Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.