Internal tides and turbulent mixing observed in the Bussol Strait

Abstract

Repeated observations with a period of about 24h of hydrography, current velocity, and microstructures were performed at three stations surrounding a seamount in the middle of the Bussol Strait, the deepest and widest one of the Kuril Straits, to reveal spatial and temporal variability of internal tides and associated turbulent mixing. It is found that isopycnal displacements are dominated by diurnal tidal components, which show phase differences (namely, time lags) between the three stations that can be explained by a first mode topographically trapped wave (TTW) propagating clockwise around the seamount. Furthermore, at the station located near the center of the strait where energy dissipation rates are largest, diurnal variations of isopycnals and velocities are amplified toward the ocean bottom, consistent with the vertical structure of the first mode TTW. At that station, vigorous turbulent mixing with the energy dissipation rate exceeding 10-6m2s-3 and diapycnal diffusivity exceeding 10-1m2s-1 was observed in deep layers when the diurnal tidal current consisting of the first mode TTW flows from the Okhotsk Sea to the North Pacific, enhancing the mean current. These spatial and temporal variation patterns are confirmed to be reproduced by a previous numerical model successfully for the isopycnals and velocities, and partially for the turbulent mixing. The total energy dissipation rate is, however, by up to a factor of 3–10 smaller than predicted by the numerical model although the observations were performed during spring tides, suggesting that the actual diapycnal mixing is overall weaker than the previous model estimate and/or that extremely strong mixing occurs within highly localized areas.