Time-space characteristics of bubble clouds in the open ocean

Abstract

In the spring of 1995, a ‘‘Doppler secscan’’ sonar system (200 kHz) was deployed from FLIP off Pt. Conception (1 month), San Clemente island (2 weeks), and Monterey (1 month), CA. This instrument provides backscatter intensity and radial velocity every 0.75 s, over a 450-m range by 30-deg azimuth. Dense temporal coverage means surface wave orbital velocities can be averaged out, and breaking events are not missed. A feature-tracking average was implemented, so longer averages can further reduce surface wave velocities without smearing features advecting past FLIP. In the first two segments, the instrument surveyed a horizontal sector, and winds ranged from 0 to 25 m/s. The evolving fields of intensity and velocity display some surprising relations, especially during higher winds. For example, the size and even orientation of intensity versus velocity features can differ. Off Monterey, a 2-D ‘‘vertical slice’’ was surveyed, yielding a new view of the time-space evolution of bubble clouds. In general, the clouds move sideways out of view before their temporal evolution can be tracked. However, it appears feasible to characterize some aspects of initial injection under breaking waves, and to characterize statistically the shapes of the clouds. A preliminary look indicates substantial asymmetry.