Effect of Tropical Cyclones on the Strength of Cold Filament in the South China Sea

Cold filaments (CFs) in the southwestern South China Sea (SCS), extend eastwards from the Vietnamese coast during summer and autumn, significantly influence hydrodynamic and ecological distributions in the region. By integrating multi-source observations and reanalysis data from 2000 to 2023 (including sea surface temperature, sea level anomalies, surface currents, and wind fields), this study proposes an objective method for identifying CF areas and intensity using sea surface isotherms. Based on the changes of CFs, the impact of typhoons passing through the northern SCS on southwestern CFs is investigated. Statistical results revealed that 66 typhoons over the past 24 years have exerted enhanced effects on CFs. These findings demonstrate that typhoons traversing the northern SCS can enhance the CFs located over 10 latitudes south of the typhoon tracks. This indicates that typhoons not only directly affect oceanic dynamic processes along their paths but also exert significant remote influences on coastal dynamics in other regions. The mechanism analysis shows that typhoons primarily enhance the CFs through two pathways: (a) Intensifying southwest winds over Vietnam's eastern coast, thereby increasing the wind stress curl and triggering stronger-than-climatological upwelling that brings deep cold water to the surface; (b) altering the dipole distribution pattern in the southwestern SCS through enhanced southwesterly winds, which strengthen cold eddies and intensify offshore currents, transporting more coastal cold water to the open sea. These combined effects ultimately amplify the intensity of the CFs and expand their coverage.