The buoy-based reversion of regional thermocline integrated with satellite observed SST in the margins off the Changjiang Estuary

摘要

Hypoxia has been widely observed in estuarine area and some reports have focused on the East China Sea over the past decade. With the increasing nutrient load from Changjiang (Yangtze) River, a severe hypoxia zone was found in summer. The mechanism and maintenance of hypoxia is due to the large density stratification caused by the significant salinity difference between the fresh plume and salty water. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Changjiang are affected the most by summer hypoxia. Physical thermohaline stratification plays an important role in the Changjiang shelf during summer. In this study, we discusses the relationship between hypoxia and the stratification according to the surface temperature reversed from satellite, in situ observed data and time series of profile data obtained from hypoxia buoy, which was especially designed for hypoxia identification. We examined the occurrence of seasonal hypoxia in the bottom waters of river-dominated ocean margins off the Changjiang River and compared the stratification procedure leading to the depletion of oxygen. A simulation for stratification was performed to calculate the seawater temperature vertical profile. By collecting the historical investigated data, we constructed a parametric structural model between surface and bottom temperature. Based on the parameterization of the layered structure of seawater temperature vertical profile, the simulation method was used to calculate the parameter distributions of stratification structure. When the real time outputs of SST and buoy-based profile were received, the parametric model figured a set of major characteristic parameters of each profile directly: sea surface temperature, mixed layer depth, thermocline depth, and temperature gradient. Hence, the approach would achieve the goal of reconstructing the regional thermocline profile directly. The thermocline reflects the ocean temperature field's important physics characteristics, and can be used in analysis of the influence on the exchange of the oxygen.