The temporal variability of acoustic backscattering from the region near the sea surface is examined for frequencies in the 30-70 kHz range. A variance spectrum of the scattering strength exhibits effects associated with three different processes. Below about 0.1 Hz, the spectrum contains a large contribution associated with temporal variations in the advection of bubble clouds through the measurement volume by large-scale processes. At high frequencies, the spectrum asymptotes to a level characteristic of a Gaussian backscattered pressure field from randomly moving bubbles within the scattering volume. The overall variability is treated as a slow modulation of this Gaussian process by larger-scale processes and a probability density function is derived for the scattering strength using Bayes' theorem. Finally, in some cases, the spectrum exhibits a peak at the frequency of the dominant surface waves. Attempts to compute coherence functions between the backscattered acoustic power and surface wave orbital velocities, measured by a microwave system observing the same spot as the acoustic system, resulted in very low values. This leads to the belief that the wave-induced peak in the acoustic backscatter variance spectrum is caused by highly nonlinear processes. A time series of acoustic backscatter from a vertically pointing system confirms the existence of this modulation at the dominant wave frequency and also suggests its nonlinear character.
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