Latitudinal dependence of diapycnal diffusivity in the thermocline estimated using a finescale parameterization

The spatial distribution of diapycnal diffusivity in the thermocline over a large area in the interior of the North Pacific is obtained by incorporating the finescale vertical shear of horizontal velocity measured by expendable current profilers into the empirical formula proposed by Gregg [1989]. It is clearly shown that diapycnal diffusivity in the North Pacific strongly depends on the latitude. For example, the estimated diapycnal diffusivities near the Hawaiian Ridge and the Izu-Ogasawara Ridge exceed 10(-4) m(2) s(-1), whereas those near the Aleutian Ridge and the Emperor Seamounts are of the order of 0.1 x 10(-4) m(2) s(-1), even though the available semidiurnal internal tidal energy is similar among all of these prominent topographic features. The observed results are consistent with the numerical prediction that the cascade of internal tidal energy down to dissipation scales is dominated by parametric subharmonic instability which is expected to occur only below 30 degreesN.