Forerunner storm surge under macro-tidal environmental conditions in shallow coastal zones of the Yellow Sea

Typhoons affecting the west coast of Korea (WCK) in the Yellow Sea (YS) typically enter the continental shelf across the Ryukyu Islands, where surges can develop in the shallow water and interact with the macro-tidal environment. In this study, we investigate the generative mechanism of forerunner surge (FS) occurring before the main surge of two typhoons in the WCK, Prapiroon, and Bolaven. Sensitivity analyses of FS generation to bottom friction, Coriolis effects, tides, wind, and pressure forcing were conducted. The effect of the Coriolis force, which depends on latitudinal position, is not an influential factor in the FS along the shallow inner shelf of the WCK. However, Kelvin waves move along the WCK and cause nonlinear characteristics that amplify or dampen the FS, depending on the timing of flooding or ebbing tides. We found that the FS appeared at least 12.6 h prior to the main surge, which is almost the same as the geophysical resonance period. Moreover, the FS was not only influenced by the geostrophic setup, represented by the distance from the coast, but may also be amplified by topographic effects, such as the concavity of the coastline. When a typhoon enters the Ryukyu Islands at an angle < 20° and moves at a relatively slow speed (< 20 km h−1), a FS is more likely to occur.