Stability of planetary waves in a two-layer system

A stability of planetary waves on an infinite beta-plane is investigated in an idealized two-layer fluid system for the large local Rossby numberM. When a primary wave is barotropic, two kinds of barotropic instability modes are found. One of them was previously discussed byGill (1974). When a primary wave is baroclinic, two different kinds of modes that enable barotropic and baroclinic energy transfers are found. The one that has the larger growth rate gains its energy mainly from the mean shear of the primary wave when the internal rotational Froude numberF is smaller than 1/2. WhenF is larger than 1/2, however, the available energy conversion of the primary wave is dominant. This mode has a fairly large part of its energy in the barotropic motion although the primary wave is purely baroclinic.The effect ofO(M ^–2) corrections is found to have a stabilizing influence on all symmetrical modes. The geophysical applications of the present analysis are suggested in the context.     

A model for Meddy formation

A particular feature of “Meddies” is that the water within them has stable gradients of salinity and temperature. In this paper a model for a dense turbulent plume falling in a stationary, linearly-stratified environment is described. The density of the plume is assumed to be a linear function of two components, while the ambient density gradient is due to a gradient in only one of these components (labelled component one). The ratio of the contributions to the source buoyancy flux of the two components within the plume is identified as the important parameter for this type of flow. After deriving the steady equations for the plume, a simplified model for the filling process which creates the Meddy is described, which shows how a layer of fluid may be formed with stable gradients in both components in the lower part of the intrusion. The intrusion is shown to be less stratified than the surrounding fluid, with relatively weak gradients in component one (identified with temperature in the Meddy) that vary from unstable at the top to stable at the bottom of the intrusion and a fairly constant (stable) gradient in the second component (salinity for a Meddy). If the stable gradients in a Meddy are due solely to the filling process then, during formation, the top of the Meddy must become cooler than the surrounding water at the same level. The observed stable temperature gradient in Meddies is likely to be due to mixing processes after their formation, whereas the stable salanity gradient is mainly due to the filling process during formation.     

Seasonal variability of the Mindanao Current determined using mooring observations from 2010 to 2014

A mooring was deployed east of Mindanao Island at 8°N, 127°3′E from December 2010 to August 2014 to collect direct measurements of the Mindanao Current (MC). The Acoustic Doppler Current Profiler (ADCP) fixed on the main float shows that the MC is a strong and stable southward flow with a standard deviation less than 21 cm/s in the upper 500 m. The core flows between depths of 50 and 100 m with a maximum mean speed of 78 cm/s at 100 m. The seasonal variability of MC varies interannually and is depth-dependent. Although it takes a double-peak structure in the upper 200 m with two maxima in April and June and one minimum in October, the MC velocity has its maximum during boreal summer (June) and a minimum in autumn (September) when a 100-day low-pass filter is applied to remove intraseasonal signals. The semiannual signals are mainly limited between 200 and 350 m. The Asian monsoon intensifies the wind-driven sea-surface height anomaly (SSHA) east of Mindanao Island, and the resulting sharp slope induces meridional flow with large variability. Rossby waves and the boundary effect weaken the contribution of wind, stabilizing the flow of MC. The MC is determined by the zonal gradient of the SSHA rather than the SSHA itself, suggesting a possible inconsistency in seasonality between the Mindanao Eddy (ME) and MC. The semiannual ME signal plays an important role in the seasonal variability of MC.     

Sloshing waves in electrified fluid layers in an excited rectangular container

This work aims to demonstrate the dynamical properties of the free interface separating two bounded layers of electrified Newtonian fluids inside a parametrically excited boxed basin. The mathematical model of the investigated problem is contacted with the linearized Navier–Stokes equation of viscous fluids and Maxwell equations together with the boundary conditions that are solved utilizing Laplace transform. Solutions of the governing equations in time domain are then numerically calculated by using Durbin's numerical inverse Laplace transform scheme. Graphical illustrations of the accomplished numerical results are provided to examine the influence of some selected parameters on the stability picture of the interfacial waves as well as the electric surface charge distribution. The numerical results proposed that the electric Euler number Eu, promotes the instability of the wave motion whatever the values of the frequency of oscillation and the destabilizing effect of Eu becomes weaker at larger values of the frequency; while a dual effect (in) stabilizing of the frequency of oscillations is reported whether the electric effects exist or not. This irregular influence of the frequency oscillation has also been monitored regarding to the forces exerted on the side wall of the container.     

