From continuous high-frequency and high-resolution observations of temperature, salinity, current and microstructure in the northern South China Sea during 28–31 May 2021, we found that the transition layer (TL) experienced a regular diurnal vertical fluctuation of ∼20–30 m, likely associated with the displacements of the mixed layer (ML) and tidal shear in the TL. Specifically, surface cooling (heating) caused an erosion (enhancement) of thermal stratification and facilitated (hindered) convection, rapidly deepening (shoaling) the ML and causing corresponding vertical displacements in the TL. The mixture of diurnal and semidiurnal tides, together with internal tides and tidal rectification, resulted in a diurnally varying large shear in the TL. This enhanced shear caused the TL to further descend along with the rapid thickening of the ML, while the relatively weaker shear slowed the uplift of the TL with a rapid shoaling of the ML and resulted in its gradual shoaling.

transition layer,tidal shear