Decoding Deep Ocean Turbulence: Bottom Mixed Layer Dynamics in the South China Sea and Western Pacific

Turbulence in deep ocean environments, particularly bottom mixing, plays a critical role in multiple disciplines such as regulating energy transport, sediment resuspension, and biogeochemical exchanges. Despite its importance, bottom turbulence remains one of the least understood components of oceanography, largely due to observational challenges and the inherent complexity of seabed environments. Meanwhile, the Luzon Strait, which connects the northern South China Sea and the western Pacific Ocean, is recognized as a global hotspot for internal wave generation to the South China Sea from the Pacific Ocean. Therefore, this study investigates the structure and variability of the bottom mixed layer (BML) and its associated turbulence mechanisms across the Luzon Strait. Specifically, we aim to characterize the height of the bottom mixed layer (HBML), identify dominant physical drivers of bottom turbulence mixing, and compare mixing regimes between the northen South China Sea and the western Pacific Ocean.

Between July 27 and August 22, 2022, an oceanographic survey was conducted along both sides of the Luzon Strait. A total of 23 temperature profiles were successfully collected from two sections, 10 from the western Pacific Ocean and 13 from the northern South China Sea. The results reveal significant spatial inhomogeneity in BML characteristics across the strait. Preliminary analysis reveals that HBML is modified by a distinct mechanism on either side of the strait. In the western Pacific Ocean, HBML is positively correlated with ocean depth, suggesting that deeper regions support thicker BMLs due to weaker stratification. In the norther South China Sea, HBML appears more sensitive to seabed roughness, with thicker layers observed over complex topography. A more detailed examination of turbulence intensity and mixing efficiency is planned to further investigate these mechanisms.

In summary, by comparing mixing behavior across the norther South China Sea and western Pacific Ocean, this study advances our understanding of bottom mixed layer dynamics and contributes to the development of more accurate models for ocean circulation, which is important to improve the understanding of turbulent mixing in the deep ocean.