Tsunami observations in the open ocean

Deep-sea tsunami measurements play a major role in understanding the physics of tsunami wave generation and propagation, and in the creation of an effective tsunami warning system. The paper provides an overview of the history of tsunami recording in the open ocean from the beginning (about 50 years ago) to the present day. It describes modern tsunami monitoring systems, including the Deep-ocean Assessment and Reporting of Tsunamis (DART), innovative Japanese bottom cable projects, and the NEPTUNE-Canada geophysical bottom observatory. The specific peculiarities of seafloor longwave observations in the deep ocean are discussed and compared with those recorded in coastal regions. Tsunami detection in bottom presure observations is exemplified based on analysis of distant (22000 km) records of the 2004 Sumatra tsunami in the northeastern Pacific.     

Near-neutral drag coefficients over open-ocean waves

Reanalysis of open-ocean eddy-correlation drag coefficients observed at Argus Island Tower near Bermuda indicates that near-neutral drag coefficients decrease with increasing mean horizontal wind velocity when the dominant (spectral peak) ocean wave phase velocity is much greater than the mean wind velocity. Near-neutral drag coefficients attain constant values when the dominant wave phase velocity approaches mean wind velocity, and increase again when mean wind velocity is greater than or equal to dominant wave phase velocity.     

Methods for processing the data of wind-profile observations over the ocean

We describe the in-line methods for processing the results of observations of the vertical distribution of wind speed with an aim of evaluation of the parameters characterizing the stable state of the surface layer of the atmosphere and the roughness of the sea surface. It is proposed to use these data for the interpretation of the results of satellite scatterometric measurements of the surface wind speed in order to increase their accuracy.     

风浪和海洋飞沫对海表面拖曳系数和风廓线的影响

基于埃克曼理论,本文将波致应力和飞沫应力引入到海-气边界层的界面应力中,来研究海表面风浪和海洋飞沫对海-气边界层动量交换的影响,并得到修改后的埃克曼模型的理论解。波致应力是由风浪谱和波增长函数估计,并得到在中低风速下,波致应力、飞沫应力与湍流应力相比,对海表面拖曳系数和风廓线的影响非常小。当风速高于25米/秒时,海洋飞沫通过飞沫应力对海-气界面应力的作用远高于波致应力,以至于波致应力可以忽略。海表面拖曳系数在高风速下,随着风速的增大而减小。通过采用风浪谱的不同波龄,得到海洋飞沫的产生会导致海-气边界层风速的增加。最后,理论解与现场的观察数据进行了对比。对比后的数据表明,在中高风速下,飞沫对海-气边界层的影响远大于表面风浪。     

台湾海域海面粗糙度及曳力系数的研究

利用福建沿海海域南日岛、金门岛与广东南澳岛的近海面气象、水文观测资料及台湾海峡西南部与南部的浮标观测资料,通过COARE(Coupled-Ocean Atmosphere Response Experiment)算法(V3.0),计算了台湾海域近海面空气动力粗糙度、曳力系数及海气表面动量通量,并研究它们关于水平风速的分布规律。结果得出:摩擦速度与10m风速基本上呈线性分布,线性拟合系数在0.042～0.045之间。曳力系数对10m风速的线性分布与过去研究结果相近,线性拟合系数在0.064～0.067之间。海面空气动力粗糙度对10m风速的线性拟合系数在0.053～0.058之间。海面动量通量与10m风速呈明显的二次关系,其线性与非线性拟合系数对于进一步了解台海地区大气湍流结构,建立和完善台海地区数值预报模式可提供重要的参数依据。     

Buoy observations of the influence of swell on wind waves in the open ocean

The influence of longer (swell) on shorter, wind sea waves is examined using an extensive database of directional buoy measurements obtained from a heave-pitch-roll buoy moored in deep water in the South Atlantic. This data set is unique for such an investigation due to the ubiquitous presence of a young swell component propagating closely in direction and frequency with the wind sea, as well as a longer, opposing swell. Our results show, within the statistical limits of the regressions obtained from our analysis when compared to measurements in swell free environments, that there is no obvious influence of swell on wind sea growth. For operational purposes in ocean engineering this means that power-laws from fetch limited situations describing the wind sea growth can be applied in more realistic situations in the open sea when swell is present.     

