DETECTION METHOD OF METEOTSUNAMI EVENTS AND CHARACTERIZATION OF HARBOUR OSCILLATIONS IN WESTERN MEDITERRANEAN. Abstract

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1 DETECTION METHOD OF METEOTSUNAMI EVENTS AND CHARACTERIZATION OF HARBOUR OSCILLATIONS IN WESTERN MEDITERRANEAN Gaël André 1, Marta Marcos 2, Camille Daubord 3 Abstract A detection algorithm aimed at analysing localized intermittent oscillations in sea level time series and identifying meteotsunami events is presented. The algorithm is based on wavelet analysis of high frequency sea level oscillations recorded by tide gauges. Detection thresholds are fixed both for oscillation amplitude and averaged power spectral density for each tide-gauge station. This method is tested on different historic meteotsunami events and during the large event of 26 July This method is also used to characterize harbour seiche oscillation periods in the North-Western Mediterranean Sea. Key words: tide, sea level oscillations, meteotsunami detection, harbour seiche 1. Introduction Meteotsunamis are atmospherically induced long ocean waves in the tsunami frequency band (with periods ranging from a few minutes to a few tens of minutes) that affect coasts in a destructive way. These waves are not originated by any seismic activity but by air pressure disturbances moving over the sea (Monserrat et al., 2006; Rabinovich and Monserrat, 1998). In the open ocean the disturbances generate smallamplitude barotropic waves that, near the coast, are amplified by topographic resonance in bays and harbours to produce seiche phenomena (Rabinovich, 2009). They occur in oceans all over the world, and are quite common in the North-Western Mediterranean Sea and specifically near the Balearic Islands, where they are locally known as rissaga and where large amplitude seiche oscillations with sea level heights larger than 4-5 m are occasionally observed at Ciutadella Harbour (Menorca Island). 2. Detection of meteotsunami events 2.1 Data and methods The analysis of meteotsunami events was focused on the North-Western Mediterranean Sea (Fig. 1), a region where large amplitude seiches are commonly observed, especially near the Balearic Islands. Sea level data were collected during summer periods (1997, 2006, 2007 and 2012) for all available stations in the area (see Table 1) Tide gauges records A total of 11 tide gauges located in the North-Western Mediterranean Sea were used for this study (Figure 1). Sea level time series are located along the continental shores and on the Balearic Islands. Data were available at 1 min temporal sampling during the summer 2012 period and at 5 minutes over the other periods. These tide gauges are operated by Puertos del Estado and the Balearic Islands Coastal Observing and Forecasting system for Spanish stations and by SHOM for French stations. 1 SHOM, 13 rue du Chatellier, Brest 29200, France. gael.andre@shom.fr 2 IMEDEA (CSIC-UIB), Miquel Marquès, 21, Esporles, Spain. marta.marcos@uib.es 3 SHOM, 13 rue du Chatellier, Brest 29200, France. camille.daubord@shom.fr 83

2 Figure 1. Tide gauges location in the Western Mediterranean. Sea level observations for summer 2012 were available for all stations, which allowed the study of the recent meteotsunami event on 26 July Additionally, data were also collected for periods corresponding to June-July 1997, June 2006 and July 2007 for Barcelona, Valencia, Sagunto and Ibiza stations in order to investigate other reported historical events. Finally, long time-series from 2009 to 2012 were studied for Palma and Pollensa stations. Table 1. Synthesis of available tide gauge data tested in this study. Available periods are indicated in grey with the temporal sampling. Tide gauge stations Periods used for the study Pollensa 1 min 5 min Andratx 1 min Sa Rapita 1 min Palma 1 min 5 min Ibiza 5 min 5 min 1 min Gandia 1 min Valencia 5 min 5 min 5 min 1 min Sagunto 5 min 1 min Barcelona 5 min 5 min 5 min 1 min Port Vendres 1 min Sète 1 min Detection method of meteotsunami events A detection method of meteotsunami events based on the continuous wavelet transform (Grinsted et al. 2004) was used to analyse localized intermittent oscillations in sea level time-series. A 3-hour butterworth high-pass filter was applied to the sea level time series in order to suppress the tidal signal and all lowfrequency oscillations. The amplitude of the high-frequency oscillations was then computed as the difference between two consecutive maximum and minimum values, whereas Power Spectral Density (PSD) was computed using a continuous wavelet transform. For example, the continuous wavelet transform applied to the sea level time serie at the Pollensa station during July 2012 is represented in Figure 2a. Frequency-averaged PSD (Fig. 2b) clearly shows three 84

