Tidal data inversion: interpolation and inference

Initial efforts in applying inverse methods to studies of ocean tides have focused on making the best use of a small number of observations to map tidal fields in a large area. As such, inversion can be viewed as an objective analysis scheme which uses a dynamically appropriate spatial covariance, derived from the shallow water equations, to interpolate and smooth a sparse data set. Data from recent altimetry missions are not sparsely distributed relative to tidal wavelengths in the open ocean, apparently reducing the need for complicated dynamically based interpolation schemes. Altimetric data sets are also quite large, making application of rigorous inversion methods to global tidal modeling a challenging computational problem. We describe here a new iterative solution scheme which allows us for the first time to fit the full set of TOPEX/Poseidon cross-over differences. The resulting solution (TPXO.3) fits validation tide gauges significantly better than previous inverse solutions. TPXO.3 also reduces residual cross-over variances relative to other recent inverse and empirical solutions, particularly in shallow water where improvements are dramatic. With the new solution approach very significant improvements in global tidal models should be possible in shallow areas and in the vicinity of complex bathymetry, where high-accuracy tidal modeling remains a challenging problem. With the recent improvements in the definition of tidal elevations in the open ocean it should now also be possible to resolve some long unanswered questions about tidal energetics and dynamics. Inverse methods provide a natural framework for addressing these issues, and making inferences about tidal dynamics. In particular, by bringing data and dynamics together in a single solution, we can rigorously test the consistency of the two. We present results of global and local inversions which suggest that over elongated bathymetric features oriented perpendicular to tidal flows, energy dissipation in the open ocean is significantly enhanced, presumably due to conversion of barotropic tidal motions into baroclinic modes. For the M₂ tide our preliminary results suggest that perhaps as much as 0.5 TW of energy is dissipated in this manner. However, due to the simplified linear dynamics and limited spatial resolution used for our inversion, there are significant uncertainties associated with these results. A more careful application of inverse methods to make more rigorous inferences about tidal energetics, including use of more reasonable prior dynamics, and the highest possible spatial resolution, should allow for closure of the tidal energy budget within the next few years.     

Developments in terrain-following ocean models: intercomparisons of numerical aspects

During the course of developing new numerical algorithms for a terrain-following ocean modeling system (TOMS), different numerical aspects have been evaluated through a comparison between two widely used community ocean models, the Princeton ocean model (POM) and the regional ocean modeling system (ROMS). While both models aim at modeling coastal to basin-scale problems using similar grids, their numerical algorithms, code structure, and parameterization options are very different. Sensitivity studies with an idealized channel flow and a steep seamount configuration demonstrate how different algorithms in the two models may affect numerical errors, the stability of the code and the computational efficiency. For example, new pressure gradient schemes using polynomial fits and new time stepping algorithms may reduce numerical errors and allow using longer time steps than standard schemes do. However, the new schemes may require more careful choices of time steps and the use of higher order advection schemes to maintain numerical stability.     

The Stability of Tide Gauges in the South Pacific Determined from Multiepoch Geodetic Levelling, 1992 to 2010

Tide gauge data is important for determining global or local sea level rise with respect to a global geocentric reference frame. Data from repeated precise levelling connections between the tide gauges and a series of coastal and inland benchmarks, including Continuous GPS (CGPS) benchmarks, are used to determine the stability of tide gauges at 12 locations in the South Pacific. The method for determining this stability is based on a constant velocity model which minimises the net movement amongst a set of datum benchmarks surveyed since the installation of the tide gauges. When assessed at a 95% confidence interval, and with the exception of the Solomon Islands, none of the tide gauges were found to be in motion relative to their CGPS benchmarks. The Solomon Islands estimate is considered to be unreliable since the CGPS benchmark was recently established and has been surveyed fewer than three times. In Tonga and Cook Islands, the tide gauges were found to be disturbed or affected by survey errors whereas the Vanuatu results were affected by earthquakes.     

