The wall-layer dynamics in a weakly stratified tidal bottom boundary layer

The application of the classical logarithmic layer model for wall-bounded shear flows to marine bottom boundary layer (BBL) usually leads to an overestimation of the friction velocity u* due possibly to the influence of form drag, stratification, and rotation of the flow vector. To gain insights on the BBL velocity scaling, acoustic Doppler current profiler (ADCP) measurements taken in the East China Sea were analyzed (a total of 270 sixteen-minute averaged velocity profiles). Single and double log-layer models, a log-wake model, and a modified log-layer (MLL) model that accounts for stratification in the upper part of the BBL (Perlin, Moum, Klymak, Levine et al. 2005) were explored. Although the first three models fit well for a majority of the profiles, the friction velocities appeared to be substantially overestimated, leading to unreasonably high drag coefficients. The friction velocity u*ml inferred from a slightly modified MLL, however, is half of that estimated using the classical log-layer assumption u*l. In a weakly stratified extended BBL, the dissipation rate ε decreases with the height from the seafloor ζ much faster than that in a homogeneous stationary BBL. This observation could be well approximated (in terms of r2) by an exponential ε (ζ) = ε0e–ζ/Lm or a power law decrease. The mixing length scale Lm = cLhBL, where hBL = 19–20 m is the BBL height and cL = 0.17, as well as the characteristic dissipation ε0, should vary in time, depending on the tidal currents and stratification in the BBL. The eddy diffusivity KN = 0.2ε/N2 showed an inverse dependence on the Richardson number Ri according to KN = K0/ (1 + Ri/Rc), where Rc is a constant and the diffusivity in nonstratified flow near the seafloor K0 = u*κζ is specified using u* = u*ml

Late summer stratification, internal waves, and turbulence in the Yellow Sea

Microstructure profiling measurements at two locations in the Yellow Sea (a deeper central basin and a local shelf break) were analyzed focusing on tidal and internal-wave induced turbulence near the bottom and in the pycnocline. A classical three-layer density structure consisting of weakly stratified surface and bottom boundary layers and a narrow sharp pycnocline is developed by the end of warm season. Turbulence in the surface layer was not influenced by the tidal forcing but by the diurnal cycle of buoyancy flux and wind forcing at the sea surface. The enhanced dissipation and diffusivity generated by the shear stress at the seafloor was found in the water interior at heights 10-15 m above the bottom with a phase shift of -5-6 m/h. No internal waves, turbulence, or mixing were detected in the pycnocline in the central basin, in contrast to the pycnocline near the local shelf break wherein internal waves of various frequencies were observed all the time. The thickness of the surface layer near the local shelf break slightly exceeded that of the bottom layer (20 vs. 18 m). A 5-6 m high vertical displacement of the pycnocline, which emerged during the low tide, was arguably caused by the passage of an internal soliton of elevation. During this episode, the gradient Richardson number decreased below 0.25 due to enhanced vertical shear, leading to local generation of turbulence with dissipation rates exceeding the background level by an order of magnitude. (C) 2008 Elsevier B.V. All rights reserved

Contribution of topographically-generated submesoscale turbulence to Southern Ocean overturning

The ocean’s global overturning circulation regulates the transport and storage of heat, carbon and nutrients. Upwelling across the Southern Ocean’s Antarctic Circumpolar Current and into the mixed layer, coupled to water mass modification by surface buoyancy forcing, has been highlighted as a key process in the closure of the overturning circulation. Here, using twelve high-resolution hydrographic sections in southern Drake Passage, collected with autonomous ocean gliders, we show that Circumpolar Deep Water originating from the North Atlantic, known as Lower Circumpolar Deep Water, intersects sloping topography in narrow and strong boundary currents. Observations of strong lateral buoyancy gradients, enhanced bottom turbulence, thick bottom mixed layers and modified water masses are consistent with growing evidence that topographically generated submesoscale flows over continental slopes enhance near-bottom mixing, and that cross-density upwelling occurs preferentially over sloping topography. Interactions between narrow frontal currents and topography occur elsewhere along the path of the Antarctic Circumpolar Current, which leads us to propose that such interactions contribute significantly to the closure of the overturning in the Southern Ocean

Intermittency of near-bottom turbulence in tidal flow on a shallow shelf

The higher-order structure functions of vertical velocity fluctuations (transverse structure functions (TSF)) were employed to study the characteristics of turbulence intermittency in a reversing tidal flow on a 19 m deep shallow shelf of the East China Sea. Measurements from a downward-looking, bottom-mounted Acoustic Doppler Velocimeter, positioned 0.45 m above the seafloor, which spanned two semidiurnal tidal cycles, were analyzed. A classical lognormal single-parameter (mu) model for intermittency and the universal multifractal approach (specifically, the two-parameter (C-1 and alpha) log-Levy model) were employed to analyze the TSF exponent xi(q) in tidally driven turbulent boundary layer and to estimate mu, C-1, and alpha. During the energetic flooding tidal phases, the parameters of intermittency models approached the mean values of (mu) over tilde approximate to 0.24, (C) over tilde (1) approximate to 0.15, and (alpha) over tilde approximate to 1.5, which are accepted as the universal values for fully developed turbulence at high Reynolds numbers. With the decrease of advection velocity, mu and C-1 increased up to mu approximate to 0.5-0.6 and C-1 approximate to 0.25-0.35, but a decreased to about 1.4. The results explain the reported disparities between the smaller "universal" values of intermittency parameters mu and C-1 (mostly measured in laboratory and atmospheric high Reynolds number flows) and those (mu = 0.4-0.5) reported for oceanic stratified turbulence in the pycnocline, which is associated with relatively low local Reynolds numbers R-lambda w. The scaling exponents xi(2) of the second-order TSF, relative to the third-order structure function, was also found to be a decreasing function of R-lambda w, approaching the classical value of 2/3 only at very high R-lambda w. A larger departure from the universal turbulent regime at lower Reynolds numbers could be attributed to the higher anisotropy and associated intermittency of underdeveloped turbulence.U.S. Office of Naval Research [N00014-05-1-0245]; Spanish Ministry of Education and Science [FIS2008-03608]; Major State Program of China for Basic Research [2006CB400602]; Catalan Institute for Water Research (ICRA