Conclusion: Stable Partitions in a Varying Ocean?
129
perhaps exhibit sufficiently uniform oligotrophic conditions as to be thought of as a
single ecological entity. But even in such cases, although seasonal production cycles will
be strongly modified within them, the coastal boundary provinces (California Current,
Central American Coastal, and Humboldt Current Coastal Provinces) will retain their
identity and unique characteristics as part of the Coastal Boundary biome of the ocean.
There may be some change in the effective coordinates of the eastern part of the zonal
province representing the transition between subarctic and subtropical regimes (North
Pacific Transition Zone Province). In the Atlantic Ocean, the distinction between the two
zonal trade wind provinces (WTRA and ETRA) might become so slight that we could,
perhaps, recognize only a single entity having the usual poleward boundaries.
But, in practice, a simple comparison of pairs of global SeaWiFS images, one for each
extreme situation, is comforting to the idea that the oceanic provinces discussed here
are relatively permanent, and may be identified usefully at either state of the Southern
or Pacific Decadal Oscillations. I illustrate this (see Color plate 4) for two cases, January
1998, during an El Niño event, and January 2001, when cool water and strong trade
wind (or La Niña) conditions obtained. You will see, I think, that it is only in the eastern
Pacific that significant anomalies occurred in 1998. Elsewhere, at all latitudes, the pattern
of sea surface chlorophyll was essentially normal, indicating that the processes that induce
the transport of nutrients into the photic zone were performing normally. Everywhere in
the southern hemisphere the chlorophyll field in the two years is essentially similar, and
all the features that we expect to observe are readily identifiable. The chlorophyll field
of the central Atlantic Ocean shows very little difference between these two situations,
and although the surface chlorophyll values are admittedly somewhat lower during the
El Niño year, their pattern is perfectly normal. The same is true for the Indian Ocean,
except that there the zonal band of chlorophyll along the equator was rather stronger
than in some other years. In the eastern Pacific itself, it is the lack of equatorial upwelling
that is most striking. But although characterized by weaker chlorophyll than normal,
the locations of upwelling cells in the eastern boundary currents are perfectly clear, as
also are the locations of the minor, Central American coastal upwellings in the province
characterized by that process.
Despite the significant lack of chlorophyll biomass along the equator, the location of
the equatorial currents can still be identified, and so can—by inference—the locations of
the equatorial provinces. Evidently, the Equatorial Divergence province (PEQD) is defined
northward by a zonal increase in surface chlorophyll that corresponds to the processes
we expect to meet within the Equatorial Countercurrent Province (PNEC). Although the
conditions within PEQD are highly anomalous along the equator itself, nevertheless conditions with that province are distinguished from the much more oligotrophic conditions
within the Subtropical Gyral Province (SPSG) to the south. An appropriate and familiar
discontinuity along about 15
S serves, as usual, to locate a suitable boundary between
PEQD and SPSG.
129
perhaps exhibit sufficiently uniform oligotrophic conditions as to be thought of as a
single ecological entity. But even in such cases, although seasonal production cycles will
be strongly modified within them, the coastal boundary provinces (California Current,
Central American Coastal, and Humboldt Current Coastal Provinces) will retain their
identity and unique characteristics as part of the Coastal Boundary biome of the ocean.
There may be some change in the effective coordinates of the eastern part of the zonal
province representing the transition between subarctic and subtropical regimes (North
Pacific Transition Zone Province). In the Atlantic Ocean, the distinction between the two
zonal trade wind provinces (WTRA and ETRA) might become so slight that we could,
perhaps, recognize only a single entity having the usual poleward boundaries.
But, in practice, a simple comparison of pairs of global SeaWiFS images, one for each
extreme situation, is comforting to the idea that the oceanic provinces discussed here
are relatively permanent, and may be identified usefully at either state of the Southern
or Pacific Decadal Oscillations. I illustrate this (see Color plate 4) for two cases, January
1998, during an El Niño event, and January 2001, when cool water and strong trade
wind (or La Niña) conditions obtained. You will see, I think, that it is only in the eastern
Pacific that significant anomalies occurred in 1998. Elsewhere, at all latitudes, the pattern
of sea surface chlorophyll was essentially normal, indicating that the processes that induce
the transport of nutrients into the photic zone were performing normally. Everywhere in
the southern hemisphere the chlorophyll field in the two years is essentially similar, and
all the features that we expect to observe are readily identifiable. The chlorophyll field
of the central Atlantic Ocean shows very little difference between these two situations,
and although the surface chlorophyll values are admittedly somewhat lower during the
El Niño year, their pattern is perfectly normal. The same is true for the Indian Ocean,
except that there the zonal band of chlorophyll along the equator was rather stronger
than in some other years. In the eastern Pacific itself, it is the lack of equatorial upwelling
that is most striking. But although characterized by weaker chlorophyll than normal,
the locations of upwelling cells in the eastern boundary currents are perfectly clear, as
also are the locations of the minor, Central American coastal upwellings in the province
characterized by that process.
Despite the significant lack of chlorophyll biomass along the equator, the location of
the equatorial currents can still be identified, and so can—by inference—the locations of
the equatorial provinces. Evidently, the Equatorial Divergence province (PEQD) is defined
northward by a zonal increase in surface chlorophyll that corresponds to the processes
we expect to meet within the Equatorial Countercurrent Province (PNEC). Although the
conditions within PEQD are highly anomalous along the equator itself, nevertheless conditions with that province are distinguished from the much more oligotrophic conditions
within the Subtropical Gyral Province (SPSG) to the south. An appropriate and familiar
discontinuity along about 15
S serves, as usual, to locate a suitable boundary between
PEQD and SPSG.