3 Coastal Dynamics 2013 distinct large amplitude oscillations events on 1, and July 2012, with sea level oscillations (Fig. 2c) higher than 10 cm and a frequency-averaged PSD higher than 10 m²/cpm. Figure 2. Continuous wavelet transform spectra (a), corresponding frequency-averaged power spectrum density (b), oscillations amplitude (c) and sea level oscillations (d) computed from the sea level time-serie recorded at Pollensa station during July Oscillation amplitude and frequency-averaged PSD thresholds must be defined and used to highlight only amplified seiche oscillations that are unusual and could be related to a meteotsunami event. Thresholds are harbour-specific, and should be defined for each site independently. Hence, thresholds values are computed in order to have 95% of oscillation amplitude and frequency-averaged PSD values lower than thresholds. This is done only on times-series longer than one year. Unfortunately, available tide gauge time series are not all as long as needed to compute thresholds (Table 1), which were only fixed for Palma and Pollensa stations over the period Tests of the detection algorithm The detection method detailed above is tested on different periods. It is first applied on summer 1997, 2006 and 2007 periods for which historical meteotsunamis affecting the North-West Mediterranean Sea have been reported in the literature (Vilibić et al., 2008; Jansa et al., 2007; Marcos et al., 2009). It is then applied on July 2012, since a meteotsunami-like event took place on 26 July 2012 and affected all the Spanish and French Mediterranean coasts Historical events Many destructive flood events were observed and documented in the region of the Balearic Islands (Vilibić et al., 2008). In particular, Ciutadella harbour is one of the sites where larger events occur with rissaga waves reaching up to 4-5 m (Gomis et al., 1993; Garcies et al., 1996, Jansa et al., 2007). Strong events were reported on 7-9 June 1997, July 1997, 15 June 2006 and on 22 June Unfortunately we do not have access to these tide gauge data, so far. Our detection method was therefore applied to Barcelona, Sagunto and Valencia tide gauge observations in the Spanish coast and to Ibiza station in the nearby island. The resulting analyses for summer periods of years 1997, 2006 and 2007 are shown in Figure 3. Dates for which large oscillations were detected are summarized in the Table 2. Large amplitude seiches were detected on 4-11 June and on July 1997 at Valencia station. On 2006, events are detected on and June 2006 and during summer 2007, three consecutive events are identified from 22 to 26 June These tests of the method prove that it is efficient to detect historical events reported in the literature. 85

4 Coastal Dynamics 2013 a b c d Figure 3. Detection method applied at Valencia station for summer 1997 (a), Barcelona for summer 2006 (b) and Ibiza for summer 2006 (c) and 2007 (d). 86

