A comprehensive discussion on the tsunami amplification effect of the 2022 Tonga volcanic tsunamis

The 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano, located near the Tonga Islands, resulted in a massive volcanic explosion that triggered a transoceanic atmospheric tsunami. While satisfactory scientific analyses have been conducted regarding the minor tsunami generated by the initial atmospheric pressure wave, there is still insufficient scientific discussion regarding the amplification of tsunami wave amplitudes between the first pressure tsunami wave and the volcanic gravity tsunami wave.

 

This study focuses on the analysis and discussion of the second group of large-amplitude tsunami waves, in addition to the first pressure wave. Our findings indicate that the oceanic disturbances in this event were primarily driven by atmospheric shock waves traveling at different velocities. The most prominent atmospheric pressure fluctuation, which reached the observation stations fastest, was a Lamb wave with an amplitude of approximately 2 hPa and a wave speed of around 308 m/s. When compared to tide gauge records from Taiwan, the sea level variation caused by this pressure wave was only about 2–5 cm. However, the sea level oscillation did not decrease but instead amplified approximately five times 2–4 hours after the first pressure wave. Through a series of numerical simulations and analyses, we found that the first pressure wave was insufficient to cause the sustained amplification of the tsunami wave amplitude. Given that atmospheric pressure propagation is much faster than that of tsunami waves, Proudman resonance is not the factor responsible for the amplification of the tsunami wave amplitude.

 

In this study, simulations and analyses were performed for the Taiwan region using atmospheric pressure data from the Central Weather Administration (CWA) and the COMCOT tsunami model. The pressure stations in Taiwan recorded the arrival of the first pressure wave followed by secondary, tertiary, and quaternary atmospheric gravity waves with speeds of approximately 280 m/s, 250 m/s, and 220 m/s, respectively, about 4 hours after the initial wave. By using atmospheric observational data to construct a linear model for the atmospheric gravity waves, we successfully reproduced the observed phenomenon of tsunami wave amplification approximately five times. The simulation results showed a high degree of agreement with the observed amplitude and period. The tsunami propagation simulations revealed that the amplification was caused by Proudman resonance between the second, third, and fourth atmospheric gravity waves, following the Lamb wave, and oceanic gravity waves. This effect caused the slower Pekeris wave to propagate through the deep western Pacific, significantly increasing the tsunami wave amplitude.