The seiche in Nagasaki-Bay, i.e. ABIKI in Japanese, is discussed from three standpoints. Firstly, the statistical aspects of the seiche in Nagasaki-Bay are studied by use of the observed data from 1961 to 1979. Secondly, the seiche which occurred on March 31, 1979, the largest one in the history of tidal observation in Nagasaki-Bay, is studied. The maximum amplitude of the seiche observed at Matsugae-quay, located at the middle part of the bay, was 278 cm and the period was about 35 minutes, while it was 478 cm at the mouth of the Urakami-river located at the northern end of the bay. This seiche may be due to the severe barometric pressure jump associated with the apparent cold air front which advanced over the sea west of Kyushu. Thirdly, the response of Nagasaki-Bay to the forcing at the mouth of the bay is examined by means of a two-dimensional numerical model. Results show that the bay has an eigen period of 35 minutes, 20 minutes and 10 minutes as the uni-, bi-, tri-nodal oscillations of the bay, respectively. These periods agree with the predominant periods observed. According to the report of Hibiya and Kajiura (1981), the dominant periods of long waves that reached the Goto-Nada sea area are 64, 36 and 24 minutes. Since the latter two periods are very close to the eigen periods of Nagasaki-Bay, it is reasonable to consider that the seiches of Nagasaki-Bay are caused by the resonance to the long waves due to the barometric pressure jump.
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