Vertical and horizontal eddy diffusivities and oxygen dissipation rate in the subtropical Northwest Pacific

A method to estimate vertical and horizontal eddy diffusivities KV and KH was devised with the steady conservation equations of potential temperature and salinity on an isopycnal surface, which approximates a neutral surface. Using this method, which presents the advantage of not having to estimate vertical velocity, the diffusivities at A0 (33°N), B0 (29°N), C0 (25°N), and D0 (21°N) along 165°E in the subtropical Northwest Pacific were estimated with conductivity–temperature–depth profiler (CTD) data. The values of KV at A0 and B0 are characterized by marked vertical changes with the maximum at a depth of 2000–2500 m; they are approximately 0.3 cm2 s−1 at depths of several hundred meters, increase to 1.1–1.2 at 2000 and 2500 m, and decrease to 0.03 and less than approximately 0.1 at depths greater than 4000 m. The decrease of KV with increasing depth was noted in the deep layer at C0. On the other hand, KV is relatively constant at 0.97–1.1 cm2 s−1 in the intermediate layer at C0 and is approximately 1.1 at full depth at D0. The large KV at D0 is probably 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 characteristics of wave field that causes turbulence. The value of KV in the intermediate layer (typically 500–2000 m) is proportional to N−1.0 at A0 and B0 because of internal gravity waves that are in a narrow band with nearly a single frequency. The intermediate-layer KV at C0 and the full-depth KV at D0 are little dependent on N because of internal gravity waves that are in a multi-wave field described by the Garrett–Munk spectrum. The value of KV in the deep layer (2250–4000 m) at A0, B0, and C0 is proportional to nearly N4.1 because of internal Rossby waves. The difference in waves causing turbulence between the intermediate and deep layers may produce the difference in the N-dependence of KV and may form the maximum of KV at 2000–2500 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) is a downwelling of 0.5×10−5 cm s−1 in the deep layer at A0–C0, which may depress vertical advection because of the upwelling of deep water. The value of KH decreases with increasing depth: 1.7×107 cm2 s−1 at 750–2000 m, 3.5×106 at 2250–4000 m, and 7.3×105 at 4250–5000 m. The oxygen dissipation rate, estimated using the obtained KV and KH, also decreases with increasing depth; it is 0.33 ml l−1 year−1 at the oxygen minimum and 0.12, 0.020, and 0.0090 in the upper, middle, and lower deep layers, respectively. These values are larger than past results by 1 order of magnitude or more at depths less than 3750 m and a few times at depths greater than 4250 m.