Cyclone of the deep sits off Sydney

This scientific chart shows the deep blue pools sitting off the coast of NSW.

The marine equivalent of a cyclone is sitting off Sydney's coast, causing a massive whirlpool which is turning the ocean green and spewing chilly water towards beaches.

A 200 kilometre-wide cyclonic eddy has pulled the sea's surface down by 70 centimetres and sucked cold deep ocean water onto Australia's eastern continental shelf.

The whirlpool is 1000 metres deep, reaching the ocean floor, CSIRO oceanographers say.

The centre is 100 kilometres off the coast and could stay there for several months.

Satellites can measure the drop in ocean surface caused by the eddy's pull, clearly showing how it is pushing aside the powerful East Australian Current running off our coast.

Another eddy of similar proportions is sitting further off the coast.

The eddies were like underwater cyclones, and the latitude of the coast near Sydney put it right in the marine cyclone belt, CSIRO oceanographer Dr David Griffin said.

"This eddy is a particularly strong one - it's caused cold water to slop up onto the continental shelf and in places that water's available to come to the surface," he said.

"It's not happening right now off Sydney, but earlier this summer people will recall very cold beach temperatures - that was associated with the early phase of this eddy.

"It doesn't seem to be impacting beach temperatures at the moment, but that doesn't mean it won't happen again."

A run of warm water temperatures last September was due to an eddy spinning the other way, Dr Griffin said.
He said the current cyclonic eddy could chill water at the edge of the continental shelf from 18 degrees to 14 degrees.

"It has a big impact on ocean ecology - the cold water coming up has a lot of nutrients and when that's combined with sun, the phytoplankton grows like crazy," he said.

"That's why the water goes green, you can see that from a boat."