Two weeks of hide-and-seek in polar pack ice by the Australian research ship Aurora Australis has ended in success with the retrieval of instruments moored deep in the seas off Antarctica’s coast and the valuable scientific data they contained.
The instruments were moored near the Amery Ice Shelf a year ago in depths of between 500 and 1200 metres, close to where the Australian supply ship Polar Bird was trapped by the ice for over a month in December.
They were placed there to collect oceanographic data from the flow of ocean across the face of the Amery Ice Shelf, where they were covered with sea ice for much of the year.
“The instruments are very expensive and retrieving them was always going to be difficult,” said CSIRO’s Dr John Church, onboard leader of the oceanographic program that deployed the moorings.
“We knew the data would be invaluable, so it was important for us to get them if we could.”
“One of them, an upward looking sonar that looks at the underside of the floating sea ice, became embedded in an iceberg and was moved several kilometres,” he said.
The other instruments, called CTDs, measure and record current, salinity, conductivity, temperature and depth, and provide a record of the ocean’s conditions.
The instruments are designed so that they can be released remotely and float to the surface as the ship approaches, where they can be scooped up onto the ship in a net for their data to be downloaded by scientists. In practice though, their retrieval wasn’t so straightforward.
Unseasonal weather with a lack of the usual southerly winds had prevented ice surrounding the moorings from being blown out to sea. Aurora Australis undertook other marine science projects while waiting for an opportunity to retrieve the instruments.
A helicopter was used for reconnaissance flights, reporting back to Aurora the current state of the ice.
At one stage, Aurora spent some ten hours pushing ice out of the way to reach the moorings. “As fast as we could clear a patch of water, it would fill in with floes again,” said Rob Easther, Voyage Leader. “On several occasions we came frustratingly close to retrieving the instruments, we were concerned new ice was forming and we wouldn’t see a breakout of ice this season at all. But on our fifth attempt we finally bagged the last instrument — we were pretty happy,” he said.