On an inverse problem of ship-induced internal waves

An inverse problem for trapped internal waves is considered in an attempt to provide a practical tool for estimating the density stratification in the sea from the wake pattern behind a moving vessel. The ambient stratification is represented by Barber's (1993) exponential series and the coefficients are found by matching the first mode dispersion relation to the one found from the wake data. A fast algorithm for calculating the dispersion relation is derived. It is shown that when the series converges with a low number of coefficients, the inverse profile is adequate, as happens for example for typical sea loch profiles. In more general circumstances the predicted maximum stratification still provides a reasonable approximation as a result of Barber's (1993) theorem.     

Assimilation of TOPEX/POSEIDON Altimeter Data with a Reduced Gravity Model of the Japan Sea

Sea level data measured by TOPEX/POSEIDON over the Japan Sea from 1993 to 1994 is analyzed by assimilation using an approximate Kalman filter with a 1.5 layer (reduced-gravity) shallow water model. The study aims to extract signals associated with the first baroclinic mode and to determine the extent of its significance. The assimilation dramatically improves the model south of the Polar Front where as much as 20 cm² of the observed sea level variance can be accounted for. In comparison, little variability in the northern cold water region is found consistent with the model dynamics, possibly due to significant differences in stratification.     

Effects of pressure ridge sheltering on the spatial and temporal variability of boundary layer stability under first year sea ice

We present series of data describing the small-scale structure of the boundary layer stratification in the vicinity and away from a 1.5 m pressure ridge keel. We then analyze the influence of pressure ridges on the spatial and temporal variability of the ocean boundary layer. Our experiment was done during the melt in order to use small-scale salinity gradients to study the variability in intensity of mixing processes. High-resolution salinity records sampled at 0+, 25 and 50 cm from the ice-water interface were analyzed at two stations, one at 5 m and the other at 75 m from a pressure ridge keel. We observed that, in the immediate vicinity of the ridge, melting is associated with the periodic development and dissipation of a melt layer in relation with the orientation of tidal currents relative to the ridge. In contrast, away from the region under the sheltering influence of the ridge keel, the boundary layer did not show any evidence of stratification during the whole experiment. Data recorded in the vicinity of the ridge are then used to compute eddy diffusivity coefficients for sheltered and non-sheltered events. The observation of a stable melt layer in the vicinity of a ridge keel confirms that pressure ridges drastically modify the dynamics of vertical exchanges in their wake. Besides, our data show a significant spatial and temporal eddy diffusivity variability with a factor 10 between sheltered and non-sheltered areas. Thus, considering that ridges and rubble fields form a substantial fraction of ice-covered areas, our results suggest that effects in their wake may be important in the global budget of ocean-ice exchanges.     

用TOPEX/POSEIDON高度计数据研究南极绕极流流域海面高度的低频变化

采用TOPEX/POSEIDON(T/P)卫星高度计 1 993年 1月— 2 0 0 0年 1 2月海面高度数据 ,研究包含了整个南极绕极流流系 (40°— 6 0°S)的海面高度低频变化。首先采用EOF分解方法获取南大洋时空分布的主要模态 ,前 3个EOF模态分别占总方差的 2 4 .8%、1 3 .8%和 1 0 .7%。然后采用EMD方法分别分析了各个EOF模态的时间系数曲线的组成成分 ,对南极绕极流海域的各种时间尺度变化给出了清晰的描述 ,对于不同尺度变化所占的比例得到了定量的结果。研究结果表明 ,EOF的各个模态不仅在形态上存在差异 ,而且具有相互独立的物理背景。EOF的第一模态主要体现了以太阳辐射冬夏差异形成的年周期变化 ,另一个显著的特征就是南极绕极流从 1 993— 2 0 0 0年海面的整体上升趋势。EOF的第二模态体现了陆地地形对南极绕极流的约束作用 ,同时也显示了ENSO过程对南极绕极流 ,特别是对南太平洋的海面高度变化的影响。EOF的第三模态则体现了南极绕极流对南大洋表面风场东西方向不均匀变化的响应。同时 ,本文的研究也证明了EOF与EMD方法联合使用对揭示大范围时空变化有重要的实际意义     

