Weakening trend of Bohai Sea cold water mass revealed by a ten-year regional ocean simulation with data assimilation

The high-resolution Oceanic Regional Circulation and Tide Model (ORCTM) with a compatible Ensemble Adjustment Kalman Filter (EAKF) assimilation module is implemented to investigate the general evolution and long-term trend of the Bohai Sea cold water mass (BHSCWM). Through comparison with multiple observational data, it can be proved that the reliability of model results is significantly improved after the data assimilation. The assimilated results from 2011 to 2020 show that the BHSCWM occurs from May to August, and its formation is closely related to the seasonal thermocline at a depth of about 10 m. There is a vertically uniform high-temperature water column in the central bank, which divides the BHSCWM into the south part (SBHSCWM) and the north part (NBHSCWM). It is noteworthy that there is a remarkable warming trend in the BHSCWM core, the warming rate reaches 0.24 °C/yr and 0.15 °C/yr for the SBHSCWM and the NBHSCWM, respectively. Meanwhile, whether in terms of cold water mass affected area or volume, a significant decreasing trend for both the SBHSCWM and the NBHSCWM is revealed by our assimilated results, suggesting that the Bohai Sea may be dramatically impacted in the context of global warming.