浅化波浪层流边界层流速分布特性的数值分析

建立了同时考虑波致雷诺应力和时均水平压强梯度影响的二阶波浪边界层数学模型,模型计算得到的浅化波浪层流边界层内瞬时流速剖面、振荡速度幅值和时均流速剖面均与水槽实验数据吻合较好,在此基础上探讨了浅化波浪边界层流速分布特性及其影响机制。随着波浪的浅化变形,边界层内时均流速剖面"底部向岸、上部离岸"的变化特征越来越明显。这是二阶对流项引起的波致雷诺应力和离岸回流引起的时均水平压强梯度共同作用的结果,在床面附近由波致雷诺应力占主导作用并趋于引起向岸流动,在上部区域由时均水平压强梯度占主导作用并趋于引起离岸流动。     

Longitudinal dispersion due to the boundary layer in an oscillatory current: Theoretical analysis in the case of an instantaneous line source

The longitudinal dispersion effect due to the boundary layer formed by a tidal oscillatory current is examined theoretically. This analysis reveals the process whereby the dispersion coefficient becomes steady after the release of the diffusing substance. Though the dispersion due to an oscillatory current has so far been investigated mostly in the case of a linear velocity profile, the following result was found by taking account of the boundary layer in the oscillatory current. The depth-averaged dispersion coefficient for the case of a current having a boundary layer can be a few times larger than in the case of a linear velocity profile when the characteristic mixing time is long; the phase lead in the boundary layer induces nearly 20 percent of the longitudinal dispersion effect.     

琼东南盆地华光凹陷天然气水合物稳定带厚度的影响因素

为了探讨琼东南盆地华光凹陷海底天然气水合物稳定带的分布规律，定量研究了静水压力、底水温度、地温梯度和气源组分对水合物稳定带的影响程度。在此基础上，分析了华光凹陷现今甲烷水合物稳定带的厚度分布。最后，综合各因素的历史演化过程，初步探讨了华光凹陷1.05 Ma BP以来天然气水合物稳定带的演化。结果表明:(1)气源组分和海底温度的变化对研究区内水合物稳定带的影响较大；水合物稳定带厚度与海底温度呈良好的线性负相关性。(2)水深超过600 m的海域具备形成天然气水合物的温压条件；超过600 m水深的海域水合物稳定带厚度大部分超过 100 m，其中西北部稳定带的最大厚度超过300 m，是有利的水合物勘探区。(3)华光凹陷1.05 Ma BP以来天然气水合物稳定带厚度经历了快速增厚–窄幅变化–快速减薄和恢复的过程。麻坑群与水合物稳定变化敏感区在空间上具有较好的叠合关系。结合前人的研究成果，推测其形成与天然气水合物的分解释放有关。     

Application limit of Tide-induced residual current theory

We showed that, in all of the previous studies made of the horizontal circular “Tide-induced residual current (TIRC)”, the velocity field was far beyond the application limit of the TIRC theory and the discussions in previous studies had no physical reality, because in the circular TIRC, the pressure gradient force balances with the centrifugal force and a weak Coriolis' force. We also showed that the Euler numberAP, which is defined by the ratio of the advection term to the pressure gradient term, is useful as a measure of nonlinearity of the temporal tidal velocity field in a numerical model experiment, and that the measureAP must be less than 0.05 or at least less than 0.1 for us to discuss the TIRC as a physical entity of “steady current” in physical space. We proposed another measure,KE, which is defined as the ratio of the sum of kinetic energies of compound- and over-tides to kinetic energies of diurnal- and semi-diurnal tides. We showed that this measure,KE, is also useful as an indicator of the magnitude of nonlinearity of tidal velocity field. We presented the distribution ofKE in the Seto Inland Sea of Japan, showing that the nonlinearity is very large in many places of the basin, which must be considered when analyzing tidal current velocities and the dynamics of the current system or material transport in the Seto Inland Sea.     

