The structure of pressure anomalies generating 1979 meteotsunami in Nagasaki Japan

The March 1979 meteotsunami event in Nagasaki is the most famous one among the meteotsunami research communities. The recorded maximum wave height was as high as 2.79 m in the middle section of the Nagasaki Bay of Kyushu, Japan. (Hibiya and Kajiura, 1982; Akamatsu, 1982). It has been thought that the meteotsunami was caused by the single pressure jumps with the anomaly as high as 5.8 hPa within 20 minutes observed at Meshima Island about 150 km southwest from Nagasaki Bay. However, there are some unexplainable behavior of the sea level change in the numerical experiment of the ocean long wave assuming that the single pressure jump model. The simulated height of ternary wave was much smaller than as observed at tidal gauge. Hence, the objective of the present study is to investigate the atmospheric structure of the pressure jump(s) propagated over the East China Sea in March 1979 meteotsunami event in Nagasaki.

Weather Research and Forecasting (WRF) model was used to compute the pressure jumps over the East China Sea. The JRA-55 reanalysis dataset was used for giving initial condition and for four dimensional data assimilation (4DDA) in synoptic scale motion. Three levels of domain were given: whole East Asia region, west Japan and southeast China, and the East China Sea including coastal area with the horizontal resolution of 50 km, 10 km, 2.5 km respectively.

As the results of WRF run, the cold dry air along southeast edge of the polar front subjet and the warm air along north edge of the subtropical subjet genarated the strong horizontal wind shear and large horizontal gradient of equivalent potential temperature nearby Shanghai, China. The sinking current of cold dry air has generated the anomalous pressure jumps propagating the East China Sea with the traveling distance longer than 800 km. Behind 100-150 km (west) of the pressure front, a train of the pressure wave had occurred with the amplitude of 1-3 hPa. The train of the pressure wave had almost faded before reaching to the coastal area of west Kyushu, with the travelling distance of about 400-500 km and with propagating speed was slower (~100km/h) than pressure front (~140 km/h). The time lag between two different pressure anomalies was about 70~80 minutes, nearly twice of the oscillation period (i.e. the interval between first and ternary wave) in the Nagasaki Bay. The resonant ocean long wave generated by the pressure wave would propagate toward the west Kyushu however the pressure wave was faded on the way.