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.