Spatiotemporal Distribution and Climate Change Sensitivity of Thermoclines in a Semi-Enclosed Sea

The Bohai Sea, a semi-enclosed inland sea located in China, has experienced a notable decline in bottom water oxygen levels over the past decade. This phenomenon is linked to the inadequate replenishment of oxygen, which is constrained by the formation of summer thermoclines that impede water renewal. The impact of global climate change on oceanic thermoclines has been pronounced. This research employs a sophisticated three-dimensional hydrothermal model in conjunction with a vertical water age model to investigate the formation and spatiotemporal characteristics of thermoclines in the Bohai Sea, as well as their response to climate change, including shifts in wind patterns and air temperature. Water age is conceptualized as the duration since a water parcel last contacted the free surface. Findings indicate that the bottom water age in the Bohai Sea remains less than 2 days in spring, suggesting that the cold bottom waters are not remnants from the winter season. The intensified surface heat flux during summer points to a thermal lag as the underlying mechanism for thermocline formation, with bottom waters warming at a slower rate than surface waters. The study reveals marked spatial heterogeneity and seasonal fluctuations in the thermocline’s distribution within the Bohai Sea. Over time, the thermoclines have exhibited a vertical descent towards the seafloor and a horizontal shift from the continental slope towards the central basin. Regarding the impacts of climate change, a trigonometric function fitting method was utilized to discern a trend of increasing wind speeds and temperatures in the Bohai Sea over the past forty years. The temperature rise leads to a downward shift of the thermocline and an intensification of its strength. Moreover, enhanced wind speeds facilitate greater vertical mixing of water masses, culminating in a weakening of the strength of the thermocline.