Enhanced Diapycnal Mixing in the Deep Ocean Around the Island of Taiwan

• Published in: Journal of geophysical research. Oceans 2022-05, Vol.127 (5), p.n/a
• Main Authors: Tang, Qunshu, Jing, Zhiyou, Li, Jiabiao, Sun, Jie
• Format: Article
• Language: English
• Subjects: Boundary currents; Cascading; Continental slope; Cruises; Diapycnal mixing; Diffusion coefficients; Diffusivity; Eddy currents; Energy dissipation; Energy exchange; Finestructure; Flow stability; Geophysics; Hot spots; Methods; Numerical simulations; Oceans; Offshore; Reflection; Resolution; Ridges; Seismic activity; Seismic reflection profiling; Seismic stability; Seismic surveys; Slopes; Spatial discrimination; Spatial resolution; Stratified water; Thermocline; Tides; Topographic effects; Topography; Turbulence; Turbulent mixing
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC018034

The deep ocean to the east and south of Taiwan Island‒including the Okinawa Trough, Ryukyu Arc, Philippine Sea, Luzon Strait, and the northeast South China Sea‒is one of the most prominent mixing hotspots of the global ocean. The high‐resolution seismic reflection technique can provide detailed views of the water's finestructure and near‐transient turbulent mixing therein. Using legacy data from two seismic cruises, we present the finestructure and quantify turbulent mixing within the thermocline and sub‐thermocline (100–1,000 m depths) to the east and south offshore Taiwan. We derive dissipation rate ε and diapycnal diffusivity Kρ by analyzing the inertial convective turbulence regime of horizontal slope spectra from the seismically auto‐tracked wave‐fields. Dissipation and diffusivities are locally enhanced close to the topography exceeding 10−6 W⋅kg−1 and 10−2 m2⋅s−1, and gradually decrease to 10−9 W⋅kg−1 and 10−4‒10−5 m2⋅s−1 beyond tens of kilometers. The mean diffusivity distribution pattern suggests that a large fraction of the energy dissipates close to the topography and a significant fraction is radiated into the open ocean. Both the tides and west boundary current drive the significantly enhanced mixing close to the topography of ridges, isolated islands and continental slopes. Kuroshio and eddy activities are responsible for the local anomalously enhanced mixing patches far away from the topography. Comparisons among seismic results, numerical simulations and eddy analyses suggest that the energy cascades downward from large‐scale motions to small‐scale turbulence via multiple mechanisms of tide‐topography interaction, flow‐topography interaction, and frontal instability around the island of Taiwan. Plain Language Summary Rough topography behaves like stirring rods that help to mix the stratified water in the open ocean. The deep ocean area around the island of Taiwan is a well‐known mixing hotspot due to its extremely strong energy dissipation effect close to complex topography via interactions with tides. The seismic reflection method has been widely used to study various oceanic phenomena in the global ocean at a high spatial resolution of ∼10 m. Here we use this method to characterize the mixing across density surfaces for the vast deep‐sea area around the island of Taiwan. We find that strong tidal activity is the main cause of the strong mixing around topography. In addition, flow‐topography interaction of Kuroshio or eddy currents is anot