On 15 January 2022, the eruption of the Hunga-Tonga undersea volcano generated tsunamis of a unique and complex nature. Unlike usual tsunamis, this natural disaster generated two distinct types of waves, from atmospheric and oceanic sources. This rare and spectacular phenomenon affected both the Pacific and Atlantic coasts of the United States. The impact of these waves, both rapid and extensive, has aroused the interest of scientists in understanding the complex dynamics of their formation.
by Laurie Henry
Cover photo : Satellite view of the eruption of the Hunga-Tonga volcano © NOAA / CSU/CIRA et JAXA/JMA
A recent study, led by Oleg Zaytsev of the Geophysical Research Institute (Mexico), Alexander B. Rabinovich of Moscow State University, and Richard E. Thomson of the Canadian Institute of Ocean Sciences, and published in the Journal of Geophysical Research: Oceans, analyses these dual waves in detail. Their research provides a better understanding of the interactions between atmospheric and oceanic phenomena in tsunami generation, and highlights the scale and diversity of tsunami generation mechanisms.
Atmospheric waves: travelling at the speed of sound
When Hunga-Tonga erupted, the violent explosion triggered Lamb waves, atmospheric pressure waves propagating at the speed of sound, around 315 m/s. These ‘atmospheric tsunamis’ differ from traditional tsunamis in their origin: they are not generated by a massive displacement of water, but by the rapid compression of air caused by the volcanic eruption. This rare phenomenon enabled these waves to cross the oceans and encircle the planet several times, their speed allowing them to reach the American coast long before the oceanic tsunamis.
In fact, the atmospheric waves were recorded up to five hours before the arrival of the slower ocean waves, travelling at around 200 m/s. These atmospheric waves, although not as high as the ocean waves (3 to 4 times smaller), nevertheless caused significant variations in sea level. In the Gulf of Mexico, for example, these waves were the only ones detected, and their second arrival was stronger than the first, highlighting the complexity of their propagation across the planet. This phenomenon remains poorly documented. But it highlights the importance of atmospheric waves in tsunami dynamics.
Thanks to the high temporal resolution of the satellite data, it has been possible to follow the path of these waves precisely, in real time, by analysing the perturbations in atmospheric temperature and correlating them with measurements of pressure on the ground, notably in Pago Pago, Samoa, a region close to the epicentre, which was the subject of a study by Horváth et al. (2024).
Geostationary satellites have detected temperature anomalies caused by the eruption of the Hunga volcano. These data show temperature variations (left), their speed (centre) and acceleration (right) at 05:14 and 05:24 UTC on 15 January 2022. A star marks Pago Pago, where pressure disturbances were recorded, and lines represent distances of 200 km around the volcano to track wave propagation © Horváth et al., 2024
This meticulous monitoring has enabled us to better understand not only the precise chronology of the eruption, but also the behaviour of these large-scale atmospheric waves. These new data have shown that the atmospheric waves were not uniform and varied in frequency and intensity as they progressed around the globe.
Ocean waves: a traditional tsunami amplified
The second type of wave, generated directly by the eruption of Hunga-Tonga, is more typical of traditional tsunamis. These waves, known as ‘ocean waves’, are caused by a sudden and massive displacement of water due to the underwater volcanic eruption. They travelled at a speed of around 200 m/s, which is around two-thirds the speed of atmospheric waves generated by Lamb waves. These oceanic tsunamis spread across the Pacific, reaching much greater heights than their atmospheric counterparts. At some coastal stations, notably Port San Luis in California, and Manzanillo and Ensenada in Mexico, these waves exceeded 2 metres. Their impact was felt all along the Pacific coasts of the Americas, causing significant flooding and disruption in the areas affected. These ocean waves were impressive not only for their height, but also for their wide geographical distribution.
These oceanic tsunamis are also characterised by a particularly wide range of frequencies, from 0.2 to 30 cycles per hour. More precisely, 0.2 cycles per hour means that a complete wave takes around 5 hours to form and pass over. This corresponds to very long waves, with slow oscillations.
30 cycles per hour means that 30 complete waves are formed in an hour, i.e. approximately one wave every 2 minutes. This corresponds to much shorter waves, with rapid oscillations.
Maps of the US sides showing the maximum wave heights recorded (in centimetres) during the 2022 Tonga tsunami from: (a) atmospheric sources; and (b) ocean sources. The size of a circle is proportional to the maximum wave height, and the white lines indicate the travel times (in hours) across the Pacific Ocean from the source area.© Zaytsev et al., 2024This observation came as a surprise to scientists who had initially underestimated the true extent of the region affected by these oceanic tsunamis. Indeed, the initial tsunami propagation models did not take into account the immensity of the area affected by the eruption, nor the capacity of these waves to conserve their energy over such a long distance.
This wide range of frequencies reveals that the oceanic tsunamis generated by the Hunga-Tonga eruption originate from a much wider source than the immediate epicentre of the eruption. In other words, the ‘effective’ region from which these waves emerged far exceeds the area of the volcanic explosion itself, extending over a much larger area of ocean. This phenomenon of long-range propagation was observed in several regions of the Pacific, where the waves continued to make themselves felt long after their initial arrival.
Data analysis for a better understanding
Analysis of the data collected during the Hunga-Tonga eruption has clarified the complex mechanisms involved in the generation and propagation of tsunamis. Tide gauges, which record changes in sea level, and microbarographs, which measure changes in atmospheric pressure, provided key data for isolating the two types of wave and analysing their distinct characteristics.
Oleg Zaytsev and his team therefore studied in detail the sea level records along the American coastline, from sensors located both on the coast and in the open sea. They were able to track their propagation, showing that ocean waves had a prolonged effect on sea levels, with oscillations that could persist for several days.
This analysis has many implications for coastal risk management. Scientists’ ability to differentiate between atmospheric and oceanic waves is improving tsunami early warning systems. In this particular case, the atmospheric waves reached the coast well before the ocean waves, which could provide additional time to alert coastal populations.
However, although these atmospheric waves were less destructive in terms of height, they are no less dangerous, as they can still cause significant maritime disruption. This distinction between the two types of waves and their respective impacts makes it easier to anticipate the response to tsunamis, particularly in vulnerable areas, and to adapt evacuation and protection measures according to the nature of the waves expected.
Source : Zaytsev, O., Rabinovich, A. B., & Thomson, R. E. (2024). “The 2022 Tonga tsunami on the Pacific and Atlantic coasts of the Americas”. Journal of Geophysical Research: Oceans, 129, e2024JC020926