Brian Dushaw – dushaw@apl.washington.edu
Applied Physics Laboratory
University of Washington
1013 N.E. 40th Street
Seattle, WA 98105-6698
Popular version of paper 1pAO2
Presented Monday afternoon, December 2, 2013
166th ASA Meeting, San Francisco
We hear regular reports in the news these days of the numerous effects of climate change. The world's oceans are playing a major role in moderating the Earth's climate changes, since they are a major sink for the heat and carbon that is warming the earth. Observations of the world's oceans over the past decades show that, in fact, the oceans are warming up, from the waters of the upper kilometer of the ocean, through the middle depths of the ocean, to the abyssal ocean at a 4-5 km (13,123-16,404 ft) depth. These observations are important (and a cause of great concern), since determining precisely how our planet is responding to climate change gives insight into why these changes are taking place and how the world's climate system may change over the next decades or centuries. This knowledge allows society to anticipate and plan for these changes and set policies that might mitigate their effects.
By its very definition, climate change is a long-term concept, referring to steadily increasing deviations from the Earth's normal weather and seasonal variations, deviations that occur over decades or centuries. Our ability to observe the Earth precisely is still evolving, however, and only in the past decade have oceanic observations achieved anything like a comprehensive global observing system, with sufficient sampling through satellites and in-water observations to accurately determine how the world's oceans are responding to climate change. As one goes back in time, the quantity and quality of observations become less and less, so that accurately determining the climatic state of the ocean in past decades is challenging. Even with present-day observations, it is recognized that there are significant gaps in the observing system, for example, in the deep ocean below 2 km (6562 ft) and the polar regions under sea ice and ice shelves, so that uncertainties are still unacceptably large. It is therefore important to examine, rescue, and retrieve available historical observations of the ocean. Any historical observations that can be brought to light can improve our ability to determine and quantify the historical ocean. Knowing the nature of the historical ocean is essential to knowing the nature of the climate changes that are occurring today and will occur in the future.
This paper reports on historical research directed toward pinning down essential scientific details of an acoustic experiment conducted in 1960. The motivation for this work is that acoustic observations have been shown to provide naturally averaging, precise measurements of ocean temperature. The speed of sound depends on temperature, so measuring the travel times of acoustic pulses sent across ocean basins provides, after careful analysis, an accurate measure of basin-wide, average temperature. This technique was the basis for the decade-long (1996-2006) Acoustic Thermometry of Ocean Climate (ATOC) program. In the case of the 1960 experiment, it is clear the primary aim of the scientists at the time was just to verify that sound could travel halfway round the world. This romantic notion had been anticipated from scientific work conducted during World War II, which showed that sound could travel great distances in the oceans with little attenuation. If the parameters of the 1960 experiment could be pinned down, the data obtained from the test would offer a remarkable measurement of the historical ocean. The 1960 data represent a measure of the average temperature of the ocean - averaged over the antipodal distance from Perth, Australia, to Bermuda. The 1960 test consisted of three 300-lb surplus depth charges detonated off Perth, Australia, at 3 am, 22 March (local) deployed by an Australian navy ship, the HMAS Diamantina. These depth charges were dropped just off the Australian continental shelf in deep water (3400 m, or 11,155 ft). As mentioned above, the goal of this experiment was to determine if those sound signals could propagate the antipodal distance. The explosions were, of course, just a means of generating sharp acoustic pulses. Acoustic experiments employing such explosions as sound sources were common at the time. The time and location of the explosions was carefully planned and coordinated with American scientists from Lamont Geological Observatory, now Lamont-Doherty Earth Observatory of Columbia University. Lamont scientists operated a permanent acoustic receiving station at Bermuda, the Bermuda SOFAR station, which was antipodal to Perth, Australia. This station was the precursor to the US Navy SOSUS system.
The acoustic data recorded by the Bermuda SOFAR station offer a rare measure of the ocean temperature a half century ago, averaged across large stretches of the Southern, South Atlantic, and North Atlantic Oceans. Measurements in the world's oceans were few and far between 50 years ago, and measurements in the Southern, South Indian, and South Atlantic Oceans were practically nonexistent. The accuracy of these data are only as good as the accuracy of the essential parameters of the experiment, particularly the time and position of the explosions. By virtue of the internet, Google, and the policies of the Australian government to make historical documents available to the public, this research turned up considerable new information about the experiment. After considerable and careful detective work analyzing the available historical documentation, the story of the HMAS Diamantina during the night of 21 March 1960 was reconstructed. Available documentation includes the ship's log, the captain's Report of Monthly Proceedings to the Australian navy, and other information. It is clear the experiment was conducted with care to obtain a precise measurement, subject to the resources available to the ship at the time. The largest uncertainty is in the position of the shots, determined by triangulation from shore landmarks in the evening, celestial navigation at dawn after the experiment, and dead reckoning in between. Recall that in 1960 there was no way to determine one's position while at sea, other than accurate time keeping and observations of stars. In addition, the depth was measured at the time of the shots by using the ship's echo sounder. This measurement was compared to present day data for the bathymetry along the Australian coast.
Fig. #1: Global Sound Speed: Sound speed of sea water at 300-m depth over the world ocean. The sound speed in this case was computed from the results of a numerical model for the ocean determined by the "Estimating the Circulation and Climate of the Ocean, Phase II" (ECCO2) project of the Jet Propulsion Laboratory (http://ecco2.jpl.nasa.gov). The black line shows the geodesic path over the surface of the earth connecting the locations of the Perth detonations and the Bermuda SOFAR station. The acoustic pulses traversed large areas of the Indian, Southern, South Atlantic, and North Atlantic Oceans.
The essential conclusion of this work was that the position of the 1960 detonations was measured to an equivalent travel-time accuracy of about three seconds. From the depth data, the uncertainty is biased towards closing the range to Bermuda; that is, the position may have been slightly further offshore and towards Bermuda. From separate considerations of the warming of the world's oceans that has been reported elsewhere, if the 1960 experiment were repeated today, the travel time might be expected to be about nine seconds less than it was in 1960. Since the expected signal from warming (-9 s) is considerably greater than the estimated uncertainty (3 s), this historical research has established that the acoustic data obtained in 1960 is an accurate, potentially important measurement of the ocean's climate state at that time.
The other half of this research, not discussed here, is the attempt to compute a present-day travel time using available techniques for computing acoustic propagation in the ocean, together with numerical ocean models. But that's another story.
N.B. The term "History Detectives" in the title is a tongue-in-cheek invocation of the PBS television show of the same name. While perhaps inappropriate for a scientific title, the aim was to invoke the sense of adventure and detective work in this historical research. This historical research was a lot of fun, particularly when CDs with scanned documents from a half century ago began to appear in the mail from Australia.
This Youtube link shows an animation of acoustic paths and ocean sound speed for the Perth-Bermuda experiment: http://www.youtube.com/watch?v=YLDA5YbOn4s.
References
Dushaw, B. D. (2008). Another look at the 1960 Perth to Bermuda long-range acoustic propagation experiment, Geophys. Res. Lett., 35, L08601, doi: 10.1029/2008GL033415.
Dushaw, B. D. and D. Menemenlis (2013). Antipodal acoustic thermometry: 1960, 2004, Deep-Sea Res. I, submitted.
Munk, W.H., O'Reilly, W.C., Reid, J.L., (1988). Australia-Bermuda sound transmission experiment (1960) revisited. J. Phys. Oceanogr. 18, 1876-1267.