Observations of Tidal Modulation on Diurnal Vertical Displacements of the Oceanic Transition Layer

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 similar to 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. Oceanic transition layer (TL) is the interface between the highly turbulent surface mixed layer (ML) and the weakly turbulent interior layer. The daily variations in the TL are significantly related to near-surface and interior processes, such as thermal stratification and tidal motions. However, the driving mechanisms cannot be quantitatively evaluated without high-resolution spatial and temporal observations. In this study, hourly observations were comprehensively recorded over 3 days in the northern South China Sea, and new results were obtained in regard to the dual effects of surface forcing and interior dynamics driving the vertical displacements of the TL. Specifically, diurnal variations in the upper boundary of the TL, that is, the base of the surface ML, are mainly controlled by surface forcing. However, the lower boundary of the TL is significantly modulated by tidal shear in association with a mixture of diurnal and semidiurnal tides, internal tides and tidal rectification. Our study provided observational evidence and viewpoints regarding the daily TL variations. Vertical displacements of the transition layer aligned with those of mixed layer driven by surface heating/cooling and the Langmuir cellA mixture of diurnal and semidiurnal tides, internal tides and tidal rectification resulted in large shear in the transition layerTidal shear modulated the depth of the transition layer by slowing the ascent or causing the layer to descend further