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 (epsilon) 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), epsilon values were typically on the order of 10(-8)& SIM;10(-7) W kg(-1), and an obvious diurnal cycle existed in the upper 40 m of the surface mixing layer. Below the ML, epsilon was reduced to 10(-9)& SIM;10(-8) W kg(-1) with some patches of high epsilon 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 epsilon & SIM;S-0.40N0.20. Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.