A well-mixed water column is observed over a 20 m ridge in the central Bohai Sea, surrounded by stratified water of much colder temperatures. We use a three-dimensional model to investigate its formation mechanisms. The results show that both tidal and surface wave mixing are important for homogenizing the water column, the former reaching 10 m above the bottom and the latter penetrating 10 m beneath the sea surface. The ridge enhances the intensity and vertical extent of tidal mixing, allowing it to connect with the downward penetration of wave mixing. Removing either the bathymetric ridge or wave mixing fails to reproduce this well-mixed water column. The inclusion of the surface wave mixing warms the bottom water by 5--6¿C in both the warm water column and surrounding cold water. Wave mixing displays a maximum in the central Bohai Sea basin, which is confirmed by satellite altimeter data and a wave model simulation.