Propagation characteristics of tsunamis in an atmosphere-ocean coupled system revealed by wave gradiometry; the 2022 Hunga Tonga-Hunga Ha'apai eruption case

A densely cabled-type ocean-bottom pressure observation network is suitable for array analyses that enable derivation of the details of the tsunami propagation as well as early detection of tsunamis. The 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano excited the atmospheric Lamb and Pekeris waves and accompanying sea-surface disturbances, and the coupled atmospheric and sea-surface disturbances traveled worldwide. To understand the propagation characteristics of sea-surface disturbances in an atmosphere-ocean coupled system, we applied wave gradiometry, one array analysis technique, to the records of the dense, offshore, ocean-bottom pressure gauge array, and a land-based array of barometers on the Japanese archipelago. We examined the propagation properties of the disturbances using spatial variations of amplitudes as well as propagation directions and velocities. The wave gradiometry analyses provided several interesting results. Coherent sea-surface disturbances accompanied by the atmospheric Lamb and Pekeris waves propagated mainly in directions and velocities that reflected the bathymetry, that is, they propagated as tsunamis, whereas the barometric disturbances propagated as simple plane waves. When the atmospheric Lamb waves propagated, we observed spatially heterogeneous variations of the amplitudes of tsunamis within the observation network that reflected the different stages during the process of splitting the forcibly excited tsunamis into two wave trains. In contrast, the amplitudes of the tsunamis generally increased during the atmospheric Pekeris waves. Considering the propagation characteristics of barometric disturbances, these results may have reflected the amplification in accordance with Green's law and the resonance between atmospheric and sea-surface disturbances. Wave gradiometry with dense ocean-bottom pressure records provides information that can facilitate monitoring tsunami propagation even if the propagation process is different from that of oceanic free waves.