Since ideas for monitoring the oceans acoustically were first voiced in the mid-1970’s, ocean acoustic tomography has evolved into an effective tool for remote sensing of the ocean interior on a wide variety of time and space scales. Regional tomographic arrays have been employed at scales of up to about 1000 km for measuring changes in integrated heat content, for observing regions of active convection, for measuring transports through the Strait of Gibraltar, for observing the evolving ocean mesoscale, for measuring barotropic currents, for directly observing oceanic relative vorticity, and for measuring barotropic and baroclinic tidal signals. Basin-scale tomographic arrays have been employed out to ranges of 5000 km for measuring large-scale changes in temperature and heat content in the North Pacific, Mediterranean, and Arctic oceans. At these ranges acoustic methods give integral measurements of large-scale ocean temperature that provide the spatial low-pass filtering needed to observe small, gyre-scale signals in the presence of much larger, mesoscale noise, offering a signal-to-noise capability for observing ocean variability with climate relevance that is not readily attainable by an ensemble of point measurements. The remote sensing capability has proven particularly suitable for measurements such as those in the Arctic and in the Strait of Gibraltar, where the application of conventional in situ methods is difficult.

The appropriate roles for acoustic tomography in an ocean observing system appear to be (1) to exploit the unique remote sensing capabilities for regional programs otherwise difficult to carry out, (2) to be a component of process-oriented programs in regions where integral or large-scale heat content or transport data are desired, and (3) to move toward deployment on basin to global scales as the acoustic technology becomes more robust and simplified. Tomography is naturally complementary to other ocean measurement techniques. Altimetry senses the ocean surface, while tomography senses the ocean interior. Profiling floats provide high vertical resolution of the upper ocean, while tomography suppresses internal wave and mesoscale noise and reaches the deep ocean. The operational costs of a tomographic network are low, making the amortized cost of the technique attractive. Permanent open-ocean stations (e.g., planned as part of the Integrated Ocean Observing System (IOOS) and the NSF Ocean Observatories Initiative) will provide supporting infrastructure for the application of acoustical techniques on basin to global scales.