Mesoscale eddies are common in the ocean and their surface characteristics have been well revealed based on altimetric observations. Comparatively, the knowledge of the three-dimensional (3D) structure of mesoscale eddies is scarce, especially in the open ocean. In the present study, high-resolution field observations of a cyclonic eddy in the Kuroshio Extension have been carried out and the anatomy of the observed eddy is conducted. The temperature anomaly exhibits a vertical monopole cone structure with a maximum of −7.3 • C located in the main thermocline. The salinity anomaly shows a vertical dipole structure with a fresh anomaly in the main thermocline and a saline anomaly in the North Pacific Intermediate Water (NPIW). The cyclonic flow displays an equivalent barotropic structure. The mixed layer is deep in the center of the eddy and thin in the periphery. The seasonal thermocline is intensified and the permanent thermocline is upward domed by 350 m. The subtropical mode water (STMW) straddled between the seasonal and permanent thermoclines weakens and dissipates in the eddy center. The salinity of NPIW distributed along the isopycnals shows no significant difference inside and outside the eddy. The geostrophic relation is approximately set up in the eddy. The nonlinearity-defined as the ratio between the rotational speed to the translational speed-is 12.5 and decreases with depth. The eddy-wind interaction is examined by high resolution satellite observations. The results show that the cold eddy induces wind stress aloft with positive divergence and negative curl. The wind induced upwelling process is responsible for the formation of the horizontal monopole pattern of salinity, while the horizontal transport results in the horizontal dipole structure of temperature in the mixed layer. species composition (EDDIES) in the Sargasso Sea off Bermuda [5], meso-and submesoscale processes in an intense front (AlborEx) [6] and the northwestern Pacific eddies, internal waves and mixing Experiment (NPEIM) [7], and so forth. Nowadays, main observation methods for field experiment include Argo floats, moored instrumentation, underwater gliders and shipboard survey.Argo profiling floats, which are designed to measure underwater ocean temperature and salinity (T/S) [8], are one of the effective methods to obtain 3D mesoscale eddy structures [9][10][11][12][13]. However, due to the drift features, the detailed structure of a specific eddy is difficult to acquire unless a large number of Argo floats are deployed inside the targeted mesoscale eddy. Reference [14] deployed 17 rapid-sampling Argo floats to study the anticyclonic eddies effects on mode water subduction in the south of the Kuroshio Extension to the east of Japan. In situ mooring observations can usually collect temperature, salinity and current data from the bottom to the surface at a fixed-point where the eddy moves through. Reference [15] designed and conducted a multi-month field campaign to capture the full-depth 3D structure of a pair of anticyclonic and c...

Mesoscale eddies are ubiquitous in the world's oceans. They are rotated with a 10-100 km length scale. The relatively isolated water mass trapped with mesoscale eddies can propagate a long distance while maintaining the source thermohaline characteristics. Thus, there are significant differences in the thermohaline structures between the inside and outside eddies, which have profound effects on underwater acoustic propagation.The earliest studies on the impact of eddies on sound propagation were conducted by Vastano and Owens (1973) and Weinberg and Zabalgogeazcoa (1977) based on hydrological field data measured from a cyclonic Gulf Stream (GS) ring detected in 1967. Vastano and Owens (1973) first observed the significant acoustic field perturbation caused by the cyclonic ring in the relatively uniform acoustic environment of the Sargasso Sea and found a strong dispersion phenomenon of the ray path in the area where the sound channel depth changed with the ray computations. In the following year, Gemmill studied the eddy's effects on the convergence mode of sound propagation and revealed that the cold eddy destroys the cyclic distribution of the convergence zones and refracts the sound rays into the deep sound channel. Subsequently, the eddy effects on ray travel time, which reflects the sound arrival structure, were studied with a range-dependent model presented by Weinberg and Zabalgogeazcoa (1977). Although the above studies are valuable, the corresponding sensitivity investigations of sound propagation on the eddy property variation cannot be conducted because of the rarity of eddy field data. To address this problem, Henrick et al. (1977) established a parametric eddy model qualitatively validated by GS ring observation data. Through the model, the effects of eddy intensity, geometric size and peak rotation speed on sound transmission are studied. Due to the good performance of such a model in describing eddy sound

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