Temporal Variability in Bottom Water Structures of the Continental Slope in the Northern South China Sea

Abstract

Temporal variability in bottom water structures has been observed in 37‐h high‐resolution temperature data at the ocean bottom up to 67.4 m on the continental shelf of the northern South China Sea. The water temperature is generally hot (cold) in the downslope (upslope) flow, but rapid temperature change is associated with the cross‐slope flow intensity changes. Downslope, wandering, and upslope phases are classified depending on the flow directions. Ellison scale method is used to evaluate the turbulent mixing. Intensive dissipation is observed in the wandering phase, the vertically integrated dissipation Eɛ ∼ 322 mW/m2 and the eddy diffusivity 〈Kz〉 in 0.01–0.15 m2/s, which is attributed to the critical interaction between the semidiurnal tides and the slope boundary. Eɛ is gradually reduced to 38.1 mW/m2 in the upslope phase, in which the topography is subcritical to the internal tides, while 〈Kz〉 is occasionally promoted to 0.077 m2/s owing to the weaker stratification. The bottom mixed layer is persistently exhibited, but highly intermittent around 13.9 m and varies from 0.2 m to greater than 67.4 m. Its thickness depends strongly on the water stratification and seems not to be directly linked with the turbulent mixing. The bottom unstable layer occurs mainly in the period slightly before and in the first half of the upslope flow phase. The induced convection makes a substantial contribution to the turbulent mixing when the overall mixing is relatively mild. In the intense mixing, even though the convectively driven mixing becomes much stronger, its contribution is less significant.