The Impacts of Far-Field Typhoon-Generated Coastal Trapped Waves on the Hydrodynamics in the Northern South China Sea: A Case Study of Typhoon In-Fa

Coastal Trapped Waves (CTWs) have the potential to transmit significant energy from atmospheric wind systems, causing dramatic coastal changes over vast areas, even far from the wind systems. This study investigated the impacts of CTWs generated by Typhoon In-fa (2021) on the northern South China Sea (NSCS) hydrodynamics using tide gauge observations, reanalysis data, and numerical model outputs. The analysis results indicate that the CTWs induced by Typhoon In-fa exhibit distinct characteristics in different phases of wave crest and trough. During the crest phase, coastal currents generated by CTWs flow opposite to the background circulation, while during the trough phase, they flow in the same direction. This process is accompanied by changes in cross-shore currents, such that during the crest phase of CTWs, the cross-shore currents are landward at the surface and seaward at the bottom, while during the trough phase of CTWs, the cross-shore currents become reversed. These changes further lead to downwelling in temperature, salinity, and density during the crest phase of CTWs, and upwelling during the trough phase of CTWs. Results from a linear CTW model demonstrated that the above characteristics agreed with the traditional CTW theory. Sensitivity experiments with the Regional Ocean Model System (ROMS) explored key factors influencing NSCS hydrodynamics, including local winds and CTWs from Typhoon In-fa. The local winds and CTWs have different effects: they compete during the crest phase and co-work during the trough phase, with local winds dominating the sea surface and CTWs dominating the seabed.