Three-dimensional structure of summer circulation in the Bohai Sea and its intraseasonal variability

The summer circulation in the Bohai Sea (BHS) has been a wide concern since the 1980s, but the 3D structure of it remains unclear. The present study implemented a regional numerical model to address this knowledge gap. The model was validated with the observations and evidence from the literature, demonstrating high reliability in reproducing the general circulation pattern. For the first time, we proposed the complete 3D structure of this complex circulation system. It indicates that the BHS summer circulation primarily consists of an anticyclonic gyre over the central shoal, along with two-layer overturning circulations in the coastal bays. The central gyre, which is robust throughout the water column, can supply water to the bottom of adjacent bays, where the supplied water subsequently overturns onto the surface. This is an important means of water supply to the bays. Furthermore, the circulation exhibits remarkable intraseasonal variability, largely regulated by the intensity of southerly winds and intrusion water from the North Yellow Sea (NYS). The circulation structures and water transport can be quite different even in the summer months. In the early summer, the prevailing southerly winds are typically stronger, while the intrusion water from the NYS is weaker. The stronger southerly winds can inhibit the southward current along the west coast, while the intrusion water can affect the water supply within the BHS basin, particularly in the southern regions. This study enriches the current understanding of the BHS circulation and is of significance to the material transports in this region.