Time series measurements of turbulent flows near an area of hydrothermal vents in the Okinawa Trough, Japan

In time series measurements of turbulent flows near the sea floor were taken at hydrothermal fields in the Okinawa Trough. The purpose of this study was to understand the dynamics of the bottom mixed layer (BML), reveal fluctuations in the time series of deep sea turbulent flows (the turbulent energy dissipation rates) using VMP-X, and correlate them with in-situ flow, turbidity, and conductivity, temperature, and depth data obtained from a monitoring station on the sea floor. At the Kumejima west field, time series fluctuations in turbulent energy dissipation rate (epsilon) between the sea surface and the seafloor were successfully measured. The a near the sea bottom changed from 10(-10) to 10(-7) (W kg (-1) ) and the maximum a was observed from a flood tide between ebb tides, and decreased to 10(-9) (W kg(-1)) during the ebb tide on June 24. Moreover, the turbulent flow was relatively strong from the seafloor to a height of around 80 m above it, and water temperature rose 0.1-0.2 degrees Celsius. The measurement points were located approximately 900 m to the northwest of the hydrothermal vent area. Deep-sea current data were successfully recovered for the layers at depths between the seafloor and 30 m above it, and a periodic fluctuation with an amplitude of approximately 10 cm/s for the east-west and north-south components, which was synchronized with variations in the water level, was also observed. According to these environmental and complex bottom topography data, this characteristic water temperature fluctuation may occur as a result of hydrothermal venting (transport or intrusion of warm water). Therefore, the intensity of turbulence may also he affected by hydrothermal venting. However, further continuous in-situ observational data are necessary to understand the dynamics of the BML. In a future study, we will collect and analyze more spatiotemporal turbulent flow data to understand the dynamics of the BML and improve the accuracy of the numerical model.