Toshiyuki Asano, Toru Yamashiro & Norihiro Nishimura

Transcription

1 Field observations of meteotsunami locally called abiki in Urauchi Bay, Kami- Koshiki Island, Japan Toshiyuki Asano, Toru Yamashiro & Norihiro Nishimura Natural Hazards Journal of the International Society for the Prevention and Mitigation of Natural Hazards ISSN X Volume 64 Number 2 Nat Hazards (2012) 64: DOI /s

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3 Nat Hazards (2012) 64: DOI /s ORIGINAL PAPER Field observations of meteotsunami locally called abiki in Urauchi Bay, Kami-Koshiki Island, Japan Toshiyuki Asano Toru Yamashiro Norihiro Nishimura Received: 21 September 2011 / Accepted: 28 July 2012 / Published online: 11 August 2012 Ó Springer Science+Business Media B.V Abstract A seasonal scale field observation extending over a period of 82 days was conducted in Urauchi Bay on Kami-Koshiki Island, to record meteotsunami events, disastrous secondary oscillations locally known as abiki. The bay has an elongated T-shape topography with a narrow mouth opening westward to the East China Sea. The area has suffered the effects of meteotsunami causing flooding in residential area and damage to fishing fleets and facilities. A comprehensive observation system for sea level, ocean currents and barometric pressure was deployed to cover the regions within and offshore from Urauchi Bay and the open sea near the island of Mejima in the East China Sea. Vigorous meteotsunami events, where the total height exceeded 150 cm, were observed over five-day periods during the observation period. One or two hours prior to the arrival of meteotsunami events at Kami-Koshiki Island, abrupt 1 2 hpa pressure changes were observed at the Mejima observation site. Pressure disturbances were found to travel eastward or northeastward. The propagation speed was found to nearly coincide with that of ocean long waves over the East China Sea, and as a result, resonant coupling should be anticipated. The incoming long waves were also amplified by geometric resonance with eigen oscillations inherent in the T-shape topography of Urauchi Bay. Keywords Meteotsunami Seiche Coastal disaster Air-ocean coupling process Harbor resonance 1 Introduction Meteotsunami are atmospherically induced destructive ocean waves in the tsunami frequency band (Rabinovich et al. 2009). They are not caused by underwater seismic action, T. Asano (&) T. Yamashiro Division of Ocean Civil Engineering, Graduate School of Science and Technology, Kagoshima University, , Korimoto, Kagoshima , Japan asano@oce.kagoshima-u.ac.jp N. Nishimura Research and Development Center, Tokyo Kyuei Co., , Shiba, Kawaguchi , Japan

4 1686 Nat Hazards (2012) 64: but rather by meteorological forces on the sea surface. The origin of the term meteotsunami can be traced to an oceanography textbook by Defant (1961). Other terms such as secondary undulations of tide, harbor oscillation, large seiche and forced seiche have been used for the atmospherically induced long waves, depending on how and where they occurred. Recently, the term meteotsunami has become widely recognized to be the appropriate general term for these waves (Rabinovich and Monserrat 1996; Šepić et al. 2009). Meteotsunami are known in Japan by the local term abiki, literally meaning netdragging waves in Japanese. Large abiki occurs in bays and harbors along the west coast of Kyushu (Fig. 1a). Almost every year during winter and early spring, several events happen that cause flooding, hamper ship maneuvering and capsize fishing boats. The study of the abiki has a long history mainly centering on Nagasaki Bay (e.g., Ishiguro and Fujiki 1955; Nakano and Unoki 1962; Akamatsu 1982). Several mechanisms have been proposed for the generation of meteotsunami. Hibiya and Kajiura (1982) examined the abiki event in Nagasaki Bay in 1979 using a numerical simulation and proposed quantitative explanations of how the original atmospheric pressure jump of a few hpa was manifested in waves of up to 278 cm. They showed that a traveling atmospheric pressure system over the ocean surface resonated with the long ocean-wave propagation (Proudman resonance). The long waves then increased their height by shoaling and subsequently by geometric resonance with the eigen period of Nagasaki Bay. In contrast, de Jong and Battjes (2004) have attributed the cause of the meteotsunami to low-frequency wind fluctuations associated with trailing convection cells traveling behind a cold front where cold air moves over a relatively warm sea surface. Another atmosphere-to-ocean energy transfer can occur when the speed of edge waves coincides with an along-shore velocity component of an atmospheric disturbance. This Fig. 1 Maps of a the western Kyushu Islands, b Koshiki Islands, and c Urauchi Bay of Kami-Koshiki Island

