Feb. 23, 1999 Dramatic ocean eddies larger than Tasmania and a kilometre deep have been discovered in the Indian Ocean, north-west of Australia.
Research by Australian and US oceanographers has revealed a system of turbulent ocean eddies that peel off a major ocean current flowing across the Indian Ocean towards Africa.
Carrying as much water as 250 Amazon Rivers, the South Equatorial Current is fed by waters from the northern and southern Indian Oceans, as well as water flowing from the Pacific Ocean through the Indonesian archipelago.
“This mixing process alters the characteristics of the ocean in a region which is a source for rainfall across southern and western Australia" says Dr Susan Wijffels, Chief Scientist on two CSIRO research expeditions that identified the changing ocean conditions.
Dr Wijffels says that an understanding of how the Indonesian throughflow and the local currents interact should contribute to improved rainfall predictions for Australia, and a recognition of El Nino and La Nina features extending into the Indian Ocean.
"This is another feature to take account of in the regional climate jugsaw puzzle. Yet, it is amazing that such an enormous natural feature of Australia's oceans has remained undiscovered until now," she says.
Dr Wijffels says scientists confirmed the ocean features occurred once a comparison was made using observations taken from the French/US ocean monitoring satellite TOPEX-Poseidon, 1300 kilometres from the earth’s surface.
“The observations came from a sophisticated instrument called a satellite altimeter which is so sensitive that it can detect elevation changes of just a few centimetres across thousands of kilometres of ocean surface.
“Elevations indicate warm water patches that are piled up by the wind or fed by flow from the Pacific Ocean via the Indonesian throughflow. But the elevations also indicate the location of these massive ocean eddies formed at certain times of the year and which act to mix ocean waters from distinctly different sources."
Dr Wijffels says the eddies move at two knots and form every two months when the South Equatorial Current, on its westward journey across the Indian Ocean, builds up immense power.
“As the flow increases in volume, the current becomes unstable, particularly from June to December, and eddies simply peel off and develop a life of their own,” says Dr Wijffels.
She explained that eddy formations also occur in other strong currents around Australia, such as the East Australian Current, which flows from the Coral Sea to the Tasman Sea. Eddies from this system are watched closely by fishermen, as well as competitors in the Sydney to Hobart yacht race, with both groups interested in sea surface conditions.
Scientists on the research voyages - aboard CSIRO’s oceanographic vessel Franklin and the Scripps Institute of Oceanography Vessel, Knorr, were taking part in the World Ocean Circulation Experiment, a project to build a decade-long snapshot of the world’s oceans.
The voyages also aimed to build a profile of the Indonesian throughflow, an important feature of global climate.
Completing the work with Dr Wijffels have been Dr Nan Bray, Chief Scientist on the Knorr and now the Chief of CSIRO Marine Research, and Mr Jackson Chong, also of Scripps Institute of Oceanography.
More information: Nick.Goldie@nap.csiro.au
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