Pacific Coastal Biome
431
imports respectively. Similar processes have been described for the Papuan Barrier Reef,
off which both mixed layer and nutricline are shallower (25–75 m). Here, upwelling at
the reef edge occurs during both NE and SE winds (parallel and normal to the reef axis,
respectively), although surface outcropping of upwelled water seems not to occur (Furnas
and Mitchell, 1996a).
South of Sandy Cape, the East Australia Current is no longer a slow southerly stream,
but its velocity is markedly increased, reaching a maximum flow rate at 25–30
S and in
austral summer. This lies over the outer shelf and velocity is progressively reduced toward
the coast; on the outer shelf, velocity fluctuates strongly at scales of 100 d and 100 km,
inducing topographic Rossby waves that drift south at about 9 km d
−1 . Where the East
Australian Current spreads onto the shelf, it drives an Ekman boundary layer of nutrientrich water shoreward that may be upwelled and advected to the south. This may induce
a plume of nutrient-rich water over several hundreds of kilometers (Cresswell, 1994).
The offshore eddy field of the East Australian Current is relatively weak during austral
winter (May–July) but at other seasons its intensive meandering induces enrichment of
the coastal water mass. When a meander or eddy approaches the shelf break, nutrient-rich
slope water is driven onto the shelf by the peripheral anticyclonic current associated with
the shoreward side of the feature. Such flow creates an Ekman boundary layer that would
flow northward, displacing the surface coastal waters. This process results in intrusions
of slope water to the 60-m isobath.
Cyclonic frontal eddies spin off from the landward side of the velocity maximum of the
coastal current and become entrained in the coastal water mass (Tranter et al., 1987); similar elongated cyclonic features are termed “shingles” on the landward side of the Florida
Current by Atkinson et al. (1984) and are an important source of nutrients over the shelf.
The major wind events on this coast (“Southerly Busters”) generate equatorward stress
and downwelling, but are of short duration. On the other hand, strong current events are
of longer duration. Therefore, bottom Ekman-layer forcing produces upwelling events of
long duration, whereas wind stress produces downwelling of shorter duration (Griffiths
and Middleton, 1992). On the narrow shelf of eastern Australia there are several sites
where coastal upwelling cells are persistent, notably at Evans Head (29
S), Laurieton
(31
S), and Sydney (34
S). In part, the location of these cells is due to topographic
influence, such as the narrowing of the shelf just north of Laurieton (Rochford, 1975)
or where the flow of the East Australia Current separates from the coast (Tranter et al.,
1987). Nutrient-rich water may be uplifted at about 45-d intervals from 125 to 275 m
(at 15–17
C) either to a bottom Ekman layer over the shelf, as is usually the case off
Sydney, or to the surface, as occurs farther north. The primary source of nutrients on
the shelf from 29
S to 38
S appears to be intrusions of slope water of this type onto shelf
during the spring and summer that is subsequently mixed up into the surface layers by
wind stress in autumn and winter.
South of the retroflection of the main transport of the East Australian Current,
conditions are much influenced by the passage of water through the Bass Straits, which
is then entrained northward toward the well-known “Bass Strait Cascade” off Victoria,
where cold, dense water from the straits slips below lighter Tasman Sea water.
I have already mentioned (see TASM) the long time series of observations made in
South Bay, on the Tasmanian coast at midshelf (50 m), that have relevance also for
the processes in the oceanic regions (Harris et al., 1991). Here, during a 4-year period
(1985–1989) a 25
C warming of the regional SST was observed; this is apparently a notunusual occurrence that is associated with the balance between intrusive subtropical and
subantarctic influences. During this warming period, intrusions of subtropical water were
detected by their high salinity and low nutrient content. The periods of increased subtropical intrusions are also periods of lower westerly wind stress and shallower penetration
of winter mixing offshore.
431
imports respectively. Similar processes have been described for the Papuan Barrier Reef,
off which both mixed layer and nutricline are shallower (25–75 m). Here, upwelling at
the reef edge occurs during both NE and SE winds (parallel and normal to the reef axis,
respectively), although surface outcropping of upwelled water seems not to occur (Furnas
and Mitchell, 1996a).
South of Sandy Cape, the East Australia Current is no longer a slow southerly stream,
but its velocity is markedly increased, reaching a maximum flow rate at 25–30
S and in
austral summer. This lies over the outer shelf and velocity is progressively reduced toward
the coast; on the outer shelf, velocity fluctuates strongly at scales of 100 d and 100 km,
inducing topographic Rossby waves that drift south at about 9 km d
−1 . Where the East
Australian Current spreads onto the shelf, it drives an Ekman boundary layer of nutrientrich water shoreward that may be upwelled and advected to the south. This may induce
a plume of nutrient-rich water over several hundreds of kilometers (Cresswell, 1994).
The offshore eddy field of the East Australian Current is relatively weak during austral
winter (May–July) but at other seasons its intensive meandering induces enrichment of
the coastal water mass. When a meander or eddy approaches the shelf break, nutrient-rich
slope water is driven onto the shelf by the peripheral anticyclonic current associated with
the shoreward side of the feature. Such flow creates an Ekman boundary layer that would
flow northward, displacing the surface coastal waters. This process results in intrusions
of slope water to the 60-m isobath.
Cyclonic frontal eddies spin off from the landward side of the velocity maximum of the
coastal current and become entrained in the coastal water mass (Tranter et al., 1987); similar elongated cyclonic features are termed “shingles” on the landward side of the Florida
Current by Atkinson et al. (1984) and are an important source of nutrients over the shelf.
The major wind events on this coast (“Southerly Busters”) generate equatorward stress
and downwelling, but are of short duration. On the other hand, strong current events are
of longer duration. Therefore, bottom Ekman-layer forcing produces upwelling events of
long duration, whereas wind stress produces downwelling of shorter duration (Griffiths
and Middleton, 1992). On the narrow shelf of eastern Australia there are several sites
where coastal upwelling cells are persistent, notably at Evans Head (29
S), Laurieton
(31
S), and Sydney (34
S). In part, the location of these cells is due to topographic
influence, such as the narrowing of the shelf just north of Laurieton (Rochford, 1975)
or where the flow of the East Australia Current separates from the coast (Tranter et al.,
1987). Nutrient-rich water may be uplifted at about 45-d intervals from 125 to 275 m
(at 15–17
C) either to a bottom Ekman layer over the shelf, as is usually the case off
Sydney, or to the surface, as occurs farther north. The primary source of nutrients on
the shelf from 29
S to 38
S appears to be intrusions of slope water of this type onto shelf
during the spring and summer that is subsequently mixed up into the surface layers by
wind stress in autumn and winter.
South of the retroflection of the main transport of the East Australian Current,
conditions are much influenced by the passage of water through the Bass Straits, which
is then entrained northward toward the well-known “Bass Strait Cascade” off Victoria,
where cold, dense water from the straits slips below lighter Tasman Sea water.
I have already mentioned (see TASM) the long time series of observations made in
South Bay, on the Tasmanian coast at midshelf (50 m), that have relevance also for
the processes in the oceanic regions (Harris et al., 1991). Here, during a 4-year period
(1985–1989) a 25
C warming of the regional SST was observed; this is apparently a notunusual occurrence that is associated with the balance between intrusive subtropical and
subantarctic influences. During this warming period, intrusions of subtropical water were
detected by their high salinity and low nutrient content. The periods of increased subtropical intrusions are also periods of lower westerly wind stress and shallower penetration
of winter mixing offshore.