Excitation Location and Seasonal Variation of Transoceanic Infragravity Waves Observed at an Absolute Pressure Gauge Array

Abstract

An array of 10 absolute pressure gauges (APGs) deployed in deep water 50 km east of Aogashima, an island in southern Japan, observed several isolated signals in the infragravity wave (IGW) frequency band (0.002–0.03 Hz) during boreal summer, whereas relatively high IGW energy persisted during boreal winter. The isolated IGW shows dispersion with a delay time of 4–5 days as a function of frequency. Here we estimate the excitation locations of IGWs for the two seasons with estimated incoming direction of IGW, calculation of transoceanic IGW trajectories and propagation times, and spatiotemporal variations of significant wave heights from WAVEWATCH III. In boreal summer, the isolated IGWs are primarily caused by IGW energies excited at the shoreline of South America, based on the following three observations: IGWs observed at the array originated from the east: the easterly ray path from the array reaches South America: and an event‐like IGWs were observed at the array when a storm approaches eastward to the shoreline of South America, in which the observed delay time of 4–5 days was also supported by the frequency‐dependent calculation of IGW propagation times. In boreal winter, the incessant IGWs consist of transoceanic IGW energies leaked from the shoreline, primarily from North America, and secondly from South America and the western Aleutian Islands.

Infragravity Wave Generation by Wind Gusts

“…To gain a more in-depth understanding of IG wave properties, measured and modeled spectra are presented in Figure 3. The measured IG wave spectra show rapid oscillations (observed also in Mediterranean measurements, and in Tonegawa et al, 2018), which can be deduced from the plot of significant IG wave heights. Therefore, we show the running mean of measured IG wave spectra in Figure 3a, the one obtained by the combined reflection-gustiness model during winter months in Figure 3b, and the spectra obtained by the individual source terms are shown in Figures 3c and 3d.…”

“…We also investigate a more recent analysis of IG waves which was done using a measurement obtained by an array of absolute pressure gauges about 70 km off the coast of Aogashima (Figure S1b), Japan (Tonegawa et al, ). The modeled and measured significant IG wave heights during the summer months were mostly in agreement.…”

Seismic Observation of Tsunami at Island Broadband Stations

“…Although tsunami in this frequency range is ocean infragravity waves excited by an earthquake, ocean infragravity waves are also excited by the other geophysical processes. For example, they are excited persistently along shorelines by incident ocean swell through nonlinear processes and travel across the ocean with a typical height on the order of 1 cm in pelagic regions (Rawat et al, ; Tonegawa et al, ). The background ocean infragravity wave activities are also key for understanding background seismic wavefields know as seismic hum, because they are the primary excitation source (Ardhuin et al, ; Nishida, , ; Rhie & Romanowicz, ).…”

Seismic observation of tsunami at island broadband stations

“…All tsunami waveforms with amplitudes of approximately 2 cm are similar to each other ( Figure 6). A tsunami earthquake with a surface wave magnitude of Ms 5.6 in the same area occurred on June 13, 1984 (Kanamori, Ekström, Dziewonski, Barker, & Sipkin, 1993;Satake & Kanamori, 1991); their focal mechanisms suggest magma injection with the submarine volcano (Fukao et al, 2018;Kanamori et al, 1993).…”

“…Although tsunami in this frequency range is ocean infragravity waves excited by an earthquake, ocean infragravity waves are also excited by the other geophysical processes. For example, they are excited persistently along shorelines by incident ocean swell through nonlinear processes and travel across the ocean with a typical height on the order of 1 cm in pelagic regions (Rawat et al, 2014;Tonegawa et al, 2018). The background ocean infragravity-wave activities are also key for understanding background seismic wavefields know as seismic hum because they are the primary excitation source (Ardhuin, Gualtieri, & Stutzmann, 2015;Nishida, 2013Nishida, , 2017Rhie & Romanowicz, 2004).…”