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

Abstract

AbstractThe 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.