Variability of turbulent mixing observed by high-resolution Acoustic Doppler Current Profilers in the deep South China Sea

Hao Xun¹Chun Zhou^1*Xin Xiao¹Shoude Guan¹Xiaodong Huang¹Tangdong Qu²Wei Zhao¹Jiwei Tian¹

• ¹Ocean University of China, Qingdao, China
• ²Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, United States

Despite turbulent mixing plays a crucial role in sustaining the global meridional overturning circulation, direct observations of turbulent mixing are sparse, especially in the deep ocean. In this study, we used 1200 kHz Acoustic Doppler Current Profilers (ADCPs) with high temporal (O(1s)) and spatial (O(2cm)) resolution to obtain continuous measurements of turbulent mixing at a depth of 1086 m in the northeastern South China Sea. Through comparative experiments and the evaluation of four different analysis methods, we provided optimal configurations and effective methodologies for continuous observations of turbulent mixing accompanied by shear background flow in the deep ocean. Our continuous observations revealed large temporal fluctuations in the near-bottom turbulent kinetic energy dissipation rates, varying by up to two orders of magnitude within a single day. These fluctuations were dominated by shear instability associated with high-mode diurnal internal tides. These findings contribute to developing optimized observational methodologies, enhancing our understanding of deep-ocean turbulence and its implications for large-scale oceanic dynamics.