Sea Level Prediction in the Yellow Sea From Satellite Altimetry With a Combined Least Squares-Neural Network Approach

Accessible high-quality observation datasets and proper modeling process are critically required to accurately predict sea level rise in coastal areas. This study focuses on developing and validating a combined least squares-neural network approach applicable to the short-term prediction of sea level variations in the Yellow Sea, where the periodic terms and linear trend of sea level change are fitted and extrapolated using the least squares model, while the prediction of the residual terms is performed by several different types of artificial neural networks. The input and output data used are the sea level anomalies (SLA) time series in the Yellow Sea from 1993 to 2016 derived from ERS-1/2, Topex/Poseidon, Jason-1/2, and Envisat satellite altimetry missions. Tests of different neural network architectures and learning algorithms are performed to assess their applicability for predicting the residuals of SLA time series. Different neural networks satisfactorily provide reliable results and the root mean square errors of the predictions from the proposed combined approach are less than 2?cm and correlation coefficients between the observed and predicted SLA are up to 0.87. Results prove the reliability of the combined least squares-neural network approach on the short-term prediction of sea level variability close to the coast.     

Reconstruction of the Linear Distribution of the Squared Buoyancy Frequency According to the Known Dispersion Properties of the Field of Internal Waves

We propose a method for the solution of the inverse problem of reconstruction of the vertical stratification of density in the ocean according to the known dispersion curves for internal gravity waves. For the stratification of density modeled by a linear distribution, we determine the accuracy of its reconstruction for values of the frequency of oscillations and wave numbers given with different degrees of accuracy. The posed problem is studied in the Boussinesq approximation for two traditionally used types of boundary conditions on the surface of the fluid. We deduce dispersion equations and focus our attention on their asymptotic analysis. An asymptotic solution of the inverse problem is constructed and its sensitivity to the degree of accuracy of the input data is investigated.     

Padé coefficients for the solution of the wave dispersion equation under ice

The equations governing the propagation of linear gravity waves in ice-covered waters of finite depth is delineated for the linear elastic deformation of the ice plates that are modeled as elastically supported. The possible limiting condition for the validity of the assumptions involved in the formulation of the problem is discussed. A solution procedure for the solution of the wave dispersion equation under ice is discussed and a set of coefficients synthesized using the properties of infinite series and Padé approximants. Direct application of these coefficients for the calculation of wave characteristics in ice-covered sea will eliminate the need for iterative procedure, and hence will reduce the computational time. The derived coefficients were used for the computation of the wave characteristics of laboratory simulated waves and compared with the values obtained through iteration and the error was found to be less than 2%.     

Detection of ocean waves using satellite altimetry: Application to equatorial Kelvin waves

A new method for wave motion detection from satellite altimetric measurements of sea surface height is presented. The essence of the approach is to construct a two‐dimensional traveling‐wave Fourier series representation of the amplitude field within a prespecified oceanic region. The method employs an iterative, nonlinear least‐squares technique based on the Marquardt‐Levenberg algorithm to solve for model parameters describing characteristic features of the evolving wave system. The Marquardt‐Levenberg Fourier series (MLFS) algorithm was applied to Kelvin waves active during the 1986–1987 El Nino event in the equatorial Pacific ocean using GEOSAT Exact Repeat Mission altimetry data. Characteristics of the wave system were found to be in essential agreement with earlier field measurements and the observations of Cheney and Miller (1987) obtained using time series developed from GEOSAT data. The advantage of the present detection scheme lies in its speed and ability to determine a wave system's dispersion relation over a finite range of wavenumbers, and hence the group velocity of that system.     

