Three-dimensional structures of mesoscale eddies in the subtropical countercurrent and Kuroshio extension regions and their vertical normal modes analysis

Abstract

This study investigates the three-dimensional structures of temperature, salinity, density, and pressure anomalies of cyclonic and anticyclonic eddies in the Subtropical Countercurrent (STCC) and Kuroshio Extension (KE) regions using composite analysis and vertical normal modes methods. The results show that maximum temperature and density anomalies in the upper mixed layer are displaced westward for cyclonic eddies and northwestward for anticyclonic eddies relative to the eddy center, while anomalies below the mixed layer display quasi-isotropic structures. Sea surface salinity anomalies are shifted northwest of the eddy center and exhibit a dual-core vertical structure influenced by the background salinity field. In both the KE and STCC regions, anticyclonic eddies cause greater maximum temperature and salinity anomalies but at shallower depths compared to cyclonic eddies. Analysis of pressure anomalies reveals that eddy vertical structures are surface-intensified in the STCC region, with an average influence depth of 508 m, but extend deeper in the KE region, with an average influence depth of 613 m. Cyclonic eddies generally penetrate deeper than anticyclonic eddies. The vertical structure of eddies is dominated by the barotropic mode and the first baroclinic mode, with the first two modes accounting for 80 % of the variability. Contributions of the barotropic mode increase from the STCC to the KE region, with cyclonic eddies having higher barotropic mode ratio than anticyclonic eddies. The vertical influence depth of eddies is largely determined by the barotropic component, with greater barotropic contributions corresponding to deeper penetration.