Satellite remote sensing is normally confined to the surface or skin of the ocean but a new technique, acoustic thermometry, currently being demonstrated in the Pacific Ocean, is capable of remotely, acoustically sensing the temperature of the interior of the ocean. Acoustic thermometry, a derivation of acoustic tomography, has the potential to repeatedly collect basin-scale, long path-averaged temperatures from depths, typically 1 km, at previously un-imagined rates. Established in early 1996, the Acoustic Thermometry of Ocean Climate program (ATOC) has accumulated long path-averaged temperature data along a dozen refracted geodesic paths across the North Pacific, creating the first time series of temperature variability of its kind in the world. Comparisons of ATOC's basin-scale acoustic and direct thermal measurements of heat content in the Pacific, satellite altimeter measurements of sea surface height, and results from a general circulation model show that only about half of the seasonal and inter-annual changes are attributable to thermal expansion. Building on the experience and success of ATOC in the Pacific, an acoustic thermometer in the Indian Ocean with path lengths of over 5,000 km is capable of detecting seasonal and inter-annual temperature changes of a few milli-degrees. Even small ocean temperature variations result in large changes in heat content of the upper ocean, which in turn can greatly influence the rainfall over very extensive areas of the Australian continent. As well as reviewing the results of ATOC in the Pacific, this paper describes the next logical step in the application of acoustic thermometry - instrumenting the Indian Ocean.
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