两层粘性流体中圆柱体受迫振荡数值模拟

研究两层粘性流体中无限长水平圆柱体的受迫振荡问题。在湍流模式下,采用VOF方法追踪两层流体的内界面,基于动网格技术模拟圆柱体的运动边界,对均匀流中横向振荡圆柱体的绕流场进行了数值模拟。计算受迫振荡圆柱体的升力系数、阻力系数随时间的演化曲线和圆柱体的尾涡分布,以及圆柱体的受迫振荡激发两层流体内界面的扰动,并与均匀流体的情况进行了比较分析。研究表明,流体的两层分层效应对受迫振荡圆柱体的升阻力系数和尾涡分布特性都有显著影响,在水下输油气管道涡激振动特性的工程评估中,应考虑流体的密度分层效应。     

Peculiar dispersion of salt by wind-driven fluctuations in the stratified water near the coast

A peculiar dispersion of salt, which was found in a partially mixed estuary by long-term continuous measurements of current and salinity and directed outward in the upper layer and inward in the lower layer in summer, is reported and discussed. The cause is estimated to be salinity stratification and wind-driven fluctuations in water near the coast. The general formulation is presented and a great possibility of negative dispersion against the horizontal gradient is pointed out. Dispersion coefficients are also estimated.     

Dispersion relation of internal waves in the western equatorial Pacific Ocean

Based mainly on TOGA COARE data, that is, the CI''D data from R/V Xiangyanghong No.5 (Pu et al.,1993),the temperature and current data from the Woods Hole mooring and other deep current data, the layered numerical profiles of buoyancy frequency and mean current components are figured out.A numerical method calculating internal wave dispersion relation without background shear current, used by Fliegel and Hunkins (1975),is improved to be fit for the internal wave equation with mean currents and their second derivatives.The dispersion relations and wave functions of the long crested internal wave progressing in any direction can be calculated inveniently by using the improved method.A comparison between the calculated dispersion relation in the paper and the dispersion relation in GM spectral model of ocean internal waves (Garret and Munk, 1972) is performed.It shows that the mean currents are important to the dispersion relation of internal waves in the western equatorial Pacific Ocean and that the currents make the wave progressing co-directional with (against) the currents stretched (shrink).The influence of the mean currents on dispersion relation is much stronger than that of their second derivatives, but that on wave function is less than that of their second derivatives.The influences on wave functions result in the change of vertical wavenumber, that is, making the wave function stretch or shrink.There exists obvious turning depth but no significant critical layer absorption is found.     

近海海洋对热带风暴线性响应的底坡度和底摩擦效应

一个简单的二维线性模式用于考查近海海洋对过境热带风暴的动力响应过程中的底坡度和底摩擦效应。选择了自东向西和向西后转向的两个模式风暴路径。数值试验结果表明，海底坡度与风暴引起的水位波动、风暴中心后部的尾流以及路径右侧后方面的涡旋密切相关。而底摩察是阻尼风暴潮和风暴流持续增长的重要因子。结果还表明，近海对风暴的强响应偏向于风暴轨迹的右侧，这与Ｃｈａｎｇ和Ａｎｔｈｅｓ（１９７８）对深海响应的研究结论是一     

Stratification-induced hypoxia as a structuring factor of macrozoobenthos in the open Gulf of Finland (Baltic Sea)

