Confluence zone of Kuroshio and Oyashio to the east of Japan is known as subarctic frontal area of the Northwestern Pacific. Because of high spatial gradients of temperature and salinity this area is considered as one of the most important energy-active zones of the World Ocean, where vertical fluxes between ocean and atmosphere as well as horizontal fluxes of heat, salt and momentum are extremely strong. In addition to meandering flow of Kuroshio Extension mesoscale eddies are important drivers of water dynamics and material exchange across the front. Highly energetic anticyclonic eddies, known as Kuroshio warm-core rings, are formed to the north of the main stream and transport trapped warm and saline subtropical water far to the north. POI observations demonstrated that KWCR may travel more than 4 years and reach area of central Kuril Islands. Joined studies with Canadian (in 1990 th ) and Japanese (in 2000s) colleagues confirmed close relation of the Kuril eddies with warm-core rings of Kuroshio. It was also demonstrated a complicated vertical structure of the eddy with, in some cases, a few cores of trapped water. Such structure influences on vertical motion in the eddy and thus on nutrients and carbon fluxes as well as on primary production and biological species distribution. Strong cyclonic eddies are formed from southward elongated meanders of Kuroshio Extension and provide cross-frontal transport of cold and less saline subarctic mode water into subtropical area. They are less studied in compare with anticyclonic rings but no less important in terms of heath, energy and material transport. Our recent studies of the Kuroshio rings are associated with Fukushima-1 NPP incident in 2011 and recent discharge of accumulated water to understand which part of this water may reach Russian waters and with what content of radioisotopes. POI cruises of 2023-2024 proved slightly higher tritium concentration in the main stream of Kuroshio Extension and the eddies. However it was close to natural level. No notable excess of tritium was found.