Field Observations of Meteotsunami in Kami-koshiki Island, Japan

BACKGROUND Meteotsunami; atmospherically induced destructive ocean waves in the tsunami frequency band, are known in Japan by the local term "abiki", literally meaning "net-dragging waves" in Japanese. Large abiki occur in bays and harbors along the west coast of Kyushu almost every year during winter and early spring. On 24-25 February, 2009, Urauchi Bay, located on west coast of Kami-Koshiki Island on the southeast coast of Kyushu, 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 bay. At least 18 boats capsized and eight houses were flooded. This event surpassed the previous record height for an abiki in Japan: 278 cm in Nagasaki Bay, also located west coast of Kyushu, in 1979. Generally, such an elongated inlet with narrow mouth as Urauchi bay 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 meteotsunami events is of concern, especially because aquaculture is the main industry in the isolated islands. Forecasting of meteotsunami is a serious request from the local people. AIMS 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; 12 January to 4 April, 2010, aiming to record large secondary oscillations. 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. MAIN RESULTS 1) Large meteotsunami events with total height in excess of 150 cm were observed five times during the 82-day observation period. On 1 February, 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 anti-node 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 hours. 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.