The “slip law” of the free surface

A “slip law” connects the excess velocity or “slip” of a wind-blown water surface, relative to the motion in the middle of the mixed layer, to the wind stress, the wind-wave field, and buoyancy flux. An inner layer-outer layer model of the turbulent shear flow in the mixed layer is appropriate, as for a turbulent boundary layer or Ekman layer over a solid surface, allowing, however, for turbulent kinetic energy transfer from the air-side via breaking waves, and for Stokes drift. Asymptotic matching of the velocity distributions in inner and outer portions of the mixed layer yields a slip law of logarithmic form, akin to the drag law of a turbulent boundary layer. The dominant independent variable is the ratio of water-side roughness length to mixed layer depth or turbulent Ekman depth. Convection due to surface cooling is also an important influence, reducing surface slip. Water-side roughness length is a wind-wave property, varying with wind speed similarly to air-side roughness. Slip velocity is typically 20 times water-side friction velocity or 3% of wind speed, varying within a range of about 2 to 4.5%. A linearized model of turbulent kinetic energy distribution shows much higher values near the surface than in a wall layer. Nondimensional dissipation peaks at a value of about eight, a short distance below the surface.     

2004年9月南海北部移动船载温盐剖面仪观测结果初步分析

在2004年9月南海北部开放航次中,中国科学院南海海洋研究所首次引进使用了民用移动船载温盐剖面仪（Moving Vessel Profiler,MVP）,并在珠江口外海和越南东岸外海2个断面进行了拖曳测量。通过对MVP和定点温盐深仪所测数据的比较分析,发现MVP下降阶段数据较为可信。对所获温、盐、深数据进行处理分析,发现2个断面的水团性质存在明显差异。越南东岸外海断面的混合层平均深度（27m左右）深于珠江口外海断面（17．5m左右）;在表层和近40m深度处,珠江口外海的盐度值都高于越南东岸外海,呈现高盐的特性。     

1998年5-6月南海上混合层、温跃层 不同定义的比较

本文根据混合层和温跃层的两种不同定义，对1998年南海南北部和西太平洋混合层和温跃层的定点观测资料进行了计算和分析，比较了不同定义确定的混层，温跃层的共同点和差异。在此基础上，得出南海和西太平洋的障碍层厚度。     

0220号热带风暴(米克拉)特征分析

南海是热带气旋活动较为频繁的地区之一。该地区因其独特的地理环境和复杂的热带大气环流系统，在热带气旋预报工作上存在一定的困难。本文在分析一些气象图表资料的基础上，对2002年第20号热带风暴(米克拉)的形成、发展及消亡进行了探讨和研究。主要从形成的背景环流，气旋的结构等方面入手，总结出一些该气旋所具备韵特有流场，从而为以后的南海自生热带气旋预报工作积累一定经验。     

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.     

Patterns and controls of surface sediment distribution: west-central Florida inner shelf

The west-central Florida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf. Surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large-scale sediment redistribution. The west Florida shelf is the submerged extension of the Florida carbonate platform, consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover. A total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition. Results show that sediment consists of a combination of fine quartz sand and coarse, biogenic carbonate sand and gravel, with variable but subordinate amounts of black, phosphorite-rich sand. The carbonate component consists primarily of molluskan fragments. The distribution is patchy and discontinuous with no discernible pattern, and the transition between sediment types is generally abrupt. Quartz-rich sediment dominates the inner 15 km north of the entrance into Tampa Bay, but south of the Bay is common only along the inner 3 km. Elsewhere, carbonate-rich sediment is the predominate sediment type, except where there is little sediment cover, in which cases black, phosphorite-rich sand dominates. Sediment sources are likely within, or around the periphery of the basin. Fine quartz sand is likely reworked from coastal units deposited during Pleistocene sea-level high stands. Carbonate sand and gravel is produced by marine organisms within the depositional basin. The black, phosphorite-rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area. The distribution pattern contains elements of both storm- and tide-dominated siliciclastic shelves, but it is dictated primarily by the sediment source, similar to some carbonate systems. Other systems with similar sediment attributes include cool-water carbonate, sediment-starved, and mixed carbonate/siliciclastic systems. This study suggests a possible genetic link among the three systems.     

