Sound-speed measurements in the surface wave zone during ASREX

Abstract

Wave breaking at the ocean–atmosphere interface injects air into the surface waters in the form of bubbles, altering the acoustical properties of the water. In situ sound-speed measurements at 3, 5, and 10 kHz were obtained in this dynamic near-surface layer using a moored sound-speed profiling buoy. The buoy was deployed in the North Atlantic during the winter of 1993-94 and obtained more than 40 days of data as part of the ASREX experiment. The record spans several storms, many with winds and significant wave heights in excess of 15 m/s and 7 m, respectively. Surface sound-speed measurements made in the first 7 m of the water column reveal large sound-speed reductions that are a result of air injection due to breaking waves. The data reveal the variable nature of the surface layer, both in short time scales O(seconds-minutes) that result from individual air-injection events, as well as longer time scales O(days) from one storm to another. Results of correlating the sound-speed field with environmental conditions will be discussed. The relationship between low-frequency, nondispersive sound speed and void fraction of air in water [A. B. Wood, A Textbook of Sound (1941)] make the resulting data useful for both oceanographers and acousticians. [Work supported by ONR.]