On leaky modes on a buoyancy interface

When the fluid below a buoyancy interface such as the seasonal thermocline in the ocean is stably stratified as it is often, energy radiation by internal waves into regions deep below is possible. In this paper, the dispersion relation for a sharp thermocline separating a well-mixed layer from a weakly but stably stratified fluid below is derived and examined for such energy leakage into the fluid below. It is found that the interfacial waves running along such a buoyancy interface leak their energy to radiating internal waves and are rapidly attenuated, if their frequency is less than the buoyancy frequency of the fluid below. The implications the leaky nature of the modes have on resonant excitation of internal waves in the ocean by atmospheric pressure fluctuations and turbulence in the mixed layer are also examined. It is found that efficient resonant build up of interfacial waves is possible only for nonleaky modes and even then their growth rate is modified slightly by the presence of stratified fluid below the interface.     

A note on modeling double diffusive mixing in the global ocean

Though ubiquitous in the global oceans, double diffusive mixing has been largely ignored or poorly represented in the models of turbulent mixing in the ocean and in 3-D ocean models, until recently. Salt fingers occur in the interior of many marginal seas and ocean basins, the Tyrrhenian Sea and the subtropical Atlantic being two examples. Diffusive convection type of double diffusion occurs in the upper layers of many sub-polar seas and polar oceans due to cold melt water from sea ice. Consequently, it is important to be able to properly parameterize double diffusive mixing in basin scale and global ocean models, so that the water mass structure in the interior of the ocean can be properly simulated. This note describes a model for double diffusive mixing in the presence of background shear, based on Mellor–Yamada type second moment closure, more specifically Kantha, 2003, Kantha and Clayson, 2004 second moment closure models of resulting turbulence, following Canuto et al. (2008a) but employing a different strategy for modeling the pertinent terms in the second moment equations. The resulting model is suitable for inclusion in ocean general circulation models.     

Geotechnical properties of stabilised Indian red earth

Locally available soils amended with sufficient bentonite are generally used for construction of liners for water and waste retention facilities. The amount of bentonite required to keep the hydraulic conductivity low varies with the nature of the local soil. Many studies have shown that bentonite content higher than 20% by weight is not usually required. This is also the case with Indian red earth containing predominantly quartz and kaolinite minerals. Incorporating bentonite, though keeps the hydraulic conductivity of soil low, increases the swelling and shrinkage potential; increases the loss of strength due to reduction in cohesion. This paper aims to improve the geotechnical properties of red earth and bentonite mixture with lime or cement. The studies reveal that the geotechnical properties of red earth with 20% by weight bentonite stabilised with 1% by weight of lime or cement are greatly enhanced, particularly after curing for 28 days. it has been shown that the early gain in strength is better with cement whereas its long-term strength gain is better with lime.     

阿尔泰南缘克朗盆地铁木尔特花岗岩体年龄、地球化学特征及成因

铁木尔特黑云母花岗岩出露于阿尔泰造山带南缘克朗盆地北西侧,岩体主要为黑云母花岗岩。锆石的SHRIMPU-Pb定年结果表明该岩体结晶年龄为459±4.9Ma。岩石具有高的SiO_2(71.92%～76.58%)和Al_2O_3(12.92%～14.55%)含量,富碱(K_2O+Na_2O=7.09%～8.11%)、高钾(K_2O/Na_2O=1.1～1.62)、低P_2O_5(0.14%)和MgO+FeO(5%)含量,强过铝质(ACNK=1.13～1.34);富集LREE和Th、U、Pb,贫Ba、P、Sr、Ti、Nb,呈现明显的Eu负异常(δEu=0.33～0.53);并具有低的Al_2O_3/TiO_2(100)比值和高的CaO/Na_2O(0.3)比值。以上特征表明该岩体属高钾钙碱性高温型强过铝花岗岩。同时岩石具有低的Sr初始值(~(87)Sr/~(86)Sr=0.701727～0.709951)和相对高的ε_(Nd)(459Ma)(-1.43～-0.98)值,两阶段Nd模式年龄较为均一(t_(2DM)=1.2～1.3Ga)。岩体的原始岩浆可能是富含白云母和黑云母的变泥质岩在大于875℃的条件下脱水熔融的产物,在部分熔融过程中有磷灰石、斜长石、钛铁矿等矿物的残留,并混入少量的幔源物质。结合阿尔泰南缘的区域地质背景综合分析,推测岩体形成于俯冲作用的初期,可能是在挤压背景下由于局部伸展减压以及异常地幔和深部热流不均匀上升提供的热影响发生的部分熔融产生。     

Tides—a modern perspective

Progress in tidal science has been rapid in recent years. The advent of precision altimetry has enabled, for the very first time in tidal history, an accurate measurement of tides in most of the global oceans. This has revolutionized our knowledge of tides and tidal processes. Combined with high‐resolution numerical models of tides (and other recent advances in astronomy and geodesy), this increased knowledge is providing valuable assistance in effecting closure on many outstanding problems in this three‐centuries‐old science. For example, we now know the dissipation rate of lunar tides to be 3.17 TW to within 2%. However, there do remain some outstanding issues. While we know the rate at which tidal energy is being dissipated in the global oceans, there is still considerable uncertainty as to the mechanisms, locations, and magnitudes of various tidal energy sinks. Imminent advances in shallow‐water barotropic and deep‐water baroclinic tides hold the prospect of a better understanding of these also. Improved knowledge of oceanic tides and high‐precision satellite measurements of tides are enabling better assessment of some matters of geophysical interest, such as the anelasticity and the length‐of‐day fluctuations of the Earth's mantle. It has been possible to map long‐period lunar tides more accurately and derive their contribution to the Earth's rotation rate fluctuations and its anelasticity at these frequencies. We discuss various aspects related to tides, including tidal dissipation and its consequences, as well as several other topics such as tidal energetics, internal tides, and long‐period tides, where considerable progress has been made in the last decade. Both oceanographic and geophysical implications are mentioned.     

Drift currents induced by reflection of propagating inertial and internal waves at rigid boundary

Turbulent fluctuations in active mixed layers can excite internal waves in stably stratified fluid regions adjoining them. Expressions are derived for the energy and momentum fluxes radiated away by internal waves from an oceanic mixed layer, in terms of the spectrum of the static pressure fluctuations imposed at the base of the mixed layer by the turbulent eddies. The role of these internal wave fluxes in questions such as the determination of the rate of deepening of the layer due to an applied surface stress and the origin of internal waves in the deep ocean is discussed.     

On leakage of energy from turbulence to internal waves in the oceanic mixed layer

In this paper, we address the question of energy leakage from turbulence to internal waves (IWs) in the oceanic mixed layer (OML). If this leakage is substantial, then not only does this have profound implications as far as the dynamics of the OML is concerned, but it also means that the equation for the turbulence kinetic energy (TKE) used in OML models must include an appropriate sink term, and traditional models must be modified accordingly. Through comparison with the experimental data on grid-generated turbulence in a stably stratified fluid, we show that a conventional two-equation turbulence model without any IW sink term can explain these observations quite well, provided that the fluctuating motions that persist long after the decay of grid-generated turbulence are interpreted as being due to IW motions generated by the initial passage of the grid through the stably stratified fluid and not during turbulence decay. We conclude that there is no need to postulate an IW sink term in the TKE equation, and conventional models suffice to model mixing in the OML.