Pacific Trade Winds Biome
375
Such westerly wind bursts, generated north of New Guinea, occur mostly in boreal
winter, and their impulsive forcing generates Kelvin waves so that, if these straddle
the equator, convergence and equatorial downwelling are induced in just a few days:
upwelling is thus induced along the pycnocline ridges at 2–3
from the equator, especially
that to the north. When the Southern Oscillation is relaxed and westward trade-wind
stress is persistent, divergence occurs along the equator, just as in the eastern Pacific.
But because the nitracline is so deep, this process enhances euphotic-zone nutrients only
during periods of strong westward wind stress; obviously, during El Niño events when
trade winds are reversed or absent here, equatorial upwelling is suppressed (Blanchot
et al., 1992).
Regional Response of the Pelagic Ecosystem
There are scatterings of atolls and numerous high islands and island groups throughout
this province, around which consequences for phytoplankton growth may be looked for,
and perhaps observed, in satellite chlorophyll fields although the effect is multiple and
complex (Dandonneau and Charpy, 1985).
Beyond such effects there are two regional blooms within the WARM province,
each responding to a different process, and each only rather weakly seasonal; the most
important of these responds to divergence at the equator and narrows progressively
westward, toward the northern coast of New Guinea. The other, farther yet to the west
and much smaller, is the narrow, meandering chlorophyll enhancement associated with
the northern edge of the NECC, where this begins its flow eastward across the ocean.
Christian et al. (2004) describe this as a “ribbon of dark water” in the western ocean
and comment that it was especially well developed during El Niño periods, when it
includes higher chlorophyll concentrations than at other times. It is associated with the
formation of the Mindenau Dome in which nutrient flux is higher in the NECC that
forms its southern boundary than in the Dome itself, despite the fact that it has a
cyclonic circulation pattern. Christian et al. attribute the chlorophyll enhancement along
the NECC variously (i) to upwelling associated with meandering, (ii) to seasonal Ekman
pumping, and (iii) to the interannual differences in nutricline depth associated with
ENSO events. There was a confluence of all these factors in 1997–98 that produced a
stronger linear feature than in the previous 5 years of observations.
However, the major part of the WARM region lies well to the east of the Mindenau
Dome, and to understand the nature of the seasonal and between-year variability of the
biological response here, we may refer to a study by Le Borgne et al. (2002b) and to
two sets of repeated meridional transects: the Australian JGOFS studies in 1992–93 along
10
S–10
N at 155
E (Mackey et al., 1997), and the French sections from 20
S to 10
N
along 165
E from 1985–89 (Radenac and Rodier, 1996). These studies encountered all
conditions of the ENSO cycle and enabled useful comparisons to be made of processes
typical of different ENSO states.
Because of the great depth of the nutricline, and the presence of the barrier layer
above it, surface nitrate is depleted to <0008 M down to 70 m although silicate remains
always >20 M; very little deeper, nitrate is typically 90 M at 100 m. There is, as
LeBorgne et al. (2002a) remark, no significant zonal gradient in phytoplankton response
across this province, and productivity is controlled entirely by flux across the nutricline.
It might be expected, therefore, that the pelagic ecosystem would resemble that of the
downwelling central gyres, but Le Borgne et al. suggest that there is—despite the barrier
layer—sufficient upward flux of nutrients to ensure that depth-integrated chlorophyll
biomass is greater than in the oligotrophic midocean gyral regions, and this suggestion
is supported by observation.
An unusually large fraction (<75%) of total production occurs within the DCM, under
low illumination, where the chl:C ratio of phytoplankton is very high; nevertheless, there
375
Such westerly wind bursts, generated north of New Guinea, occur mostly in boreal
winter, and their impulsive forcing generates Kelvin waves so that, if these straddle
the equator, convergence and equatorial downwelling are induced in just a few days:
upwelling is thus induced along the pycnocline ridges at 2–3
from the equator, especially
that to the north. When the Southern Oscillation is relaxed and westward trade-wind
stress is persistent, divergence occurs along the equator, just as in the eastern Pacific.
But because the nitracline is so deep, this process enhances euphotic-zone nutrients only
during periods of strong westward wind stress; obviously, during El Niño events when
trade winds are reversed or absent here, equatorial upwelling is suppressed (Blanchot
et al., 1992).
Regional Response of the Pelagic Ecosystem
There are scatterings of atolls and numerous high islands and island groups throughout
this province, around which consequences for phytoplankton growth may be looked for,
and perhaps observed, in satellite chlorophyll fields although the effect is multiple and
complex (Dandonneau and Charpy, 1985).
Beyond such effects there are two regional blooms within the WARM province,
each responding to a different process, and each only rather weakly seasonal; the most
important of these responds to divergence at the equator and narrows progressively
westward, toward the northern coast of New Guinea. The other, farther yet to the west
and much smaller, is the narrow, meandering chlorophyll enhancement associated with
the northern edge of the NECC, where this begins its flow eastward across the ocean.
Christian et al. (2004) describe this as a “ribbon of dark water” in the western ocean
and comment that it was especially well developed during El Niño periods, when it
includes higher chlorophyll concentrations than at other times. It is associated with the
formation of the Mindenau Dome in which nutrient flux is higher in the NECC that
forms its southern boundary than in the Dome itself, despite the fact that it has a
cyclonic circulation pattern. Christian et al. attribute the chlorophyll enhancement along
the NECC variously (i) to upwelling associated with meandering, (ii) to seasonal Ekman
pumping, and (iii) to the interannual differences in nutricline depth associated with
ENSO events. There was a confluence of all these factors in 1997–98 that produced a
stronger linear feature than in the previous 5 years of observations.
However, the major part of the WARM region lies well to the east of the Mindenau
Dome, and to understand the nature of the seasonal and between-year variability of the
biological response here, we may refer to a study by Le Borgne et al. (2002b) and to
two sets of repeated meridional transects: the Australian JGOFS studies in 1992–93 along
10
S–10
N at 155
E (Mackey et al., 1997), and the French sections from 20
S to 10
N
along 165
E from 1985–89 (Radenac and Rodier, 1996). These studies encountered all
conditions of the ENSO cycle and enabled useful comparisons to be made of processes
typical of different ENSO states.
Because of the great depth of the nutricline, and the presence of the barrier layer
above it, surface nitrate is depleted to <0008 M down to 70 m although silicate remains
always >20 M; very little deeper, nitrate is typically 90 M at 100 m. There is, as
LeBorgne et al. (2002a) remark, no significant zonal gradient in phytoplankton response
across this province, and productivity is controlled entirely by flux across the nutricline.
It might be expected, therefore, that the pelagic ecosystem would resemble that of the
downwelling central gyres, but Le Borgne et al. suggest that there is—despite the barrier
layer—sufficient upward flux of nutrients to ensure that depth-integrated chlorophyll
biomass is greater than in the oligotrophic midocean gyral regions, and this suggestion
is supported by observation.
An unusually large fraction (<75%) of total production occurs within the DCM, under
low illumination, where the chl:C ratio of phytoplankton is very high; nevertheless, there