It is reported that turbulent mixing is enhanced in the South China Sea (SCS), and it is highly variable in both space and time. Generation and breaking of internal tides has been identified as the main process to drive turbulent mixing in the SCS, while the contributions from other processes are not clear enough. Here we investigate the potential contribution from mesoscale eddies to turbulent mixing in the SCS using a high resolution numerical simulation. Our results show that mesoscale eddies in the SCS effectively dissipate over complex rough topography and indicate that the generation of submesoscale motions and lee waves are two pathways for the transfer of mesoscale eddy energy down to small dissipation scales. The energy loss from mesoscale eddies near the Xisha Islands is estimated to be sufficient to sustain turbulent kinetic energy dissipation rate of O (10⁻⁸) W/kg. This study suggests an alternative and potentially efficient mechanism to internal tides for the local maintenance of turbulent mixing in the SCS.

Water‐mass properties and circulation in the deep and abyssal Philippine Sea are investigated using repeatedly occupied full‐depth hydrographic data. The abyssal water‐mass, the Lower Circumpolar Deep Water (LCDW), is captured. After crossing the Yap Mariana Junction, the LCDW spreads to the Parece Vela Basin, the southern part of the Shikoku Basin, the Minami‐Daito Basin, and the northern Philippine Basin, with its upper interface sinking from ∼4,000 m to ∼5,000 m. Blocked by the Central Basin Fault, the LCDW cannot penetrate further southward into the southern Philippine Basin. An abyssal current across the Kyushu‐Palau Ridge connecting the eastern and western parts of the Philippine Sea is identified at 22.5°N–29.5°N, which is suggested to be the main branch transporting LCDW from the Yap Mariana Junction westward into the northern Philippine Basin. Synoptic analysis of hydrographic observations suggests the existence of a cyclonic circulation in the abyssal Philippine Sea. During 1990–2020, a warming trend of 0.33 ± 0.19 × 10⁻³°C yr⁻¹ is identified at depths below 2,000 m, while a SiO₂ maximum core at 30°N implies a possible cooling of the Mixed Deep Water (more influenced by North Pacific Deep Water) in the Philippine Sea. Under 4,000 m, freshening together with warming south of 20°N indicate contraction of LCDW after entering the Yap Mariana Junction.

Water‐mass transports in the vast and seemingly quiescent abyssal ocean, basically along topographically‐guided pathways, play a pivotal role in the Earth's climate. The pulse of abyssal circulations can be taken with observations at topographic choke points. The Yap‐Mariana Junction (YMJ) is the exclusive choke point through which the Lower Circumpolar Deep Water (LCDW) enters the Philippine Sea. Here, we quantify the LCDW transport and its variability based on mooring observations at the YMJ and the Mariana Trench (MT). The LCDW flows northward toward the Philippine Sea as an intensified current on the western side of the YMJ, with maximum mean velocity reaching 7.6 cm/s. The mean LCDW transports through the MT and the YMJ are 2.2 ± 1.0 Sv and 2.1 ± 0.4 Sv, respectively. Reversal flow at autumn in both the YMJ and MT is captured, indicating seasonal variability of the abyssal flow.

Clarifying contributions to the surface mixed layer (SML) dissipation from dynamic processes including winds, waves, buoyancy forcing and submesoscales is of significance for quantifying exchanges between the atmosphere and the ocean. Based on two observation sections across an anticyclonic eddy in the South China Sea, the contributions from different dynamic processes to the SML dissipation rate of turbulence are quantified. The potential vorticity indicates instability events including symmetric instability (SI), gravitational instability and centrifugal instability at the eddy. Despite of a dominant role of wind‐ and wave‐induced dissipation rates, SI is highlighted by a mean estimated depth‐integrated dissipation rate of 4.3 × 10⁻⁶ W m kg⁻¹ with a maximum up to 3.2 × 10⁻⁵ W m kg⁻¹. The SI dissipation is believed to play a role in the eddy kinetic energy budget by extracting energy from the vertical geostrophic shear at the eddy.