5 Table 2. Results of the detection method tested on tide gauge data Periods Tide gauge Stations Valencia Barcelona Valencia Barcelona Ibiza Valencia Barcelona Ibiza Valencia Sagunto Barcelona Detected dates Historical events References 4-11 June June June July July July June June June June June June June 1997 Vibilic et al July June 2006 Vibilic et al Jansa et al June 2007 Marcos et al The 26 July 2012 event A meteotsunami-like event occurred on 26 July 2012 in the North-Western Mediterranean Sea. During this event, large sea level amplitude oscillations were observed at coastal tide gauges, along the Spanish, French and Balearic Islands coasts. The origin of large and unusual harbour oscillations in this area such as observed on the 26 July 2012 is often described in the literature as an oceanic response to some atmospheric gravity waves and/or to convective pressure jumps (Tintoré et al., 1988, Monserrat et al., 1991; Monserrat el al., 2006, Renault et al 2011). In the western Mediterranean, travelling atmospheric pressure oscillations could generate these long oceanic surface waves that can become amplified and produce strong seiche oscillations inside harbours. A 6 hpa abrupt jump of sea surface air pressure (between 1018 and 1012 hpa) was recorded at the same time by offshore wave buoys and coastal stations in the Balearic area (Figure 4). Figure 4. Atmospheric pressure recorded at Sa Rapita, Andratx and Pollensa stations from 24 to 31 July Figure 5 shows 1-min sea level oscillations after application of the 3-hour high pass filter. On this figure, the signature of this event was evidenced by sea level oscillations of about 40 cm along Spanish and Balearic coasts, with periods ranging between a few minutes and some tens of minutes. Meteotsunami waves seem to propagate from Valencia region eastward to the Balearic Islands and north-eastward along 87

6 Catalan and French coasts. Then, harbour seiches of cm were recorded at Port-Vendres and Sète, likely as a consequence of the same event. Figure 5. Sea level oscillations from 1 minute tide gauge measurements. The detection method of meteotsunami events was applied to the sea level oscillations time-series represented on Figure 5. Results for the Pollensa station during July 2012 are visible on Figure 2 which clearly shows a large amplitude oscillation event on 26 July The same event is also detected for all sites analysed in this study. 3. Characterization of harbour seiche oscillation periods The meteotsunami detection method can also be used to characterize harbour seiche oscillation periods in the North-Western Mediterranean Sea. This analysis is based on long time-series, spanning the period , which have been collected for Pollensa and Palma stations on Mallorca Island (see location on Figure 1). First of all, thresholds values are computed over 4-year time-series of oscillation amplitude and frequency-averaged PSD. Computed histograms are presented on Figure 6. Hence, thresholds values are fixed to 12.6 and 21 cm for the oscillation amplitude and to 8.7 and 13.9 m²/cpm for the frequencyaveraged PSD at Pollensa and Palma stations respectively. The characterization of harbour seiche oscillation periods is then based on the meteotsunami detection method used to identify large amplitude oscillations. 88

7 a b c d Figure 6. Histograms of oscillation amplitude and frequency-averaged PSD at Pollensa (upper) and Palma (lower) stations. Finally, spectra were calculated during calm periods of 4 consecutive days, corresponding to weak sea level oscillations. Spectra of background sea level oscillations computed as the average spectra over 4 years ( ) are represented on Figure 7, for Pollensa and Palma stations. The background spectra represent the natural periods of harbour seiche oscillations. Hence, main harbour seiche occurs at periods of 16, 21, 34 and minutes in Pollensa harbour and at 24 and 73 minutes in Palma harbour. Figure 7. Spectral density of sea level oscillations computed on calm periods at Pollensa (left) and Palma (right) stations. Average spectra are represented as a black line. 89

