Census of mesoscale eddies in the Northwestern Japan Sea based on Lagrangian analysis and ship observations

Mesoscale eddies are intrinsic features in the Japan Sea northwest of the Subpolar Front, some of which have been sampled by the authors in different years. Using an automatic eddy-detection algorithm and altimetry-based Lagrangian maps, we have found two domains with increased occurrence frequency of anticyclones and three domains with increased occurrence frequency of cyclones, where the mesoscale eddies regularly form, circulate and decay. The census of long-lived eddies with a surface signature in 1993 – 2022 has been conducted. The distributions of these eddies over the values of nonlinearity parameter, lifetime, radius, swirl and translational velocities and the months of formation and decay have been calculated and analyzed. Using the shipboard and satellite observations and Lagrangian analysis, a few sampled anticyclones and cyclones have been studied in detail with the focus on the thermohaline properties and vertical structure, the genesis and variability. These properties differ for the eddies in the discovered formation domains. Using the Lagrangian particle-tracking method, the content of waters of subtropical and subarctic origin in the surface cores of the sampled eddies and its temporal variability have been estimated. It has been found that cyclonic eddies are associated with a dome-like lifting of isolines which can be traced down to the bottom, but they do not transport trapped water over a long distance. Anticyclonic eddies contain low-salinity water in their cores. Extremely low saline water is observed in the eddies located in the western domain, northwest of the Subpolar Front. The eddies, located at the front and to the south of it, have warm and high-salinity upper cores formed by modified subtropical water and also contain secondary cores of low-salinity water at an intermediate depth. By trapping and transporting low-salinity water, these eddies contribute to distribution of low-salinity intermediate water in the Japan Sea.