A synoptic study on tsunami-like sea level oscillations along the west coast of Korea using an unstructured-grid ocean model

Abstract

Abnormal tsunami-like sea level oscillations were recorded along the west coast of Korea on March 29-31, 2007, and May 4, 2008. These oscillations had maximum amplitudes of 1.6 m and 1.4 m, respectively. Analysis of observation data revealed that sea level oscillations coincided with atmospheric pressure disturbances moving eastward over the Yellow Sea. The atmospheric pressure disturbance in 2007 was characterized by a 5.3 hPa air pressure jump at Yeonggwang whereas that in 2008 was characterized by a 2-3 hPa change at Boryeong. To understand the generation and amplification mechanisms of the two events, we carried out synoptic numerical experiments using a two-dimensional unstructured-grid ocean model. From analyses of weather radar images and weather charts, we estimated the direction and speed of the atmospheric pressure jump over the Yellow Sea. Atmospheric pressure disturbances during the 2007 and 2008 events moved with average speeds of 24 m.s(-1) and 28 m.s(-1) respectively, which are nearly equal to the propagation speed of the long ocean wave. As this movement occurred, it appears that sea level oscillations were enhanced via Proudman resonance over the Yellow Sea. Numerical simulations reproduced, reasonably well, the occurrence time and amplitude of sea level oscillations during 2008 event. However, simulations were unable to reproduce adequately the oscillation peaks during the 2007 event because they failed to reach near-resonant conditions over spatially varying water depths. For the synoptic experiments, we designed four types of atmospheric pressure jumps, each having a different size as defined by differences in amplitude, width, and length. In a variety of synoptic experiments, the gradient of atmospheric disturbance plays a significant role in amplifying long ocean waves under a near-resonant condition. The effects of a complicated bathymetry and geometry on long wave amplification still remain to be studied, including those of shoaling and local eigenfrequencies.