ORGANIZED STRUCTURES IN SUBSURFACE BUBBLE CLOUDS - LANGMUIR CIRCULATION IN THE OPEN OCEAN

Observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface. The measurements were obtained in the open ocean from a freely drifting, self-contained acoustical instrument equipped with side scan sonar, dual-frequency echo sounders, and ambient sound recording systems. The bubble clouds are organized into long, narrow plumes aligned with the wind, consistent with windrows caused by Langmuir circulations. They have widths of about 3 to 5 m and lengths up to 100 m, with multiple scales coexisting. Their mean separation depends upon wind speed: below 5 m s-1, mean spacings were about 5 m, increasing to 10 m when winds exceeded 10 m s-1. The maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed, with the greatest penetrations of 12 m occurring at the highest wind speeds (13 m s-1). However, the time-averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed. Whenever winds exceeded 5 m s-1, a mean penetration depth of 6 m was observed. Associated with these bubble plumes were maximum downward velocities of about 0.06 m s-1. These speeds were detected at 8 m depth in the center of the plumes; magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes.