Eric Lamarre
R. M. Parsons Lab., MIT, Cambridge, MA 02139
W. Kendall Melville
Scripps Inst. of Oceanogr., La Jolla, CA 92093-0213
It is well known that wave breaking events create a bubbly layer a few meters thick below the water surface. The most important characteristic of this shallow channel is its sound-speed profile. Results are presented from two recent field experiments in which the sound-speed profile was directly measured in the first few meters below the surface for frequencies in the range 5--40 kHz. Preliminary results in moderate sea states (wind speeds less than 10 m/s) indicate that significant departures from the sound speed in bubble-free water occur in a shallow [O(1 m)] layer and are intermittent in time. Two dominant time scales are present. A fast time scale associated with the surface waves, which advect the bubble clouds horizontally with respect to the buoy; and a slow time scale O(100 s). Sound-speed reductions up to 150 m/s have been observed at 50-cm depth with 10-min means well above the 5 to 15 m/s previously cited in the literature. Other results including ambient sound, acoustic attenuation, and upward looking sonar measurements of the bubble clouds will be discussed. [Work supported by ONR.]