5 Nat Hazards (2012) 64: Greenspan resonance (1956) has been confirmed to be the source mechanism for seiche motion by field observations on a shelf region with a straight coastline (Yanuma and Tsuji 1998). Monserrat et al. (2006) summarize the particular conditions required for meteotsunami generation: strong small-scale atmospheric disturbance, propagation of the disturbance toward the entrance of a harbor or bay, atmosphere ocean resonance (Proudman, Greenspan or shelf resonance), and harbor resonance. Vilibić et al. (2008) and Vilibić (2008) used a numerical model considering Proudman resonance, topographical amplification and harbor resonance to reproduce a meteotsunami that occurred in Menorca, in the Balearic Islands of Spain, in They found that the model results quantitatively reproduced the observed sea-level heights. Regarding the barometric pressure perturbation that induces a meteotsunami, Tanaka (2010) pointed out two important mechanisms that enable gravity waves, once they are generated, to maintain their energy while propagating away from the source region over a long distance. These mechanisms are termed wave-duct (Monserrat and Thorpe 1996) and wave-cisk (Lindzen 1974). Both mechanisms can generate the necessary sea-level pressure disturbance to cause a meteotsunami. In his study, advanced numerical simulations of these mechanisms at mesoscale meteorological fields are considered. On February 2009, Urauchi Bay, located on west coast of Kami-Koshiki Island on the southeast coast of Kyushu (Fig. 1b), was subjected to a destructive meteotsunami. In this event, a maximum sea surface height of 3.1 m was observed at the inner part of the fishing port of Oshima (Fig. 1c). At least 18 boats capsized and eight houses were flooded. This February 2009 event, as estimated from numerous observations, surpassed the previous record height for an abiki in Japan: 278 cm in Nagasaki Bay in During the same 2009 event, high water-level anomalies were also recorded as 157 cm in Nagasaki and 141 cm in Makurazaki. Generally, Urauchi Bay, with its elongated inlet and narrow mouth, provides calm water conditions even when offshore weather is stormy. Therefore, the area is regarded as a suitable place for the farming of large fish with a high market value. Possible damage to the extensive fish cage system as a result of abiki events is of concern, especially because aquaculture is the main industry in the isolated islands. Therefore, forecasting abiki events is a serious request from the local people. The authors team has investigated meteotsunami in Urauchi Bay since 2004 (Shirahashi et al. 2008). A series of field observations was conducted there in October 2008, just 4 months prior to the destructive event in February 2009 (Asano et al. 2010). The observations, however, were conducted over a period of only 15 days, and the total number of measuring points for water levels and current velocities was restricted. The maximum secondary oscillation obtained during the observation period reached only around 50 cm. The objectives of the present study are to detect a meteotsunami event in Urauchi Bay and to clarify the meteorological and hydrodynamic conditions related to its occurrence. This work attempts to observe the whole process of a meteotsunami event: generation offshore, resonance while it propagates and finally amplification in the bay. Observations were conducted over a period of 82 days (January 12 to April 4, 2010), aiming to record large secondary oscillations occurring in winter and early spring. A comprehensive measuring system for sea level, current and barometric pressure fluctuations was deployed covering not only inside and near Urauchi Bay but also further offshore in the vicinity of Mejima in the East China Sea. Based on the measured data, we discuss (a) the atmospheric and oceanographic characteristics of the origin of the meteotsunami, (b) the amplification mechanisms during its movement in the offshore area, and (c) the resonance mechanisms in Urauchi Bay.

