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Thirty-four individual thermohaline sheets are identified at depths of 170–400 m in the Canada Basin of the Arctic Ocean by using the hydrographical data measured with the Ice-Tethered Profilers (ITPs) between August 2005 and October 2009. Each sheet is well determined because the salinity within itself remains very stable and the associated salinity anomaly is markedly smaller than the salinity jump between neighboring sheets. These thermohaline sheets are nested between the Lower Halocline Water (LHW) and Atlantic Water (AW) with lateral coherence of hundreds of kilometers and thickness varying from several to dozens of meters. The physical properties, including temperature, heat flux, and vertical turbulent diffusivity, in the sheet are found to be averagely associated with the AW propagation. Spatially, the thermohaline sheet is in a bowl-shaped distribution, which is deepest in the basin center and gradually becomes shallower toward the periphery. The interaction between the LHW and AW could be demonstrated through the property variances in the sheets. The temperature variances in the upper and lower sheets are correlated with the LHW and AW, respectively, transited at the 15th sheet, whereas the depth variance in the sheet is strongly correlated with the LHW. It is proposed that the sheet spatial distribution is mainly dominated by the Ekman convergence with the Beaufort Gyre, slightly modulated with the AW intrusion. Significance Statement: The diffusive convection staircases, composed of consecutive steps containing thick mixed layers and relatively thin interfaces, are prominent between the Lower Halocline Water (LHW) and the Atlantic Water (AW) throughout the Canada Basin. This sheet-like structure is in a bowl shape with lateral coherence over hundreds of kilometers. It is proposed that the distribution of the thermohaline sheet is mainly dominated by the Ekman convergence with Beaufort Gyre, as well as the AW intrusion. The present method of thermohaline-sheet identification would have more implications beyond this work. Since the thermohaline sheet remains mostly stable and coherent on a very large spatial–temporal scale, it might play a similar role as the water mass analysis in numerous applications, e.g., climate change. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The ocean bottom boundary layer (BBL) is a main area of interaction of physical, geochemical and biological processes. It is characterized by a bottom mixed layer (BML). A mooring equipped with densely distributed thermistors near the seafloor was deployed at a site on the continental shelf in the northern South China Sea for 37 h. The temporal variability of BBL was examined with high‐resolution temperature loggers (sampling rate of 8 Hz). It was observed that the bottom water structures are modulated by internal tides and the bottom current. The bottom mixed layer was persistently exhibited in accompaniment with the occasional appearance of an unstable bottom layer (UL). The thickness of BML was highly intermittent and varied from 0.2 m to greater than 67.4 m. The active bottom mixing was observed either in the energetic dissipation induced by the critical interaction of semidiurnal tide and slope topography or in the weak stratification induced by strong upslope flow. The influence of the convective flow to the bottom mixing was evaluated. It was found that convection makes a substantial contribution to bulk mixing when the bottom mixing is relatively mild, and its contribution is less significant with intense bottom mixing. Key Points: A mixed layer is persistently exhibited with dense‐distributed measurements near bottom, occasionally accompanied with the convective layerActive bottom mixing occurs either in energetic dissipation under critical interaction of tides and topography or in weak stratificationConvection makes substantial contribution to bottom mixing when mixing is mild, and its contribution is less significant with intense mixing [ABSTRACT FROM AUTHOR]