8 4. Conclusion and discussion A detection method of meteotsunami events based on continuous wavelet transform (Grinsted et al. 2004) has been used to analyze localized intermittent oscillations in sea level time-series. This method is tested on different historic meteotsunami events which occurred near the Balearic Islands and on the recent 26 July 2012 event which affected all the Spanish and French Mediterranean coasts. It has been shown that the method is efficient to detect the occurrences of meteotsunami events, but it is not able to separate them from other events that induce similar amplified high frequency oscillations. Improvements could be done to try to better isolate meteotsunamis from other amplified seiches generating events. This detection method of large sea level amplitude oscillations is finally used to characterize harbour seiche oscillation periods. The knowledge of harbour oscillation periods is important because seiches may cause problems for the safety of navigation and may prevent proper operation of ports. These harbour seiche oscillation periods are represented by the mean background spectra estimated as the average spectra at each tide gauge for all available calm periods of 4 consecutive days. The frequency response to the external forcing for each site can be estimated as the ratio between the spectrum of an event and the spectrum of the corresponding background following the method described in Marcos et al, Therefore, the spectral ratio may be considered as the estimation of the energy content of the incoming long waves during the event. Figure 8. Spectral density of sea level oscillations (left) and spectral ratio (right) computed during the 26 July 2012 event at Pollensa and Palma stations Figure 8 shows the spectral density and the corresponding spectral ratio computed during the meteotsunami event over 4 days at Pollensa and Palma stations. For this event, the sea level forcing consists of waves with periods between 16 and 28 min. Indeed, high energy peaks appear within this frequency band on the spectral ratio of the two sites. An increase of energy is also observed at periods around 47 and 73 min at Palma site. This method should also be tested in macro-tidal area such as in the English Channel and along the Atlantic coast. Then, it could be implemented on the real-time data acquisition process of SHOM to build a real-time monitoring system for large amplitude seiches. Finally, the characterization of harbour seiche oscillation periods should be extended to all RONIM tide gauge observatories which provide long timeseries of sea level height. Acknowledgements The authors would like to thanks CETMEF, Météo-France and Puertos del Estado for providing wave buoys and meteorological data. The sea level observations of Port-Vendres and Sète are the property of SHOM, DREAL-LR, CG Pyrénées Orientales and Région Languedoc-Roussillon and are available on the REFMAR website (refmar.shom.fr). Other sea level stations are downloaded from the Sea Level Station Monitoring Facility website (ioc-sealevelmonitoring.org) and from the Balearic Islands Coastal Observing 90

9 and Forecasting System (socib.es). References Garcies, M., Gomis, D., Monserrat, S., Pressure-forced seiches of large amplitude in inlets of the Balearic Islands. Part II: observational study. J. Geophys. Res., 101: Gomis, D., Monserrat, S., Tintoré, J Pressure-forced seiches of large amplitude in inlets of the Balearic Islands, J. Geophys. Res., 98(C8): Grinsted, A., Jevrejeva, S., and Moore, J., Application of the cross wavelet transform and wavelet coherence to geophysical time series. Nonlinear Proc. Geophys., 11: Jansá, A., Monserrat, S., and Gomis, D., The rissaga of 15 June 2006 in Ciutadella (Menorca), a meteorological tsunami, Advances in Geosciences:,12: 1-4. Marcos, M., Monserrat, S., Medina, R., Orfila, A., Olabarrieta, M., External forcing of meteorological tsunamis at the coast of the Balearic Islands, Physics and Chemistry of the Earth, 34(17-18): Monserrat, S., Ramis, C., and Thorpe, A. J., Large amplitude pressure oscillations in the Western Mediterranean, Geophys. Res. Lett., 18: Monserrat, S., Vilibić, I., and Rabinovich, A. B., Meteotsunamis: atmospherically induced destructive ocean waves in the tsunami frequency band, Nat. Hazards Earth Syst. Sci., 6: Rabinovich, A. B., Monserrat, S., Generation of meteorological tsunamis (large amplitude seiches) near the Balearic and Kuril Islands, Natural Hazards, 18(1): Rabinovich, A. B., 2009, Seiches and harbour oscillations. In: Handbook of Coastal and Ocean Engineering (ed. Y.C. Kim), World Scientific, Singapore. Renault, L., Vizoso, G., Jansá, A., Wilkin, J., and Tintoré, J., Toward the predictability of meteotsunamis in the Balearic Sea using regional nested atmosphere and ocean models, Geophys. Res. Lett., 38: L Tintoré, J., Gomis, D., Alonso, S., Wang, D. P., A theoretical study of large sea level oscillations in the Western Mediterranean. J. Geophys. Res., 93: Vilibić, I., Monserrat, S., Rabinovich, A., Mihanović, H., Numerical Modelling of the Destructive Meteotsunami of 15 June, 2006 on the Coast of the Balearic Islands. Pure and Applied Geophysics, 165(11-12):

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