362
Chapter 11: The Pacific Ocean
values occurring during boreal winter when productivity is minimal. This is perhaps an
effect of the seasonal migration to depth of herbivores in the northern part of the province.
North Pacific Equatorial Countercurrent
Province (PNEC)
Extent of the Province
The North Pacific Equatorial Countercurrent Province (PNEC) is much smaller than the
NPTG Province and lies between the northern and southern Doldrum salinity fronts,
that is to say around 5–10
N in the central Pacific, widening toward the American
continent. Thus, PNEC includes the triangular region of weak currents that lies between
the influence of the equatorial divergence and the California Current extension that flows
into the North Equatorial Current and the NPTG province. To the west, a meaningful
limit to PNEC can be set around 180
E.
Defining Characteristics of Regional Oceanography
The flow across this province of the NECC takes its origin in the Mindanao Dome at the
termination of the western boundary current and terminates in the cyclonic flow around
the Costa Rica Dome, off central America.
The western boundary of PNEC is set at the date line for the following reasons: despite
the trans-Pacific nature of the North Equatorial Countercurrent (NECC), west of 180
E
this flow lies below the western Pacific atmospheric convection cells, where excess of
precipitation over evaporation induces a different ecological regime, that characteristic
of the WARM province. Near the date line, a significant surface salinity front was clearly
identified in the JGOFS Flupac sections (Le Borgne et al., 2002a); here, salinity rises
eastward rather abruptly by about 0.5‰, although surface temperature shows no change
in its gradual eastward cooling trend. Here also, concentrations of NO 3 and chlorophyll
change rather abruptly to higher values to the east. The boundary between WARM and
PNEC is, therefore, significant for pelagic ecology.
The southern boundary lies at the oceanic Equatorial or Doldrum Front that crosses the
equator near the Galapagos and is marked at the surface by a discontinuity in temperature,
salinity, and in mixed-layer nitrate that is delivered by divergence at the equator.
The NECC flows eastward across the ocean, lying above the north slope of the equatorial thermocline ridge, all the way from 120
E to Central America where it turns
northward. Flow of this countercurrent is weaker and more intermittent progressively
toward the east; at 135
E it is 25 Sv, but near its termination in the eastern Pacific it
is reduced to only about 10 Sv. Accordingly, mixed-layer depth must shallow toward
the east. Generic NECCs are the product more of wind stress curl than of wind stress
(Sverdrup, 1947; Philander, 1985), and when the Intertropical Convergence Zone (ITCZ)
is in its northerly, boreal summer position, the NECC lies below a zone of exceptionally
strong positive wind-stress curl that lies across the whole ocean, from the Indo-Pacific
conjunction all the way to Central America above the thermal ridge that is associated
with flow of the NECC (Lagler and O’Brien, 1980). From July to January, positive values
of Ekman vertical velocity are higher in the eastern part of the ocean (from 140
W to
the American continent), reaching local maxima of 60 × 10
−6 m sec
−1 .
For these reasons, flow of the NECC is strongly seasonal in the eastern part of this
province. During boreal winter, when the atmospheric ITCZ is at its most southerly, the
countercurrent can still be detected in thermocline topography, but is sufficiently weak
that flow at the surface does not occur east of about 110–120
W (Tomczak and Godfrey,
1994). In the eastern tropical Pacific, the ridge-trough system of the thermocline occupies
a wider, triangular region between the coast and the convergent fronts of the North and
Chapter 11: The Pacific Ocean
values occurring during boreal winter when productivity is minimal. This is perhaps an
effect of the seasonal migration to depth of herbivores in the northern part of the province.
North Pacific Equatorial Countercurrent
Province (PNEC)
Extent of the Province
The North Pacific Equatorial Countercurrent Province (PNEC) is much smaller than the
NPTG Province and lies between the northern and southern Doldrum salinity fronts,
that is to say around 5–10
N in the central Pacific, widening toward the American
continent. Thus, PNEC includes the triangular region of weak currents that lies between
the influence of the equatorial divergence and the California Current extension that flows
into the North Equatorial Current and the NPTG province. To the west, a meaningful
limit to PNEC can be set around 180
E.
Defining Characteristics of Regional Oceanography
The flow across this province of the NECC takes its origin in the Mindanao Dome at the
termination of the western boundary current and terminates in the cyclonic flow around
the Costa Rica Dome, off central America.
The western boundary of PNEC is set at the date line for the following reasons: despite
the trans-Pacific nature of the North Equatorial Countercurrent (NECC), west of 180
E
this flow lies below the western Pacific atmospheric convection cells, where excess of
precipitation over evaporation induces a different ecological regime, that characteristic
of the WARM province. Near the date line, a significant surface salinity front was clearly
identified in the JGOFS Flupac sections (Le Borgne et al., 2002a); here, salinity rises
eastward rather abruptly by about 0.5‰, although surface temperature shows no change
in its gradual eastward cooling trend. Here also, concentrations of NO 3 and chlorophyll
change rather abruptly to higher values to the east. The boundary between WARM and
PNEC is, therefore, significant for pelagic ecology.
The southern boundary lies at the oceanic Equatorial or Doldrum Front that crosses the
equator near the Galapagos and is marked at the surface by a discontinuity in temperature,
salinity, and in mixed-layer nitrate that is delivered by divergence at the equator.
The NECC flows eastward across the ocean, lying above the north slope of the equatorial thermocline ridge, all the way from 120
E to Central America where it turns
northward. Flow of this countercurrent is weaker and more intermittent progressively
toward the east; at 135
E it is 25 Sv, but near its termination in the eastern Pacific it
is reduced to only about 10 Sv. Accordingly, mixed-layer depth must shallow toward
the east. Generic NECCs are the product more of wind stress curl than of wind stress
(Sverdrup, 1947; Philander, 1985), and when the Intertropical Convergence Zone (ITCZ)
is in its northerly, boreal summer position, the NECC lies below a zone of exceptionally
strong positive wind-stress curl that lies across the whole ocean, from the Indo-Pacific
conjunction all the way to Central America above the thermal ridge that is associated
with flow of the NECC (Lagler and O’Brien, 1980). From July to January, positive values
of Ekman vertical velocity are higher in the eastern part of the ocean (from 140
W to
the American continent), reaching local maxima of 60 × 10
−6 m sec
−1 .
For these reasons, flow of the NECC is strongly seasonal in the eastern part of this
province. During boreal winter, when the atmospheric ITCZ is at its most southerly, the
countercurrent can still be detected in thermocline topography, but is sufficiently weak
that flow at the surface does not occur east of about 110–120
W (Tomczak and Godfrey,
1994). In the eastern tropical Pacific, the ridge-trough system of the thermocline occupies
a wider, triangular region between the coast and the convergent fronts of the North and