Seasonal variations in the water residence time in the Bohai Sea using 3D hydrodynamic model study and the adjoint method

The Bohai Sea is a large, semi-enclosed bay of China with a transport timescale of more than 1 year. Residence time (RT) is an important indicator used to determine the water exchange rate of coastal oceans, and it has a significant influence on coastal ecosystems and the environment. In this study, the RT and its seasonal variability within the Bohai Sea were investigated using a 3-D hydrodynamic model and the adjoint method of the RT. The model results show that the annual mean RT in the Bohai Sea is 3.43 years, and the RT increases from the Bohai Strait to the northwestern coast. The seasonal variability of the RT averaged over the entire Bohai Sea is not obvious; however, the regional RT has a significant seasonal variation, which could be more than 290% on average. When the monsoon winds were removed from the model, the sensitivity experiment showed an annual mean RT increase of similar to 90% with negligible seasonal variations in most areas of the Bohai Sea. The wind-induced residual current controls the seasonal variation in ocean circulation and induces the seasonal variability in RT; thus, monsoon winds play a dominant role in the seasonal variations of the water exchange rate in the Bohai Sea. Sensitivity experiments also suggest that tides can slow the water exchange rate, whereas the baroclinic processes and river runoff can accelerate the water exchange rate in the region near the Yellow River estuary. This study highlights the critical role of monsoon winds in affecting seasonal variabilities in the coastal transport timescale and exchange rate.