[A-OS11] Ocean Mixing Frontiers

[AOS11-P10] On widespread intensified thermocline turbulence in the seasonally stratified Yellow Sea

*Wei Yang¹ (1.Tianjin University)

Keywords:Thermocline turbulence, Salt fingers, Low-salinity water intrusion, Near-inertial waves, The Yellow Sea

Thermocline turbulent mixing lies at the heart of shelf sea systems, regulating the vertical exchange of momentum, mass, and biogeochemical constituents. Based on microstructure measurements conducted in the summer of 2013 and 2017, here we show for the first time that there is widespread intensified thermocline turbulence in the summer stratified Yellow Sea. Shipboard velocity measurements demonstrate that the wind-induced near-inertial waves have driven prevailing strong velocity shear across the thermocline. However, analysis shows that the shear is not always large enough to excite shear instability according to the Richardson number criterion (Ri < 1/4). Strikingly, we show that the elevated thermocline turbulent kinetic energy dissipation rates ε O(10^-7 Wkg^-1) and microscale thermal dissipation rates χ O(10^-6 K²s^-1) are more related to the presence of low-salinity water layers (5 –15 m thick) adjacent to the main thermocline at isopycnals 24 – 25 kgm^-3. Turner angle (Tu) analysis demonstrates the presence of salt-finger favorable conditions (Tu > 45°) at the upper boundary of the intrusion. High values of ε and χ tend to occur in the salt-finger favorable layers suggesting the occurrence of double-diffusive convective instability. These results have important implications for nutrients cycling and the maintenance of primary productivity in seasonally stratified shelf seas.