Shear Instability and Turbulent Mixing by Kuroshio Intrusion Into the Changjiang River Plume

Shear instability is a dominant mechanism for mixing in the stratified oceans and coastal seas. For the first time, we present fine‐scale, direct measurements of shear instabilities in the bottom front generated by the Kuroshio intrusion into the Changjiang (Yangtze) river plume. Shear instabilities were identified using a shipboard echo‐sounder and the resulting turbulent mixing was quantified using a turbulence microstructure profiler. The shear instabilities generate vigorous turbulent mixing with dissipation rate and vertical diffusivity up to O (10−4 m2 s−3) and O (10−1 m2 s−1), respectively, comparable to values associated with shear instabilities observed in river plumes and western boundary currents but several orders of magnitude larger than typical values in the open ocean. The enhanced turbulence may contribute significantly to mixing between the Kuroshio water and coastal water and thereby alter the coastal biogeochemistry cycles. Plain Language Summary Strong velocity shear across a density interface can produce instability, causing the interface to roll up to form a train of vortices (“billows”) that subsequently break down into turbulence. This process is important in the vertical mixing of the oceanic interior and coastal seas. Using acoustic images, we find that this flow instability is induced by the interaction between the Kuroshio current and the Yangtze river plume. We also quantify the resulting turbulent mixing using high‐resolution measurements. The turbulence efficiently mixes the Kuroshio and river plume‐influence water and can change biogeochemistry cycles in the coastal seas. Key Points Interaction between Kuroshio and river plume‐influenced coastal waters generates shear instability Shear instability elevates turbulence level by 2–3 orders of magnitude compared to the fluid above and below The enhanced turbulence effectively mixes oceanic and coastal waters