Assessing the Validity of Seismo-Acoustic Predictor Equations for Obtaining Seabed and Sub-Surface Sediment Physical Properties

The interdependence between the seismo-acoustic properties of a marine sediment and its geotechnical/physical parameters has been known for many years, and it has been postulated that this should allow the extraction of geotechnical information from seismic data. Though in the literature many correlations have been published for the surficial layer, there is a lack of information for greater sediment depths. In this article, a desktop study on a synthetic seafloor model illustrates how the application of published near-surface prediction equations to subsurface sediments (up to several tens of meters burial depth) can lead to spurious predictions. To test this further, acoustic and geotechnical properties were measured on a number of sediment core samples, some of which were subjected to loading in acoustically-equipped consolidation cells (oedometers) to simulate greater burial depth conditions. For low effective pressures (representing small burial depths extending to around 10 meters subsurface), the general applicability of established relationships was confirmed: the prediction of porosity, bulk density, and mean grain size from acoustic velocity and impedance appears generally possible for the investigated sedimentary environments. As effective pressure increases through, the observed relationships deviate more and more from the established ones for the near-surface area. For the samples tested in this study, in some instances increasing pressure even resulted in decreasing velocities. There are several possible explanations for this abnormal behavior, including the presence of gas, overconsolidation, or bimodal grain size distribution. The results indicate that an appropriate depth correction must be introduced into the published prediction equations in order to obtain reliable estimates of physical sediment properties for greater subsurface depths.     

A microcosm of global trends

In a foreshortened era when the international community has undergone a number of changes and the word ‘global’ has come to mean not a limitless expanse but rather an all-too-confined and finite orb, UNCLOS has set several benchmarks in multilateral diplomacy. In this light, its accomplishments are truly remarkable.     

The Precautionary approach to fisheries management: How this is taken into account by Tuna regional fisheries management organisations (RFMOs)

For several decades it has been acknowledged that there is an urgent need for new approaches to fisheries management, embracing conservation and environmental considerations. The voluntary Code of Conduct on Responsible Fishing and the United Nations Fish Stocks Agreement provide the formal basis for the Precautionary Approach to fisheries management. Some tuna Regional Fisheries Management Organisations such as the WCPFC and IATTC make explicit mention of these codes in their conventions, whilst others, whose conventions do not explicitly address the Precautionary Approach, are searching for ways in which to take these codes into consideration. In practical terms, the scientific obligations to Precautionary Approaches are to determine the status of the stock(s) relative to limit and target reference points, to predict outcomes of management alternatives for reaching the targets and avoiding the limits, and to characterise the uncertainty in both cases. A convenient framework to conduct management evaluations is through the use of harvest control rules, for which managers agree on specific management actions under their control which are evoked according to levels of stock status relative to predefined reference points. These pre-agreed management actions are then simulated for a range of scenarios. This paper presents the ways in which tuna RFMOs are currently incorporating the precautionary approach in their fisheries management as well as suggestions for possible best practice.     

Assessing the Validity of Seismo-Acoustic Predictor Equations for Obtaining Seabed and Sub-Surface Sediment Physical Properties

The interdependence between the seismo-acoustic properties of a marine sediment and its geotechnical/physical parameters has been known for many years, and it has been postulated that this should allow the extraction of geotechnical information from seismic data. Though in the literature many correlations have been published for the surficial layer, there is a lack of information for greater sediment depths. In this article, a desktop study on a synthetic seafloor model illustrates how the application of published near-surface prediction equations to subsurface sediments (up to several tens of meters burial depth) can lead to spurious predictions. To test this further, acoustic and geotechnical properties were measured on a number of sediment core samples, some of which were subjected to loading in acoustically-equipped consolidation cells (oedometers) to simulate greater burial depth conditions. For low effective pressures (representing small burial depths extending to around 10 meters subsurface), the general applicability of established relationships was confirmed: the prediction of porosity, bulk density, and mean grain size from acoustic velocity and impedance appears generally possible for the investigated sedimentary environments. As effective pressure increases through, the observed relationships deviate more and more from the established ones for the near-surface area. For the samples tested in this study, in some instances increasing pressure even resulted in decreasing velocities. There are several possible explanations for this abnormal behavior, including the presence of gas, overconsolidation, or bimodal grain size distribution. The results indicate that an appropriate depth correction must be introduced into the published prediction equations in order to obtain reliable estimates of physical sediment properties for greater subsurface depths.     

Analysis of the differential-difference schemes of equations for the barotropic ocean

Difference schemes constructed for the problems using a barotropic approximation (Poincaré waves, Kelvin waves, Rossby waves, quasi-geostrophic normal modes, forced free oscillations) allow one to obtain exact solutions for a set of discrete equations. It is shown that the better approximation is achieved on thef-plane, using the grid different from gridsB andC. Translated by Vladimir A. Puchkin.     

