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

Abstract

The dissipation rate of turbulent kinetic energy e 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 (19m depth) shelf epsilon(nb)similar to 5 x 10(-5)W/kg was associated with the stronger flooding current of the reversing tide and the lowest epsilon(nb)similar to 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 epsilon(nb) = c(0)u*(3)/kappa zeta with c(0)=1 was found to hold well for the reversing tide, but for rotating flow c(0) = 1.5. The scaling for the dissipation rate, epsilon = c(epsilon)e(tr)/L-tr, used in the turbulent kinetic energy (e(tr)) balance equation requires c(epsilon) = 0.06 for the reversing tide, but c(epsilon) = 0.09 for the rotating flow, where the turbulent scale L-tr = kappa zeta and zeta 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. (C) 2007 Elsevier Ltd. All rights reserved