6 1688 Nat Hazards (2012) 64: Observations 2.1 Data acquisition Atmospheric pressure was measured offshore at the island of Mejima, located around 140 km west of Kami-Koshiki Island (Fig. 1a). Another pressure gauge was established at Oshima (Fig. 1b). Continuous data recording of atmospheric pressure was conducted with a short sampling rate of 60 s to detect an abrupt pressure changes. The Japan Meteorological Agency (JMA) continuously measures the atmospheric pressure of the surrounding area at Fukue and Akune (see Fig. 1). These records were used as supporting data to analyze the meteorological situations. Shirahashi et al. (2008) and Asano et al. (2010) identified the unique characteristics of the secondary oscillation in Urauchi Bay that result from its T-shaped configuration (Fig. 2). Three predominant modes exist. Mode-1 oscillations have a node at the mouth of the bay and antinodes at the inward ends of the two branched bays (the eigen period is approximately 24 min). Mode-2 oscillations have two nodes in the bay and an antinode at both ends of the branched bays with a period of around 12 min. Mode-3 oscillations alternatively move the water level up at one end of the bay and down at the other with a node at the branching point (an eigen period of around 10.5 min). The numerical simulation can reproduce the above characteristics of the eigen oscillation (Asano et al. 2010). Fig. 2 Deployment of the measuring devices inside and near Urauchi bay

7 Nat Hazards (2012) 64: Based on the above findings, measurement points for water-level fluctuations were established as follows. Semiconductor-type pressure gauges (measuring range: 25 m, accuracy: 0.3 % FS) were deployed at three points in the offshore area (Fig. 2, St. 1 St. 3) and at six points inside the bay (Fig. 2, St. 4 St. 9). In addition, a wave gauge was set to measure the open sea waves at Mejima (St. 10). Continuous data logging was undertaken with sample interval of 60 s. The pressure gauge at Mejima was fixed to the sea bottom because of severe wave conditions. For the other points, the pressure gauges were set at a determined level below the sea surface using a float anchor system. Under rough wave conditions, however, it was noticed that the float anchor system in the offshore area (St. 1 St. 3) was swung by the wave motion. Because of this disturbance, the accuracy of the data might decrease during severe wave conditions. Continuous current measurements were conducted with a 600 khz Teledyne Workhorse ADCP at St. 5, near the branching point, and at St. 8, at the inner end of the bay near the port of Oshima. Vertical profiles of the currents can be detected at 1-m layer intervals. The other specifications of the ADCP were a maximum range of 70 m and ±0.8 % FS accuracy. The data sampling was also made at a 60-s interval. 2.2 Event extraction Seiche motions were extracted by removing the tidal and wind wave components from the original water-level fluctuation data using a numerical high-pass filter. For sea-level and barometric pressure fluctuation data, fluctuations with periods greater than 1 h were eliminated. The design of the high-pass filter can be attained by setting appropriate weighting coefficients for a time series by referring the frequency response function (Chatfield 2003). Figure 3 illustrates the typical secondary oscillations observed at St. 9 on February 1, The zero-up-cross method, spectral analysis and wavelet analysis were applied to the extracted seiche-band water-level fluctuations. Seiche events where the height (i.e., crest-to-trough height) exceeds 100 cm at St. 9 happened 82 times over 15 days during the overall observation period of 82 days (Fig. 4). Greater seiche events with heights over 150 cm were observed five times on 25 January (1.6 m), 1 February (1.6 m), 3 March (2.6 m), 5 6 March ( m) and 24 March (2.3 m). The seiche on 1 February caused minor flooding (to a depth of around 10 cm) in a residential area of Senoue village (see Fig. 1c). On this occasion, the seiche and the high water level of the spring tide coincided. Figure 5 (top) shows the flooding caused by the event on February 1, 2010 at Senoue village. The photo was taken at 9:10 a.m. when the water level had almost reached its peak with an inundation depth of 0.2 m above the crown height of the jetties. The local government conducted a damage survey and measured the inundation depths just after the event. Figure 5 (bottom) shows the survey scene taken on February 2, 2010 at the same location. Fig. 3 Water-level fluctuations observed at St. 9 on February 1, 2010