基于多源信息融合的浅海超低频声源目标探测关键技术及实现

以多传感器信息融合理论为指导,结合现代信息处理技术与数据驱动建模及科学计算技术,研究浅海超低频声源目标激发共存地震波的复合声场中超低频声波传播特性和数据驱动建模的水下目标深度识别等关键技术,并论证了技术实现方案及路径。结果表明:以复合矢量水听器、地震波监测仪等多传感器信息融合理论为指导研究前海超低频声源目标可以克服传统声场建模存在的问题,有助于浅海超低频声源目标探测及改善海洋水下声学监测手段。在提高声呐探测设备的测量准确度、精度方面具有重要的理论意义,对周边海域为浅海的我国海防具有实战价值。     

Analytical solutions for storm tide codes

A sequence of analytical solutions explore aspects of response patterns expected from numerical codes for storm tides in one- and two-dimensional basins. Complete analytical details of the solutions are provided, together with specific suggestions for an associated set of analytical benchmark tests. Illustrations of predicted response patterns provide the basis for a discussion of many significant physical aspects and their representation in discrete numerical codes.     

Model of palaeoenvironmental evolution of the Black Sea region during the last glacial maximum—Holocene

A model for the palaeoenvironmental evolution of the Black Sea and its adjacent basins during the past 20.000 years, in which variations in sedimentation, erosion, and hydrologic processes as a result of climatic change are taken into consideration, is developed. The data used include those from five cruises in the Black Sea with the participation of the author, seismo-acoustic data in the possession of the Institute of Oceanology in Varna, and data from the published literature. The most important result is that the water level of the Black Sea is controlled largely by that of the Marmara Sea via the Bosporus sill. The water circulation in the south part of the Bosporus channel as natural regulative mechanism of the Black Sea level during 11800–9000 yr C¹⁴ BP is produced. A succession of climatic and water conditions for the Black Sea by pollen analysis is presented. The linear ridges on the Black Sea shelf as result of the lower Holocene regression are proved by means of a stratigraphic interpretation of the seismo-acoustic profiles. The levels of the Black Sea, Marmara Sea, and Mediterranean at regarded intervals of time are presented. An attempt at explanation of maximum number of facts from the study region with this model is made.     

模糊多属性决策模型在海洋平台选型中的应用

将模糊多属性决策理论应用于海洋平台设计选型中,根据多个属性的估计值,结合决策者的偏好,通过综合评价,在若干设计方案中选出最优方案.考虑到在概念设计阶段对于属性特征值及属性权重值的估计带有一定的模糊不确定性,将属性特征值和属性权重值表示为模糊数的形式,并且根据模糊集理论的表现定理得出各个设计方案的决策值.根据模糊数排序理论对设计方案进行排序并得到最优设计方案.总之,该研究为海洋平台设计方案的选型决策提高了一种新方法.     

The potential benefits to the basin modeling enterprise of modularization and incorporating organic and inorganic chemical change

Mineral and hydrocarbon resources in basins are produced by fluid movements whose nature is not fully understood. Flow models in realistically defined basins that predict chemical change are needed to make progress, but these models will be difficult to develop and keep vital. Two simple examples are given to illustrate what is required, and the challenges to basin modeling are then outlined. It is argued that the best strategy is to modularize existing physical and chemical codes and place as many of these modules as possible in the public domain. This will keep the modeling enterprise open to the new entry and continuing invigoration.     

Experimentation with various difference schemes for numerical solution of the equation for wind wave spectral density transport

Some test problems are presented designed for qualitative evaluation of the accuracy of the numerical solution for the transport equation. The tests are conducted for three types of difference schemes. The accuracy of numerical solutions for specific evolution cases is evaluated and recommendations are given on the application of the schemes. Translated by V. Puchkin.     

Distribution of free gas in marine sediments: a global overview

To quantify the global distribution of free gas in shallow marine sediments, we have identified and indexed over 100 documented cases in the scientific and engineering literature. Our survey confirms previous assumptions, primarily that gas bubbles are ubiquitous in the organic-rich muds of coastal waters and shallow adjacent seas. Acoustic turbidity, as recorded during seismo-acoustic surveys, is the most frequently cited evidence used to infer the presence of seafloor gas. Biogenic methane predominates within these shallow subbottom deposits. Because of their global prevalence, growing interest in gassy marine sediments is understandable, as their presence can have profound scientific, engineering and environmental significance.     

