| A method to estimate vertical and horizontal eddy diffusivities KV and KH with the steady conservation equation of salinity on an isopycnal surface was devised. Using this method that presents the advantage of not having to estimate vertical velocity, the diffusivities at 33°, 29°, 25°, and 21°N, 165°E in subtropical Northwest Pacific were estimated with conductivity-temperature-depth profiler (CTD) data. KV at 33° and 29°N is characterized by marked vertical changes with a maximum at the depth of 2250-2500 m; it is approximately 0.2 cm^2 s^-1 at depths of several hundred meters, increases to 0.63 (33°N) and 0.90 (29°N) cm^2 s^-1 at 2250-2500 m, and decreases to approximately 0.02 cm^2 s^-1 at depths greater than 4000m. The decrease of KV with increasing depth was noted in the deep layer at 25°N. On the other hand, KV is relatively constant at 0.6-0.7 cm^2 s^-1 in the intermediate layer at 25°N and is approximately 1.0 cm^2 s^-1 at full depth at 21°N. The large KV at 21°N is due to the generation and reflection of internal gravity waves at the Mid-Pacific Seamounts. The vertical changes indicate that KV depends on the Brunt-Väisälä frequency N, and this dependence on N shows the wave field which causes turbulence. KV in the intermediate layer (500-2000 m) is proportional to N-1.0 (33°N) and N-0.8 (29°N), due to internal gravity waves in a narrow band with nearly single frequency. The intermediate-layer KV at 25°N and the full-depth KV at 21°N are little dependent on N, due to internal gravity waves in a multi-wave field described by the Garrett-Munk spectrum. KV in the deep layer (2250-4000 m) at 33°, 29°, and 25°N is proportional to N4.2, due to internal Rossby waves. The difference in waves causing turbulence between the intermediate and deep layers produces the difference in the N dependence of KV, and forms the maximum of KV at 2250-2500 m. The contribution of Rossby waves to KV should be examined further, although instability of the bottom current formed by Rossby waves was suggested. The diffusive pseudo-velocity Wd (≡∂KV/∂z) brought about by the vertical change of KV is a downwelling of 3.7 × 10e-6 cm s^-1 in the deep layer at 33°, 29°, and 25°N, which may depress vertical advection due to the upwelling of deep water. KH was estimated to be 0.42, 1.1, and 3.4 × 10e6 cm^2 s^-1 at 33°, 29°, and 25°N, respectively. The oxygen dissipation rate was estimated using the results of KV and KH and dissolved oxygen data, and a step-like profile was found. The oxygen dissipation rate at 33°N showed a step-like reduction, being 0.39 ml l^-1 year^-1 in the oxygen minimum layer (750-1250 m), 0.078 at 2000-2750 m, 0.0036 at 3000-3500 m, and 0.0026 in the lower deep layer below 3750 m. These values are larger than past results by one order of magnitude or more at depths less than 2750 m, and are similar to them at depths greater than 3000 m. |