Direct Measurements of Turbulence in the Upper Western Pacific North Equatorial Current over a 25-h Period

Measurements of the turbulent kinetic energy dissipation rate (ε) were conducted by a free-fall microstructure profiler in the western Pacific North Equatorial Current (WPNEC) during a continuous period of 25 h, from the sea surface to about 160 m depth. In the mixed layer (ML), ε values were typically on the order of 10(−8)∼10(−7) W kg(−1), and an obvious diurnal cycle existed in the upper 40 m of the surface mixing layer. Below the ML, ε was reduced to 10(−9)∼10(−8) W kg(−1) with some patches of high ε reaching 10(−7.5) W kg(−1). The barrier layer was identified in the nighttime with a maximum thickness of 20 m, and it was eroded by the advection of freshwater within the lower part of the isothermal layers associated with an anticyclonic eddy in the afternoon. A simple scaling relevant to shear (S(2)) instability and stratification (N(2)) that can predict turbulent dissipation rates in the transition layer, between the well-mixed layer and the thermocline below, was obtained through the scaling [Formula: see text]. Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.