Wave dispersion study for tsunami propagation in the Sea of Marmara

In this paper the aim is to investigate whether there are differences between the dispersion and non-dispersion solutions on tsunami propagation. For this purpose, two numerical models of tsunami propagation are compared. One of these numerical models is a nondispersive model that uses Saint Venant equations and the other is a dispersive model that uses Boussinesq equations. The tsunamis resulting from a submarine mass failure (SMF) which is settled at the bottom of the north eastern Sea of Marmara are examined. An analytical solution considering wave dispersion is developed for obtaining near-field tsunami amplitudes above the submarine mass failure. Numerical modeling is used at the sea surface from the common boundary called as liquid boundary with incident waves up to the coastal regions to get the tsunami amplitudes. The output of the analytical model is taken as the disturbances for the numerical method. In the numerical solutions TELEMAC-2D software system is used for both dispersive and nondispersive modeling. The results of the dispersive and nondispersive models are compared to each other. Both temporal and spatial differences in the amplitudes and wave shapes are examined. The obtained results demonstrate that there are no noticeable differences between the dispersion and non-dispersion solutions except some special cases and some special landslide velocities.     

Evaluation of various vertical mixing parameterizations in a tropical Pacific Ocean GCM

Coupled general circulation models (GCMs) have had weak El Niño/Southern Oscillation variability that has been attributed to a diffuse thermocline in the modeled equatorial Pacific Ocean. Consequently, there have been many attempts to improve the thermocline by developing new or improved ocean vertical mixing schemes. This paper investigates the influence of gradient Richardson Number-based vertical mixing scheme profiles in a tropical Pacific Ocean GCM. It has been common for vertical mixing schemes to be assessed in tropical Pacific Ocean models that have a limited latitudinal domain bounded by zonal walls with sponge layers. However, recent work has shown that warm surface water can accumulate in these models and stop them from achieving the observed sharp equatorial thermocline. The present model employs a parameterized wall heat transport scheme that prevents warm surface water from accumulating. Thus we are able assess the influence of vertical mixing profiles in an ocean model that does not allow warm surface water to accumulate and influence the thermocline.In this paper we evaluate the equatorial performance of three different Richardson number (Ri)-based vertical mixing profiles: an integer power (IP) profile based on the observations of Peters, Gregg and Toole; a form of the Pacanowski and Philander profile modified to have low background mixing; and the Max Planck Institute profile. With the accumulation of warm surface water prevented, each of these profiles is able to achieve a sharp thermocline. When compared with observations, the IP profile achieves a better upwelling velocity distribution. We also examine the influence on equatorial performance of very high mixing coefficients at low Richardson number, and of low background mixing coefficients.     

Transfer matrix of linear water wave scattering over a stepwise bottom

For a two-dimensional linear wave scattering problem, a new transfer matrix incorporating evanescent modes is developed, from which the solution of a scattered wave field over a stepwise topography is directly obtained. The present method is shown to be capable of solving practical scattering problems with complicated bedforms, for which application of the conventional methods has been fairly limited. Highly accurate numerical solutions for sufficiently large systems are presented, and the computational efficiency of the present method is demonstrated. The interaction of bars in periodic beds is examined, and changes in the reflection coefficient with increasing number of bars are illustrated.     

Stability analysis of high-order finite-difference discretizations of the linearized forward-speed seakeeping problem

A high-order finite-difference method solution of the linearized, potential flow, seakeeping problem for a ship at steady forward speed was recently presented by Amini-Afshar et al. [1,2]. In this paper, we provide a detailed matrix-based eigenvalue stability analysis of this model, highlighting the sources of instability and the effects of possible remedies. In particular, we illustrate how both boundary treatment and grid stretching are important factors which are not typically captured by a von Neumann-type analysis. The new analysis shows that when grid stretching is used together with centered finite difference schemes, the method is generally unstable. The source of the instability can in some cases be traced to an effective downwinding of the convective terms. Stable solutions can be obtained either by introducing upwind-biased schemes for computing the convective derivatives on the free-surface, or by application of a mild filter at each time-step. A second source of instability is associated with the treatment of the convective derivatives of the free-surface elevation at points close to the domain boundaries. Here it is necessary to consider whether the surrounding fluid points lie in an upwind or a downwind direction. For upwinded points, ordinary one-sided differencing can be used, but for downwinded points we instead impose a Neumann-type boundary condition derived from the body and free-surface boundary conditions. As an example application to complement those already given in [1], [2], the method is applied to solve the steady wave resistance problem and comparison is made to reference solutions for a two-dimensional floating cylinder and a submerged sphere. Estimates of the wave resistance of the Wigley hull are also compared with experimental measurements.