Impact Assessment of Extreme Freshwater Inflow on the Formation and Dissipation of Hypoxia in the Ariake Sea Using Large Ensemble Climate Simulation Database

It is apparent that climate change is progressing, increasing the frequency of heavy rainfall events. In coastal areas, halocline formation caused by increased freshwater inflow can inhibit vertical mixing, reducing oxygen concentration in the bottom layer. The Ariake Sea, an inner bay in Japan with a large tidal range, has faced serious issues such as red tides and hypoxia. Previous studies have revealed that increased river discharge can prolong hypoxia duration in the bay. For instance, the longest hypoxic event was observed during the 2020 flood. This study investigates how extreme freshwater inflow influences hypoxia dynamics using a large ensemble climate simulation database including both present and future scenarios. Results indicate that the influence of discharge weakens under extremely severe flood events. Although freshwater volume strongly correlates with stratification intensity, hypoxia duration does not exhibit a clear relationship with discharge. Qualitative analyses based on time series data and vertical profiles suggest that hypoxia formation is mainly controlled by discharge-driven stratification, whereas its dissipation depends on subsequent disturbances such as strong winds and spring tides. These differing controls can increase the uncertainty and variability of hypoxia duration under extreme conditions and in future climate projections.