Pacific Trade Winds Biome
369
down to 350 m at 29
S, by nitrate-depleted water (<01 M). Therefore, the nitrate of the
upper layer (60 M near the equator and 10–20 M near 12
S) has been transported
poleward in the diverging surface layer from the equatorial upwelling, which passes above
the more saline nitrate-depleted water, rather than originating in vertical mixing in situ.
It will be appropriate to return to the high-S aspects of this province shortly.
Regional Response of the Pelagic Ecosystem
The PEQD is one of the favorite study regions of the satellite image community because
of the very spectacular consequences of the equatorial divergence that occurs here. One of
the iconic images from the SeaWiFS sensors is that of the bloom that occurred along the
equator at the termination of the unusually strong 1997–98 Niño episode in July–August
1998 already discussed in Chapter 8 (see Color plate 4). Other images of this bloom that
may dominate the eastern equatorial Pacific show clearly how the narrow band of high
chlorophyll values (<30 mg m
−3 ) reveals the form of planetary waves propagating toward
the west in the eastward flow of the undercurrent (see Color plate 18). The transition
event that terminated the Niño conditions in 1998 dominates the long-term, satellitederived chlorophyll record for the entire period 1997–2002 shown in the regional synopsis;
otherwise, seasonality is relatively weak—as was originally suggested by the CZCS images.
Ryan et al. (2002) discuss the different mechanisms that forced the transition bloom in
1998 when the regional thermocline had become extremely shallow; toward the west,
the bloom appears to have been forced by nutrients delivered by turbulent wind mixing
and wind-driven upwelling, whereas further east it was planetary wave-induced shoaling
of nutrient-rich source waters. Zonal advection within the equatorial undercurrent and
meridional spreading by tropical instability waves were the major factors in determining
the spatial extent of the transition bloom.
Otherwise, this province has attracted much attention for two reasons: not only because
it is a high-S region, but also because it is very important in the global flux of carbon
dioxide across the sea surface. I have dealt at some length with some of the high-S aspects
of this province in earlier sections; unfortunately, the Fe-fertilization experiments done
here did not develop significant new understanding of the structure and functioning of
the pelagic ecosystem; nevertheless, I shall briefly discuss the results later.
But another initiative of the early 1990s, responding to concerns over increasing
atmospheric CO 2 concentrations, did produce a major new body of significant research.
The equatorial eastern Pacific attracted special attention from oceanographers at that time
because of the critical role that this region plays in the global carbon cycle. It is the largest
single natural source of atmospheric CO 2 (of order 1.0 Gt C y
−1 ) and is also the site of
a major fraction of global oceanic primary production (0.8–1.9 Gt C y
−1 )—and hence
of the uptake of CO 2 from the atmosphere. It was thought urgent to understand the
balance between these two fluxes, and to predict how each would respond to a changed
global climate pattern; so, a primary objective of JGOFS was to understand more fully
the nature of carbon flux in the PEQD province (Murray et al., 1995). To this end,
the JGOFS EqPac studies were undertaken in February–March and August–September,
1992, during which time both states of the ENSO regime were experienced. The U.S.
NSF supported a time-series study at 140
W, between Hawaii and Papeete, and the U.S.
NOAA mounted five meridional sections across the PEQD province between 140
and
95
W. Four special volumes of Deep-Sea Research II were devoted to the results of EqPac
between 1995 and 2002.
The excess of macronutrients in PEQD is often presented in simple terms, but the
reality of the ecological response to the imbalance is different, and more complex. The
source of excess NO 3 is, as noted above, the undercurrent that runs along the equatorial
geostrophic ridge, although only the upper part of the EUC is involved in surface
369
down to 350 m at 29
S, by nitrate-depleted water (<01 M). Therefore, the nitrate of the
upper layer (60 M near the equator and 10–20 M near 12
S) has been transported
poleward in the diverging surface layer from the equatorial upwelling, which passes above
the more saline nitrate-depleted water, rather than originating in vertical mixing in situ.
It will be appropriate to return to the high-S aspects of this province shortly.
Regional Response of the Pelagic Ecosystem
The PEQD is one of the favorite study regions of the satellite image community because
of the very spectacular consequences of the equatorial divergence that occurs here. One of
the iconic images from the SeaWiFS sensors is that of the bloom that occurred along the
equator at the termination of the unusually strong 1997–98 Niño episode in July–August
1998 already discussed in Chapter 8 (see Color plate 4). Other images of this bloom that
may dominate the eastern equatorial Pacific show clearly how the narrow band of high
chlorophyll values (<30 mg m
−3 ) reveals the form of planetary waves propagating toward
the west in the eastward flow of the undercurrent (see Color plate 18). The transition
event that terminated the Niño conditions in 1998 dominates the long-term, satellitederived chlorophyll record for the entire period 1997–2002 shown in the regional synopsis;
otherwise, seasonality is relatively weak—as was originally suggested by the CZCS images.
Ryan et al. (2002) discuss the different mechanisms that forced the transition bloom in
1998 when the regional thermocline had become extremely shallow; toward the west,
the bloom appears to have been forced by nutrients delivered by turbulent wind mixing
and wind-driven upwelling, whereas further east it was planetary wave-induced shoaling
of nutrient-rich source waters. Zonal advection within the equatorial undercurrent and
meridional spreading by tropical instability waves were the major factors in determining
the spatial extent of the transition bloom.
Otherwise, this province has attracted much attention for two reasons: not only because
it is a high-S region, but also because it is very important in the global flux of carbon
dioxide across the sea surface. I have dealt at some length with some of the high-S aspects
of this province in earlier sections; unfortunately, the Fe-fertilization experiments done
here did not develop significant new understanding of the structure and functioning of
the pelagic ecosystem; nevertheless, I shall briefly discuss the results later.
But another initiative of the early 1990s, responding to concerns over increasing
atmospheric CO 2 concentrations, did produce a major new body of significant research.
The equatorial eastern Pacific attracted special attention from oceanographers at that time
because of the critical role that this region plays in the global carbon cycle. It is the largest
single natural source of atmospheric CO 2 (of order 1.0 Gt C y
−1 ) and is also the site of
a major fraction of global oceanic primary production (0.8–1.9 Gt C y
−1 )—and hence
of the uptake of CO 2 from the atmosphere. It was thought urgent to understand the
balance between these two fluxes, and to predict how each would respond to a changed
global climate pattern; so, a primary objective of JGOFS was to understand more fully
the nature of carbon flux in the PEQD province (Murray et al., 1995). To this end,
the JGOFS EqPac studies were undertaken in February–March and August–September,
1992, during which time both states of the ENSO regime were experienced. The U.S.
NSF supported a time-series study at 140
W, between Hawaii and Papeete, and the U.S.
NOAA mounted five meridional sections across the PEQD province between 140
and
95
W. Four special volumes of Deep-Sea Research II were devoted to the results of EqPac
between 1995 and 2002.
The excess of macronutrients in PEQD is often presented in simple terms, but the
reality of the ecological response to the imbalance is different, and more complex. The
source of excess NO 3 is, as noted above, the undercurrent that runs along the equatorial
geostrophic ridge, although only the upper part of the EUC is involved in surface