振荡流层移输沙条件下悬沙层泥沙扩散系数垂向分布特征研究

层移输沙是海岸带泥沙运动的主要形式之一,其垂向悬沙浓度分布规律的研究一直是海岸工程关心的重点。一般情况下,经典的纯扩散模型被用来描述和解释悬沙浓度的试验数据,该模型认为周期平均悬沙浓度主要由参考浓度、泥沙沉速和泥沙扩散系数确定。泥沙扩散系数可以由泥沙沉速和悬沙浓度的垂向梯度反演得到。既往研究大多直接给出泥沙扩散系数的结果,对于不同反演计算方法间结果差别的研究较少。本研究汇总了已有振荡流层移输沙试验数据,采用曲线拟合方法和直接差分方法计算了相应的泥沙扩散系数,研究表明两种方法得到的计算结果在垂向位置z 0.15 m处差异不大,随着垂向位置的升高,差分方法的计算结果略微大于拟合方法。考虑到拟合方法可以得到连续的泥沙扩散系数垂向分布,本研究推荐使用幂函数形式的曲线拟合方法求解悬移泥沙扩散系数。基于此,对比分析了层移输沙悬沙层泥沙扩散系数随泥沙粒径、振荡流周期、均方根流速和振荡流类型等物理参数的变化规律。在纯振荡流层移输沙条件下,泥沙扩散系数随泥沙粒径的增大而增大,而振荡流周期和均方根流速几乎不影响泥沙扩散系数。在振荡流和定常流共同作用下,泥沙扩散系数受振荡流周期和定常流流速的影响,泥沙扩散系数随着振荡流周期的增大或定常流流速的减小而增大。     

Ocean-atmosphere momentum coupling in the Kuroshio Extension observed from space

Using a combination of Quick Scatterometer (QuikSCAT), Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E), and Lagrangian drifter measurements, we demonstrate that wind data alone are not sufficient to derive ocean surface stress (momentum flux) over mid-latitude ocean fronts, specifically the Kuroshio Extension. There was no continuous and large-scale stress measurement over ocean until the launch of the scatterometers. Stress had been derived from winds through a drag coefficient, and our concept of stress distribution may be largely influenced by our knowledge of wind distribution. QuikSCAT reveals that the variability of stress could be very different from wind. The spatial coherence between the magnitude of stress and sea surface temperature (SST), between the divergence of surface stress and the downwind SST gradient, and between the vorticity of stress and crosswind SST gradient, are the inherent characteristics of stress (turbulence production by buoyancy) that would exist even under a uniform wind field. The coherence between stress vorticity and SST gradient is masked by the rotation of ocean currents over the Kuroshio meanders. Surface stress rotates in the opposite direction to surface currents because stress is the vector difference between wind and current. The results are in agreement with a previous study of the Agulhas Extension and confirm the unique stress measuring capability of the scatterometer.     

Discrete-time quasi-sliding mode control of an autonomousunderwater vehicle

This paper presents a discrete-time quasi-sliding mode controller for an autonomous underwater vehicle (AUV) in the presence of parameter uncertainties and a long sampling interval. The AUV, named VORAM, is used as a model for the verification of the proposed control algorithm. Simulations of depth control and contouring control are performed for a numerical model of the AUV with full nonlinear equations of motion to verify the effectiveness of the proposed control schemes when the vehicle has a long sampling interval. By using the discrete-time quasi-sliding mode control law, experiments on depth control of the AUV are performed in a towing tank. The controller makes the system stable in the presence of system uncertainties and even external disturbances without any observer nor any predictor producing high rate estimates of vehicle states. As the sampling interval becomes large, the effectiveness of the proposed control law is more prominent when compared with the conventional sliding mode controller     

