368
Chapter 11: The Pacific Ocean
the Equatorial Undercurrent (EUC) lies in the upper part of the equatorial thermocline
ridge noted earlier. Townsend Cromwell was the first to describe this flow, shortly before
his untimely death in an air crash on his way to join a Scripps ship on the Mexican coast.
Here, in the PEQD province, the EUC extends to within 40–50 m of the surface, whereas
further to the west it lies at 100–300 m.
The linearity of zonal flow in the equatorial Pacific, including both the NECC and
the SEC, is testified to by remarkable images representing mesoscale sea-level anomalies obtained by TOPEX-POSEIDON sensors; the ridge-trough system at the sea surface
observed subsequent to the 1997–1998 El Niño showed a ridge precisely aligned along the
equator and another, stronger one along the poleward side of the NECC; a trough separated the two. During the event itself, the equatorial zone of positive sea-level anomalies
was almost entirely absent, while that associated with the NECC was exceptionally strong
(see Color plate 18). The multiple threads of the SECC are represented in the relatively
poorly defined linear zones of sea surface elevation anomalies to the south of the equator.
Within this general circulation, linear divergence and upwelling occur along the equator forced by the change of sign of Coriolis parameter because, in each hemisphere,
westward motion must generate a poleward force. In many transequatorial sections from
135
W to 160
W (Wyrtki and Kilonsky, 1984; Colin et al., 1987; Carr et al., 1992) the
divergence—as indicated by the surfacing of isotherms and the nitrate distribution—is
aligned almost precisely along the equator and extends to about 2
N–2
S. East of 120
W,
however, the divergence lies somewhat to the south of the equator at all seasons; the
EASTROPAC sections show that 25
C water is exposed at the surface from 2
N to 5
S
in August and from 1
N to 3
S in February. Satellite thermal imagery shows that the
boundary between upwelled and surface water carries 1000-km wavelength instability
waves that propagate westward.
The ECF follows these waves, and this strongly convergent front may form a spectacular
(the word is carefully chosen) feature at the sea surface, because the transition between
cold, clear upwelled water within the SEC and the warm, greener water of the NECC
may be only a few tens of meters wide. This phenomenon is associated with current
shear and eddying and may also be marked by a field of whitecaps or an aggregation of
floating Thalassiosira mats. Very high concentrations of chlorophyll (background ×3) and
extremely high rates of autotrophic production (14–18 g C m
2 d
−1 ) have been observed
along such a convergent front at 140
W (Barber, 1992). The ECF contains the surface
isohalines for 34.4–34.9 ppt, whereas further south and sometimes at the edge of the
divergence zone there is usually a second salinity front containing the isohalines for 35.0–
35.4 ppt. These indicate progressive transitions in the relative influence of low-salinity
water of the NECC and the denser water of the SEC.
In the eastern part of the province, the ECF passes close around the north of the Galapagos Islands so that important anomalies in circulation and hence in the surface chlorophyll field must occur here. Under normal conditions, the island wakes lie to the west or
northwest of the islands and may exhibit plumes of high chlorophyll biomass, induced
within the turbulent wake. Under El Niño conditions, when flow at the surface may even
be temporarily reversed, such plumes may occur to the east and northeast of the islands.
This is, of course, one of the original high-S regions and fully to understand the nature
of the nitrate enigma, it is necessary to consider the three-dimensional distribution of
nitrate here (Thomas, 1972, 1978). I have already noted how sharp the boundary between
nitrate-replete and nitrate-depleted water is at the ECF. We must examine both the eastern
and western parts of the province: in the east, at 110–120
W, the EASTROPAC surveys
found a zone of nitrate-replete water between about 4
N and 10–15
S, or far to the south
of the divergence zone. North of this region, only in the CRD is 10-m nitrate higher
than 01 M. A nitrate section across the province shows that the mixed-layer nitrate
from about 8
S is separated from the deep nitracline that lies at 150 m at 10
S and slopes
Chapter 11: The Pacific Ocean
the Equatorial Undercurrent (EUC) lies in the upper part of the equatorial thermocline
ridge noted earlier. Townsend Cromwell was the first to describe this flow, shortly before
his untimely death in an air crash on his way to join a Scripps ship on the Mexican coast.
