4 – Oceanography
29
the GBR and favourable currents may sweep them towards reefs, either other coral reefs (meaning the reefs
are then connected) or their natal reefs (meaning the
reef is self-seeded).
Oceanography affects the GBR at large and small
spatial scales. At the largest spatial scales (thousands of
kilometres) major oceanic currents of the Coral Sea bath
the GBR, affecting patterns of connectivity between
reefs and the likelihood of coral bleaching. At very
small spatial scales (centimetres to metres) small scale
turbulence can affect the settlement patterns of organisms such as corals. The oceanography of the GBR is
affected by oceanic currents, tides that flow on and off
the shelf, complex topography that generates jets, eddies, stagnation, and convergences, and wind that varies in direction and strength with time of year. Variation
in vertical physical structure (i.e. thermoclines and
haloclines) and the input of freshwater and mud from
rivers also affect water density and related currents.
Across the shelf, near-shore waters are often impacted
by freshwater whereas outer reefs are more affected by
upwelling.
N CURRENTS IN THE CORAL SEA
South Equatorial Current and south flowing
tropical currents
The strong, westward flowing, South Equatorial Current
(SEC) prevails in the Coral Sea and is directed towards
the GBR (Fig. 4.2A, B). The strength of this flow is indicated by the Southern Oscillation Index (SOI), or, the
difference in atmospheric pressure between French
Polynesia and Darwin. The large scale features of this
oceanic circulation (Fig. 4.2A) are readily measured by
satellite-mounted altimeters and scatterometers. Shipborn CTD data are necessary to measure and map
smaller scale circulation features (Fig. 4.2B) such as the
bifurcation of the SEC that generates the southward
BOX 4.1 GLOSSARY
Eckman transport. Wind-related transport where the upper water layer drags the layer
beneath. The rotation of the Earth ( Coriolis effect) results in a slight change in current
direction with depth. In the southern hemisphere, average transport in the Eckman layer
is to the left (opposite for northern hemisphere).
ENSO. El Nino Southern Oscillation (defined in the text)
Halocline. The region of transition between low salinity waters and high salinity waters.
Internal wave. The result of a deformation or perturbation of the thermocline. Internal
waves can be solitary (solitons), or often occur in groups (packets). Internal waves can channel cool water to reefs and influence transport of organisms in slicks at the surface.
Plankton. All organisms that are considered ‘wanderers’ or ‘drifters’. Plankton includes
viruses, autotrophs and heterotrophs, phytoplankton and zooplankton.
Pycnocline. The region of transition between low density waters and high density waters.
Sigma-T. A measure of water density. Water density is explained by water temperature
(about 65% of the relationship) and salinity.
sv. Volumetric measure of flow (1 million cubic metres per second)
Thermocline. The region of transition between warmer surface waters and colder deep
oceanic waters.
Upwelling. A process whereby cold, generally nutrient rich waters from the ocean rise
to the surface.
29
the GBR and favourable currents may sweep them towards reefs, either other coral reefs (meaning the reefs
are then connected) or their natal reefs (meaning the
reef is self-seeded).
Oceanography affects the GBR at large and small
spatial scales. At the largest spatial scales (thousands of
kilometres) major oceanic currents of the Coral Sea bath
the GBR, affecting patterns of connectivity between
reefs and the likelihood of coral bleaching. At very
small spatial scales (centimetres to metres) small scale
turbulence can affect the settlement patterns of organisms such as corals. The oceanography of the GBR is
affected by oceanic currents, tides that flow on and off
the shelf, complex topography that generates jets, eddies, stagnation, and convergences, and wind that varies in direction and strength with time of year. Variation
in vertical physical structure (i.e. thermoclines and
haloclines) and the input of freshwater and mud from
rivers also affect water density and related currents.
Across the shelf, near-shore waters are often impacted
by freshwater whereas outer reefs are more affected by
upwelling.
N CURRENTS IN THE CORAL SEA
South Equatorial Current and south flowing
tropical currents
The strong, westward flowing, South Equatorial Current
(SEC) prevails in the Coral Sea and is directed towards
the GBR (Fig. 4.2A, B). The strength of this flow is indicated by the Southern Oscillation Index (SOI), or, the
difference in atmospheric pressure between French
Polynesia and Darwin. The large scale features of this
oceanic circulation (Fig. 4.2A) are readily measured by
satellite-mounted altimeters and scatterometers. Shipborn CTD data are necessary to measure and map
smaller scale circulation features (Fig. 4.2B) such as the
bifurcation of the SEC that generates the southward
BOX 4.1 GLOSSARY
Eckman transport. Wind-related transport where the upper water layer drags the layer
beneath. The rotation of the Earth ( Coriolis effect) results in a slight change in current
direction with depth. In the southern hemisphere, average transport in the Eckman layer
is to the left (opposite for northern hemisphere).
ENSO. El Nino Southern Oscillation (defined in the text)
Halocline. The region of transition between low salinity waters and high salinity waters.
Internal wave. The result of a deformation or perturbation of the thermocline. Internal
waves can be solitary (solitons), or often occur in groups (packets). Internal waves can channel cool water to reefs and influence transport of organisms in slicks at the surface.
Plankton. All organisms that are considered ‘wanderers’ or ‘drifters’. Plankton includes
viruses, autotrophs and heterotrophs, phytoplankton and zooplankton.
Pycnocline. The region of transition between low density waters and high density waters.
Sigma-T. A measure of water density. Water density is explained by water temperature
(about 65% of the relationship) and salinity.
sv. Volumetric measure of flow (1 million cubic metres per second)
Thermocline. The region of transition between warmer surface waters and colder deep
oceanic waters.
Upwelling. A process whereby cold, generally nutrient rich waters from the ocean rise
to the surface.