双涡互旋的数值模拟试验

本文应用准地转正压无辐散模式对双涡的相互作用进行了数值模拟试验。试验结果表明：（1）只考虑相对涡度平流的情况下双涡以互旋为主，而且随着初始间距的缩小，互旋越来越明显。双涡的相互作用与它们的初始间距以及它们的强度和结构有密切的关系。（2）双涡的移动加速和减速都是出现在它们移动方向改变的时刻，当移动方向向逆时针方向变化时移速减慢，否则加快。     

近海底高分辨率地震探测系统设计与实现

受大深度水体的影响,传统的海面拖曳式多道地震技术在进行深水地层探测时,目标地层深度处的菲涅耳半径非常大,水平分辨率低,难以满足海域天然气水合物高精度探测需求。针对海面拖曳式地震探测技术存在的上述问题,设计了一套可以在2000 m水深近海底作业的地震探测系统。应用耐压透声发射阵技术,克服了20 MPa外压环境和瞬时内压冲击对等离子体震源子波幅频特性的不利影响,研制的深拖等离子体震源的声源级达到214 dB,主频低于1000 Hz;水下控制中心采用集成SoC片上系统设计,可以对震源进行定距激发控制,进行近海底多道地震数据的连续采集。系统在2019年深海试验拖曳最大深度达2025 m,测试剖面数据显示最大地层穿透深度达380 m,纵向分辨率<2 m,横向分辨率<10 m,为深水海域沉积地层的深拖高分辨率地震探测提供了技术支撑。     

声跃层结构变化对深海汇聚区声传播的影响

根据射线理论建立了线性声速结构条件下的声跃层强度与深海汇聚区关系模型,用最小位移角讨论了海洋环境变化（如声跃层强度变化、声跃层位置变化及季节性跃层生消等）与汇聚区距离和宽度变化的相关性.结果表明,声跃层的结构变化对汇聚区特征影响很大.声跃层强度增大使汇聚区向远离声源的方向变化,跃层强度每增加0.01 s-1对应的汇聚区位移增大约为3.5～5.0 km.声跃层位置变化对汇聚区的影响小于声跃层强度,与两层结构的声速剖面相比,上行结构使汇聚区向靠近声源的方向变化,声跃层上升200 m对应的汇聚区位移减小约为1.0～1.5 km,声跃层越浅,汇聚区距离越近;下行结构使汇聚区向远离声源的方向变化,混合层加深200 m对应的汇聚区位移增大约为1.0～1.5 km,混合层越深,汇聚区距离越远.季节性跃层的生消使近表层有负梯度、零梯度和正梯度的变化.负梯度结构的变化规律与两层结构条件下的声跃层强度变化类似,但对汇聚区的影响程度相对较小;正梯度结构使汇聚区在近表层出现表面声道,梯度值的增强将使汇聚区向靠近声源的方向变化.     

Circulation and teleconnection mechanisms of Northeast Brazil droughts

The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March–April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October–January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient.     

Preliminary results about the stability of an intermediate water current

Experiments were run on a 14 m diameter rotating platform to study the stability conditions for a constant volume flow rate current of intermediate water. The flow was introduced in a two-layer system initially at rest in solid body rotation, along the sidewall of the tank, and allowed freely to evolve. A sink collected the intermediate water and thus ensured that the free surface height was constant. Thus the upstream conditions were the rotation rate, the volume flow rate, the density and the initial width of the intermediate current, which was in geostrophic equilibrium when it left the source; i.e. its thickness along the wall at the source was fixed by this condition. The relevant parameters appear to be the Ekman and the Burger numbers. The data collected from the experiments are very consistent, and it appears that there were five typical flow regimes: (1) a stable current along the whole basin; (2) a series of cyclonic vortices attached to the outer edge of the current, with an upstream stable current; (3) a large cyclonic vortex attached to an anticyclonic instability; (4) dipoles shed from the current into the interior fluid; and (5) generation of lenses of intermediate water, similar to meddies. This last result shows that no topographical effect is required to generate such long-lived lenses, which then drift slowly upstream as the dipoles do.     