Here, in the PEQD province, the EUC extends to within 40–50 m of the surface, whereas
further to the west it lies at 100–300 m.
The linearity of zonal flow in the equatorial Pacific, including both the NECC and
the SEC, is testified to by remarkable images representing mesoscale sea-level anomalies obtained by TOPEX-POSEIDON sensors; the ridge-trough system at the sea surface
observed subsequent to the 1997–1998 El Niño showed a ridge precisely aligned along the
equator and another, stronger one along the poleward side of the NECC; a trough separated the two. During the event itself, the equatorial zone of positive sea-level anomalies
was almost entirely absent, while that associated with the NECC was exceptionally strong
(see Color plate 18). The multiple threads of the SECC are represented in the relatively
poorly defined linear zones of sea surface elevation anomalies to the south of the equator.
Within this general circulation, linear divergence and upwelling occur along the equator forced by the change of sign of Coriolis parameter because, in each hemisphere,
westward motion must generate a poleward force. In many transequatorial sections from
135
W to 160
W (Wyrtki and Kilonsky, 1984; Colin et al., 1987; Carr et al., 1992) the
divergence—as indicated by the surfacing of isotherms and the nitrate distribution—is
aligned almost precisely along the equator and extends to about 2
N–2
S. East of 120
W,
however, the divergence lies somewhat to the south of the equator at all seasons; the
EASTROPAC sections show that 25
C water is exposed at the surface from 2
N to 5
S
in August and from 1
N to 3
S in February. Satellite thermal imagery shows that the
boundary between upwelled and surface water carries 1000-km wavelength instability
waves that propagate westward.
The ECF follows these waves, and this strongly convergent front may form a spectacular
(the word is carefully chosen) feature at the sea surface, because the transition between
cold, clear upwelled water within the SEC and the warm, greener water of the NECC
may be only a few tens of meters wide. This phenomenon is associated with current
shear and eddying and may also be marked by a field of whitecaps or an aggregation of
floating Thalassiosira mats. Very high concentrations of chlorophyll (background ×3) and
extremely high rates of autotrophic production (14–18 g C m
2 d
−1 ) have been observed
along such a convergent front at 140
W (Barber, 1992). The ECF contains the surface
isohalines for 34.4–34.9 ppt, whereas further south and sometimes at the edge of the
divergence zone there is usually a second salinity front containing the isohalines for 35.0–
35.4 ppt. These indicate progressive transitions in the relative influence of low-salinity
water of the NECC and the denser water of the SEC.
In the eastern part of the province, the ECF passes close around the north of the Galapagos Islands so that important anomalies in circulation and hence in the surface chlorophyll field must occur here. Under normal conditions, the island wakes lie to the west or
northwest of the islands and may exhibit plumes of high chlorophyll biomass, induced
within the turbulent wake. Under El Niño conditions, when flow at the surface may even
be temporarily reversed, such plumes may occur to the east and northeast of the islands.
This is, of course, one of the original high-S regions and fully to understand the nature
of the nitrate enigma, it is necessary to consider the three-dimensional distribution of
nitrate here (Thomas, 1972, 1978). I have already noted how sharp the boundary between
nitrate-replete and nitrate-depleted water is at the ECF. We must examine both the eastern
and western parts of the province: in the east, at 110–120
W, the EASTROPAC surveys
found a zone of nitrate-replete water between about 4
N and 10–15
S, or far to the south
of the divergence zone. North of this region, only in the CRD is 10-m nitrate higher
than 01 M. A nitrate section across the province shows that the mixed-layer nitrate
from about 8
S is separated from the deep nitracline that lies at 150 m at 10
S and slopes