Characterization of internal waves and their links to chlorophyll in the northern South China Sea (2003–2023)

Internal waves are a critical oceanic process that strongly influence on nutrient transport and phytoplankton distribution. The northern South China Sea (SCS) is among the most dynamically active regions for internal waves globally. However, the mechanisms by which internal waves modulate chlorophyll patterns in this region remain poorly understood. In this study, we developed an integrated remote sensing framework that synergistically combines multi-sensor satellite imagery (synthetic aperture radar and optical sensors), deep learning-based automatic detection algorithms, and the Extended Korteweg-de Vries (EKdV) theory. This framework was applied to generate a consistent, 20–year (2003–2023) dataset of internal wave amplitudes in the northern SCS. The resulting amplitude distribution shows good consistency with previous observational records. Quantitative analysis reveals that internal waves systematically disrupt surface chlorophyll distributions by fragmenting homogeneous chlorophyll patches and forming convergence zones characterized by alternating chlorophyll anomalies along their crests. Further analysis of chlorophyll anomalies indicates an immediate positive response to internal wave events. The strongest response is associated with internal waves with amplitudes below 30 m, while the magnitude of the chlorophyll response generally decreases with increasing wave amplitude, following a negative exponential trend for amplitudes less than 100 m. For amplitudes exceeding 100 m, the relationship becomes more scattered due to complex interactions with mesoscale eddies, upwelling, water depth and seasonal variability. Moreover, depression-type internal waves induce a stronger immediate chlorophyll response than elevation-type waves, whereas elevation waves exhibit a more pronounced delayed effect. This study establishes a robust long-term remote sensing framework for monitoring internal waves in dynamic marginal seas. It also elucidates a persistent link between internal waves and heterogeneous chlorophyll patterning, thereby highlighting the significant role of internal waves in shaping marine ecosystems in dynamic oceanic environments.