5:15 PM - 6:45 PM
[SSS03-P04] Ocean infragravity waves at seafloor DONET stations
Keywords: Infragravity waves, Tropical cyclones, Ocean dynamics, Spectral analysis
Ocean waves play a vital role in ocean dynamics and significantly impact the environments from coastal to deep sea areas. The infragravity waves (IGWs), dominated in periods exceeding 30 seconds, directly cause the vertical mixture of stratified ocean water layers and the continuous oscillation of the solid Earth. Due to the property of low-frequency waves, IGWs notably traverse vast oceanic distances with minimal energy loss in long wavelengths, which are commonly detected at seafloor seismic stations. We aim to systematically observe IGWs, focusing on their distinct frequency bands and spatial propagation. Tropical cyclones (TCs) have been considered an important energy source of IGWs, whose influence is discussed in this study.
We conducted an extensive analysis of IGWs using recordings from 46 ocean bottom seismometers of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) deployed in the Tonankai and Nankai areas in Japan (offshore forearc basin north to the Naikai trough). The predominant energy of IGWs at the DONET is identified at the frequency band from 0.005 to 0.03 Hz. In general, the strength of IGWs slightly exhibits seasonal variations, but is significantly provoked in the periods when TCs cross over the Pacific Ocean.
The cross-correlation beamforming method determines the propagation velocities and direction of IGWs arriving at DONET. Analysis within a frequency band of 0.005–0.03 Hz has uncovered two prominent wave energy sources bearing back-azimuths of approximately 114° and 151°, with phase velocities around 108.4 m/s. When TCs propagate in the Pacific Ocean, these sources originate from multiple directions, and their energies are enhanced relative to those observed in non-TC periods, suggesting the importance of TC activity in wave generation and propagation paths. The rose diagrams constructed from these results highlight energy in the directions of approximately 58°, 132°, 240°, and 310° within the lower frequency range of 0.005 to 0.01 Hz. In the higher frequency band of 0.01 to 0.02 Hz, the rose diagrams do not exhibit any substantial incoming wave directions. The marked increase in energy in these directions during TC periods could indicate the significant impact of TCs on the IGW propagation paths. The data suggests that IGWs could potentially be used as a proxy for assessing the seafloor energetic environment in response to TC activity, as their energy and directionality appear to be influenced by the presence of these powerful storms.
We also applied a back-projection technique to retrieve the IGWs’ propagation in space, particularly for frequencies from 0.005 to 0.01 Hz. Our findings reveal that TCs have a pronounced impact on the stacked amplitude of IGWs from various incoming wave directions, possibly due to the enhanced wind energy and ocean surface activity associated with these meteorological events. During the non-TC periods, the stacked amplitude of IGWs is relatively low, demonstrating significant consistency in incoming wave directions as CCF beamforming results.
Our study demonstrates that TCs generate the apparent IGWs that reach the seafloor with an energy 10 dB higher than the circumstance without TC. TC particularly enhances resonant wave oscillations, which are controlled by the water depths of stations and the frequency of IGWs. These IGWs are found to propagate along diverse trajectories shaped by the underlying bathymetric features. IGW dynamics during TC periods provide the interaction between atmospheric phenomena, oceanic responses, and seafloor, offering critical insights into the broader implications of storm-induced ocean wave behavior.
We conducted an extensive analysis of IGWs using recordings from 46 ocean bottom seismometers of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) deployed in the Tonankai and Nankai areas in Japan (offshore forearc basin north to the Naikai trough). The predominant energy of IGWs at the DONET is identified at the frequency band from 0.005 to 0.03 Hz. In general, the strength of IGWs slightly exhibits seasonal variations, but is significantly provoked in the periods when TCs cross over the Pacific Ocean.
The cross-correlation beamforming method determines the propagation velocities and direction of IGWs arriving at DONET. Analysis within a frequency band of 0.005–0.03 Hz has uncovered two prominent wave energy sources bearing back-azimuths of approximately 114° and 151°, with phase velocities around 108.4 m/s. When TCs propagate in the Pacific Ocean, these sources originate from multiple directions, and their energies are enhanced relative to those observed in non-TC periods, suggesting the importance of TC activity in wave generation and propagation paths. The rose diagrams constructed from these results highlight energy in the directions of approximately 58°, 132°, 240°, and 310° within the lower frequency range of 0.005 to 0.01 Hz. In the higher frequency band of 0.01 to 0.02 Hz, the rose diagrams do not exhibit any substantial incoming wave directions. The marked increase in energy in these directions during TC periods could indicate the significant impact of TCs on the IGW propagation paths. The data suggests that IGWs could potentially be used as a proxy for assessing the seafloor energetic environment in response to TC activity, as their energy and directionality appear to be influenced by the presence of these powerful storms.
We also applied a back-projection technique to retrieve the IGWs’ propagation in space, particularly for frequencies from 0.005 to 0.01 Hz. Our findings reveal that TCs have a pronounced impact on the stacked amplitude of IGWs from various incoming wave directions, possibly due to the enhanced wind energy and ocean surface activity associated with these meteorological events. During the non-TC periods, the stacked amplitude of IGWs is relatively low, demonstrating significant consistency in incoming wave directions as CCF beamforming results.
Our study demonstrates that TCs generate the apparent IGWs that reach the seafloor with an energy 10 dB higher than the circumstance without TC. TC particularly enhances resonant wave oscillations, which are controlled by the water depths of stations and the frequency of IGWs. These IGWs are found to propagate along diverse trajectories shaped by the underlying bathymetric features. IGW dynamics during TC periods provide the interaction between atmospheric phenomena, oceanic responses, and seafloor, offering critical insights into the broader implications of storm-induced ocean wave behavior.