Observing the Ocean in the 2000s: A Strategy for the Role of Acoustic Tomography in Ocean Climate Observation

Since it was first proposed in the late 1970s (Munk and Wunsch, 1979, 1982), ocean acoustic tomography has evolved into a multipurpose remote-sensing measurement technique that has been employed in a wide variety of physical settings. In the context of longterm oceanic climate change, acoustic tomography provides integrals through the mesoscale and other high-wavenumber noise over long distances. In addition, tomographic measurements can be made without risk of calibration drift; therefore these measurements have the accuracy and precision required for large-scale ocean climate observation. The transbasin acoustic measurements offer a signal-to-noise capability for observing ocean climate variability that is difficult to attain by an ensemble of point measurements. On a regional scale, tomography has been employed for observing regions of active convection, for measuring changes in integrated heat content, for observing the mesoscale with high resolution, for measuring barotropic currents in a unique way, and for directly observing oceanic relative vorticity. The remote-sensing capability has proven effective for measurements under ice in the Arctic (in particular the recent well-documented temperature increase in the Atlantic layer) and in regions such as the Strait of Gibraltar, where conventional in-situ methods are problematic. As oceanographic science moves into an era of global-scale observations, the niches for these acoustic techniques appear to be (1) to exploit the unique remote-sensing capabilities for regional programs which are otherwise difficult to carry out, (2) to be a component of processmonitoring efforts in regions where integral 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.