Long-term (1965–2000) changes of macrozoobenthos and hydrography have been studied in the Gulf of Finland (GoF). For the first time, statistical multivariate time series analysis is applied to Baltic Sea data to verify the relationship between biota and interacting environmental factors causing large-scale hypoxia in the open sea. For macrozoobenthos, a consistent long-term development of the assemblages was found over the study area. In the period before the 1990s, very sparse macrozoobenthos prevailed, followed by a notable expansion of macrofauna between the late 1980s and early 1990s and leading to a maximum of total abundance and species number between 1991 and 1996. After that, a sudden collapse of the communities took place in 1996–1997. The hydrographical changes included a continuous decrease in salinity and density stratification until the early 1990s, after which an increase took place again. In contrast, low mean and minimum dissolved oxygen concentrations were observed at the beginning of the study period, followed by increasing values in the late 1980s and early 1990s, and a simultaneous decline of oxygen conditions in 1996. Based on non-linear trends estimated by dynamic factor analysis (DFA), high and significant correlations were found between total macrofauna abundance, number of species, salinity, oxygen conditions, strength of stratification and freshwater run-off. The results confirm that oxygen is obviously a fundamental factor that determines the state of the macrozoobenthos in the deep GoF, overruling other abiotic factors. However, the improvement of the oxygen conditions is apparently caused by the long-term decrease of salinity and loss of stratification in the relatively shallow GoF, reflecting large-scale changes in hydrography of the Baltic Sea during the long 1977–1993 stagnation period. Thus the development in GoF is opposite to the deeper basins in the central Baltic. We conclude that salinity and stratification are probably linked with climatic variability via freshwater run-off, which may be important in regulating the oxygen conditions and state of macrozoobenthos in GoF.     

The role of the gravity-wave dispersion relation in HF radar measurements of the sea surface

Recent experimental and theoretical findings raise interesting questions about the applicability of the normal gravity-wave dispersion relation at wave frequencies that exceed the spectral peak frequency. The use of the dispersion relation in analysis of HF radar Doppler sea echo is examined in this paper. Drawing on the results of perturbation theory for wave-wave nonlinear interactions, we show that this relation, so essential to echo interpretation in terms of current and wave information, can be employed with no degradation in accuracy for current measurement when the dominant wave frequency is considerably less (by as much as 10) than the radar Bragg resonance frequency. This finding is supported by comparisons of currents measured by HF radar with "surface truth;" the first-order echo must only be identifiable in order to be used accurately. Wave-height directional spectral information can be extracted from the second-order echo at a given radar frequency up to the point (in wave height) where the perturbation solution employed in the inversion process fails; then a lower radar frequency must be used. On the other hand, most conventional wave measuring instruments should not use the dispersion relation for interpretation of data well beyond the spectral peak, because they do not observe wave height as a function of both space and time independently, as does HF radar.     

A 2DH nonlinear Boussinesq-type wave model of improved dispersion,shoaling, and wave generation characteristics

A modified Boussinesq-type model is derived to account for the propagation of either regular or irregular waves in two horizontal dimensions. An improvement of the dispersion and shoaling characteristics of the model is obtained by optimizing the coefficients of each term in the momentum equation, expanding in this way its applicability in very deep waters and thus overcoming a shortcoming of most models of the same type. The values of the coefficients are obtained by an inverse method in such a way as to satisfy exactly the dispersion relation in terms of both first and second-order analyses matching in parallel the associated shoaling gradient. Furthermore a physically more sound way to approach the evaluation of wave number in irregular wave fields is proposed. A modification of the wave generator boundary condition is also introduced in order to correctly simulate the phase celerity of each input wave component. The modified model is applied to simulate the propagation of breaking and non-breaking, regular and irregular, long and short crested waves in both one and two horizontal dimensions, in a variety of bottom profiles, such as of constant depth, mild slope, and in the presence of submerged obstacles. The simulations are compared with experimental data and analytical results, indicating very good agreement in most cases.     

海域立体分层使用的现实困境与制度完善

近年来,随着我国海洋开发利用规模的持续增长,不同项目重叠用海的问题开始凸显,海域立体分层使用模式得到广泛关注。海域立体分层使用以海域的立体性、多宜性等自然属性为基础,以协调竖向空间的用海活动为目的,是未来海洋空间管理制度体系变革的方向之一。现行的海域空间“平面化”管理思路,使海域立体分层使用面临竖向空间边界不清、二维海籍管理模式不适用、缺少立体空间规划及利益相关者协调难度大等问题。通过借鉴土地立体分层使用的经验,结合海域空间自然特征与开发利用特点,分别从海域分层方法、三维海籍信息表达方式、海域立体空间规划、设立海域役权等方面提出对策建议,以期为海域空间立体分层使用提供借鉴。     

