南海北部海域跃层上部的湍流耗散

频发的非线性内波(内孤立波)与内潮活动是南海北部上层海洋动力的重要特征,特别是在南海东北部东沙群岛邻近海域,内孤立波与内潮活动非常活跃,大量的内孤立波与内潮能量在此耗散掉,产生很强的湍流混合.然而,目前国内外对南海北部海域湍流耗散与混合的直接观测研究还非常有限,且主要局限于对吕宋海峡邻

Upper pycnocline turbulence in the northern South China Sea

The first regional mapping of the averaged turbulent kinetic energy dissipation rate aOE (c) E > (p) > in the upper pycnocline of the northern South China Sea is presented and discussed. At phi = 20A degrees N and to the north of this latitude, aOE (c) E > (p) > appears to be more than two times larger than that to the south of 20A degrees N. It is suggested that this asymmetry is associated with the predominant northwestward propagation and dissipation of the internal waves originated in the Luzon Strait area. An approximately linear relationship between aOE (c) E > (p) > and the available potential energy of the waves P (IW), suggests a characteristic time of the P (IW) dissipation of about 6 h.State Key Laboratory of Marine Environmental Science (Xiamen University); National Basic Research Program of China [2009CB421200, 2007CB411803]; National Natural Science Foundation of China [41006017, 41076001]; Fundamental Research Funds for the Central Universities of China [2010121030]; U.S. Office of Naval Research [N00014-05-1-0245

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 97 (2016): 1859–1884, doi:10.1175/BAMS-D-14-00197.1.Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.This work was sponsored by the U.S. Office of Naval Research (ONR) in an ONR Departmental Research Initiative (DRI), Air–Sea Interactions in Northern Indian Ocean (ASIRI), and in a Naval Research Laboratory project, Effects of Bay of Bengal Freshwater Flux on Indian Ocean Monsoon (EBOB). ASIRI–RAWI was funded under the NASCar DRI of the ONR. The Indian component of the program, Ocean Mixing and Monsoons (OMM), was supported by the Ministry of Earth Sciences of India.2017-04-2

Statistics of microstructure patchiness in a stratified lake

[1] Statistics of microstructure patches in a sheared, strongly stratified metalimnion of Lake Banyoles (Catalonia, Spain), which occupied ∼40% of the total lake depth of 12 m, are analyzed. Light winds ( 0.25 m, the ratio LTp/hp depends on the patch Richardson and mixing Reynolds numbers following the parameterization of Lozovatsky and Fernando (2002). Analysis of the dynamics of mixing reveals that averaged vertical diffusivities ranged between ∼1 × 10−4 m2 s−1 and ∼5 × 10−5 m2 s−1, depending on the phase of the internal waves. Episodic wind gusts (wind speed above 6 m s−1) transfer ∼1.6% of the wind energy to the metalimnion and ∼0.7% to the hypolimnion, generating large microstructure patches with hp of several meter

Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence

Major State Program of China for Basic Research [2006CB400602]; US Office of Naval Research [N00014-05-1-0245]; National Natural Science Foundation of China [41006017, 41076001]; Fundamental Research Funds for the Central Universities [2010121031]; Arizona State University; Spanish Ministry of Education and Science [FIS2008-03608]Velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column using acoustic Doppler current profiler (ADCP) at a test site in the western part of the East China Sea. The friction velocity u (*) and the turbulent kinetic energy dissipation rate epsilon (wl)(zeta) profiles were calculated using log-layer fits; zeta is the height above the bottom. During a semidiurnal tidal cycle, u (*) was found to vary in the range (1-7) x 10(-3) m/s. The law-of-the-wall dissipation profiles epsilon (wl)(zeta) were consistent with the dissipation profiles epsilon (mc)(zeta) evaluated using independent microstructure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50-m seafloor) located to the east of the measurement site is responsible for the latter. Calculation of the depth integrated internal tide generating body force in the region showed that the flanks of the rise are hotspots of internal wave energy that may locally produce a significant turbulent zone while emitting tidal and shorter nonlinear internal waves. This distant topographic source of turbulence may enhance the microstructure-based dissipation levels epsilon (mc)(zeta) in the bottom boundary layer (BBL) beyond the dissipation epsilon (wl)(zeta) associated with purely locally generated turbulence by skin currents. Semi-empirical estimates for dissipation at a distance from the bathymetric rise agree well with the BBL values of epsilon (mc) measured 15 km upslope

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

[1] 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 (μ) model for intermittency and the universal multifractal approach (specifically, the two-parameter (C1 and α) log-Levy model) were employed to analyze the TSF exponent ξ(q) in tidally driven turbulent boundary layer and to estimate μ, C1, and α. During the energetic flooding tidal phases, the parameters of intermittency models approached the mean values of µ˜ ≈ 0.24, C˜1 ≈ 0.15, and ᾶ ≈ 1.5, which are accepted as the universal values for fully developed turbulence at high Reynolds numbers. With the decrease of advection velocity, μ and C1 increased up to μ ≈ 0.5–0.6 and C1 ≈ 0.25–0.35, but α decreased to about 1.4. The results explain the reported disparities between the smaller “universal” values of intermittency parameters μ and C1 (mostly measured in laboratory and atmospheric high Reynolds number flows) and those (μ = 0.4–0.5) reported for oceanic stratified turbulence in the pycnocline, which is associated with relatively low local Reynolds numbers Rλw. The scaling exponents ξ(2) of the second-order TSF, relative to the third-order structure function, was also found to be a decreasing function of Rλw, approaching the classical value of 2/3 only at very high Rλw. A larger departure from the universal turbulent regime at lower Reynolds numbers could be attributed to the higher anisotropy and associated intermittency of underdeveloped turbulenceThis study was supported by the U.S. Office of Naval Research (grant N00014‐05‐1‐0245), the Spanish Ministry of Education and Science (grant FIS2008‐03608), and the Major State Program of China for Basic Research (grant 2006CB400602). The first author also received financial support during his temporary affiliation with the Catalan Institute for Water Research (ICRA

The TKE dissipation rate in the northern South China Sea

State Key Laboratory of Marine Environmental Science (Xiamen University); National Basic Research Program of China [2009CB421200, 2007CB411803]; US Office of Naval Research [N00014-05-1-0245]; National Natural Science Foundation of China [41006017]; Fundamental Research Funds for the Central Universities of China [2010121030]The microstructure measurements taken during the summer seasons of 2009 and 2010 in the northern South China Sea (between 18A degrees N and 22.5A degrees N, and from the Luzon Strait to the eastern shelf of China) were used to estimate the averaged dissipation rate in the upper pycnocline aOE (c)epsilon (p)> of the deep basin and on the shelf. Linear correlation between aOE (c)epsilon (p)> and the estimates of available potential energy of internal waves, which was found for this data set, indicates an impact of energetic internal waves on spatial structure and temporal variability of aOE (c)epsilon (p)>. On the shelf stations, the bottom boundary layer depth-integrated dissipation reaches 17-19 mW/m(2), dominating the dissipation in the water column below the surface layer. In the pycnocline, the integrated dissipation was mostly similar to 10-30 % of . A weak dependence of bin-averaged dissipation on the Richardson number was noted, according to , where epsilon (0) + epsilon (m) is the background value of for weak stratification and Ri (cr) = 0.25, pointing to the combined effects of shear instability of small-scale motions and the influence of larger-scale low frequency internal waves. The latter broadly agrees with the MacKinnon-Gregg scaling for internal-wave-induced turbulence dissipation