A satellite image captures the 200km-wide dip (shown here in red) in the ocean surface caused by the spinning water body off the Sydney coast.
Credit: CSIRO
SYDNEY: A mysterious, huge and dense mass of cold water is milling off the coast of Sydney, baffling researchers and delighting fishermen.
Experts currently know very little about what causes this large eddy or the influence it is having on the ecosystem of the surrounding Tasman Sea.
"What we do know is that this is a very powerful natural feature which [is tending] to push everything else aside - even the mighty East Australian Current,” said oceanographer David Griffin with the government research body CSIRO in Sydney.
About 100km from the shore, the huge mass is 200km in diameter and 1,000m deep. As well as a puzzle for researchers, the phenomenon brings with it cold temperatures for swimmers and a fishing bonanza.
Eddies are upwellings of cold water that rotate horizontally from depths of 400m up to 200m. At that depth, the temperature can be 6° C cooler than normal. Originating in the deep ocean, the water brings with it huge amounts of nutrients to otherwise starved phytoplankton, creating a bloom of life and a feeding frenzy for fish.
As the cold water rises within the eddy, the sea surface is conversely lowered by about 70cm. Though invisible to the naked eye, this dip in the ocean can be accurately measured by satellites. With that technology, eddies appear as blemishes on the ocean’s surface, said Griffin, “similar to fogged glass”.
Satellite images of the cold water eddy can be found here, on the CSIRO’s web site.
“Fishermen know successful fisheries are closely tied to these eddies,” said Griffin. “They’re making the most of what knowledge we do have and check ocean current maps much like we’d check a weather forecast before going out.” Tuna is one species commonly fished off the coast of Sydney.
Smaller cold water eddies appear most years near Sydney. Griffin said that the last strong eddy - around 2 years ago - generated many enquiries from swimmers about why the water was so cold. This time around, water temperatures at the beach have plunged as low as 14° C. “What’s interesting is this eddy is in same place as warm core eddy last September which also affected beach temperatures,” said Griffin.
"Until 20 years ago we would not have known these eddies even existed without accidentally steaming through them on a research vessel," he said. "However, now that we can routinely identify them from space, marine scientists can evaluate their role as a source of life in the marine ecosystem."
The cold-water eddy phenomena is one of the issues being discussed this week at the CSIRO, the French Space Agency and U.S. space agency NASA’s Ocean Surface Topography Science Team Meeting in Hobart, Tasmania.
Ocean eddies can have a life of 2-3 weeks, although similar eddies identified off South Australia and Western Australia are known to have survived for several months.
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