Observational characterization of Balearic Islands meteotsunamis at an unprecedented high spatial resolution

Abstract

[eng] Meteotsunamis are atmospherically induced sea level oscillations with energy at the same frequency band that seismically induced tsunamis. Ciutadella harbour in Menorca is known to be one of the locations with more frequent occurrence of severe meteotsunamis worldwide: every year several events that exceed 1 meter of amplitude (trough-to-crest) are registered. This has motivated extensive research on this phenomenon in the Balearic Islands that has established the basis of the physical generation mechanisms responsible for the meteotsunami occurrence: the synoptic environment enables the formation of high-frequency atmospheric pressure disturbances that generate a sea level response which is amplified both at open sea, mainly by effect of the Proudman resonance, and at the coast, mainly by harbour resonance. Even though the proposed generation mechanisms can explain the energy transfer between the atmospheric perturbation and the sea wave, it is still not possible to infer the amplitude of the sea level oscillations from the atmospheric perturbation characteristics. This is due to the complex relationship between the atmospheric forcing and the sea wave amplification not being fully comprehended yet. It is also hard to correctly guess the formation of an atmospheric disturbance capable of provoking a meteotsunami from the synoptic conditions. The goal of this thesis is to revise the established meteotsunami generation mechanism and to outline the features whose effects on the meteotsunamis amplitude are still uncertain. To do so, a series of meteotsunami events is analysed by means of the high spatial and temporal resolution data provided by a new ultra-dense observational network. Advanced techniques to estimate the atmospheric perturbation propagation velocity and the external forcing acting on the harbours are also implemented. From the analysed events the idea that a certain established synoptic pattern is necessary to the occurrence of meteotsunamis is reinforced and the possible mechanism of generation and propagation of the atmospheric disturbance is suggested for every event. Furthermore, the amplification of the sea wave is found to be highly influenced by the propagation speed and direction of the atmospheric perturbation and the presence of atmospheric energy around the 10.5 min period (the normal mode of Ciutadella harbour) seems to be a requisite to the meteotsunami occurrence. Other factors not considered previously, as the phase difference between the incoming wave and the standing wave in the harbour and the perturbation duration, are observed to be crucial to determine the meteotsunami amplitude.