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 exi