Pacific Trade Winds Biome
363
South Equatorial Currents where these turn westward away from the continent. Within
this region of weak flow and shallow mixed layer, the eastward countercurrent bifurcates
and the influence of the continent on regional wind stress and curvature results in
several cyclonic domes of ecological significance: the Costa Rica Dome (CRD), offshore
in the PNEC Province, and the Tehuantapec and Panama Bight domes in the coastal
boundary province CAMR.
The doming of the thermocline off Costa Rica, with seasonal intensification, has
regional ecological importance. Recent analysis of archived data and satellite imagery
has now revealed the real nature and seasonal evolution of the CRD (Fiedler, 2002).
Previously, it had been considered to be a quasipermanent feature located some 1000 km
southwest of Honduras and Nicaragua, but we now know that it is formed de novo each
winter. In February to April, it takes the form of a coastal shoaling of the thermocline in
the Gulf of Papagaya, forced by Ekman pumping to the south of the wind jet through
the sierra mountains behind the Gulf of Papagaya. In May–June it separates from the
coast as the ITCZ moves to the north in boreal summer, and then, in July–November
the CRD merges into the ridge associated with the NECC as this shoals beneath cyclonic
wind stress on the northern side of the ITCZ. Finally, in December–January, the CRD
deepens in response to strong trade winds as the ITCZ again moves to the south. Because
of the ecological importance of the CRD, it is good finally to have a satisfactory account
of its formation.
There is very little regular seasonality in the 20–35 m mixed layer of this province, and
irradiance at the surface is sufficiently strong that the entire mixed layer and much of
the thermocline lies perpetually within the euphotic zone. However, there is an eastward
decrease in mixed-layer temperatures right along the axis of the NECC, which is strongest
in boreal summer when very significant warming occurs in the western Pacific. A westward
flow of cooler water, associated with the NEC, is encountered below about 200 m.
It is in this province that we meet the extensive seasonal survey data obtained by
EASTROPAC in 1967–68 that, although not produced by the methods of today, still yield
valuable insights. Consider Fig. 11.9, which shows the near-instantaneous location of the
front between nitrate-depleted and nitrate-replete mixed-layer water on either side of the
boundary between PNEC and PEQD provinces, or along about 2–5
N. The sharpness of
this feature (which is, of course, the “Great Front” observed by Barber and his colleagues
on R/V Thompson almost 30 years later) is quite obscured in data representations such
as the well-known Ocean Atlas 2001, although it was mapped along 30
of longitude in
both extreme seasons in the EASTROPAC data.
Regional Response of the Pelagic Ecosystem
A seasonally and annually variable linear region of chlorophyll enhancement characterizes
the NECC region, and may occasionally—in some satellite images—be traced clear across
the Pacific Ocean to the origin of the NECC. Here, enhancement has been attributed to
upwelling associated with current meandering (Christian et al., 2004). But the curvature
of wind stress that forces the flow of the NECC also induces enhanced algal growth in that
flow (Longhurst, 1993), and Ekman suction is perhaps a sufficient explanation for the
linear zone of chlorophyll enhancement associated with the oceanic NECC. Maximum
curl values in winter produce a vertical velocity at the pycnocline of 075 m d
−1 , which is
a significant effect where the mixed-layer depth is only 25–50 m. The surface chlorophyll
enhancement seen in satellite images in the eastern part of the NECC does not normally
extend west of about 110
W, consistent with the progressive westward deepening of the
mixed layer. We may expect enhancement of subsurface blooms along the NECC toward
the west, but have no evidence for this.
Fiedler (1994) has analyzed monthly and between-year variability for the countercurrent from CZCS and wind-field data and has confirmed that wind stress, Ekman suction,
363
South Equatorial Currents where these turn westward away from the continent. Within
this region of weak flow and shallow mixed layer, the eastward countercurrent bifurcates
and the influence of the continent on regional wind stress and curvature results in
several cyclonic domes of ecological significance: the Costa Rica Dome (CRD), offshore
in the PNEC Province, and the Tehuantapec and Panama Bight domes in the coastal
boundary province CAMR.
The doming of the thermocline off Costa Rica, with seasonal intensification, has
regional ecological importance. Recent analysis of archived data and satellite imagery
has now revealed the real nature and seasonal evolution of the CRD (Fiedler, 2002).
Previously, it had been considered to be a quasipermanent feature located some 1000 km
southwest of Honduras and Nicaragua, but we now know that it is formed de novo each
winter. In February to April, it takes the form of a coastal shoaling of the thermocline in
the Gulf of Papagaya, forced by Ekman pumping to the south of the wind jet through
the sierra mountains behind the Gulf of Papagaya. In May–June it separates from the
coast as the ITCZ moves to the north in boreal summer, and then, in July–November
the CRD merges into the ridge associated with the NECC as this shoals beneath cyclonic
wind stress on the northern side of the ITCZ. Finally, in December–January, the CRD
deepens in response to strong trade winds as the ITCZ again moves to the south. Because
of the ecological importance of the CRD, it is good finally to have a satisfactory account
of its formation.
There is very little regular seasonality in the 20–35 m mixed layer of this province, and
irradiance at the surface is sufficiently strong that the entire mixed layer and much of
the thermocline lies perpetually within the euphotic zone. However, there is an eastward
decrease in mixed-layer temperatures right along the axis of the NECC, which is strongest
in boreal summer when very significant warming occurs in the western Pacific. A westward
flow of cooler water, associated with the NEC, is encountered below about 200 m.
It is in this province that we meet the extensive seasonal survey data obtained by
EASTROPAC in 1967–68 that, although not produced by the methods of today, still yield
valuable insights. Consider Fig. 11.9, which shows the near-instantaneous location of the
front between nitrate-depleted and nitrate-replete mixed-layer water on either side of the
boundary between PNEC and PEQD provinces, or along about 2–5
N. The sharpness of
this feature (which is, of course, the “Great Front” observed by Barber and his colleagues
on R/V Thompson almost 30 years later) is quite obscured in data representations such
as the well-known Ocean Atlas 2001, although it was mapped along 30
of longitude in
both extreme seasons in the EASTROPAC data.
Regional Response of the Pelagic Ecosystem
A seasonally and annually variable linear region of chlorophyll enhancement characterizes
the NECC region, and may occasionally—in some satellite images—be traced clear across
the Pacific Ocean to the origin of the NECC. Here, enhancement has been attributed to
upwelling associated with current meandering (Christian et al., 2004). But the curvature
of wind stress that forces the flow of the NECC also induces enhanced algal growth in that
flow (Longhurst, 1993), and Ekman suction is perhaps a sufficient explanation for the
linear zone of chlorophyll enhancement associated with the oceanic NECC. Maximum
curl values in winter produce a vertical velocity at the pycnocline of 075 m d
−1 , which is
a significant effect where the mixed-layer depth is only 25–50 m. The surface chlorophyll
enhancement seen in satellite images in the eastern part of the NECC does not normally
extend west of about 110
W, consistent with the progressive westward deepening of the
mixed layer. We may expect enhancement of subsurface blooms along the NECC toward
the west, but have no evidence for this.
Fiedler (1994) has analyzed monthly and between-year variability for the countercurrent from CZCS and wind-field data and has confirmed that wind stress, Ekman suction,