On the natural variability of sound field in long-range ocean propagation

A mechanism underlying the variability of sound fields on ocean paths of different lengths is considered. The mechanism is related to the interaction of sound with natural low-frequency hydrodynamic waves, namely, the surface gravity and internal waves observed in the frequency band 10(-4) to 10(-1) rad/s; Within this band (limited by the frequencies of the Rossby waves and surface wind waves), in the absence of internal waves, the natural hydrodynamic field is close to the potential one and weakly interacts with sound. The estimates are based on the fact that both surface and internal gravity waves form a periodic structure of the refraction index within the ocean waveguide for the sound modes propagating in it. This structure is especially characteristic for shallow-water ocean regions. It is shown that, on long paths, the interaction of sound with surface waves leads to additional phase changes in the sound modes along the path. This effect is most significant for low-frequency (less than 100-200 Hz) sound propagating over long (more than 80-100 km) paths in shallow-water regions; here, the effect may be comparable to that of phase fluctuations usually attributed to natural internal waves. Spectral components of the mode phase modulation for the sound field that interacts with the surface waves at small angles of incidence are analyzed and compared with those for sound interacting with internal waves. On this basis, conditions are formulated that allow one to observe a minimal regular variability of few-frequency sound. Path lengths are estimated that are most favorable for these effects to be observed. It is shown that, at given sound frequency and path length, the extent to which the wave held affects the phase structure of the sound modes depends solely on the averaged refraction index on short paths; In contrast, on long paths, this extent is determined by the product of the refraction index and the hydrodynamic wavelength. Generalized estimates of this parameter are presented for the gravity waves that are typical for different ocean regions. In the estimation, experimental data are used on the spectrum of near-bottom pressure for the waves in the frequency band at hand. The results obtained can be useful to predict the variability of sound fields on long ocean paths, mainly, in shallow-water regions.