Applicability of the Prandtl mixing length model for estimating diapycnal diffusivity in the Kuroshio is evaluated by comparing dissipation rates of turbulent kinetic energy, ɛ, at a depth of ~195 m with independent measurements. We first calculate the Ellison length scale and the structure function (SF) from the moored temperature and velocity data, respectively, downstream of a small reef called Hira-se in the Tokara strait, where the Kuroshio leaves the East China Sea for the North Pacific. Then, two time series of ɛ are derived: one by dividing diapycnal diffusivity estimated from the Ellison length scale by a constant mixing efficiency of 0.2, and the other by fitting the SF to the two-thirds law. Both estimates of ɛ vary almost synchronously in the range of 10^-6 - 10^-5 W kg^-1, being positively correlated with the semidiurnal variation of the mean streamwise velocity. In addition, the values of ɛ estimated from the Ellison length scale agree fairly well with microstructure measurements around the mooring site for ɛ ≈ 10^-7 W kg^-1, the range in which the slope of the fitting line to the SF is not statistically significant. These results support the use of the Prandtl mixing length model for obtaining continuous and long-term estimates of diapycnal diffusivity in the Kuroshio from conventional moored observations.

dissipation rate,Ellison length scale,structure function,turbulence,diapycnal diffusivity