Tsugaru Gyre development in summer 2025

The Tsugaru Warm Current (TWC) flows eastward through the Tsugaru Strait from the Japan Sea to the North Pacific, influencing the water-mass distribution, marine ecosystems, and fisheries in the confluence region east of Japan. In this study, we conducted shipboard observations during the transition phase of the TWC from its coastal mode to its gyre mode in early July 2025. Subsurface information obtained from these in-situ observations at key locations is highly valuable for validating model simulation products, together with satellite observations. These datasets were comprehensively analyzed to investigate the processes involved in the development of the Tsugaru Gyre. Across the shallow region at the eastern mouth of the Tsugaru Strait, relative to the source water in the strait, the TWC water became colder and saltier at the surface but warmer and fresher in the subsurface due to vertical mixing. Further downstream, both the temperature and salinity of the TWC decreased markedly due to horizontal mixing, for which the simulation data suggested barotropic and baroclinic instabilities as possible driving mechanisms. In addition, we investigated the deep-gyre development (down to ~ 1000 m) that occurs below the surface-layer TWC outflow (~ 0–200 m depth) by applying a mean kinetic energy (MKE) budget analysis to the simulation data. Our results suggest that the mean pressure effect provides the deep-gyre MKE as the primary forcing, counteracted by the eddy-driven form drag, and that the advection horizontally redistributes the MKE. These results provide new insights into the vertical momentum transfer mechanisms governing the Tsugaru Gyre.