Current variability of the Kuroshio near the separation point from the western boundary

Direct current measurements were conducted from April 2003 to March 2004 to investigate the current variability of the Kuroshio near the separation point from the western boundary. During the study period, the main stream of the Kuroshio flowed northeastward in the upper layer slightly offshore from the array of four mooring systems, while a southwestward current was observed in the intermediate layer on the coastal side of the array. Analyses based on the velocity and temperature measurement of the mooring meter array revealed eddy‐to‐mean kinetic and mean‐to‐eddy potential energy conversion in both the upper and intermediate layers. Downstream‐propagating waves detected as significant extended empirical orthogonal functions (EEOFs) are predominant over velocity fluctuations of periods shorter than 50 days, explaining 67% of the total variance. The five apparent wave groups have periods of 7–18 days, wavelengths of 220–380 km, and phase velocities of 22–30 cm s −1 , respectively, similar to the values reported in previous studies of upstream regions. Although the phase velocity at a given wavelength in the Kuroshio is lower than that in the Gulf Stream, the dispersion tendency (i.e., that phase velocity increases with decreasing period and wavelength) is the same for both currents. The relatively low phase velocity of the Kuroshio is considered to reflect its relatively low background velocity. Data regarding growth rate, vertical phase lags, and energetics suggest that kinetic energy in this region is transferred from small to large scales mainly via eddies resulting from baroclinic instability, which is possibly related to synoptic‐scale path variability and the penetration of areas of high kinetic energy into the Kuroshio Extension.