Pressure fluctuations induced by the nonlinear interaction of ocean surface waves.

Abstract

Pressure fluctuations caused by the nonlinear interaction of ocean surface waves are, in second order, related to sums and differences of the phases of pairs of surface waves. Terms with frequencies above those of the surface waves result from the sum type. When the sea waves include nearly oppositely directed components these pressure fluctuations arrive on the deep seafloor as acoustic waves (horizontal wave numbers k≤ω/c), at double the frequency of the generating waves, and can couple with Rayleigh waves in the ocean crust [Longuet-Higgins, Philos. Trans. R. Soc. London Ser. A 243, 1–35 (1950)]. At depths between the sea surface and c/ω the horizontal wave-number distribution is broader. Since each wave-number component decreases exponentially with depth, the integrated pressure intensity increases toward the sea surface, approximately inversely as depth squared. This behavior is to be contrasted with pressure fluctuations induced in the sea by microseismic Rayleigh waves from distant sources. Here, the sea surface acts as a pressure release and the intensity of pressure fluctuations diminishes toward the sea surface. Inferences on the magnitude of local generation of microseisms have been made by measurement of pressure spectra over a range of depths. Local generation was dominant when a storm swell was incident on, and reflected by, a steep, rocky shore. Generation was generally weaker when storm waves were incident on a gently sloping sandy shore. [Work supported by ONR.]