Turbulent bottom Ekman boundary layer measured over a continental shelf

Abstract

An acoustic Doppler current profiler was deployed in summer and autumn in 2007–2009 at two stations on the East China Sea shelf. Clear velocity spirals that basically correspond to theoretical Ekman spirals were identified for both mean and tidal currents. From these spirals and the corresponding Ekman equation, we estimated the time‐averaged eddy viscosity (μ) profiles. The estimated μ was largest (2–3 × 10−3 m2 s−1) around 5 m from the bottom and decreased almost exponentially with height. A qualitatively similar profile of the eddy diffusivity was also inferred from the acoustic Doppler current profiler data and microstructure profiler data. The flux Richardson number was estimated as 0.11 ± 0.10 ∼ 0.46 ± 0.17, indicating relatively large buoyancy contribution to the turbulent kinetic energy budget.