Pacific Trade Winds Biome
385
monsoon, 60 m July–August in southerly monsoon) while photic depth is almost invariant at 50–55 m, so that the thermocline is permanently illuminated except briefly in
austral winter (July–August). A very slight productivity rate increase occurs during onset
of southerly monsoon winds, when the mixed layer also deepens through the photic
depth. The dynamic range of chlorophyll biomass is very small, about a mean of about
015 mg m
−3 . Accumulation and chlorophyll closely matches the seasonal increase in productivity, but decline sets in almost at once and continues until the next rate increase of the
following year; this appears to be consistent with an overall very close match between consumer biomass and production rate. Overall, seasonality closely resembles that of MONS.
South Pacific Subtropical Gyre Province,
North and South (SPSG)
Extent of the Province
The limits adopted for this province represent a pragmatic response to the fact that this
is the least-well-known region of the oceans, about which it is very difficult to locate
sufficient information for a reasoned description. For this reason I make no apologies for
an arrangement that is contrary to the definition of the Trades and Westerlies biomes—
obviously, this province includes both wind systems, as I shall discuss later, and therefore
includes two oceanographic regimes that I believe are otherwise fundamentally different.
Thus, the SPSG province comprises the central and southern part of the subtropical gyre
of the South Pacific Ocean, the northwestern regions are the WARM province just discussed.
SPSG is bounded to the south by the far-field effects on chlorophyll enhancement of the
Subtropical Convergence Zone (see SSTC) and to the north by the southern edge of the
chlorophyll enhancement caused by equatorial divergence (see PEQD). To the east, SPSG is
bounded by the offshore eddy field of the Humboldt Current (see Humboldt Current Coastal
Province) and to the west by the 29
C surface isotherm at the edge of the western Pacific
warm pool and the line of the New Hebrides (now Vanuatu) that enclose the Coral Sea.
This is therefore an entirely oceanic province, but the presence of the Polynesian
Islands from Easter Island at 120
W to the Samoas at 170–180
W must be noted: the
Marquesas, the Tuamotos, the Cook Islands, Pitcairn, Ducie, and many other isolated
islands and atolls populate this otherwise empty ocean.
Defining Characteristics of Regional Oceanography
This is the most data-poor region of the ocean, a situation that may finally be redeemed by
the state-of-the-art drifting oceanographic floats now deployed globally in large numbers. A
glance at any global maps showing where observations have been made of any variable will
attest to the relative paucity of our knowledge and understanding of the South Pacific gyre.
The canonical single-gyre model has the SEC passing along its equatorward side (and
flowing more strongly in boreal summer) and a South Pacific Current along its poleward
side, associated with the oceanic Subtropical Convergence Zone. The western part of the
gyre includes circulation into the Coral Sea and the Western Pacific Warm Pool (WARM
and ARCH provinces). As noted in the discussion of the PEQD Province, an SECC,
weaker and more variable than its counterpart of the Northern Hemisphere, flows across
the northern part of the province, embedded within the westward flow of the SEC from
7
S to 14
S at 155
W, being farther south and stronger in austral winter (Wyrtki and
Kilonsky, 1984; Eldin, 1983).
However, such anticyclonic surface circulation models that are based on a single
gyre rotating around an axial point at about 25
S in the western part of the ocean are
certainly a misleading oversimplification of the real pattern: flows within the southern
385
monsoon, 60 m July–August in southerly monsoon) while photic depth is almost invariant at 50–55 m, so that the thermocline is permanently illuminated except briefly in
austral winter (July–August). A very slight productivity rate increase occurs during onset
of southerly monsoon winds, when the mixed layer also deepens through the photic
depth. The dynamic range of chlorophyll biomass is very small, about a mean of about
015 mg m
−3 . Accumulation and chlorophyll closely matches the seasonal increase in productivity, but decline sets in almost at once and continues until the next rate increase of the
following year; this appears to be consistent with an overall very close match between consumer biomass and production rate. Overall, seasonality closely resembles that of MONS.
South Pacific Subtropical Gyre Province,
North and South (SPSG)
Extent of the Province
The limits adopted for this province represent a pragmatic response to the fact that this
is the least-well-known region of the oceans, about which it is very difficult to locate
sufficient information for a reasoned description. For this reason I make no apologies for
an arrangement that is contrary to the definition of the Trades and Westerlies biomes—
obviously, this province includes both wind systems, as I shall discuss later, and therefore
includes two oceanographic regimes that I believe are otherwise fundamentally different.
Thus, the SPSG province comprises the central and southern part of the subtropical gyre
of the South Pacific Ocean, the northwestern regions are the WARM province just discussed.
SPSG is bounded to the south by the far-field effects on chlorophyll enhancement of the
Subtropical Convergence Zone (see SSTC) and to the north by the southern edge of the
chlorophyll enhancement caused by equatorial divergence (see PEQD). To the east, SPSG is
bounded by the offshore eddy field of the Humboldt Current (see Humboldt Current Coastal
Province) and to the west by the 29
C surface isotherm at the edge of the western Pacific
warm pool and the line of the New Hebrides (now Vanuatu) that enclose the Coral Sea.
This is therefore an entirely oceanic province, but the presence of the Polynesian
Islands from Easter Island at 120
W to the Samoas at 170–180
W must be noted: the
Marquesas, the Tuamotos, the Cook Islands, Pitcairn, Ducie, and many other isolated
islands and atolls populate this otherwise empty ocean.
Defining Characteristics of Regional Oceanography
This is the most data-poor region of the ocean, a situation that may finally be redeemed by
the state-of-the-art drifting oceanographic floats now deployed globally in large numbers. A
glance at any global maps showing where observations have been made of any variable will
attest to the relative paucity of our knowledge and understanding of the South Pacific gyre.
The canonical single-gyre model has the SEC passing along its equatorward side (and
flowing more strongly in boreal summer) and a South Pacific Current along its poleward
side, associated with the oceanic Subtropical Convergence Zone. The western part of the
gyre includes circulation into the Coral Sea and the Western Pacific Warm Pool (WARM
and ARCH provinces). As noted in the discussion of the PEQD Province, an SECC,
weaker and more variable than its counterpart of the Northern Hemisphere, flows across
the northern part of the province, embedded within the westward flow of the SEC from
7
S to 14
S at 155
W, being farther south and stronger in austral winter (Wyrtki and
Kilonsky, 1984; Eldin, 1983).
However, such anticyclonic surface circulation models that are based on a single
gyre rotating around an axial point at about 25
S in the western part of the ocean are
certainly a misleading oversimplification of the real pattern: flows within the southern