Stratification in central Bohai Sea and how it has shaped hypoxic events in summer

In the last 10 years (2012–2021), five hypoxic events have been observed in summer in the central Bohai Sea (CBS). Frequent and persistent hypoxia will have an impact on the ecosystem of the CBS. In this paper, historical sea temperature (ST), salinity (SAL), density (Den), and dissolved oxygen (DO) concentration data from three stations in the CBS are analyzed via the linear regression method, and the correlations between the stratification factors (ST, SAL, and Den) and DO concentration are determined. The thresholds of the stratification factors at the three stations in June in the year in which hypoxia occurred were determined and applied to survey data from 29 stations in late May to early June in 2022 in the CBS; this assessment found that the data from 19 stations indicated that hypoxia was about to occur. In August, the survey data showed that 14 out of the 29 stations indicated hypoxic conditions, of which 12 were from the predicted 19 stations, meaning that the estimation accuracy reached 63%. The same approach was applied to data from June 2023. The data for August from a bottom-type online monitoring system in the CBS verified the occurrence of hypoxic events around Sta. M2. The results show that the strength of the seawater stratification plays a leading role in hypoxic events in the summer in the CBS, and the thresholds of the stratification factors can be used to predict the occurrence of hypoxic events.