Pacific Trade Winds Biome
367
ridge in the pycnocline west of the Costa Rica Dome, and accordingly the squid occur
in high abundance and are spatially associated with a well-developed deep chlorophyll
maximum. This did not occur in autumn 1999, when La Niña conditions prevailed.
The strong salinity front (see earlier discussion) that characterizes the edge of the NECC
retains the squid along the ridge, and also yellowfin tuna (Thunnus albacares) that—like
Dosidocus—feed largely on micronekton.
Synopsis
Case 4—Small-amplitude response to trade-wind seasonality—Mixed-layer depth has
insignificant seasonality when meaned over the whole area of the province (20–30 m
year-round) and because Z eu is consistently deeper (50–60 m), the thermocline is illuminated in all months. Productivity is seasonally almost invariant (030–045 mg C m
−2 d
−1 )
with a minor depression in June–July during strongest trade-wind season (Fig. 11.10);
between-year differences are significant and respond to the value of the SOI. Chlorophyll
biomass shows only slight enhancement in the first half of each year. The pattern is
consistent with close balance in tropical seas between consumers and production.
Pacific Equatorial Divergence Province (PEQD)
Extent of the Province
PEQD lies between the Equatorial Convergent Front (ECF) to the north and the irregular
flow of the South Equatorial Counter Current to the south. These were termed the
Northern and Southern Doldrum fronts by Roden (1975). The northern front is, as we
have seen when discussing the PNEC province, the southern boundary of the eastwardflowing NECC and lies above a thermocline trough at distances north of the equator that
increase eastward: it lies at about 5
N at the date line, and 12
N in the eastern Pacific.
The less distinct front of the SECC defines the southern limit of PEQD at about 5
S in the
west and almost 20
S in the east. This front—or series of weak fronts—lies above a region
where the thermocline deepens rapidly to the south; in the eastern Pacific it separates
the warmer equatorial water from the cooler water of the Peru Current. PEQD is thus
symmetrical about the equator, only from 115
W to 180
W, where it is narrowest (5
N
to 5
S). Eastward from here it extends increasingly toward the south until it encounters
the eastern coastal boundary current (CHIL province). The westward extent of PEQD
is time-dependent, and the limit at 180
W is set simply as the climatological average
condition, and for the same reasons as discussed earlier for the PNEC province.
Defining Characteristics of Regional Oceanography
The surface circulation here is dominated by the flow of the South Equatorial Current
(SEC) westward across the ocean, forced by the southeast trades of the South Pacific
Ocean, a response well described by Wyrtki (1966). This westward stream extends across
the equator to about 5
N and thus requires a ridge in the topography of the thermocline
along the equator so that, in each hemisphere, it flows along the poleward slope of this
ridge. At the equator, flow in the SEC is relatively shallow, extending only to as little as
20–50 m although flow is strongest, reaching 50 cm sec
−1 . South of the equator, the SEC
is interrupted by variable threads of eastward flow attributable to the SECC; that part of
the SEC that lies equatorward of the SECC is designated SEC 01, while that lying south
of the SECC is designated SEC 02 (Wyrtki, 1984). The latter, of course, forms the flow
around the northern part of the South Pacific Subtropical Gyre, the SPSG discussed later.
The flow of the SEC westward across the ocean represents about 50 × 10
6 m
3 sec
−1 but, of
this, only about 25% flows within SEC 02. The narrow (<200 km), long (14,000 km) jet of
367
ridge in the pycnocline west of the Costa Rica Dome, and accordingly the squid occur
in high abundance and are spatially associated with a well-developed deep chlorophyll
maximum. This did not occur in autumn 1999, when La Niña conditions prevailed.
The strong salinity front (see earlier discussion) that characterizes the edge of the NECC
retains the squid along the ridge, and also yellowfin tuna (Thunnus albacares) that—like
Dosidocus—feed largely on micronekton.
Synopsis
Case 4—Small-amplitude response to trade-wind seasonality—Mixed-layer depth has
insignificant seasonality when meaned over the whole area of the province (20–30 m
year-round) and because Z eu is consistently deeper (50–60 m), the thermocline is illuminated in all months. Productivity is seasonally almost invariant (030–045 mg C m
−2 d
−1 )
with a minor depression in June–July during strongest trade-wind season (Fig. 11.10);
between-year differences are significant and respond to the value of the SOI. Chlorophyll
biomass shows only slight enhancement in the first half of each year. The pattern is
consistent with close balance in tropical seas between consumers and production.
Pacific Equatorial Divergence Province (PEQD)
Extent of the Province
PEQD lies between the Equatorial Convergent Front (ECF) to the north and the irregular
flow of the South Equatorial Counter Current to the south. These were termed the
Northern and Southern Doldrum fronts by Roden (1975). The northern front is, as we
have seen when discussing the PNEC province, the southern boundary of the eastwardflowing NECC and lies above a thermocline trough at distances north of the equator that
increase eastward: it lies at about 5
N at the date line, and 12
N in the eastern Pacific.
The less distinct front of the SECC defines the southern limit of PEQD at about 5
S in the
west and almost 20
S in the east. This front—or series of weak fronts—lies above a region
where the thermocline deepens rapidly to the south; in the eastern Pacific it separates
the warmer equatorial water from the cooler water of the Peru Current. PEQD is thus
symmetrical about the equator, only from 115
W to 180
W, where it is narrowest (5
N
to 5
S). Eastward from here it extends increasingly toward the south until it encounters
the eastern coastal boundary current (CHIL province). The westward extent of PEQD
is time-dependent, and the limit at 180
W is set simply as the climatological average
condition, and for the same reasons as discussed earlier for the PNEC province.
Defining Characteristics of Regional Oceanography
The surface circulation here is dominated by the flow of the South Equatorial Current
(SEC) westward across the ocean, forced by the southeast trades of the South Pacific
Ocean, a response well described by Wyrtki (1966). This westward stream extends across
the equator to about 5
N and thus requires a ridge in the topography of the thermocline
along the equator so that, in each hemisphere, it flows along the poleward slope of this
ridge. At the equator, flow in the SEC is relatively shallow, extending only to as little as
20–50 m although flow is strongest, reaching 50 cm sec
−1 . South of the equator, the SEC
is interrupted by variable threads of eastward flow attributable to the SECC; that part of
the SEC that lies equatorward of the SECC is designated SEC 01, while that lying south
of the SECC is designated SEC 02 (Wyrtki, 1984). The latter, of course, forms the flow
around the northern part of the South Pacific Subtropical Gyre, the SPSG discussed later.
The flow of the SEC westward across the ocean represents about 50 × 10
6 m
3 sec
−1 but, of
this, only about 25% flows within SEC 02. The narrow (<200 km), long (14,000 km) jet of