Coastal deoxygenation is intensifying globally, yet the mechanisms governing its variability across timescales remain poorly understood. In the Yangtze Estuary and the East China Sea, hypoxia exhibits pronounced fluctuations in severity and duration, driven by coupled physical and biogeochemical processes. In this study, we employ the unstructured-grid hydrodynamic model SCHISM coupled with the biogeochemical model CoSiNE to investigate the physical and biogeochemical controls on deoxygenation in the East China Sea. Spatiotemporal variability analysis suggests that seasonal deoxygenation responds rapidly to physical forcing, particularly temperature, whereas biogeochemical influences, including chlorophyll variability, exhibit delayed effects on system variability. Hypoxia occurs on top of seasonal deoxygenation as episodic events. To further characterize the episodic nature of hypoxia, we introduce an event-based analysis that identifies and classifies hypoxic events as short- or long-duration, enabling systematic quantification of hypoxia dynamics. The nonlinear relationships between dissolved oxygen and key drivers are characterized using a flexible statistical framework. Driver importance and response functions vary across event types, indicating distinct control mechanisms governing the development and persistence of hypoxia. These findings reveal timescale-dependent controls on coastal deoxygenation and provide a unified framework linking event evolution to underlying drivers, with implications for predicting future hypoxia under climate change.
 

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