There has been an increased interest in ocean phenomena with horizontal scales comparable to the radius of the Earth, and timescales of years and beyond. These phenomena occur in the presence of intense processes of higher spatial and temporal frequency. An observational programme for the large-scale phenomena has an inherent advantage if it can rely on measurements that are, by their very nature, integrated moments over the prerequisite scale.

The oceans provide an excellent medium for transmitting sound waves of low frequency, as demonstrated in the closing days of World War II, and subsequently confirmed by a 20000 km acoustic transmission between Perth, Australia and Bermuda. For the last six years we have been developing a method (Ocean Acoustic Tomography) to take advantage of the favourable ocean acoustic properties. We measure travel time Δ+ from mooring m to mooring n (positive x), and Δ− from n to m. The sum Δ+ + Δ− then gives information about the sound speed C (e.g. temperature) averaged along the acoustic ray path; the difference Δ+ – Δ− gives information about the x-component u of current velocity. The recorded acoustic signal can be decomposed into 10-20 distinct ray arrivals Δi, each with a distinct ray path and associated depth-weighting of the ocean column; the ray travel times can be inverted to yield information about the depth profiles C(z) and u(z). The product 〈C〉 〈u〉 of these range-averaged quantities is related to the climatological large-scale heat flux; the space-time average $\overline{\langle \delta C\,\delta u\rangle}$ is related to the eddy heat flux, and can be estimated by measuring the difference variance (Δ+) – variance (Δ−).