Interferometry of infragravity waves off New Zealand

Abstract

Wave interferometry is a remote sensing technique, which is increasingly employed in helioseismology, seismology, and acoustics to retrieve parameters of the propagation medium from twopoint cross-correlation functions of random wavefields. Here we apply interferometry to yearlong records of seafloor pressure at 28 locations off New Zealand's South Island to investigate propagation and directivity properties of infragravity waves away from shore. A compressed cross-correlation function technique is proposed to make the interferometry of dispersive waves more robust, decrease the necessary noise averaging time, and simplify retrieval of quantitative information from noise cross correlations. The emergence of deterministic wave arrivals from cross correlations of random wavefields is observed up to the maximum range of 692 km between the pressure sensors in the array. Free, linear waves with a strongly anisotropic distribution of power flux density are found to be dominant in the infragravity wavefield. Lowest-frequency components of the infragravity wavefield are largely isotropic. The anisotropy has its maximum in the middle of the spectral band and decreases at the high-frequency end of the spectrum. Highest anisotropy peaks correspond to waves coming from portions of the New Zealand's shoreline. Significant contributions are also observed from waves propagating along the coastline and probably coming from powerful sources in the northeast Pacific. Infragravity wave directivity is markedly different to the east and to the west of the South Island. The northwest coast of the South Island is found to be a net source of the infragravity wave energy.

Acoustic-gravity waves in the atmosphere generated by infragravity waves in the ocean

“…Background IGWs, which are believed to be generated by the nonlinear interaction of sea swell with bathymetry close to shore, have much smaller amplitude than destructive tsunamis but are continuously present throughout the ocean (Webb et al 1991;Herbers et al 1995;Aucan and Ardhuin 2013;Godin et al 2014). Over extended time periods, the contribution of the background IGWs into the flux of mechanical energy from the ocean into the upper atmosphere is expected to dominate over the contributions of tsunamis.…”

“…But tsunamis are only a transient feature of a more general phenomenon: infragravity waves (IGWs) (Webb et al 1991;Herbers et al 1995;Aucan and Ardhuin 2013;Godin et al 2014). IGWs are permanently present surface gravity waves in the ocean with periods longer than the longest periods (approximately 30 s) of wind-generated waves.…”

Source regions and reflection of infragravity waves offshore of the U.S.s Pacific Northwest

“…Using an array of pressure gauges in the southwestern Pacific off the South Island of New Zealand, Godin et al [2014] observed strong directionality of the infragravity wavefield with the northwest coast of the South Island acting as a net source of infragravity wave energy. Webb et al [1991] studied deep water infragravity waves in the Pacific during November 1988 and identified infragravity waves originating from the Gulf of Alaska, the northwest Pacific and the southern tip of South America, but little from the southern ocean or tropical western Pacific.…”

Passive acoustic measurement of flow velocity in the Straits of Florida

“…Power spectra of the recorded noise have multiple sharp peaks, which is typical of shipping noise. To suppress the effects of a rapid variation of the noise power spectrum with frequency, we followed Godin et al [21] and evaluated the noise cross-correlation as a Fourier transform of the coherence function, rather than the cross-spectrum. This is equivalent to pre-whitening and normalization of the noise spectrum in each data segment and can be viewed as a frequency-domain counterpart of "one-bit correlation" in the time domain [12], which has been extensively utilized in noise interferometry.…”