Tidal modulation of infragravity waves across the Oregon shelf

Infragravity waves (IGWs) play a key role in nearshore processes and the excitation of long-period microseisms (the so-called “hum”), yet the dynamics of IGW generation, propagation, and dissipation across diverse beach/shelf morphologies and sea states remain sparsely documented. Using three days of distributed acoustic sensing (DAS) data from the Ocean Observatories Initiative Regional Cabled Array (OOI RCA) offshore Pacific City, Oregon, we quantify the time-dependent IGW energy budget across the continental shelf, which varies dramatically with tidal stage. At low tide, the nearshore IG wavefield is nearly unidirectional with incident waves dissipating at the beach; whereas at high tide, subequal incident and reflected waves produce strong interference patterns. The apparent reflection coefficient R^2 increases from <0.1 at low tide to ~1 at high tide, consistent with a transition in beach slope from 1:100 to 1:10 over the tidal cycle. The observed correlation between incident IGW energy and local sea-swell forcing, a high-resolution nearshore energy balance, and nonlinear phase-coupling illuminated with bispectral analysis and interferometry all support bound wave release as the dominant generation mechanism. Because high-tide conditions enhance reflection and allow locally generated waves to radiate offshore, the nearshore release and reflection of bound waves effectively sets the IGW climate across the entire shelf, with up to 10 times higher total IGW energy in deep water at the local high tide. Trapped IGWs generated elsewhere along the Oregon coast and refracted back towards shore are also tidally modulated in phase with the local high tide, suggesting that this is a regionally uniform process. These observations demonstrate that beach morphology exerts a first-order control on IGW energetics and that the regional IGW climate can evolve dramatically over short time scales, even in the absence of changes in offshore forcing. Such dynamics are absent from current-generation spectral wave models.

Room 350/372

Speaker: Ethan Williams, Stanford University

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