Tides and mixing in the northwestern East China Sea, Part II: Near-bottom turbulence

The dissipation rate of turbulent kinetic energy ε and friction velocity u* was studied in reversing and rotating tidal flows in the East China Sea near the northeastern coast of China using ADV and ADCP measurements. The highest near-bottom dissipation rate on a shallow (19 m depth) shelf εnb∼5×10−5 W/kg was associated with the stronger flooding current of the reversing tide and the lowest εnb∼10−7 W/kg with the weaker ebb current. The log-layer (ADCP-based) and the skin-layer (ADV-based) near-bottom estimates of friction velocities, u*(log) and u*(cor), showed close correspondence for the reversing tidal flow, but when the tidal vector rotated over a sloping bottom u*(log) was approximately two times larger than u*(cor). The inapplicability of the Prandtl–Karman log-layer scaling for energetic rotating flow is considered as the major source for this discrepancy. The classical wall-layer parameterization ε nb = c 0 u * 3 / κ ζ with c0=1 was found to hold well for the reversing tide, but for rotating flow c0=1.5. The scaling for the dissipation rate, ε = c ε e tr 3 / 2 / L tr , used in the turbulent kinetic energy (etr) balance equation requires cε=0.06 for the reversing tide, but cε=0.09 for the rotating flow, where the turbulent scale Ltr=κζ and ζ is the distance from the seafloor. Significant departure from the wall layer parameterization was noted when advection of warm water affected the testing site at a sloping shelf (38 m depth), possibly causing convective mixing in addition to boundary-induced turbulence.