8 1690 Nat Hazards (2012) 64: Fig. 4 Time series data of the extracted seiche component observed at St. 9 The other great seiche events did not cause flooding because they occurred during low tide conditions. Present observations have revealed, however, that the occurrence frequency of great seiche events is much higher than we expected. Even if seiche-induced water elevation does not result in flooding, attention should be paid to seiche-induced strong currents, which may cause destruction to fishing boats and cages. 3 Resonant characteristics in Urauchi Bay Figure 6 illustrates the energy spectra of seiche-band water-level fluctuations measured at the bay mouth (St. 4) and inside the bay (St. 5 St. 9). As mentioned, Urauchi Bay has three dominant modes of oscillation: (1) mode-1, with a node at the bay mouth and antinodes at both of the inner ends, and with an eigen period of 24 min, (2) mode-2 having higher

9 Nat Hazards (2012) 64: Fig. 5 Photograph taken during the inundation event on February 1, 2010 at Senoue villege (top) and just afterward of the event on February 2, 2010 at the same place (bottom) (provided by Satsuma-Sendai city) harmonics with a period of 12 min, and (3) mode-3 oscillating between the two inner ends making its node at the branching point with a period of 10.5 min. The period range in band- 1 indicated in Fig. 6 corresponds to the mode-1 component. The order of magnitudes for seiche amplitudes in this mode is arranged as St. 9 [ St. 8 [ St. 7 = St. 6 [ St. 5 [ St. 4. This means that the seiche amplitude becomes greater toward both of the inner bays, and smaller approaching the mouth of Urauchi Bay. The magnitudes in the band-2 range correspond to mode-2 oscillations. They can be arranged as St. 9 [ St. 8 [ St. 4 [ St. 5 [ St. 6. Under this mode, two nodes will be generated, one at the bay mouth and the other in the middle of the bay. The magnitude of oscillation becomes smaller near St. 6 where the inner node is formed. The band-3 range corresponds to mode-3 oscillations. The magnitude order in this case is St. 9 [ St. 8 [ St. 7 [ St. 6, which implies a seesaw-like oscillation taking its node at the branching point and antinodes at both bay ends. The wavelet spectrum of the water-level fluctuation observed at St. 9 on February 1, 2010 is shown in Fig. 7. There are intense energy areas in the min period band corresponding to mode-1 oscillations, and the min period band corresponding to mode-2 and mode-3 oscillations. The and 6 17-min period band components are extracted from the time series data using numerical filters. The time series for the min components is shown in Fig. 8a. In-phase oscillations are observed. The

10 1692 Nat Hazards (2012) 64: Fig. 6 Power spectra of sea-level fluctuations observed at the bay mouth and inner points of the bay on February 1, 2010 magnitude of the oscillations becomes greater toward the inner ends of both bays and reaches a maximum at St. 9. The time series of the latter components corresponding to mode-2 and mode-3 oscillations is illustrated in Fig. 8b. The oscillations at St. 3 and St. 4 located near the mouth of the bay are found to be out-of-phase from those of inner bay at St. 8 and St. 9. The water level in one bay (St. 8 and St. 9) oscillates out-of-phase with the other bay (St. 7). Note that the oscillation at St. 6, located near the branching point, becomes small indicating that a nodal point has formed in this region. These results support the existence of mode-3 oscillation. The same characteristics are found for almost all of the other observations. The offshore data taken at Mejima (St. 10), which include the intensive energy of windand swell-wave bands, reached its peak energy at 18 and 45 min on 6 March, and 15, 26 and 58 min on 24 March (figure not shown here). Fig. 7 Wavelet spectrum of water-level fluctuations observed at St. 9 on February 1, 2010

