Mechanism of Delayed Storm Surge in Straits: Seiche-Induced Oscillations Triggered by Typhoon Passage

Abstract. A storm surge is a phenomenon in which the sea level rises significantly due to low-pressure systems, such as typhoons, accompanied by strong winds. Once storm surge-induced flooding occurs, it can rapidly inundate low-lying areas. Generally, the primary contributor to storm surge is wind set-up, where wind forces the sea towards the coast. As such, it is well-known that severe storm surges occur at typhoon's closest approach because of strong wind set-up. However, when Maysak (2020) struck the northern coast of Kyushu Island (NCKI), located on the south side of the Tsushima Strait, the sea level rose and flooding occurred approximately half a day after the typhoon had passed. At NCKI, both atmospheric pressure and wind had already weakened at the time of the flooding. Thus, the storm surge could not be explained by wind set-up or the inverted barometer effect. We examined storm surge observations for typhoons that impacted NCKI over the past 20 years and revealed a tendency for two peaks in storm surge when typhoons passed through the western channel of the strait. The second peak was identified as the maximum storm surge height, occurring approximately 10 hours after the typhoon had passed. The first peak occurred when the typhoon was closest to NCKI, coinciding with the time of minimum atmospheric pressure. This was attributed to the sea level rise caused by the inverted barometer effect. After the first peak, oscillations with a period of approximately 10 hours were observed, resulting in the second peak. NCKI, located along the Tsushima Strait, is subject to the geographical characteristics of the strait, which likely caused the oscillations leading to the maximum storm surge. To identify the oscillations that occurred after the typhoon’s passage, a continuous wavelet transform was applied to the results of storm surge simulations for time-frequency analysis. As a result, it was found that two types of seiches in a two-dimensional spatial domain of the strait (5-hour and 10-hour periods) occurred after the typhoon's passage. These seiches were triggered by the release of potential energy as external forces weakened following the typhoon's transit through the strait. Furthermore, the seiches were observed to occur approximately two hours earlier when the external force was wind, compared to when it was atmospheric pressure. This is because the time variation of atmospheric pressure drop is slower than that of wind direction. In this study, we identified the occurrence of anomalous storm surges caused by typhoons passing through a strait under specific conditions and conducted a detailed investigation of their generation mechanisms, and demonstrated storm surges can occur even after a typhoon has passed and improved understanding of storm surge characteristics in straits.