One of the EAC moorings is recovered so that 18 months of ocean data can be obtained. (Photo: Bernadette Sloyan/Marine National Facility)


East Australian Current measurements provide clues on climate and more

The East Australian Current (EAC) moves massive amounts of water down the east coast of Australia, each second transporting more than 40 million cubic metres, the equivalent of 16,000 Olympic swimming pools, southwards.

This southward movement of warm EAC water influences Australian weather and climate and its interaction with the coastal ocean has a profound effect on the region’s marine ecology.

Dr Bernadette Sloyan, CSIRO researcher and leader of the IMOS Deepwater arrays sub-facility, has been studying properties of the current system by means of a series of sub-surface instruments off the coast of southern Queensland. These instruments are attached to six moorings, distributed from the continental slope to the deep ocean, reaching up to 4km through the water column from the sea-floor to just 30 metres beneath the surface.

Dr Sloyan and her colleagues have just returned from a voyage on the RV Investigator to retrieve the moorings, which have been collecting data for 18 months. Each instrument along the mooring line is brought into the ship and returned to the CSIRO laboratory in Hobart where the data are downloaded, quality controlled and then made available via the Australian Ocean Data Network portal for researchers to analyse.

During the voyage, a replacement set of new moorings with calibrated instruments are redeployed so that they can continue to collect data for another 18 month period. By turning over the moorings in this way, IMOS is collecting a sustained time-series of observations of the East Australian Current across its entire extent, and of sufficient duration, to understand seasonal, interannual and decadal signals.

Dr Sloyan was pleased with the recovery rate of intact, functioning instruments and said that initial indications are that the data obtained from the moorings is of high quality.

“In the 18 months from now until we go out again to retrieve the instruments we’ve just put out, we’ll be working hard to get the just recovered data into shape so that we can begin to make sense of it,” said Dr Sloyan.

“And it’s not only my group who will be looking at the results; anyone, anywhere in the world will be able to access this data and perform their own analyses on it, drawing their own conclusions about the implications for Australia and beyond," she said.

Retrieving and redeploying the six moorings was not the only task of this particular voyage out into the EAC. The RV Investigator is equipped with an instrument that enables tracking of the real-time upper ocean (0-500m) ocean currents – an acoustic Doppler current profiler (ADCP). Maps produced by the ADCP provide a record of sub-surface conditions at a given time which can then be correlated with other underway observations. On this voyage, a team of three biologists kept a record of the birdlife encountered at sea. Correlating bird sightings with other ocean data collected can provide information about the interaction of ocean conditions with the distribution and behaviour of sea birds.

Describing the voyage as successful, Dr Sloyan praised the skills of the ship’s crew and the collaboration between IMOS, CSIRO and the Marine National Facility which operates the RV Investigator.

“To pull off a successful operation like this, you need all of the elements to come together and that means the marine observation infrastructure, the scientists who design and carry out the deployment and of course the ship and its talented Master, officers and crew,” said Dr Sloyan.

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Figure showing various aspects of the EAC at the time of the voyage. The background colour map is sea surface temperature, black dots indicate the location of the moorings, white contours show altimeter sea level and thin black arrows show estimated velocity from sea surface height. Blue arrows along the mooring line represent the near surface velocity as measured from the ship's instrumentation (ADCP).