34 / 2023-07-24 15:07:57

Observations of tidal modulation on diurnal vertical displacements of the oceanic transition layer

Transition layer, Mixed layer, Heat flux, Shear

With continuous high-frequency and high-resolution observations of temperature, salinity, current and microstructure in the northern South China Sea during May 28-31, 2021, the diurnal vertical displacements of the transition layer (TL), as well as the driving mechanisms, were first investigated. Defined by the buoyancy frequency N2 = 0.0001 s-2, the TL base descended to 35-45 m in the early morning and gradually rose to ~15-20 m at midnight. The gentle wind basically blew unidirectionally northward, and the wind-driven mixing was restricted to above ~10 m, indicative of its minor contribution to the diurnal vertical displacements of the TL. The diurnal cycle of the TL resulted from the combination of the vertical displacements of the mixed layer (ML) and tidal shear in the TL. Specifically, due to the surface cooling (e.g., at night), the erosion of thermal stratification rapidly descended the ML base, which correspondingly pushed down the TL. Additionally, the mixture of diurnal and semidiurnal tides, together with internal tides and tidal rectification, resulted in enhanced shear in the TL, which caused the TL to descend further. With the surface heating in the daytime, the thermal stratification led to rapid shoaling of the ML base. However, due to the tidal shear in the TL, the ascent of the TL base along with the ML base was decelerated, resulting in its gradual shoaling.