Pacific Westerly Winds Biome
351
Regional Response of the Pelagic Ecosystem
The NPPF and NPST provinces have not attracted direct attention from oceanographers
in recent decades so, lacking direct information on the characteristic seasonal evolution
of the pelagic ecosystem, the obvious first step is to consult satellite imagery. This was
first done by Glover et al. (1994), who assembled data from the CZCS sensors in which
a steep transition zone was observed, between high chlorophyll to the north and low
to the south, across the ocean at 30–40
N. This feature was also simulated by their
simple model in which the critical process was the winter nitrate recharge of the upper
layers, responding to mixed-layer seasonal geography according to Levitus’ climatology.
This shows a transition between mixing depths of >75 m and <50 m at about 30–35
S,
corresponding to an end-of-winter surface nitrate transition from around 50 M m
−3 to
the north and <04 M m
−3 to the south of the transition. Polovina et al. (2001) have
expanded the study of this transition, showing that it can be traced 8000 km across the
ocean and that it seasonally migrates 1000 km from 30–35
N to 40–45
N—or, essentially,
across the Transition Zone as defined above.
All this may be so, but inspection of the available seasonal and monthly images reveals
a more complex situation than could be described simply as a front. During the first half
of the year, and most strongly in March–May, a narrow zone of high chlorophyll stretches
across the ocean, continuous with the enhanced chlorophyll in the zone of interaction
between Oyashio and Kuroshio east of Japan; its location corresponds very well with
that of the NPPF province. The chlorophyll images may be matched with co-registered
TOPEX-POSEIDON sea surface elevation images to confirm that the individual high
chlorophyll features are indeed associated with meanders and eddies (see Color plate 16).
The seasonal intensity of chlorophyll in this feature, as observed in the SeaWiFS 30-day
images, matches climatological data aggregated for the NPPF province, which maintains
somewhat higher chlorophyll values than in the NPST province. Although the CZCS
images suggested significant differences between eastern and western halves of the NPST
province, these are no longer observed in SeaWiFS or MODIS data. What is sustained,
however, is the earlier chlorophyll accumulation in NPST(W) (peaks in February–March)
than in NPST(E) (peaks in April–May).
Some previously unpublished sections for April and May along 158
and 172
W are
offered in the PICES publication discussed earlier; these clearly show the anticipated
DCM at between 50 and 100 m depth, deepening equatorward and, perhaps, shoaling progressively as the season advances. The meridional, late-summer URSA MAJOR
chlorophyll section near 150
W shows two features of interest to this discussion: a rapid
change to lower values in the DCM at 34–35
N (near the southern boundary as anticipated) and a discontinuity at 42–43
N in the slope of the DCM and of the chlorophyll
values.
The zonal INDOPAC sections (July 1977) for temperature, chlorophyll, and nitrite
(NO 2 ) provide useful information on the zonal structure of NPST, and they suggest
important differences between NPST(W) and NPST(E), on either side of 160
W. Meandering is stronger in the west, as sea surface height anomalies from TOPEX-POSEIDON
also now confirm, so that the western half of the section shows several large (diameter, 5
latitude) bowls in the mixed layer with depth anomalies ∼250 m, whereas the eastern part
has a more or less uniform mixed-layer depth at 75–100 m. Not surprisingly, the western
part of the section has several deep excursions of high values of mixed-layer chlorophyll
down to 200 m, whereas in the east the DCM lies consistently at 75–150 m, conforming
to the mixed-layer depth. Finally, the nitrite section shows high values associated with the
high chlorophyll values in the west, indicating rapid remineralization of DOC, whereas
in the oligotrophic east values are very low. Passage over deep-ocean ridges (e.g., the
Emperor Seamount Chain) modifies the characteristics of the western ocean.
351
Regional Response of the Pelagic Ecosystem
The NPPF and NPST provinces have not attracted direct attention from oceanographers
in recent decades so, lacking direct information on the characteristic seasonal evolution
of the pelagic ecosystem, the obvious first step is to consult satellite imagery. This was
first done by Glover et al. (1994), who assembled data from the CZCS sensors in which
a steep transition zone was observed, between high chlorophyll to the north and low
to the south, across the ocean at 30–40
N. This feature was also simulated by their
simple model in which the critical process was the winter nitrate recharge of the upper
layers, responding to mixed-layer seasonal geography according to Levitus’ climatology.
This shows a transition between mixing depths of >75 m and <50 m at about 30–35
S,
corresponding to an end-of-winter surface nitrate transition from around 50 M m
−3 to
the north and <04 M m
−3 to the south of the transition. Polovina et al. (2001) have
expanded the study of this transition, showing that it can be traced 8000 km across the
ocean and that it seasonally migrates 1000 km from 30–35
N to 40–45
N—or, essentially,
across the Transition Zone as defined above.
All this may be so, but inspection of the available seasonal and monthly images reveals
a more complex situation than could be described simply as a front. During the first half
of the year, and most strongly in March–May, a narrow zone of high chlorophyll stretches
across the ocean, continuous with the enhanced chlorophyll in the zone of interaction
between Oyashio and Kuroshio east of Japan; its location corresponds very well with
that of the NPPF province. The chlorophyll images may be matched with co-registered
TOPEX-POSEIDON sea surface elevation images to confirm that the individual high
chlorophyll features are indeed associated with meanders and eddies (see Color plate 16).
The seasonal intensity of chlorophyll in this feature, as observed in the SeaWiFS 30-day
images, matches climatological data aggregated for the NPPF province, which maintains
somewhat higher chlorophyll values than in the NPST province. Although the CZCS
images suggested significant differences between eastern and western halves of the NPST
province, these are no longer observed in SeaWiFS or MODIS data. What is sustained,
however, is the earlier chlorophyll accumulation in NPST(W) (peaks in February–March)
than in NPST(E) (peaks in April–May).
Some previously unpublished sections for April and May along 158
and 172
W are
offered in the PICES publication discussed earlier; these clearly show the anticipated
DCM at between 50 and 100 m depth, deepening equatorward and, perhaps, shoaling progressively as the season advances. The meridional, late-summer URSA MAJOR
chlorophyll section near 150
W shows two features of interest to this discussion: a rapid
change to lower values in the DCM at 34–35
N (near the southern boundary as anticipated) and a discontinuity at 42–43
N in the slope of the DCM and of the chlorophyll
values.
The zonal INDOPAC sections (July 1977) for temperature, chlorophyll, and nitrite
(NO 2 ) provide useful information on the zonal structure of NPST, and they suggest
important differences between NPST(W) and NPST(E), on either side of 160
W. Meandering is stronger in the west, as sea surface height anomalies from TOPEX-POSEIDON
also now confirm, so that the western half of the section shows several large (diameter, 5
latitude) bowls in the mixed layer with depth anomalies ∼250 m, whereas the eastern part
has a more or less uniform mixed-layer depth at 75–100 m. Not surprisingly, the western
part of the section has several deep excursions of high values of mixed-layer chlorophyll
down to 200 m, whereas in the east the DCM lies consistently at 75–150 m, conforming
to the mixed-layer depth. Finally, the nitrite section shows high values associated with the
high chlorophyll values in the west, indicating rapid remineralization of DOC, whereas
in the oligotrophic east values are very low. Passage over deep-ocean ridges (e.g., the
Emperor Seamount Chain) modifies the characteristics of the western ocean.