Using recently collected current and hydrographic data, we provide a high resolution picture of the subinertial flow and estimate the volume transport through the Luzon Strait. The distribution of the subinertial flow shows a strong westward flow around 100 m in the northern part of the Luzon Strait, while the eastward flow is confined to the deeper layers, mostly at depths around 1000 m. The total volume transport is estimated to be 6 ± 3 Sv during the period of observations from October 4 to 16, 2005. The observations also confirm that the Luzon Strait transport has a sandwiched vertical structure. The net westward volume transport in the deep (>1500 m) layer of the Luzon Strait reaches 2 Sv.

Profiles of current velocity, temperature, and salinity were obtained in the Internal Wave and Mixing Experiment in the South China Sea (SCS), the Luzon Strait, and the North Pacific. The observations are examined for evidence of enhanced diapycnal mixing in the SCS, which reaches O(10 23 m 2 s 21 ) in magnitude. Results from independent casts reveal that diapycnal diffusivity in the SCS and the Luzon Strait is elevated by two orders of magnitude over that of the smooth bathymetry in the North Pacific, which are typical of background values in an open ocean. The vertical distribution of diapycnal diffusivity is nonuniform in the SCS, exhibiting higher values at depths greater than about 1000 m. This result compares favorably with the direct microstructure measurements at four stations in the SCS. Velocity and density profiles are combined to estimate the internal tide energy flux generated in the Luzon Strait and directed into the SCS. The energy amounts to 10 GW, most of which is rationalized to be the potential energy source for enhanced mixing in the SCS.

The Luzon Strait, with its deepest sills at the Bashi Channel and Luzon Trough, is the only deep connection between the Pacific Ocean and the South China Sea (SCS). To investigate the deep-water overflow through the Luzon Strait, 3.5 yr of continuous mooring observations have been conducted in the deep Bashi Channel and Luzon Trough. For the first time these observations enable us to assess the detailed variability of the deep-water overflow from the Pacific to the SCS. On average, the along-stream velocity of the overflow is at its maximum at about 120 m above the ocean bottom, reaching 19.9 ± 6.5 and 23.0 ± 11.8 cm s−1 at the central Bashi Channel and Luzon Trough, respectively. The velocity measurements can be translated to a mean volume transport for the deep-water overflow of 0.83 ± 0.46 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) at the Bashi Channel and 0.88 ± 0.77 Sv at the Luzon Trough. Significant intraseasonal and seasonal variations are identified, with their dominant time scales ranging between 20 and 60 days and around 100 days. The intraseasonal variation is season dependent, with its maximum strength taking place in March–May. Deep-water eddies are believed to play a role in this intraseasonal variation. On the seasonal time scale, the deep-water overflow intensifies in late fall (October–December) and weakens in spring (March–May), corresponding well with the seasonal variation of the density difference between the Pacific and SCS, for which enhanced mixing in the deep SCS is possibly responsible.

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. Potential mechanisms responsible for these spatial patterns are discussed, which include internal tide, bottom bathymetry, and near-inertial energy.

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Direct microstructure observations across three warm mesoscale eddies were conducted in the northern South China Sea during the field experiments in July 2007, December 2013, and January 2014, respectively, along with finestructure measurements. An important finding was that turbulent mixing in the mixed layer was considerably elevated in the periphery of each of these eddies, with a mixing level 5–7 times higher than that in the eddy center. To explore the mechanism behind the high mixing level, this study carried out analyses of the horizontal wavenumber spectrum of velocities and spectral fluxes of kinetic energy. Spectral slopes showed a power law of k−2 in the eddy periphery and of k−3 in the eddy center, consistent with the result that the kinetic energy of submesoscale motion in the eddy periphery was more greatly energized than that in the center. Spectral fluxes of kinetic energy also revealed a forward energy cascade toward smaller scales at the wavelength of kilometers in the eddy periphery. This study illustrated a possible route for energy cascading from balanced mesoscale dynamics to unbalanced submesoscale behavior, which eventually furnished turbulent mixing in the upper ocean.

Deep western boundary current (DWBC) was observed for the first time by an array of 6 current meter moorings southeast of the Zhongsha Islands in the South China Sea (SCS) deep basin during the period from August 2012 to January 2014. In the mean, the DWBC in the SCS flows southwestward with core velocity of 2.0 cm/s and a volume transport of 1.65 Sv (1 Sv = 1 × 106 m3/s). Its temporal variability is dominated by intraseasonal fluctuations with period around 90 days. The main axis of the DWBC, characterized by a low temperature core, tends not to shift with the 90-day fluctuation.