Mixing in the Main Thermocline

A series of profiles of velocity microstructure along 152°E in the western North Pacific Ocean were collected in May–June 1982. Large, averaged turbulent dissipation rates, ε, found in the main thermocline (400 to 1000 m) were determined by a combination of large independent estimates of ε and a greater rate of occurrence of turbulent events in the main thermocline than elsewhere. Concurrently we find that averaged values of ε exhibit a positive dependence on the buoyancy frequency, N, and that form ε = aNγ is best fit by γ = 1 when only the data below 400 m are considered. Of the more than 5000 m of data collected below 1000 m depth, 12% showed measurable turbulence and dominated the depth averages. A deep ocean estimate of an upper bound to the eddy coefficient for vertical diffusion, Kρ, is 10−4 m2 s−1 and not significantly different from the value estimated by Munk. The inferred dependence of the mass flux with depth indicates the relative significance of vertical mixing in the main thermocline. Other processes must influence the maintenance of the more weakly stratified 15°–18°C water above.