC) and the Lower and
Southern Equatorial Intermediate Currents (LEIC,
SEIC) and 3000 m current are all quite similar in
magnitude and location to Fig. 4.3.3a. At 165°E the
4000 m current is located on the equator, though
sample density below 2000 m decreases, making the
mean less reliable at such depths. The ‘Equatorial
Deep Jet’ regime, with alternating jets about 2° wide
every 200–300 m between 500 and 2500 m, is just
as evident in the mean geostrophic flow at 165°E as
in Fig. 4.3.3a. The only large differences at 165°E
from Fig. 4.3.3a lie above 300 m, where the Northern and Southern Subsurface CounterCurrents
(NSCC, SSCC; also known as Tsuchiya Jets) are
observed at 165°E about 2–3° from the equator near
270 m. In the far west Pacific, the Subsurface CounterCurrents (SCCs) typically form minor maxima
within the general band of eastward flow that
includes the EUC and the NECC; however, the
SCCs diverge poleward, developing a distinct identity further east. The south edge of the NSCC is just
seen in Fig. 4.3.3a. The direct velocity sections provide a spot sampling of variability with longitude,
season and phase of the ENSO cycle; they, too, generally confirmed the idea that the 159°W section is
representative of the mean flow, over most of the
width of the Pacific. However, departures of individual sections from the mean are substantial, and they
show no obvious relationship with season or with
the phase of the ENSO cycle.
SECTION 4 THE GLOBAL FLOW FIELD
222
Tahiti-to-Hawaii Shuttle data of Fig. 4.3.2 are also
well reproduced in XBT data at similar longitudes
(Picaut and Tournier, 1991). Drifter data confirm
the universally poleward mean nature of surface
flow in the Pacific (TOGAObs, Reverdin et al.,
1994). TAO mooring data provide mean seasonal
cycles of zonal flow and temperature along the
equator at four longitudes, with at least 7 years of
data in each average (TOGAObs). The boreal
spring decrease in the SEC and associated increase
in the upper parts of the EUC is particularly noteworthy at all longitudes; these are earliest and of
greatest magnitude in the east Pacific. There are
indications that the observed annual-cycle variations in current, SST and wind stresses may be
linked through an ENSO-like feed-back mechanism, and that the annual cycle is non-stationary –
e.g. annual thermocline variations were much
greater in the 1990s than the 1980s. TOGAObs
also reviews work on mean seasonal cycles elsewhere in the equatorial Pacific, using XBT, drifter
and tide gauge data.
For a review of variability on ENSO, Madden–
Julian Oscillations and Tropical Instability Wave
time scales, we refer the reader to TOGAObs and
the accompanying articles in the TOGA Special
Issue of the Journal of Geophysical Research, of
June 1998. However, one item of particular interest in the context of WOCE is the relation of the
Indonesian Throughflow (IT) to ENSO. Clarke and
Liu (1993, 1994) and Meyers (1996) showed that
the strength of the IT was closely related to ENSO,
using sea level and XBT data, respectively. According to Clarke and Liu, the ENSO variations of
thermocline depth in the western Pacific are transmitted by Kelvin and Rossby waves along the western coast of Australasia. Thermocline behaviour at
the Indonesian coast is determined by Indian
Ocean wind. Both have an ENSO component of
the same sign, but the latter is much smaller. As a
result, the peak-to-trough variation of the IT is
about <5 Sv – comparable to its mean value (7 Sv,
from steric heights relative to 400 db).
4.3.2.2 Mean subthermocline zonal currents in the
equatorial Pacific
Current structure below the thermocline has obviously received less attention than upper ocean flow.
However, Firing (1987, 1989) obtained 41 meridional sections of currents from top to bottom across
the equatorial Pacific, at 159°W, over 16 months in
1982–83. The resulting long-term mean section of
zonal flow (Fig. 4.3.3a) shows a remarkably complex pattern of interleaving jets. Even the nomenclature is difficult, with some significant features (e.g.
the eastward current at 3000 m near 2°S and the
westward flow near 4000 m) having no name as yet.
The observations spanned the 1982–83 ENSO event
but are nevertheless similar to earlier data sets, so
Firing (1987) suggested that these flows may be permanent. This has been largely borne out by the
results of Firing et al. (1998). They examined 12
top-to-bottom velocity sections, mostly measured on
WOCE cruises, and a geostrophic current estimate
based on the average of a large number of hydrographic sections at 165°E between 1984 and 1991
(Gouriou and Toole, 1993; Wijffels, 1993). The latter average (Fig. 4.3.3b) shows a remarkable similarity to Fig. 4.3.3a; the South and North Intermediate
CounterCurrents (SICC, NICC) and the Lower and
Southern Equatorial Intermediate Currents (LEIC,
SEIC) and 3000 m current are all quite similar in
magnitude and location to Fig. 4.3.3a. At 165°E the
4000 m current is located on the equator, though
sample density below 2000 m decreases, making the
mean less reliable at such depths. The ‘Equatorial
Deep Jet’ regime, with alternating jets about 2° wide
every 200–300 m between 500 and 2500 m, is just
as evident in the mean geostrophic flow at 165°E as
in Fig. 4.3.3a. The only large differences at 165°E
from Fig. 4.3.3a lie above 300 m, where the Northern and Southern Subsurface CounterCurrents
(NSCC, SSCC; also known as Tsuchiya Jets) are
observed at 165°E about 2–3° from the equator near
270 m. In the far west Pacific, the Subsurface CounterCurrents (SCCs) typically form minor maxima
within the general band of eastward flow that
includes the EUC and the NECC; however, the
SCCs diverge poleward, developing a distinct identity further east. The south edge of the NSCC is just
seen in Fig. 4.3.3a. The direct velocity sections provide a spot sampling of variability with longitude,
season and phase of the ENSO cycle; they, too, generally confirmed the idea that the 159°W section is
representative of the mean flow, over most of the
width of the Pacific. However, departures of individual sections from the mean are substantial, and they
show no obvious relationship with season or with
the phase of the ENSO cycle.
SECTION 4 THE GLOBAL FLOW FIELD
222