The hydrological properties of five major surface zones in the south-east
Indian Ocean are discussed. These are the Subantarctic, the Subtropical, the
Tropical, and two transition zones separating them. Two regions of upwelled water
are described.
The Subantarctic Zone lies south of the 35.00‰ isohaline and has an inorganic
phosphate P content hetween 0.30 and 0.60 μg-atom/l.
The Subtropical Zone lies between the 35.80‰ isohaline in the south and the
35.50‰ isohaline in the north and has an inorganic phosphate P content between
0.10 and 0.15 μg-atom/l. For this zone waters with salinity greater than 36.00‰ in
the south-east Indian Ocean and greater than 36.30‰ in the Great Australian
Bight constitute the major source regions of high salinity water. Both of these, but
particularly the latter, are very low in phosphate.
The Tropical Zone lies between the south equatorial salinity minimum
(34.26‰) and the Monsoon Current (salinity 34.10‰), at about 2°N. Four hydrological regions have been distinguished in this zone. From north to south these are
(1) the Monsoon Current with low salinity (34.10‰), high temperature (28.9°C),
and low phosphate P (0.12 μg-atom/l.); (2) the Equatorial Counter-Current with
moderately high salinity (34.50‰), high temperature (28.7°C), and high phosphate
(0.21 μg-atom/l.); (3) the South Equatorial Divergence with moderately high
salinity (34.48-34.55‰), moderate temperature (26-27.5°C), and high phosphate
(0.20-0.30 °g-atom/l.); (4) the South Equatorial Current with waters at its core
of salinity 34.26-34.31‰, temperature 26-27°C, and phosphate 0.20-0.25 μg-atom/l.
In the transitional zone between the Subantarctic and the Subtropical Zones
salinity changes rapidly with latitude. The centre of the zone is marked by the
position of the 35.40‰ isohaline. The centre of the transitional zone between the
Subtropical and the Tropical Zones is marked by the position of the 34.90‰ isohaline.
Two regions of upwelled water were found: (1) along the shelf and elope
regions of north-west Australia south of Timor, distinguished by high phosphate P
(0.20-0.30 μg-atom/l.) and a maximum upwelling occurring before September;
(2) along the shelf and slope region of the eastern Arafura Sea, from the Aroe Islands
to the Gulf of Carpentaria,, where the highest surface phosphate values in oceanic
waters of Australia were found (0.66 μg-atom/l.), and a maximum upwelling
probably occurring before August.
Chaetognaths of the Leeuwin Current system: oceanographic conditions drive epi-pelagic zoogeography in the south-east Indian Ocean
