Observational characterization of meteotsunami triggering in the Balearic Islands from an ultra-dense observational network

Meteotsunamis are atmospherically induced sea level oscillations with periods ranging from a few minutes to an hour, the same energy band as seismically induced tsunamis.  These phenomena have been widely studied in the Balearic Islands, as the port of Ciutadella is known for its high occurrence of extreme events (> 1 meter trough-to-crest a couple of times per year) which can cause strong damages. At present, forecasting systems struggle to produce reliable predictions of meteotsunami intensity in Ciutadella.

Past intensive research established the basis for the physical mechanism that explains how a relatively small atmospheric pressure perturbation can cause a sea level oscillation of the order of meters that can have hazardous consequences on the coast. According to the state-of-the-art knowledge, a particular meteorological synoptic pattern triggers the generation and propagation of atmospheric gravity waves resulting in high-frequency atmospheric pressure oscillations at surface; the later force a sea level response that is amplified by Proudman resonance as it travels; finally, the amplified sea surface waves force port and harbour eigenmodes, causing a further amplification by resonance. However, the complex relationship between the characteristics of the atmospheric forcing and of the ocean wave amplification is not yet fully understood. This results in a great uncertainty when operational systems attempt to infer the meteotsunami amplitude from the characteristics of the atmospheric disturbance. Part of the knowledge gaps are due to the lack of resolution of atmospheric observations.

From April to October 2021, ten events were observed in Ciutadella with sea level oscillations exceeding 50 cm amplitude. During that period, time series of atmospheric pressure have been recorded at high temporal resolution (10 s) by the ultra-dense amateur weather station network (71 high quality Davis stations in the Balearic Islands) of BalearsMeteo. We use these observations, as an example of citizen science, to characterize the propagation velocity and spectral energy distribution of the atmospheric disturbances causing the meteotsunamis at an unprecedentedly high spatial resolution covering the Balearic islands archipelago. As in previous studies, these parameters are found to be key in causing the meteotsunamis, since a high correlation has been found between the energy in the high frequency band (period < 20 min) of atmospheric pressure and the energy of sea level oscillations at Ciutadella, when both are averaged over several hours. The estimated velocity and direction of propagation during meteotsunami events are also in agreement with prior numerical and observational studies. However, when examining the time series of sea level and atmospheric pressure on a minutal time scale, the correlations dramatically drop. This has led us to propose other parameters such as the duration of the pressure perturbation, or the preconditioning of the harbour as being key to explain the magnitude of the events.

  The high spatial resolution of observations also allows the study of spatial structures within the atmospheric disturbances causing meteotsunamis, which has not been analyzed before. Preliminary results show a high degree of heterogeneity among the disturbances, although it remains unclear whether that heterogeneity can affect the final meteotsunami amplitude.