11 Nat Hazards (2012) 64: Fig. 8 Time series of mode-1 component (the upper row) and mode-2 and mode-3 components (the lower row) of the seiche oscillations To quantify the amplification in Urauchi Bay, rough estimates were made to compare the power spectra at the inner end station, St. 9, with a station outside of the bay, St. 3. Figure 9 illustrates the spectra of both stations using 1 February data, and the response p function defined as HðTÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi SðTÞ St:9 =SðTÞ St:3 : Since an antinode forms at the inner end of the bay, the resonant oscillation has a maximum at St. 9. As a reference station, we regard St. 3 as more appropriate than St. 4 because St. 3 is located offshore and, therefore, has little influence on the eigen oscillations in the bay, where the oscillations are mainly composed of incoming waves toward the bay. In contrast, because St. 4 is located at the entrance to the bay, the eigen oscillations form a nodal point there, and as a result, the amplification ratio may become excessively large if St. 4 was adopted as the reference point. Note that the response function defined above contains not only resonant amplification but also other effects such as topographical convergence, shoaling (the water depth at St. 3 is roughly 60 m, see Fig. 2) and bottom frictional loss. Figure 9 shows that the response function has a peak at 10.5 min for the mode-3 oscillation and at 24 min for the mode-1 oscillation, and the magnitudes have values of 8.2 and 7.0, respectively. 4 Propagation of barometric pressure waves Time series of air pressure data at Kami-Koshiki, Mejima, Fukue and Akune (note their locations in Fig. 1a) was analyzed to determine the conditions of barometric pressure fields at the times of the meteotsunami that were investigated. Long wave components ([60 min) were eliminated using a numerical filter. Figure 10 illustrates a barometric pressure time series for the raw (upper row) and filtered data measured on February 1, An abrupt pressure change of 2.1 hpa at the 70-min period was observed between 7:04 and 8:14 at Mejima. The eastward propagating pressure wave reached Kami-Koshiki between 8:11 and 9:04 with the magnitude, 1.6 hpa. The 140 km distance between Mejima and Kami- Koshiki was covered in around 1 h. Between 20 and 30 min after the arrival of the wave at Kami-Koshiki, a pressure peak was reached at Akune (NE-ENE and 40 km from Kami- Koshiki). The barometric pressure at Fukue, the northernmost of the four stations,

12 1694 Nat Hazards (2012) 64: Fig. 9 Power spectra of sea-level fluctuations at St. 3 and St. 9 and response function of Urauchi bay fluctuated the least and did not show significant peaks. Judging from the above results and referring to the 6-hour interval weather charts for that day, the pressure wave conceivably traveled eastward with a speed of around 140 km/h. The sea-level fluctuation time series for St. 9, located at the inner end of Urauchi Bay, is shown in the bottom part of Fig. 10.A maximum seiche height of 1.6 m was recorded between 8:50 and 9:00. Prior to the peak, however, intense sea-level fluctuations were observed between 7:00 and 8:00. Coherence between air pressure and sea-level fluctuation is not necessarily present for all seiche records. This is because sea-level motions do not directly respond to local air pressure fluctuations. The data of 3 March show that four distinct pressure waves persisted for 3 h and traveled from Mejima to Kami-Koshiki (Fig. 11). For example, an abrupt pressure rise at Mejima of 1.7 hpa over 33 min starting at 10:40 (M3 in Fig. 11) propagated and arrived at Kami-Koshiki of where a rise in pressure of 1.0 hpa occurred over 11 min starting at 12:10 (K3). The rise at K3 propagated further eastward and arrived at Akune at 12:30 (A3). Since the station at Fukue is located on a west-to-east path between Mejima and Kami-Koshiki, the arrival times of the pressure rises reflect the geographical location. The results (Fig. 11) indicate that the pressure waves propagated eastwards, on the whole, with a propagation speed of 90 km/h, significantly lower than that observed on 1 February. The data from 5 March (not shown here) illustrate that the pressure waves propagated from Mejima to Fukue to Kami-Koshiki to Akune. The speed of the pressure waves is estimated to have been 140 km/h from the time lag of 1 h between Mejima and Kami- Koshiki. The propagating patterns of 3 and 5 March are regarded to be basically the same as that of 1 February. Figure 12 illustrates the time series of barometric pressure measured on 6 March. The orders of occurrence of abrupt pressure rise and fall were different from the previous data. A sudden pressure rise at Kami-Koshiki at 2:30 reached Akune just 10 min later. Another