海床CPT和井下CPT在砂土中贯入机理差异研究

海洋石油建设中的一个关键环节是对有关海域的工程地质条件作出准确评价。海上原位静力触探(CPT)是主要的原位勘察手段,与陆地CPT不同,其贯入方式主要有Seabed和Downhole两种形式。这两种CPT由于贯入方式不同,所得到的锥端阻力存在一定的差异。采用有限元方法对这两种形式的CPT锥端阻力和贯入模式进行计算分析。研究显示,CPT入土过程中,周围土体存在两种不同位移状态,即滑动状态和排挤状态。在入土初期,土体以滑动状态为主,土体中竖向应力随贯入深度的增加而递增。在入土一定深度以后,土体的排挤状态占主导地位,竖向应力变化趋于平稳。这两种位移状态的相互转化解释了Seabed CPT和Downhole CPT实测数据之间的差异。在贯入深度较大时,Seabed CPT受排挤状态控制,而Downhole CPT仍然受滑动状态的影响。     

A Method for Analyzing the Self-Supported Earth-Retaining Structure Using Stabilizing Piles

The self-supported earth-retaining structure using stabilizing piles (SSR is used from here) has the advantages of less deformation and less internal force compared with conventional cantilever retaining structure. It is easier to conduct the excavation when SSR is used for an excavation instead of using braced excavation with struts. The SSR is better than other methods to the 10 m shallow excavation depth in terms of economical and constructional efficiency when the ground is not very soft. However, this SSR method lacks a theoretical basis in terms of geotechnical engineering. The objective of this study is to develop a method of analysis by laboratory model tests. A variety of model tests were performed in order to analyze the behavior of SSR and the ground, and to measure the stress acting on stabilizing piles relative to excavation steps and earth pressures on the wall. The analysis reveals the failure mechanism of a wedge and then suggests a method for calculating a virtual supported point. These findings were incorporated into a method for analyzing retaining wall, stabilizing piles, and beams connecting two structures. Future research is geared toward developing a design program that uses the analytical methodology for this SSR.     

广东三水盆地晚自垩世—始新世古地磁研究及构造意义

对广东三水盆地上白垩统三水组、大　山组、古新统莘庄组及始新统　心组、宝月组的系统的古地磁研究结果表明：（１）旋转运动是该区构造运动的主要形式，顺时针旋转盆地拉张发育，这时针旋转盆地挤压衰亡，发育的程度与旋转角度有关；（２）始新世该区旋转方向由顺时针旋转变为道时针旋转，对应于太平洋－库拉板块对欧亚板块作用方向的改变，说明三水盆地的形成发育与太平洋－库拉板块对欧亚板块的作用密切相关；（３）三水盆地所在的华南地块从晚白垩世至始新世在纬向上不存在明显的运动，一直停留在略低于现今纬度的位置上。     

Longitudinal and depth variation of bacterioplankton productivity and related factors in a temperate estuary

Bacterioplankton productivity (BP) spatial variation was investigated in relation to potential resources, including primary productivity and dissolved organic matter, in the micro-tidal Neuse River–Pamlico Sound estuarine system, North Carolina, USA. Estuarine BP was predicted to correlate with the trophic gradient, decreasing along the salinity gradient in parallel with the decrease in organic matter and primary productivity. This prediction was tested over four years at spatial scales ranging from kilometers to meters along the riverine axis and with depth. The general pattern of BP across the salinity gradient was unimodal and matched the phytoplankton patterns in peak location and variability. Peak locations varied with discharge, especially in 2003 when above average discharge moved peaks downstream. Spatial coherence of BP with other variables was much less at short time scales. The effect of temperature, nutrients, and phytoplankton on BP varied by location, especially fresh versus brackish stations, although only temperature explained more than 20% of the BP variation. Depth variation of BP was as great as longitudinal variation and bottom samples were often higher than surface. BP was strongly correlated with particulate organic carbon at the pycnocline and bottom, highlighting the importance of particulate matter as a resource. Station-averaged BP and phytoplankton data corresponded well with two published meta-analyses, although the offset of the freshwater station suggested longitudinal differences in community composition or resource availability.