Vertical structure of mesoscale eddies is key to the eddy-induced heat/material transport that further affects the climate and marine ecosystem. This study explores the vertical structure of mesoscale eddies in the Kuroshio-Oyashio Extension region (KOE) and its underlying dynamics. By applying the hierarchical ascending classification to the observational and reanalysis datasets, we classify mesoscale eddies with three distinct kinds of vertical structures. Each kind of eddies exhibits clear spatial aggregation along a distinct zonal band. Eddies have core depths of 100-300 m in the northern part of the KOE and core depths of 300-500 m and 0-100 m in the southern. The eddy splitting or merging does not introduce new kind of eddy vertical structure but causes large intra-kind variability. The different kinds of eddy vertical structures can be partially accounted for by the inference from the baroclinic instabilities at the eddy generation sites.

The vertical structure of the Kuroshio Extension (KE) is investigated using velocity measurements from a subsurface mooring array. Mode decomposition based on climatological Temperature/Salinity (T/S) data shows that the barotropic and first baroclinic normal modes dominate the vertical structure of the zonal flow in the KE. This structure is also well described by the leading mode of Empirical Orthogonal Functions (EOFs) that contains the first two vertical normal modes. Further analysis demonstrates that the projection coefficient of the mooring velocity onto the summed vertical mode could be well represented by the surface geostrophic velocity. Therefore, we propose a dynamic method that relates the surface geostrophic flow and the vertical structure of the zonal flow. The applicability of this method is verified with both reanalysis datasets and estimation from hydrographic data. The findings implicate that the KE transport can be well reproduced by surface geostrophic flow and climatological T/S data only.