13 Nat Hazards (2012) 64: Fig. 10 Time series of barometric pressures at observation sites (top), low cut filtered barometric pressures at Mejima, Kami-Koshiki, Fukue and Akune, and sea-level fluctuation at St. 9 at the inner end of Urauchi bay (bottom) (February 1, 2010)

14 1696 Nat Hazards (2012) 64: Fig. 11 Time series data of the low cut filtered barometric pressure at Kami-Koshiki, Mejima, Fukue and Akune observation points and sea-level fluctuation at St. 9 at the inner end of Urauchi bay (bottom) (March 3, 2010) rise at Kami-Koshiki at 19:50 arrived at Akune at around 20:05 min and then at Mejima at 20:15. These propagating patterns indicate that the pressure waves generated south of Kami-Koshiki moved northeastward at a speed of km/h. The sea-level fluctuation shown in the bottom row indicates a significant crest-trough height with 2.0 m at 20:50 and 2.8 m at 21:30 recorded at St. 9. The data from 24 March (Fig. 13) show that there were two events of abrupt pressure changes at Mejima; one was a change of 1.7 hpa over 23 min starting at 5:00, whereas the other was a change of 2.2 hpa over 22 min starting at 21:30. They propagated eastward and reached Kami-Koshiki at 6:50 with a pressure of 1.1 hpa over 30 min and at 22:40

15 Nat Hazards (2012) 64: Fig. 12 Time series data of the low cut filtered barometric pressure at Kami-Koshiki, Mejima, Fukue and Akune observation points and sea-level fluctuation at St. 9 at the inner end of Urauchi bay (bottom) (March 6, 2010) with a pressure of 1.0 hpa over 24 min, respectively. These waves traveled the distance between Mejima and Kami-Koshiki in around 1 2 h. Although the barometric pressure intensively fluctuated for the whole day and these event peaks were sometimes dispersed by the background fluctuations, the propagation pattern can be found to run from Mejima to Fukue to Kami-Koshiki to Akune. The result agrees with the eastward propagation pattern found in all the seiche-event data with the exception of 6 March. The sea-level

16 1698 Nat Hazards (2012) 64: Fig. 13 Time series data of the low cut filtered barometric pressure at Kami-Koshiki, Mejima, Fukue and Akune observation points and sea-level fluctuation at St. 9 at the inner end of Urauchi bay (bottom) (March 24, 2010) fluctuation time series (bottom row of Fig. 13) shows that the maximum crest-to-trough height of 2.3 m was recorded at 23:40 at St. 9. Next, the barometric pressure fluctuation time series was examined in frequency space in terms of wavelet spectra. This was done because the phenomenon is a highly non-stationary

