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Chapter 11: The Pacific Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf north of Sandy Cape is almost wholly occupied by the Great Barrier
Reef off Queensland that is both wide at its southern end (250 km at 22
S) and long,
extending for about 2000 km from Torres Strait to Sandy Cape. It is broad and shallow,
with no major canyons; the coral facies is not uniformly distributed upon it, as shall be
discussed later, and a coastal lagoon extends almost the entire length of the Queensland
shelf. The ribbon reef on the outer shelf is densest on the wider, southern part, whereas
in the northern, narrower sections the fringing barrier is less complete and sandbanks
and isolated reefs populate the inner shelf.
The southern coast of New Caledonia carries a similar fringing reef, and although
it is much narrower (10–25 km), it does extend northwest of the island as far as the
Entrecasteaux reefs, so the total length of the habitat is almost 600 km.
South of Sandy Cape, along the New South Wales and Tasmanian coasts, the shelf is
rarely more than 10–20 km wide and is interrupted by the shallow Bass Strait between
Tasmania and the mainland.
Defining Characteristics of Regional Oceanography
The coastal circulation in this province is the western boundary current of the SW
Pacific that has been reviewed recently by Church and Craig (1998). North of 18
S,
where westward flow of the South Equatorial Current across the Coral Sea diverges north
and south on encountering the continent, the flow at the shelf edge is highly variable
and unpredictable, though generally equatorward. This northward flow rounds the Gulf
of Papua and turns eastward along the Papuan Barrier Reef toward the coast of New
Caledonia. The southward limb, as the East Australian Current, forms a typical western
boundary current with predictable poleward flow from the central Great Barrier Reef
south to near Sydney near 30
S, where retroflection occurs into an eastward jet along the
Tasman Front (see Tasman Sea Province) that continues on into the western boundary
current down the east coast of New Zealand.
The relatively slow flow of the East Australia Current along the Queensland coast
lies above the continental slope, beyond the barrier reef, and induces similar flow along
the outer part of the wide shelf. On the inner shelf, the Southeast Monsoon of austral
winter forces a near-coastal equatorward flow, largely occupying and mixing the shelf
area behind the ribbon reef. In austral summer, the rate of southward flow over the
Queensland shelf is enhanced by the Northwest Monsoon winds and, at the same season,
the flow of the offshore East Australian Current is strongest.
The mixed layer of the Coral Sea is deeper than typical shelf depths off Queensland,
so that here the entire feature is typically covered in tropical surface water. Buoyancy is
sporadically induced by freshwater discharge after austral winter monsoon rains on the
inner shelf, while subsurface intrusions of cool, nutrient-rich slope water onto the outer
shelf also occur. The mechanism of these intrusions is that in the slope region, alongshore
winds excite barotropic motion that displaces the thermocline by as much as 50 m, and so
induces Coral Sea water to flood up onto the shelf. This process induces the development
of strong jet currents through the narrow gaps between individual reefs with which are
associated eddy systems extending over the inner continental shelf behind the reef. Little
flow occurs over the reefs themselves (Young et al., 1994), but the eddies provide a
mechanism to enhance the exchange of deep water with the inner shelf water of the lagoon.
In the central Barrier Reef region (17–20
S) this process occurs episodically several
times during each austral summer while occasional large upwelling events may displace
up to one third of the total outer shelf water mass (Furnas and Mitchell, 1996a); these
intrusions penetrate the whole width of the reef facies and form the major source of
nutrients to the central Barrier Reef, being of order 75% and 30% of gross N and P
Chapter 11: The Pacific Ocean
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf north of Sandy Cape is almost wholly occupied by the Great Barrier
Reef off Queensland that is both wide at its southern end (250 km at 22
S) and long,
extending for about 2000 km from Torres Strait to Sandy Cape. It is broad and shallow,
with no major canyons; the coral facies is not uniformly distributed upon it, as shall be
discussed later, and a coastal lagoon extends almost the entire length of the Queensland
shelf. The ribbon reef on the outer shelf is densest on the wider, southern part, whereas
in the northern, narrower sections the fringing barrier is less complete and sandbanks
and isolated reefs populate the inner shelf.
The southern coast of New Caledonia carries a similar fringing reef, and although
it is much narrower (10–25 km), it does extend northwest of the island as far as the
Entrecasteaux reefs, so the total length of the habitat is almost 600 km.
South of Sandy Cape, along the New South Wales and Tasmanian coasts, the shelf is
rarely more than 10–20 km wide and is interrupted by the shallow Bass Strait between
Tasmania and the mainland.
Defining Characteristics of Regional Oceanography
The coastal circulation in this province is the western boundary current of the SW
Pacific that has been reviewed recently by Church and Craig (1998). North of 18
S,
where westward flow of the South Equatorial Current across the Coral Sea diverges north
and south on encountering the continent, the flow at the shelf edge is highly variable
and unpredictable, though generally equatorward. This northward flow rounds the Gulf
of Papua and turns eastward along the Papuan Barrier Reef toward the coast of New
Caledonia. The southward limb, as the East Australian Current, forms a typical western
boundary current with predictable poleward flow from the central Great Barrier Reef
south to near Sydney near 30
S, where retroflection occurs into an eastward jet along the
Tasman Front (see Tasman Sea Province) that continues on into the western boundary
current down the east coast of New Zealand.
The relatively slow flow of the East Australia Current along the Queensland coast
lies above the continental slope, beyond the barrier reef, and induces similar flow along
the outer part of the wide shelf. On the inner shelf, the Southeast Monsoon of austral
winter forces a near-coastal equatorward flow, largely occupying and mixing the shelf
area behind the ribbon reef. In austral summer, the rate of southward flow over the
Queensland shelf is enhanced by the Northwest Monsoon winds and, at the same season,
the flow of the offshore East Australian Current is strongest.
The mixed layer of the Coral Sea is deeper than typical shelf depths off Queensland,
so that here the entire feature is typically covered in tropical surface water. Buoyancy is
sporadically induced by freshwater discharge after austral winter monsoon rains on the
inner shelf, while subsurface intrusions of cool, nutrient-rich slope water onto the outer
shelf also occur. The mechanism of these intrusions is that in the slope region, alongshore
winds excite barotropic motion that displaces the thermocline by as much as 50 m, and so
induces Coral Sea water to flood up onto the shelf. This process induces the development
of strong jet currents through the narrow gaps between individual reefs with which are
associated eddy systems extending over the inner continental shelf behind the reef. Little
flow occurs over the reefs themselves (Young et al., 1994), but the eddies provide a
mechanism to enhance the exchange of deep water with the inner shelf water of the lagoon.
In the central Barrier Reef region (17–20
S) this process occurs episodically several
times during each austral summer while occasional large upwelling events may displace
up to one third of the total outer shelf water mass (Furnas and Mitchell, 1996a); these
intrusions penetrate the whole width of the reef facies and form the major source of
nutrients to the central Barrier Reef, being of order 75% and 30% of gross N and P