A number of the workshop participants have alluded to the good spatial coherence of climate fluctuations, present and past, measured at separated sites. A system of climate observations should take into account the fact that variability on decadal and century time scales is generally associated with large spatial scales. In the ocean, this means gyre and basin scales of the order of say, 10 megameters (10,000 km).
I wish to discuss briefly a program of measuring climate-related fluctuations in ocean temperatures. The speed of sound is a good temperature indicator, increasing by 4 m/s per degree centigrade. Travel time between separated sites A and B yields a spatial average of temperature along the transmission path between A and B. The separation can be as great as 10 megameters, a distance well matched to climate scales. In most cases such a spatial average is preferable to traditional point measurements.
The ocean is a very good propagator of sound. It has exceptional acoustic properties because of the existence of the "sound-fixing and ranging" (SOFAR) channel, a classical wave guide, typically at a depth of 1000 m. The SOFAR channel owes its existence to a minimum in sound speed, with sound increasing upward from the SOFAR axis with increasing temperature, and increasing downward from the axis with increasing pressure. Sound is trapped in the wave guide, and the attenuation at the lossy top and bottom boundaries is largely avoided.
In January 1991 we carried out a feasibility test to determine whether 10-megameter ranges were achievable. We would like to be able to measure changes of 5 millidegrees Kelvin per year. This number derives from various model studies of greenhouse warming, from temperature changes inferred from a sea-level rise of 2 mm per year, and from a few direct observational time series. An increase of 5 millidegrees per year translates to a decrease in travel time by 0.2 s per year. To measure this we need a time precision of better than 0.05 s per year. This in turn demands that we use stable and coded acoustic signals from electrically driven (non-explosive) sources.
Our 1991 feasibility test was aimed at determining whether such electric sources were of sufficient intensity to be heard at 10-megameter ranges, and whether the codes could maintain sufficient fidelity to be read at such distances. Both questions have been affirmatively answered. In fact, usable signals were read at 17-megameter ranges, almost halfway around the globe.
We are now aiming at developing a global acoustic network that can resolve variations on a gyre and basin scale. The array will be designed to monitor ambient variability on decadal and century scales, as well as to detect a possible greenhouse-induced ocean warming.
Summary of remarks made at dinner on 24 September 1992
Scripps Institution of Oceanography, University of California San Diego, La Jolla, California