Indian Ocean Trade Wind Biome
287
ocean from western Australia to Madagascar: eddies are of order 100 km diameter, have
SLA signatures of around 15–25 cm, and—because cyclonic features lie equatorward of
anticyclonic features—the whole forms a ridge-trough system in the SLA field.
The low salinity of the water mass transported in the SEC originates partly from the
southerly, eastern limb of the Bay of Bengal gyre, partly from the low-salinity water of the
Southeast Asian archipelago, and partly from the effects of a belt of very heavy rainfall
(>200 cm y
−1
) from 3
N to 10
S, east of 60
E. Finally, there is input to SEC water from
rainfall and rivers as it passes north of Madagascar. Mixed-layer depth in the north of
the province may be influenced more by salinity distribution than by temperature.
Regional Response of the Pelagic Ecosystems
Persistently high water clarity extends over much of this province, for much of the year,
and surface chlorophyll of <005 mg chl m
−3 is widespread. Chlorophyll images suggest
four regions of higher chlorophyll: (i) a zonal region south of the equator across the
entire ocean corresponding to the SEC, (ii) in the eddy field to the west of Australia,
(iii) to the east of southern Madagascar, and (iv) at midocean locations of shoal water
and banks.
South Equatorial Current Apparently coincident with the SEC is the most persistent
region of high chlorophyll in this province, which dominates many of the serial monthly
images. It lies almost permanently across the ocean south of the equator, being initiated
in January or February and strengthening progressively as the austral winter progresses;
it is most strongly developed during the period of the Southwest Monsoon in July–
August, when it becomes continuous with the high chlorophyll in the ARAB province
in the western, coastal regions of the Arabian Sea. At its initiation, it lies zonally across
the ocean at ∼10
S, but as it strengthens it shifts progressively southward and comes
to lie somewhat diagonally from about 20
S to the west of Australia to 10
S, or even
equator-crossing in the west of the ocean. Except at this season in the extreme west, an
equatorial band of clear water, lacking chlorophyll biomass, is maintained. This bloom
must represent the consequences of persistent eddying within the permanent westward
flow of the SEC, because the entire region having chlorophyll higher than background is
rich in Rossby-scale eddies with low-chlorophyll cores and streamers of high chlorophyll
around their periphery. Examined in movie mode, the serial images show how individual
eddies in the chlorophyll field may be followed across the western part of the ocean,
subsequently crossing the equator to proceed up into the Somali Current. Although the
eddy field is very well defined in the sea surface chlorophyll images, these do not suggest
that very high concentrations are reached: around 02–05 mg chl m
−3 appears to be
normal, although 10 mg chl m
−3 may be reached in the western part of the ocean.
These simple observations are compatible with the divergent flow and potential
upwelling that occurs on the 10
S thermocline ridge and should be reflected in biological
enhancement at the surface. Recall that here the geostrophic ridge along the equatorward
side of the subtropical gyre brings the nutricline relatively close to the surface, as is
confirmed by 100-m nutrient distributions. Production/loss balance computed from the
surface satellite chlorophyll suggests that coupling between herbivores and phytoplankton
is extremely close: accumulation tracks the rise in primary production rate only very
briefly, and then declines well before productivity slackens.
The IIOE samples of higher trophic levels such as mesozooplankton were not sufficiently closely spaced to identify the local consequences of this zonal band of high
productivity. However, Tranter (1973) discusses the seasonal reproductive migrations of
southern bluefin tuna that carry them into the region of 20
S to the west of Australia,
where they must encounter sufficiently high abundance of squid and other large nekton
287
ocean from western Australia to Madagascar: eddies are of order 100 km diameter, have
SLA signatures of around 15–25 cm, and—because cyclonic features lie equatorward of
anticyclonic features—the whole forms a ridge-trough system in the SLA field.
The low salinity of the water mass transported in the SEC originates partly from the
southerly, eastern limb of the Bay of Bengal gyre, partly from the low-salinity water of the
Southeast Asian archipelago, and partly from the effects of a belt of very heavy rainfall
(>200 cm y
−1
) from 3
N to 10
S, east of 60
E. Finally, there is input to SEC water from
rainfall and rivers as it passes north of Madagascar. Mixed-layer depth in the north of
the province may be influenced more by salinity distribution than by temperature.
Regional Response of the Pelagic Ecosystems
Persistently high water clarity extends over much of this province, for much of the year,
and surface chlorophyll of <005 mg chl m
−3 is widespread. Chlorophyll images suggest
four regions of higher chlorophyll: (i) a zonal region south of the equator across the
entire ocean corresponding to the SEC, (ii) in the eddy field to the west of Australia,
(iii) to the east of southern Madagascar, and (iv) at midocean locations of shoal water
and banks.
South Equatorial Current Apparently coincident with the SEC is the most persistent
region of high chlorophyll in this province, which dominates many of the serial monthly
images. It lies almost permanently across the ocean south of the equator, being initiated
in January or February and strengthening progressively as the austral winter progresses;
it is most strongly developed during the period of the Southwest Monsoon in July–
August, when it becomes continuous with the high chlorophyll in the ARAB province
in the western, coastal regions of the Arabian Sea. At its initiation, it lies zonally across
the ocean at ∼10
S, but as it strengthens it shifts progressively southward and comes
to lie somewhat diagonally from about 20
S to the west of Australia to 10
S, or even
equator-crossing in the west of the ocean. Except at this season in the extreme west, an
equatorial band of clear water, lacking chlorophyll biomass, is maintained. This bloom
must represent the consequences of persistent eddying within the permanent westward
flow of the SEC, because the entire region having chlorophyll higher than background is
rich in Rossby-scale eddies with low-chlorophyll cores and streamers of high chlorophyll
around their periphery. Examined in movie mode, the serial images show how individual
eddies in the chlorophyll field may be followed across the western part of the ocean,
subsequently crossing the equator to proceed up into the Somali Current. Although the
eddy field is very well defined in the sea surface chlorophyll images, these do not suggest
that very high concentrations are reached: around 02–05 mg chl m
−3 appears to be
normal, although 10 mg chl m
−3 may be reached in the western part of the ocean.
These simple observations are compatible with the divergent flow and potential
upwelling that occurs on the 10
S thermocline ridge and should be reflected in biological
enhancement at the surface. Recall that here the geostrophic ridge along the equatorward
side of the subtropical gyre brings the nutricline relatively close to the surface, as is
confirmed by 100-m nutrient distributions. Production/loss balance computed from the
surface satellite chlorophyll suggests that coupling between herbivores and phytoplankton
is extremely close: accumulation tracks the rise in primary production rate only very
briefly, and then declines well before productivity slackens.
The IIOE samples of higher trophic levels such as mesozooplankton were not sufficiently closely spaced to identify the local consequences of this zonal band of high
productivity. However, Tranter (1973) discusses the seasonal reproductive migrations of
southern bluefin tuna that carry them into the region of 20
S to the west of Australia,
where they must encounter sufficiently high abundance of squid and other large nekton