黄海春季海雾的年际变化研究

利用黄海沿岸有代表性测站的常规观测资料和NCEP／NCAR资料，对黄海春季海雾年际变化进行了分析发现，雾多年份冬季环流减弱、低层流场向黄海为偏南向流入、中低层水汽充足、层结稳定；有雾时气温水温差在0．5—2．2℃范围内，地面风向以S-ESE为主。分析结果表明，在春季黄海雾形成过程中，高空环流提供了暖湿空气的输送条件，低层流场及地面风场的分布有利于来自西太平洋低纬地区的水汽向黄海海区输送；中低层水汽充沛，昙结稳定，水气温差在一定范围内有利于海雾的形成和维持。     

Spatial and temporal variability of heat content above the thermocline in the tropical Pacific Ocean

Abstract-Heat content of the upper layer above the 20℃ isotherm in the tropical Pacific Ocean isestimated by using the sea temperature data set with a resolution 2°latitude×5°longitude (1980～1993)for the water depths (every 10 m) from 0 m to 400 m, and its temporal and spatial variabilities are an-alyzed. (1) The temporal variability indicates that the total heat in the upper layer of the equatorial Pa-cific Ocean is charcterized by the interannual variability. The time series of the equatorial heat anomaly5 months lead that of the El Nino index at the best positive lag correlation between the two, and theformer 13 months lag behind the latter at their best negative lag correlation. Therefore the equatorialheat content can be used as a better predictor than the El Nino index for a warm or cold event. In addi-tion, it is also found that less heat anomaly in the equator corresponds to the stronger warm events inthe period (1980～1993) and much more heat was accumulated in the 4 years including 1992/1     

Interannual Sea Level Variations in the South China Sea Over 1950–2009

Spatial patterns of interannual sea level variations in the South China Sea (SCS) are investigated by analyzing an EOF-based 2-dimensional past sea level reconstruction from 1950 to 2009 and satellite altimetry data from 1993 to 2009. Long-term tide gauge records from 14 selected stations in this region are also used to assess the quality of reconstructed sea levels and determine the rate of sea level along the coastal area. We found that the rising rate of sea levels derived from merged satellite altimetry data during 1993–2009 and past sea level reconstruction over 1950–2009 is about 3.9 ± 0.6 mm/yr and 1.7 ± 0.1 mm/yr, respectively. For the longer period, this rate is not significantly different from the global mean rate (of 1.8 ± 0.3 mm/yr). The interannual mean sea level of the SCS region appears highly correlated with Niño 4 indices (a proxy of El Niño-Southern Oscillation/ENSO), suggesting that the interannual sea level variations over the SCS region is driven by ENSO events. Interpolation of the reconstructed sea level data for 1950–2009 at sites where tide gauge records are of poor quality (either short or gapped) show that sea level along the Chinese coastal area is rising faster than the global mean rate of 1.8 mm/yr. At some sites, the rate is up to 2.5 mm/yr.     

北极中央区海冰密集度与云量相关性分析

本文使用海冰密集度以及低云、中云、高云的日平均数据,借助滑动相关分析方法,研究了北极中央区海冰密集度与云量之间的相关性,分析了海冰与云的相互作用机制。研究表明,在春季海冰融化季节(4、5月)、秋季海冰冻结季节(10、11月),低云与海冰密集度之间表现为较好的负相关,表明在这段时间内冰区海面蒸发强烈,对低云的形成有重要贡献。在10月和11月,中云与海冰密集度也有很好的负相关,表明秋季低云可以通过抬升形成中云。高云与海冰密集度之间并没有明显的相关性,可能原因:一方面海冰的空间分布对高云无影响,另一方面,高云主要影响到达的短波辐射,从而影响海冰的融化和冻结速度,与海冰厚度有直接显著的关系,而与海冰密集度的关系不明显。此外,在海冰密集度与低云存在较好负相关的情况下会出现某些年份相关性不好的情况,我们的研究发现这是北极中央区与周边海区发生了海冰交换或云交换的结果。