Yutaka Michida

Experiment of particle dispersion on the sea surface with GPS tracked drifters

Many studies were carried out since 1960s to make direct estimation of the diffusivity in the ocean by various types of releasing experiments of dye or floats. Motions of water on the sea surface were described from a macroscopic point of view by those methods, and diffusion coefficients were estimated from the deformation of the envelope of whole particles. In the present study, we deployed GPS tracked surface drifters, which provide detailed information of the motions of water particles that result in the synoptically estimated diffusivity. Experiments were carried out in Otsuchi Bay, Iwate, Japan. The estimated values of diffusivity in the present study using drifters drogued at 5 m below the sea surface are smaller than the previous estimations using drifting particles on the sea surface. It is suggested that the diffusivity changes vertically in the surface layer. Our experiments have demonstrated the effectiveness of the drifter in observing detailed variability of surface current and applicability to visualization of the coastal current field.

Experimental study using GPS tracked drifters on depth dependency of eddy diffusivity

The eddy diffusivity, one of the most important parameters in coastal environment, has been estimated by a series of in situ observations using surface drifters in coastal waters in Japan. The parameter is dependent upon the horizontal scale, as shown by many previous studies both theoretically and experimentally. We have carried out experimental observations by deploying surface drifters drogued at different depths below the sea surface to investigate vertical distribution of eddy diffusivity, of which there has been limited knowledge. Preliminary analysis shows that the diffusivity has some depth dependency that seems to change with seasonal variation of stratification and other environmental conditions including wind field. Variations of estimated diffusivity are bigger in the surface layer than that in deeper layer, possibly because of unsteady turbulent condition in the surface layers.

Direct estimation of relative vorticity of the Kuroshio off Boso Peninsula

The horizontal structure of current velocity and relative vorticity of the Kuroshio were precisely investigated by analyzing shipmounted acoustic Doppler current profiler (ADCP) data repeatedly collected off Boso Peninsula in October-November, 2003. In previous studies, the horizontal velocity shear of the Kuroshio was estimated only with the vorticity component A (dv/dx: cross-stream shear of down-stream velocity), assuming that the vorticity component B (-du/dy: down-stream shear of cross-stream velocity) is small. And the used coordinate is usually stream coordinate, which consists in streamline and curvature direction. This coordinate has one problem, strictly speaking. If the measured velocity is not along the streamline of the current, and it is not suitable to evaluate relative vorticity. In the present study, the vorticity component B is estimated directly by measuring the velocity structure along multiple cross-sections of the Kuroshio repeatedly, and it is very accurate to evaluate relative vorticity , since the present coordinate is not stream coordinate but Cartesian coordinate. The vorticity component A of the Kuroshio on the coastal side of the main path is larger than that on the offshore side off Boso Peninsula. The cross-stream velocity changes locally and the vorticity component B cannot be ignored. The relative vorticity on the coastal side of the Kuroshio main path is high (>0.6 times 10-4 [1/s]) and cannot be neglected in comparison with the planetary vorticity (~0.8 times 10-4 [1/s]), which means that the Rossby number cannot be assumed to be small and nonlinearity may be effective in local dynamics on the coastal side of the Kuroshio main path. The vorticity component B cannot be neglected, which not vorticity component A but also vorticity component B is necessary to evaluate relative vorticity. Potential vorticity (PV) investigating relative vorticity data and expendable-CTD data is investigated. The distribution of PV is shown the vortex stretching of the parcel in Kuroshio current

Mean Sea-Level and Its Seasonal Change Observed at Syowa Station, East Antarctica

Hydrographic Department of Japan started sea level observation in 1960’s at Syowa Station as a part of Japan Antarctic Research Expedition, but its record was frequently broken. Through huge effort, we finally succeeded in taking the record continuously over years from 1981 using a pressure sensor of strain gauge. This continuous sea level record shows two remarkable phenomena. One is a long-term tendency of descending at the rate of about −0.45cm/year and another is a large seasonal change of sea level reaching about 26cm. Former is agreed with the geological fact of the ground rise assured by the geological remain of the elevated shore lines near the station. Latter is supposed as the seasonal change of coastal oceanographic condition, mainly caused by the coastal current circulation of the Weddel Gyre.