In March 1960, an ocean acoustics experiment conducted by the Lamont Geological Observatory of Columbia University recorded the travel times of sound from Perth, Australia to its antipode at Bermuda. The travel time, 13,382 s, is a measure of the ocean temperature in 1960, averaged along the sound channel axis over a path that traverses the southern Indian Ocean, the South Atlantic, and the North Atlantic. Previous work on this subject has been focused on determining the acoustic path followed by the sound. The horizontal refraction of the sound as it traversed the global ocean, and the Antarctic circumpolar current in particular, gives the perplexing result that the sound was blocked from Bermuda by the African continent. Based on primitive data for sound speed and bathymetry, previous work determined two acoustic paths that required acoustic interactions with Heard Island in the southern Indian Ocean and the north coast of Brazil. Viewed from a modern perspective, that explanation is unconvincing. An explanation for the results of the 1960 antipodal experiment remains elusive.
Modern high-resolution ocean models will be employed to (a) determine if intense, small-scale features, Agulhas rings near the Cape of Good Hope in particular, can account for the successful antipodal propagation, including the details of the acoustic arrival pattern at Bermuda, and (b) establish a present-day travel time for the antipodal acoustic propagation. The change in travel time from 1960 to the present day is a measure of a half-century of ocean climate change. Such a measurement is valuable because the acoustic path traversed regions of the southern Indian and Atlantic Oceans which were poorly observed a half-century ago. Based on recent estimates of ocean warming, travel times today are expected to be about 10s faster than in 1960.
The accurate description of acoustic propagation is of importance for such purposes as tracking or observing marine mammals acoustically, or for the detection of clandestine nuclear or conventional explosions. The challenging Southern Ocean environment is particularly relevant to both these examples. The Comprehensive Nuclear Test Ban Treaty Organization maintains several monitoring hydrophones in the Indian Ocean. The measurement of oceanic climate change is a subject of primary importance in oceanography and to society at the present time. The proposed work offers a unique quantitative measure of the net change of ocean temperature over the past half-century.
