322
Chapter 10: The Indian Ocean
the eddying flow recedes from the eastern part of this coast. However, the warm and
saline surface water mass that occupies the central and eastern part of the bight during
much of the year is probably water from the Leeuwin Current modified by the arid
and evaporative nature of the coastal climatic regime. Cold, high-salinity water of the
west-wind drift of the Southern Ocean current system lies above the slope year-round,
and when the Leeuwin Current flow is relaxed in austral summer this water mass intrudes
over the shelf break and may flood onto the continental shelf. Off the eastern shelf,
dense high-salinity water may cascade from the shelf and produce temperature inversions
(Godfrey et al., 1986).
The shelf edge here is thus a region of very active frontogenesis. The shelf edge current
is fast (<15 m sec
−1 ) and strongly baroclinic in the west, especially after it rounds Cape
Leeuwin, where its dynamics become nonlinear and current speed increases due to a
Bernoulli effect. The fronts between warm Leeuwin Current water and the offshore cold
water of the west wind drift are sharp and extend to < 200 m. Large (200- to 300-km)
cyclonic eddies and sickle-shaped vortex filaments curl back toward the west beyond the
shelf edge and are especially prominent between Cape Leeuwin and Cape Arid. Several
times each month there is a major irruption of filaments of warm water (often terminating
in eddy pairs) into the west wind drift, especially when flow of the Leeuwin Current is
strong (Griffiths and Pearce, 1985).
Biological Response and Regional Ecology
By far the most comprehensive ecological coverage of this part of the ocean was the
Australian contribution to the IIOE, which was reviewed in the description of the ISSG
province that lies offshore of AUSW. To the south of Java, at 8–12
S, primary production
rates are maximal during the southeast monsoon period (May–November) and, during the
same period, zooplankton biomass was higher in the Java upwelling area than anywhere to
the south. Though the observations are few, rates of primary production in a coastal cell
extending from Java to about 10 or 11
S results in elevated chlorophyll (<11 mg chl m
−3 )
and productivity (07 gC m
−2 day−1 ) in austral winter (Humphrey and Kerr, 1969). In
January and February, values are about 20% of those of the upwelling season May–
October. Cushing (1973) evaluated this region as among the most productive of the
entire Indian Ocean during the southerly monsoon period of boreal summer.
At the time of the IIOE, it was known that high chlorophyll biomass occurs seasonally
over the Sahul shelf of northwestern Australia, and it had been thought that this was part
of the same process as the simultaneous upwelling enrichment of the Southeast Monsoon
on the coast of Java. However, a more recent investigation revealed a quite different
mechanism (Tranter and Leech, 1987). The seasonal pulse of higher chlorophyll values
was confirmed, especially subsurface and at the shelf break, but these were found to be
unusual because the source of nutrients on the Sahul shelf is neither upwelling (as had
been previously thought) nor riverborne nutrients. The source is rather episodic intrusions
of cool, nitrate-rich slope water over the shelf edge below the warm, low-salinity surface
layer. These intrusions result in near-bottom chlorophyll maxima (>05 mg liter
−1 ) over
the shelf at 50–100 m that are continuous with the offshore DCM that lies permanently
at ∼75 m: such a situation also occurs below the Florida Current and perhaps also in
other boundary currents. Chlorophyll enhancement is coincident with the base of the
pycnocline and responds to vertical motion of this feature. Plant growth is possible in this
situation because incident solar radiation on this coast is so high that the subpycnocline
shelf water is sufficiently illuminated for algal growth to occur at all seasons. In austral
winter, stratification is reduced and a “phytoplankton-dispersed” season (April–July)
replaces the summer “phytoplankton-stratified” situation (August–March).
Another consequence of the extremely high solar irradiance is to impart sufficient
stability to the upper water column to resist tidal mixing over much of the shelf; so,
Chapter 10: The Indian Ocean
the eddying flow recedes from the eastern part of this coast. However, the warm and
saline surface water mass that occupies the central and eastern part of the bight during
much of the year is probably water from the Leeuwin Current modified by the arid
and evaporative nature of the coastal climatic regime. Cold, high-salinity water of the
west-wind drift of the Southern Ocean current system lies above the slope year-round,
and when the Leeuwin Current flow is relaxed in austral summer this water mass intrudes
over the shelf break and may flood onto the continental shelf. Off the eastern shelf,
dense high-salinity water may cascade from the shelf and produce temperature inversions
(Godfrey et al., 1986).
The shelf edge here is thus a region of very active frontogenesis. The shelf edge current
is fast (<15 m sec
−1 ) and strongly baroclinic in the west, especially after it rounds Cape
Leeuwin, where its dynamics become nonlinear and current speed increases due to a
Bernoulli effect. The fronts between warm Leeuwin Current water and the offshore cold
water of the west wind drift are sharp and extend to < 200 m. Large (200- to 300-km)
cyclonic eddies and sickle-shaped vortex filaments curl back toward the west beyond the
shelf edge and are especially prominent between Cape Leeuwin and Cape Arid. Several
times each month there is a major irruption of filaments of warm water (often terminating
in eddy pairs) into the west wind drift, especially when flow of the Leeuwin Current is
strong (Griffiths and Pearce, 1985).
Biological Response and Regional Ecology
By far the most comprehensive ecological coverage of this part of the ocean was the
Australian contribution to the IIOE, which was reviewed in the description of the ISSG
province that lies offshore of AUSW. To the south of Java, at 8–12
S, primary production
rates are maximal during the southeast monsoon period (May–November) and, during the
same period, zooplankton biomass was higher in the Java upwelling area than anywhere to
the south. Though the observations are few, rates of primary production in a coastal cell
extending from Java to about 10 or 11
S results in elevated chlorophyll (<11 mg chl m
−3 )
and productivity (07 gC m
−2 day−1 ) in austral winter (Humphrey and Kerr, 1969). In
January and February, values are about 20% of those of the upwelling season May–
October. Cushing (1973) evaluated this region as among the most productive of the
entire Indian Ocean during the southerly monsoon period of boreal summer.
At the time of the IIOE, it was known that high chlorophyll biomass occurs seasonally
over the Sahul shelf of northwestern Australia, and it had been thought that this was part
of the same process as the simultaneous upwelling enrichment of the Southeast Monsoon
on the coast of Java. However, a more recent investigation revealed a quite different
mechanism (Tranter and Leech, 1987). The seasonal pulse of higher chlorophyll values
was confirmed, especially subsurface and at the shelf break, but these were found to be
unusual because the source of nutrients on the Sahul shelf is neither upwelling (as had
been previously thought) nor riverborne nutrients. The source is rather episodic intrusions
of cool, nitrate-rich slope water over the shelf edge below the warm, low-salinity surface
layer. These intrusions result in near-bottom chlorophyll maxima (>05 mg liter
−1 ) over
the shelf at 50–100 m that are continuous with the offshore DCM that lies permanently
at ∼75 m: such a situation also occurs below the Florida Current and perhaps also in
other boundary currents. Chlorophyll enhancement is coincident with the base of the
pycnocline and responds to vertical motion of this feature. Plant growth is possible in this
situation because incident solar radiation on this coast is so high that the subpycnocline
shelf water is sufficiently illuminated for algal growth to occur at all seasons. In austral
winter, stratification is reduced and a “phytoplankton-dispersed” season (April–July)
replaces the summer “phytoplankton-stratified” situation (August–March).
Another consequence of the extremely high solar irradiance is to impart sufficient
stability to the upper water column to resist tidal mixing over much of the shelf; so,