Mechanism of delayed storm surges in straits: seiche-induced oscillations triggered by typhoon passage

Abstract. Storm surges are typically driven by a combination of low atmospheric pressure and strong winds associated with typhoons, with the maximum surge often occurring near the time of the typhoon's closest approach. However, during Typhoon Maysak (2020), delayed flooding was observed along the northern coast of Kyushu Island (NCKI), approximately 10 h after the typhoon had passed. By that time, both atmospheric pressure and wind intensity had substantially weakened, indicating that conventional mechanisms – such as wind setup and the inverted barometer effect – could not fully account for the phenomenon. This study explores the mechanisms behind delayed storm surges along the NCKI through a combination of observational data analysis, storm surge simulations, and continuous wavelet transform (CWT) analysis. The results reveal that typhoons passing through the western channel of the Tsushima Strait (WCTS) frequently generate a double-peaked storm surge anomaly, with the second peak being both larger and longer-lasting than the first. Spectral analysis identified dominant oscillations with periods of approximately 5 and 10 h, corresponding to natural modes of the Tsushima Strait. These oscillations, interpreted as low-mode seiches, are excited by the release of potential energy trapped within the strait once the external forcing has subsided. Notably, the 5 h mode appears to be strongly influenced by the Coriolis effect. Our findings reveal a previously unrecognized storm surge mechanism in which significant surges occur not during but after a typhoon’s passage through a strait. This delayed response arises from resonant amplification caused by modal oscillations within the strait, particularly in cases where typhoon tracks curve westward. The study underscores the critical roles of strait geometry, natural oscillation modes, and the timing of external forcing in shaping storm surge behavior. These insights extend beyond the NCKI and are relevant to other semi-enclosed strait regions worldwide that exhibit similar meteorological and geographical characteristics, even though the extent of influence from oscillation modes, periods, and amplitudes may vary depending on the strait's horizontal scale, water depth, and latitude.