17 Nat Hazards (2012) 64: Fig. 14 Wavelet spectra of the barometric pressure fluctuations at Mejima (top) and Kami-Koshiki (bottom) (February 1, 2010) process. Figures 14, 15, 16 and 17 illustrate the results using the data at Mejima and Kami- Koshiki measured on 1 February, and 3, 6 and 24 March. Note that the raw barometric pressure fluctuation data are analyzed here. Comparing the wavelet spectrum (Fig. 14) with that of the water-level fluctuation (Fig. 7), high-energy areas (drawn in red colors) extend over longer period bands of up to around 100 min. Also, the intensive energy areas at Mejima are found to move to Kami-Koshiki with a 1 2 h delay. These results coincide with the findings of the barometric time series data analyses (Figs. 10, 11, 12, 13). Overall, the barometric pressure wave propagation data from this study indicate that the pressure waves propagate eastward or northeastward and arrive at Kami-Koshiki Island, a distance of 140 km, within 1 2 h. The bathymetry in the study area consists of the continental shelf of the East China Sea, extending eastward for about 600 km from the coast of China. Over most of the shelf, the water depth is about m, and the corresponding ocean long wave speed is C = m/s = km/h. Thus, resonant air-sea coupling (Proudman resonance) may be expected to occur. However, the narrow but deep Okinawa Trough (900 m at its deepest) enters the region between Mejima and Kami- Koshiki. It is, therefore, difficult to assign an average water depth to estimate the propagating speed of the ocean waves. Nevertheless, it is safe to say that the propagation speed of barometric pressure waves as high as 140 km/h corresponds to the phase velocity of ocean long waves. 5 Characteristics of currents in Urauchi Bay Prior to discussing meteotsunami-induced currents, it is useful to look at the characteristics of the tidal currents in the area in general. The tidal characteristics of Urauchi Bay can be

18 1700 Nat Hazards (2012) 64: Fig. 15 Wavelet spectra of the barometric pressure fluctuations at Mejima (top) and Kami-Koshiki (bottom) (March 3, 2010) Fig. 16 Wavelet spectra of the barometric pressure fluctuations at Mejima (top) and Kami-Koshiki (bottom) (March 6, 2010)

19 Nat Hazards (2012) 64: Fig. 17 Wavelet spectra of the barometric pressure fluctuations at Mejima (top) and Kami-Koshiki (bottom) (March 24, 2010) generalized to those of a tidal inlet with an elongated topography. According to the Hydrographic and Oceanographic Department of the Japan Coast Guard, the amplitudes of the major four tide constituents in the study area are as follows: 76 cm for M 2, 33 cm for S 2, 30 cm for K 1 and 21 cm for O 1. The inflow/outflow velocity at the entrance of Urauchi Bay, U, can be roughly estimated as: U ¼ A s H t = ða c T t Þ; where H t is the spring range (two times the sum of the amplitudes of M 2 and S 2 ), A S is the sea surface area of the bay, A c is the cross-sectional area of the bay mouth, and T t is the elapsed time from the high to low tide. Given the specific values for Urauchi Bay as A s = m 2, A c = m 2, H t = 2.2 m and T t = s, we have U = 1.6 cm/s. Meanwhile, the harmonic analysis of the continuous velocity data during the period from January 19, 2010 to April 11, 2010 at St. 5 indicates that the spectrum had a predominant peak around 12 h. The amplitude corresponding to the 12 h component was found to be 2.7 cm/s. Both velocities are so small that the tidal velocity can be ignored in the following discussion of meteotsunami-induced currents. The time series of current vectors measured at St. 5 (near the bay mouth, see Fig. 2) and St. 8 (inward end of Oshima) for the 1 February event is illustrated in Figs. 18 and 19, respectively. Note that raw data without filtering are used for the current data. Figure 18 indicates that the meteotsunami-induced currents occurred between 7:00 and 12:00 with the velocity magnitudes reaching around 20 cm/s at St. 5. The velocity direction was found to alter with a period of around 25 min. Wind-driven currents were superimposed on the surface-level data at z =-2m. The current vectors do not show marked differences at measurement levels. Thus, it can be judged that the meteotsunami-induced currents have almost uniform profiles in the vertical direction. The velocity vectors measured at St. 8 at

20 1702 Nat Hazards (2012) 64: Fig. 18 Time series of current vectors measured at St. 5 on February 1, 2010 Fig. 19 Time series of current vectors measured at St. 8 on February 1, 2010

