Modulation of surface gravity waves by internal waves

Abstract

Internal waves are a regular feature of the open ocean and coastal waters. As a train of internal waves propagate, their surface induced currents modulate the surface waves, generating a characteristic rough and smooth banded structure. While the surface expression of these internal waves is well known and has been observed from a variety of remote sensing instruments, direct quantitative observations of the directional properties of the surface gravity wave field modulated by an internal wave remain sparse. In this work, we report on a comprehensive field campaign conducted off the coast of Point Sal, CA in September 2017. Using a unique combination of airborne remote sensing observations, along with in-situ surface and subsurface measurements, we investigate and quantify the interaction between surface gravity and internal wave processes. We find that surface waves are significantly modulated by the currents induced by the internal waves. Through novel observations of ocean topography, we characterize the rapid modification of the directional and spectral properties of surface waves over very short spatial scales (O(100)m or less). Over a range of wavelengths (3-9m waves), geometrical optics and wave action conservation predictions show good agreement with the observed wavenumber spectra in smooth and rough regions of the modulated surface waves. If a parameterization of wave action source terms is used, good agreement is found over a larger range of wavenumbers, down to 4rad/m. These results elucidate properties of surface waves interacting with a submesoscale ocean current, and should provide insight into more general interactions between surface waves and the fine scale structure of the upper ocean.

A Model Sensitivity Study of Ocean Surface Wave Modulation Induced by Internal Waves

“…In Figure 8, we plot the time‐averaged omnidirectional wavenumber spectra in the quasi‐stationary time duration for the three representative cases IW‐A, IW‐B, and IW‐C in Table 1. Note that the existence of the peak in the spectrum of the entire domain at the near‐zero wavenumber is induced by the internal wave, which is consistent with recent experimental observations (Lenain & Pizzo, 2021). Specifically, the abrupt change around k = 0.5 k p is caused by the surface displacement induced by the internal wave at low wavenumbers.…”

Surface Gravity Wave Interferometry and Ocean Current Monitoring With Ocean‐Bottom DAS

“…Their typical propagation speed is 0.5–2 m/s, with a wavelength (inter‐soliton distance) of 0.5–2 km and a period of 10–15 min, though these parameters vary considerably among observations and their shallow‐water shoaling behavior on the shelf has not been studied (Brandt et al., 1996; Watson & Robinson, 1990; Ziegenbein, 1970). Because the wavelength is much longer and the speed is much slower than OSGWs, the interaction of internal waves with short surface waves is comparable to advection by a surface current (Alpers, 1985; Lenain & Pizzo, 2021). The propagation time of an internal wave from the Camarinal Sill to the 3–6 km cable segment is about 2–4 hr at these speeds, so the timing of the sharp jumps in our current velocity time‐series is consistent with this interpretation, as is the sole occurrence of these features immediately following the maximum southwestward (Mediterranean outflow) current (Vázquez et al., 2008).…”

Periodic Waves and Ligaments on the Surface of a Viscous Exponentially Stratified Fluid in a Uniform Gravity Field

“…In the low frequency range, the forcibly oscillating free surface of a stratified ocean can be a source of internal and inertial waves that transfer energy into the ocean. Internal waves, in turn, interact non-linearly with each other [76], generating new wave groups and ligaments (i.e., accompanying flows that cause the observed modulation of the surface waves) [77]).…”