Effect of Decadal Kuroshio Extension Jet and Eddy Variability on the Modification of North Pacific Intermediate Water

Salinity modifications in the North Pacific Intermediate Water (NPIW) core layer of 26.7–26.8 σθ in the western North Pacific Ocean are investigated using temperature–salinity data from available profiling float and hydrographic measurements in 2002–09. During 2002–05, when the Kuroshio Extension (KE) jet was intense and zonally elongated, coherent positive salinity anomalies appeared along the inflow KE jet southeast of Japan and in the downstream Mixed Water region east of 152°E. Broad-scale negative salinity anomalies were detected south of the KE jet and in the upstream Mixed Water region west of 152°E. The signs of these observed salinity anomalies were reversed in 2006–09, when the KE jet transitioned to a weakened and zonally contracted dynamic state. By adopting an isopycnal advection–diffusion model and conducting model runs with the time-dependent advective field inferred from the eddy-resolving satellite altimeter sea surface height data, it is found that the observed salinity anomalies are oscillatory in nature and are determined not only by the decadally varying KE jet itself but also by mesoscale eddy signals that modulate temporally and longitudinally along the path of the KE jet.