21 Nat Hazards (2012) 64: Fig. 20 Velocity spectra measured at St. 5 on February 1, 2010 (solid line: north south component, dotted line: east west component) Fig. 21 Velocity spectra measured at St. 8 on February 1, 2010 (solid line: north south component, dotted line: east west component)

22 1704 Nat Hazards (2012) 64: Fig. 22 Velocity spectra for major meteotsunami events the inner end of the bay (Fig. 19) also show almost uniform profiles in the vertical direction. In the data for St. 8, large velocities were only recorded during a period of meteotsunami occurrence. The rest of the time, the currents were calm. The wind-driven currents were obscure compared to the data at St. 5, because of the location at the inward end of the bay where hilly topography protects the water surface from the wind. The velocity magnitudes at St. 8 were greater than those at St. 5, which reached around 40 cm/s. The current directions are close to 45 in a northwest direction, which coincides with the orientation of the bay topography. The current data measured during the meteotsunami events on March 3 and March 5 8 (not shown here) indicate that stronger currents over 60 cm/s were recorded at St. 8.

23 Nat Hazards (2012) 64: The spectra of the velocity data measured at St. 5 on 1 February are shown in Fig. 20. The north south component of the velocity has a maximum energy peak at around 24.4 min and a secondary peak at around 12.5 min. The east west component of the velocity has a similar spectrum structure, but with a smaller magnitude. The spectral shapes for the data measured at the other levels were almost the same (not shown here). The velocity spectra measured at St. 8 are shown in Fig. 21. The energy peaks found at 24.8 and 11.8 min are similar to the above results at St. 5. In addition, a longer period component at around 94 min is also found. Several jetties for mooring fishing boats are built near St. 8 in Senoue village (Fig. 5 shows the situation there), and they may affect the velocity field by generating vortices at the tip of the structures. The generation of the longer period component in Fig. 21 could be explained by such local flow motion, but further examination is needed. Figure 22 illustrates the spectra for the four major meteotsunami events observed. The spectrum structures and peak positions were essentially the same for the four events. 6 Conclusions A seasonal scale field observation undertaken over a period of 82 days was conducted to detect meteotsunami events in Urauchi Bay. The bay has an elongated T-shape bay opening westward to the East China Sea. A comprehensive measurement system for water level, current and barometric pressure was deployed covering both inside and outside the bay and extending out into the open sea near the island of Mejima. Based on the data collected, the dynamic characteristics of atmospheric pressure disturbances were investigated in relation to resonant water-level fluctuations and induced currents. The following conclusions were reached. 1. Large meteotsunami events with total height in excess of 150 cm were observed five times during the 82-day observation period. On February 1, 2010, one such event coincided with the high water of a spring tide, which resulted in flooding. The present observations have revealed that meteotsunami events occur more frequently than previously estimated from existing records of flooding. Even if a meteotsunami event does not result in flooding (e.g., if it coincides with a low tide), attention should be paid to the seiche-induced strong currents that may damage fishing boats or aquaculture installations. 2. Three dominant modes were found to exist in sea-level fluctuation data in Urauchi Bay using spectra analysis, wavelet analysis and phase analysis of the extracted period band components. The node and antinode structure for each node governs more energetic areas for sea-level and the current velocity fluctuations. 3. Analyses of barometric pressure data show that abrupt pressure changes of 1 2 hpa are generated in the open sea area at Mejima when major meteotsunami events occur. The pressure waves propagated eastward or northeastward to reach Kami-Koshiki within 1 2 h. The propagation speed was found to nearly coincide with ocean long waves over the East China Sea. This air-sea resonant coupling is considered to be a source mechanism of meteotsunami generation. 4. Meteotsunami-induced currents have almost uniform profiles in the vertical direction, although wind-driven currents are found to be superimposed in the surface layer. At the inner ends of the bay, significant flow only occurs during a meteotsunami events. However, as seen during the present observations, when a meteotsunami happens, the magnitude of the current reaches a maximum of over 60 cm/s.

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