Characterization of global and regional meteotsunamis using an ultra-dense observational network in the Western Mediterranean

[eng] In the Western Mediterranean, episodes of abnormally large sea-level oscillations occur regularly and can cause flooding and damage to coastal infrastructure. These phenomena are known as meteotsunamis because the oscillations have periods similar to those of seismic tsunamis (~ tens of minutes) but the forcing is atmospheric. Meteotsunamis result from the amplification of the ocean response to atmospheric disturbances which, when travelling at a speed similar to that of the ocean waves they generate, can amplify the static sea-level response (~1 cm per millibar) up to 10–20 cm per millibar. These waves can be further amplified by resonance when they enter harbours or bays, where amplitudes may exceed 4 metres. In the Western Mediterranean, Ciutadella (Menorca) is internationally recognized as a hotspot for large-amplitude meteotsunamis. However, meteotsunamis had not been examined in detail throughout the rest of the region, and the lack of long-term, high-resolution records had hindered the assessment of the spatial and temporal variability of meteotsunamis as well as of the atmospheric disturbances that generate them. This doctoral thesis addresses these gaps by making use of a new ultra-dense observational network of atmospheric pressure and sea level that has an unprecedented spatial and temporal resolution in the Western Mediterranean. Analysis of this dataset has enabled the first regional characterization of meteotsunamis in the Western Mediterranean. The results reveal a highly heterogeneous spatial impact, highlighting Ciutadella and Vilanova i la Geltrú as the most impacted locations, with events exceeding one meter recorded almost every year. In general, the ports with the strongest capacity for harbour resonance amplification are also those that experience the largest meteotsunamis and the most substantial contribution of this phenomenon to positive extreme sea levels. The regional climatology shows that most meteotsunamis occur in late spring and summer, coinciding with a synoptic pattern favorable to the generation of atmospheric gravity waves. The climatology also shows that meteotsunamis may occur simultaneously at different locations across the region, demonstrating that meteotsunamis are regional phenomena modulated by local resonance conditions. Thanks to the ultra-dense observational network, we have also been able to estimate in detail the propagation speed, spectral content, and spatial structure of the atmospheric disturbances that generated meteotsunamis in Ciutadella during 2022. These disturbances exhibit periods compatible with the natural period of the harbour and propagation speeds close to resonance in the Menorca Channel. However, events with similar atmospheric characteristics produced different ocean responses, indicating that additional factors—such as pre-existing oscillations in the harbour or subtle variations in the geometry of the disturbance—also condition the final amplitude. Finally, the thesis presents an analysis of the global meteotsunami generated by the 2022 eruption of the Hunga Tonga–Hunga Ha’apai volcano, an unprecedented event in the modern instrumental era. Observations collected worldwide show how the atmospheric wave triggered by the eruption produced a marine response comparable to a meteotsunami. During this event, however, the high speed of the atmospheric disturbance (~310 m/s) prevented the amplification through the coupling between atmospheric and oceanic waves, which has led to proposing the excitation of shelf, harbour, and bay modes as the drivers of this planetary meteotsunami. Overall, this thesis is a step forward in the understanding of the generation and impact of meteotsunamis based on the combination of a regional analyses from high-resolution atmospheric and sea level observations with a unique global case.