Journal of the Oceanographical Society of Japan Origin of the Abiki Phenomenon (a Kind of Seiche) in Nagasaki Bay*

Abstract: Large oscillations of water level in Nagasaki Bay are called Abiki and are most frequently observed in winter. The largest Abiki recorded in the past 20 years at the tide station at Nagasaki occurred on March 31, 1979. Simultaneously, a distinct atmospheric pres-sure disturbance of solitary type with an amplitude of about 3 mb was recorded at several neighbouring stations in KytIshtt, which indicated the pressure disturbance probably travelled eastward with an average speed of about 110km The quantitative relation between this pressure disturbance and notable seiches observed in Nagasaki Bay is examined by means of numerical simulation, and it is confirmed that the exceptionally large range of oscillations in the bay, which reached 278 cm at the tide station, was indeed produced by this travelling pressure disturbance. The leading part of shallow water waves induced by the atmospheric pressure disturb-ance was amplified up to about 10cm in amplitude, over the broad continental shelf region off China, because of near resonant coupling to the pressure disturbance. After leaving this continental shelf region, the amplified water wave converged into the shelf region (Goto Nada) surrounded by the north-western coast of Kyftsha and the Gote. Islands and excited eigenoscillations on the shelf. A train of waves thus: formed with a period of about 35 min entered Nagasaki Bay and was resonantly amplified at periods of 36 min and 23 min which are the eigen periods of the bay. Besides resonance, the combined effects of shoaling and reflection inside Nagasaki Bay also enhanced the amplification