Evolution and Structure of a Mesoscale Anticyclonic Eddy in the Northwestern Japan Sea and Its Exchange with Surrounding Waters: In Situ Observations and Lagrangian Analysis

Abstract

We have found a region in the northwestern Japan Sea near the Subpolar Front where mesoscale eddies regularly form and circulate. The anticyclonic eddy with a strong dynamic feature, vertically extended to the bottom (~3500 m), multilayer core structure and extreme values of temperature and salinity was sampled there in the cruise in May 2004. The altimetry-based eddy tracking showed that it had a life span of nine months with the size reaching 120 km. CTD observational data were used to explore the eddy’s features and vertical structure. The eddy had a multilayer core with different thermohaline characteristics which could be originated by eddy interaction with the surrounding waters during its life. The evolution of the eddy has been studied and analyzed with the help of altimetry-based Lagrangian indicators of water motion. Performing the particle-tracking numerical experiments, we computed the Lagrangian maps that contain information on the origin of water inside the eddy core and its ‘age’ on every day of the eddy’s life. Inspecting daily-computed Lagrangian maps, we documented the essential events in the evolution of the eddy including its formation, splitting, merger, entrainment and detrainment of water, erosion and eventual decay. All these observations have been verified with the infrared satellite images. The new Lagrangian technique has been applied to calculate day by day the content of water of various origin (coastal, subtropical and subarctic) entrained into the eddy core. It has been found that the surface core has been filled mainly with subtropical water originated in the southern flank of the Subpolar Front. The episodes with cardinal changes of ratio of different water types have been recorded and verified with the infrared satellite images and maps of the origin of water inside the eddy core.