Climate warming amplifies coastal hypoxia response to extreme river discharge

Extreme hydrological events are critical drivers of coastal hypoxia, yet their impacts remain poorly constrained under future warming. Coastal oxygen depletion occurs when physical oxygen supply fails to offset biological oxygen demand, with stratification, nutrient enrichment, and warming further intensifying oxygen loss. Here, we quantify coastal oxygen responses to extreme discharge events using a three-dimensional hydrodynamic–biogeochemical model driven by high-resolution regional climate simulations for the Ariake Sea, Japan. Under a climate scenario 4 K warmer, extreme discharge magnitudes increase by 6.9% and 10.0% for 50- and 100-year return periods, respectively, relative to the historical baseline. This leads to a 33.2% increase in mean hypoxia duration (95% confidence interval: 11.7–55.3%, p < 0.01) for events within the 50–100-year return period range, while the additional increase in hypoxia duration from +2 K to +4 K warming is limited. Climate-driven intensification of extreme discharge strengthens stratification and prolongs hypoxia, with intensified conditions near estuaries and persistent low oxygen offshore, demonstrating that warming amplifies hypoxia and underscoring the need to integrate extreme events into coastal climate adaptation. Climate-driven extreme river discharge is projected to prolong and intensify coastal hypoxia, with stronger stratification and spatially uneven oxygen loss, suggests a study using an event-based storyline framework with a 3D hydrodynamic-biogeochemical model applied to the Ariake Sea, Japan.