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Direct measurement of near-bottom turbulence in deep ocean for predicting behavior of suspended particles in Sagami Bay and the Okinawa Trough, Japan

In this study, turbulent flows were measured using the Expendable Vertical Microstructure Profiler (VMP-X) at the hydrothermal fields in the Okinawa Trough (Iheya North field and Noho site) and Sagami Bay (off Itoh and at Ohmuro Caldera) to characterize the turbulence intensity. For each observation point, the turbulent flow data were successfully measured between the sea surface and the sea floor. In the vicinity of the sea floor off Itoh port, the energy dissipation rate (ε) was measured as 10−6 (W kg−1). This value was equivalent to that frequently observed in the surface mixed layer. Real-time current measurements are obtained from the Acoustic Doppler Current Profiler (ADCP) at the Real-time Deep Seafloor Observatory off Hatsushima Island, which is operated by JAMSTEC. Strong northeastward flows at the time of turbulent flow measurement were confirmed by ADCP data. Such strong northeastward flows may intensify turbulence near the sea floor. It is important that we clarify the response with turbulent flow data and existing data in this way. As a result of the direct measurement of turbulent flow in the Okinawa Trough, the turbulent energy dissipation rate (ε) near the bottom was approximately 10−7 (W kg−1). Turbulent flow was strong from the bottom to a height of around 20 m from the bottom, and water temperature slightly rose at the Noho site in the Okinawa Trough. This fluctuation may be due to the influence of hydrothermal venting. Therefore, the intensity of turbulence (ε: energy dissipation rate) may also be affected by the hydrothermal venting. However, the results obtained using the turbulent flow meter (VMP-X) are only a snapshot, and more in-situ observational data is necessary to understand the dynamics of the bottom mixed layer (turbulent flow). Therefore, data filing in consideration of spatiotemporal dynamics is still necessary.