On Mechanisms Controlling the Seasonal Hypoxia Hot Spots off the Changjiang River Estuary

Large areas of seasonal hypoxia have been detected off the Changjiang River Estuary over the last decade. Although seasonal bottom hypoxia was transient due to complex dynamical processes, it occurred with high frequency in certain regions. Summer hypoxia hot spots off the estuary were first identified using a comprehensive, high-resolution ecosystem model and 17 years of observations collected from historical literature and research cruises. Comparisons between long-term observations, a 4-year averaged simulation, and a 1-year simulation implied that the geographical locations of summer hypoxia hot spots were generally consistent. This ecosystem model suggested that the Changjiang River plume predominantly spread either over the Yangtze Bank or the Submarine Canyon and the Mud Belt depending on the direction of the wind; the temperature, salinity, and residence time of bottom water mass off the estuary depended highly on the properties of the Kuroshio subsurface intrusion. The dual effects of stratification and prolonged bottom water residence time created favorable condition for hypoxia to develop. The analysis in this study identified wind and the Kuroshio subsurface intrusion as the factors that facilitated the migration of seasonal hypoxia and the establishment of summer hypoxia hot spots off the estuary. In spite of the persistent hypoxia hot spots, the timing of individual hypoxia can be variable because of distinct dynamical processes over a wide range of time scales. Tidal advection was quantified as a secondary factor that influences the spatial extent and migration of bottom hypoxia.