基于潜标观测的黑潮-亲潮混合区近惯性能量季节变化特征研究

近惯性内波广泛存在于全球海洋,是维持深层海洋跨等密度面湍流混合及海洋层结的重要能量来源。基于黑潮-亲潮混合区的多年深海潜标数据,分析了:(1)该海域近惯性内波及其能量的季节变化特征与影响因素,(2)上层和深层近惯性运动的频率、波数谱及垂向分布等特征。结果表明,该海域存在丰富的近惯性动能,无论海洋上层还是深层均呈现显著的冬季强、夏季弱的季节变化特征,冬季(12～2月)上层的近惯性能量可占全年能量的41%,深层近惯性信号同样显著,同潮汐信号相当。平板模型分析表明,该区域近惯性动能的季节循环特征主要受风场的季节变化所主导,同时受到黑潮延伸体流轴的摆动调制。     

Modelling the relationship between sexual reproduction and rhizome growth in Posidonia oceanica (L.) Delile

The relationship between flowering and growth performance of Posidonia oceanica (L.) Delile in meadows distributed along the south‐eastern coast of Sicily (Italy) was investigated by means of a statistical model (generalized linear mixed model) combined with the lepidochronological analysis. Over a 28‐year period, 67 floral stalk remains were observed. The highest flowering index was recorded in lepidochronological year 1998 (10.1%) and the Inflorescence Frequency per age showed a clear decrease corresponding to 15‐year‐old shoots. The sexual reproductive event had positive effects on rhizome elongation (cm year^?1) and leaf production (no. leaves year^?1) in the same flowering year, whilst no effect on the rhizome production (mg year^?1) was observed. Rhizome growth variables showed significant negative lagged responses in the two years following flowering. On the whole, we calculated that the effect exerted by flowering, in terms of loss on rhizome elongation and production, was about 27% and 38%, respectively. Although it has been demonstrated that recovery from the stress induced by sexual reproduction is limited to the two years after flowering, the magnitude of the reproductive cost may become quite considerable especially in comparison with the whole lifespan of individual shoots.     

Characteristics of the boundary layer at the bottom of a solitary wave

The results of direct numerical simulations of the boundary layer generated at the bottom of a solitary wave are described. The numerical results, which agree with the laboratory measurements of Sumer et al. (2010) show that the flow regime in the boundary layer can be laminar, laminar with coherent vortices and turbulent. The average velocity and bottom shear stress are computed and the results obtained show that the logarithmic law can approximate the velocity profile only in a restricted range of the parameters and at particular phases of the wave cycle. Moreover, the maximum value of the bottom shear stress is found to depend on the dimensionless wave height only, while the minimum (negative) value depends also on the dimensionless boundary layer thickness. Diagrams and simple formulae are proposed to evaluate the minimum and maximum bottom shear stresses and their phase shift with respect to the wave crest.     

Surface gravity wave interaction with elastic bottom

In the present paper, a hydroelastic model is developed to deal with surface gravity wave interaction with an elastic bed based on the small amplitude water wave theory and plate deflection in finite water depth. The elastic bottom bed is modelled as a thin elastic plate and is based on the Euler-Bernoulli beam equation. The wave characteristics in the presence of the elastic bed is analyzed in both the cases of deep and shallow water waves. Further, the linearized long wave equation is generalized to include bottom flexibility. A generalized expansion formula for the velocity potential is derived to deal with the boundary value problems associated with surface gravity waves having an elastic bed. The utility of the expansion formula is illustrated by demonstrating specific physical problems which will play significant role in the analysis of wave structure interaction problems. Behavior of the wave spectra are discussed in the case of closed basin having a free surface and an elastic bottom topography.