09:15 〜 09:30
[AOS15-08] Spatial and temporal variations of turbulence along the Kuroshio over a seamount
キーワード：flow-topography interaction, Kuroshio, shear instability, lee wave, turbulent mixing
Several scientific cruises were conducted to investigate the flow-topography interactions along the Kuroshio over a seamount off the eastern coast of Taiwan and near Green Island. When the Kuroshio flowed over the 200-m-depth seamount, the echo sounder captured the signals of lee waves and shear instability above the seamount and at the immediate lee of the seamount. These small-scale processes further induced vigorous turbulent mixing. Results of microstructure profiling revealed that the turbulent kinetic energy(TKE) dissipation rate varied in space and time around the seamount. That is, the TKE dissipation rate was O (10-5 W kg-1) downstream, which was 100 times larger than the upstream region of the seamount. Besides, the TKE dissipation rate was strongly modulated by the tide, i.e., strong and weak TKE dissipation rates occurred at low and high tides, respectively. Using a simplified one-dimensional diffusion model, we demonstrated that turbulent mixing plays a role in shaping the downstream hydrographic structure. On the other hand, lee waves are likely to create a circumstance favoring a higher growth rate of shear instability, which was examined using linear stability analysis. Mostly, the criterion for the occurrences of shear instability, Richardson number Ri < 0.25, was satisfied downstream. The distribution of Ri fluctuated around a central value near 0.25, which is a typical feature of marginal instability. This can be interpreted as a cyclic circumstance in a marginal balance between shear forcing and turbulent mixing. Finally, we compared parameterizations of turbulent mixing based on the Ri between previous empirical relationships and our observations. This may help to improve the parameterization of turbulent mixing induced by flow encountering abrupt topography.