Sound Of The Sea A Clue To Climate

"This Australian-led project will tell us when and where the Indian Ocean is warming, and if that warming is having an effect on Australian rainfall" says Dr Andrew Forbes, an oceanographer at CSIRO Marine Research, Hobart.

The first signals will be transmitted as early as 2001.

Dr Forbes says the Indian Ocean Acoustic Climate Initiative (IOACI) will use part of a global network of listening posts being developed by the Comprehensive Nuclear Test Ban Treaty Organisation.

The speed of sound in the ocean increases with rising ocean temperatures. By transmitting signals across the Indian Ocean an acoustic thermometer is formed that can map temperatures across thousands of kilometres of ocean.

"United States and Australian oceanographers have proved the technique during two years of acoustic thermometry trials in the Pacific Ocean and are now ready to proceed in the Indian Ocean.

Dr Forbes said the acoustic technique - generating information beyond the reach of most existing oceanographic research techniques - will aid researchers investigating the influence of monsoons and El Nino in the Indian Ocean by indicating crucial regions of warmer water centred at 1100 metre depth.

The presence of warmer or cooler pools in the Indian Ocean north-west of Australia influences rainfall across southern Australia and western Australia.

The preferred site for an acoustic transmitter is near the Cocos Islands, 2,000 kilometres north-west of Australia.

Coincidentally, the Vienna-based Comprehensive Nuclear Test Ban Treaty Organisation is about to install three listening posts in the Indian Ocean intended to discourage clandestine nuclear testing. The network will be available for use by climate researchers.

The listening stations will be off Diego Garcia in the northern Indian Ocean, near the French sub-Antarctic island of Crozet, and off Cape Leeuwin, Western Australia. The Cape Leeuwin receiver will be 100 kilometres offshore, at a depth of 1100m.

In the past six months, a receiver site survey off Cape Leeuwin has been completed, and a research cruise by CSIRO's vessel, Franklin, is scheduled for October off Cocos Is. Planning for deployment of specialist transmission and receiving equipment is also underway.

The Franklin cruise will investigate the location for a sound source and a cable route to shore in preparation for deployment of the transmitter. A transmitter used in the US project will be made available to Indian Ocean researchers.

"We already have satellites, research vessels, moored instruments and commercial shipping with special measuring instruments fitted, and this technique will give us another tool to for scientific measurements of climate variability and change," Dr Forbes says.