10:45 AM - 11:00 AM
[SSS04-07] Low-frequency ocean gravity waves triggered by tropical cyclones via seafloor distributed acoustic sensing
Keywords:Infragravity waves, Internal waves, Tropical cyclones, Distributed acoustic sensing
Low-frequency ocean gravity waves, including infragravity (IG) and internal waves, affect ocean dynamics and energy transfer within the ocean. IG waves, with periods ranging from 30 to 300 seconds, propagate across the ocean, influencing coastal regions and deep-sea environments, while internal waves propagate along density interfaces and transport energy within the ocean interior. In recent years, tropical cyclones (TCs) have become more frequent and significantly impact the generation and propagation of low-frequency ocean gravity waves. This study investigates IG and internal waves triggered by the TC Lan in 2023. To conduct this study, distributed acoustic sensing (DAS) technology was employed along a submarine fiber-optic cable off Cape Muroto, Japan. By using DAS, we achieve a fine-scale wavefield analysis with high spatial resolution, which is traditionally limited by sparse oceanographic instrumentation.
We use frequency-wavenumber analysis and slant stacking techniques to identify the approximate apparent phase velocities and stacked amplitudes of IG and internal waves. These results were collected to construct the temporal evolution throughout the TC Lan. Because the DAS cable is deployed across the fore-arc basin, topographic high, and continental slope over a sensing length of 120 km, it provides an excellent occasion to study behaviors of the deepsea low-frequency waves influenced by TC over different topographic features. In our results, two distinct IG wave patterns emerged during the approach of the TC Lan (August 11th-15th, 2023): seaward-propagating IG waves showed increased stacked amplitude at all topographic features; in contrast, landward-propagating IG waves exhibited the apparent phase velocities varying from 60 to 190 m/s along the continental slope then decreasing to ~60 m/s before making landfall. Simultaneously, internal wave energies significantly intensified across all topographic features, which was most prominent in the topographic high area. In this region, their apparent phase velocities varied between 25-46 cm/s in the landward direction and 12-25 cm/s in the seaward direction. Post-TC, the stacked amplitudes of IG waves show large continuous amplitudes. However, seaward IG wave energies decrease significantly, which is down to the background level. On the other hand, internal waves maintain strong amplitudes in the topographic high region, displaying wide-ranging phase velocities (24-50 cm/s in a seaward direction, decreasing to 22-38 cm/s afterward). These observations show that topographic features significantly influence the propagation of both IG and internal waves during the TC Lan.
Based on the investigation of the periodic features from our results, tidal modulation on both IG and internal waves was remarked in the topographic high region, which mostly enhanced the amplitudes in the periods of M2 and O1 tidal constituents. We infer that tides can amplify IG and internal waves. Moreover, our results reveal that IG waves across the topographic high may excite internal waves during TC periods, as indicated by the arrival of internal waves following the amplified IG waves driven by the TC Lan, 800 km away from the DAS cable.
Our study demonstrates that DAS technology efficiently resolves low-frequency gravity waves along the seafloor. DAS cable can serve as a valuable ocean-bottom observatory for studying the ocean dynamics in deep seas. Moreover, its high spatial resolution and continuous data provide new concepts for understanding the interplay between air-sea turbulent fluxes, low-frequency ocean gravity waves, and seafloor topography, which is important for assessing storm-driven wave hazards.
We use frequency-wavenumber analysis and slant stacking techniques to identify the approximate apparent phase velocities and stacked amplitudes of IG and internal waves. These results were collected to construct the temporal evolution throughout the TC Lan. Because the DAS cable is deployed across the fore-arc basin, topographic high, and continental slope over a sensing length of 120 km, it provides an excellent occasion to study behaviors of the deepsea low-frequency waves influenced by TC over different topographic features. In our results, two distinct IG wave patterns emerged during the approach of the TC Lan (August 11th-15th, 2023): seaward-propagating IG waves showed increased stacked amplitude at all topographic features; in contrast, landward-propagating IG waves exhibited the apparent phase velocities varying from 60 to 190 m/s along the continental slope then decreasing to ~60 m/s before making landfall. Simultaneously, internal wave energies significantly intensified across all topographic features, which was most prominent in the topographic high area. In this region, their apparent phase velocities varied between 25-46 cm/s in the landward direction and 12-25 cm/s in the seaward direction. Post-TC, the stacked amplitudes of IG waves show large continuous amplitudes. However, seaward IG wave energies decrease significantly, which is down to the background level. On the other hand, internal waves maintain strong amplitudes in the topographic high region, displaying wide-ranging phase velocities (24-50 cm/s in a seaward direction, decreasing to 22-38 cm/s afterward). These observations show that topographic features significantly influence the propagation of both IG and internal waves during the TC Lan.
Based on the investigation of the periodic features from our results, tidal modulation on both IG and internal waves was remarked in the topographic high region, which mostly enhanced the amplitudes in the periods of M2 and O1 tidal constituents. We infer that tides can amplify IG and internal waves. Moreover, our results reveal that IG waves across the topographic high may excite internal waves during TC periods, as indicated by the arrival of internal waves following the amplified IG waves driven by the TC Lan, 800 km away from the DAS cable.
Our study demonstrates that DAS technology efficiently resolves low-frequency gravity waves along the seafloor. DAS cable can serve as a valuable ocean-bottom observatory for studying the ocean dynamics in deep seas. Moreover, its high spatial resolution and continuous data provide new concepts for understanding the interplay between air-sea turbulent fluxes, low-frequency ocean gravity waves, and seafloor topography, which is important for assessing storm-driven wave hazards.