The feasibility of using long-range acoustics as a measure of global ocean warming

Abstract

The SOFAR channel in the ocean has been known for decades to be an efficient acoustic waveguide. Military applications of long-range acoustic transmissions using the SOFAR channel for communications and detection have been many, but scientific applications that capitalize on its acoustic properties have, until recently, been few. For ranges between source and receiver of order 1000 km, acoustic tomography yields the maximum return of information about both horizontal and vertical ocean structure but this sophisticated technique does not rely on the existence of the sound channel. Over megameter ranges, however, higher acoustic modes are scattered and absorbed so that only the lowest, slowest modes survive as near-axial waves in the sound channel. Following the reinterpretation in 1988 by Munk et al. of data from a long-range acoustic experiment conducted in 1960 (between Perth, Australia and Bermuda), Munk and Forbes (1989) proposed to further develop the technique for use in measuring path-averaged ocean temperatures. They intend to simply measure travel times of lowest mode signals simultaneously along many different megameter paths (refracted geodesics). They predict that, if sufficiently long time-series of such data are gathered, then many different spatial and time scales of ocean temperature variability will be captured and identified. These range from diurnal to decadal scales, including the signature of the often-predicted global warming. A feasibility test of this idea is scheduled for early 1991.