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7 Ocean Currents
South Equatorial Current is directed to the Indian Ocean through the Indonesian throughflow. The influx of the South Equatorial Current splits at a bifurcation point, located between latitudes of 14° and 18°S. One part of the current
continues to flow southward as the East Australian Current and the other as
the Coral Sea Coastal Current, which joins with the Indonesian throughflow
through the Torres Strait. Tomczak and Godfrey (1994) note that if the Indonesian passage were closed, the transport of the East Australian Current
would be doubled. Near latitude 34°S, the current separates from the Australian continent, and its path from Australia to New Zealand is known as the
Tasman Front, at which the warm waters of the Coral Sea meet the colder
waters of the Tasman Sea. The continuation of the East Australian Current
east of New Zealand is the East Auckland Current.
The East Australian Current is associated with strong instabilities which
result in meanders and eddies. Figure 7.13 (see colour plate p.565) shows
sea surface temperature distributions as observed by the satellite NOAA9 on
September 19, 1988. Red colour illustrates a formation of warm mesoscale
eddies of the current. The East Australian Current generates about three eddies
per year and at any particular time about 4-8 eddies may co-exist. When eddies
separate themselves from the main current, they maintain their speed (1.5-2.0
m/s) for long periods of time (months). The speed of the East Australian
Current varies seasonally and reaches about 1.3 m/s near 30 0 S during summer.
During winter its speed is about 0.7 m/s at the same location (Tomczak and
Godfrey, 1994). At the shelf break, in 120 m water depth, just offshore from
the Great Barrier Reef (GBR) matrix and close to Myrmidon Reef (18.3°S),
measurements at water depths of 27 m and 100 m confirm a southward current
speed of 0.55 m/s and 0.2 mis, respectively (Wolanski and Pickard, 1983).
A very important feature of the east Australian shelf is the Great Barrier
Reef, which is not a continuous barrier but a matrix of 2500 individual reefs
extending along the shelf rim between latitudes of 9.2°S to 25°S. Water depths
on the GBR shelf increase from 30 m to 140 m in the northern section (south
of 20°). The lagoon between the GBR and the coast is rather shallow (~ 60
m) and connected to the Coral Sea by numerous channels. A comprehensive
overview of the physical oceanographic processes of the east Australian shelf,
and especially the Great Barrier Reef region, can be found in Wolanski (1994)
and biological characteristics of the GBR were recently summarized by Alongi
(1998). Some aspects of the interactions between water movement on the shelf
and biological life in the GBR are discussed in Part III of this book.
Agulhas Current. Currents along the western side of the northern Indian
Ocean have important implications for biological productivity. The South
Equatorial Current, flowing westward, converges on Africa and Madagascar
during the spring and summer, and splits into the northward Somali Current
(with transport of 30 Sv) and the southward Agulhas Current. Due to strong
upwelling at the region of divergence, the primary productivity rate is very
high.
7 Ocean Currents
South Equatorial Current is directed to the Indian Ocean through the Indonesian throughflow. The influx of the South Equatorial Current splits at a bifurcation point, located between latitudes of 14° and 18°S. One part of the current
continues to flow southward as the East Australian Current and the other as
the Coral Sea Coastal Current, which joins with the Indonesian throughflow
through the Torres Strait. Tomczak and Godfrey (1994) note that if the Indonesian passage were closed, the transport of the East Australian Current
would be doubled. Near latitude 34°S, the current separates from the Australian continent, and its path from Australia to New Zealand is known as the
Tasman Front, at which the warm waters of the Coral Sea meet the colder
waters of the Tasman Sea. The continuation of the East Australian Current
east of New Zealand is the East Auckland Current.
The East Australian Current is associated with strong instabilities which
result in meanders and eddies. Figure 7.13 (see colour plate p.565) shows
sea surface temperature distributions as observed by the satellite NOAA9 on
September 19, 1988. Red colour illustrates a formation of warm mesoscale
eddies of the current. The East Australian Current generates about three eddies
per year and at any particular time about 4-8 eddies may co-exist. When eddies
separate themselves from the main current, they maintain their speed (1.5-2.0
m/s) for long periods of time (months). The speed of the East Australian
Current varies seasonally and reaches about 1.3 m/s near 30 0 S during summer.
During winter its speed is about 0.7 m/s at the same location (Tomczak and
Godfrey, 1994). At the shelf break, in 120 m water depth, just offshore from
the Great Barrier Reef (GBR) matrix and close to Myrmidon Reef (18.3°S),
measurements at water depths of 27 m and 100 m confirm a southward current
speed of 0.55 m/s and 0.2 mis, respectively (Wolanski and Pickard, 1983).
A very important feature of the east Australian shelf is the Great Barrier
Reef, which is not a continuous barrier but a matrix of 2500 individual reefs
extending along the shelf rim between latitudes of 9.2°S to 25°S. Water depths
on the GBR shelf increase from 30 m to 140 m in the northern section (south
of 20°). The lagoon between the GBR and the coast is rather shallow (~ 60
m) and connected to the Coral Sea by numerous channels. A comprehensive
overview of the physical oceanographic processes of the east Australian shelf,
and especially the Great Barrier Reef region, can be found in Wolanski (1994)
and biological characteristics of the GBR were recently summarized by Alongi
(1998). Some aspects of the interactions between water movement on the shelf
and biological life in the GBR are discussed in Part III of this book.
Agulhas Current. Currents along the western side of the northern Indian
Ocean have important implications for biological productivity. The South
Equatorial Current, flowing westward, converges on Africa and Madagascar
during the spring and summer, and splits into the northward Somali Current
(with transport of 30 Sv) and the southward Agulhas Current. Due to strong
upwelling at the region of divergence, the primary productivity rate is very
high.