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 (<i>ε</i>) 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), <i>ε</i> 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, <i>ε</i> was reduced to 10⁻⁹∼10⁻⁸ W kg⁻¹ with some patches of high <i>ε</i> 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 (<i>S</i>²) instability and stratification (<i>N</i>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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>ε</mi><mo>∼</mo><msup><mrow><mfenced close="|" open="|"><mi>S</mi></mfenced></mrow><mrow><mo>−</mo><mn>0.40</mn></mrow></msup><msup><mrow><mfenced close="|" open="|"><mi>N</mi></mfenced></mrow><mrow><mn>0.20</mn></mrow></msup></mrow></semantics></math></inline-formula>. Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.