Atmospheric drag coefficients over sea ice — validation of a parameterisation concept

The concept of drag partitioning to parameterise the surface roughness of sea ice is validated using topography data of regions with high sea ice concentrations. The parameterised drag is compared to measurements obtained by aircraft and ship. The form drag can well be expressed as a function of mean ridge heights and spacings averaged over flight legs of 12 km, if an improved approximation for the coefficient of resistance of a single ridge is used. We find a good agreement between the parameterised and observed drag coefficients. The highest sea ice roughness was encountered close to coastal regions and the lowest in the central Arctic.     

Measurement of tsunamis in the open ocean

Abstract

The spectrum of long waves, which are a background to tsunamis, is analyzed on the basis of records of near‐bottom pressure sensors obtained in the Northwest Pacific during the first and second USA‐USSR expeditions on the investigation of tsunamis in the open ocean (1975 and 1978). Instrumental trends, tidal oscillations, and quasistationary longwave noise were contained in the records. Special filters were used to pick out small waves generated by the seismicity of the ocean bottom. A decrease of noise level from 10² cm (including tides) to 10°¹ cm could be reached. The level of long‐wave noise is found to depend on the instrument's location. Minimal disturbances in the records were observed at stations situated on the edge of the continental slope. The influence of cyclones passing over the instrument's site is deduced. It shows an increase in noise level on Nyquist frequency (0.5 min°¹), which possibly is caused by the action of swell generated by the cyclone. Seismicity of the region under investigation for the second expedition (August‐October 1978) is described, and the recurrence of tsunamis is estimated, including microtsunamis. Taking into account this analysis, records were filtered and sections corresponding to probable arrivals of tsunamis from most strong earthquakes were selected. The anomalous disturbance of ocean level with a height of about 0.5 cm was found. Presumably, it was generated by an earthquake with magnitude M = 6.     

Registration of tsunamis in the open ocean

Abstract

The small tsunami of February 23, 1980, which originated near the southern part of the Kuril Islands was recorded by the bottom vibrotron sensor installed on the shelf near Shikotan Island and connected to the island observatory by underwater cable. The marigram of the tsunami is given and compared with marigrams obtained at shore tide gauges. Some spectral features of the records are discussed.     

Fidelity of ocean bottom seismic observations

The often poor quality of ocean bottom seismic data, particularly that observed on horizontal seismometers, is shown to be the result of instruments responding to motions in ways not intended. Instruments designed to obtain the particle motion of the ocean bottom are found to also respond to motions of the water. The shear discontinuity across the ocean floor boundary results in torques that cause package rotation, rather than rectilinear motion, in response to horizontal ground or water motion. The problems are exacerbated by bottom currents and soft sediments. The theory and data presented in this paper suggest that the only reliable way of obtaining high fidelity particle motion data from the ocean floor is to bury the sensors below the bottom in a package with density close to that of the sediment. Long period signals couple well to ocean bottom seismometers, but torques generated by bottom currents can cause noise at both long and short periods. The predicted effects are illustrated using parameters appropriate for the operational OBS developed for the U. S. Office of Naval Research. Examples of data from ocean bottom and buried sensors are also presented.     

Coupling winds to ocean surface currents over the global ocean

A Wind stress–Current Coupled System (WCCS) consisting of the HYbrid Coordinate Ocean Model (HYCOM) and an improved wind stress algorithm based on Donelan et al. [Donelan, W.M., Drennan, Katsaros, K.B., 1997. The air–sea momentum flux in mixed wind sea and swell conditions. J. Phys. Oceanogr. 27, 2087–2099] is developed by using the Earth System Modeling Framework (ESMF). The WCCS is applied to the global ocean to study the interactions between the wind stress and the ocean surface currents. In this study, the ocean surface current velocity is taken into consideration in the wind stress calculation and air–sea heat flux calculation. The wind stress that contains the effect of ocean surface current velocity will be used to force the HYCOM. The results indicate that the ocean surface velocity exerts an important influence on the wind stress, which, in turn, significantly affects the global ocean surface currents, air–sea heat fluxes, and the thickness of ocean surface boundary layer. Comparison with the TOGA TAO buoy data, the sea surface temperature from the wind–current coupled simulation showed noticeable improvement over the stand-alone HYCOM simulation.     

Seasonal cycle of phosphate in the open ocean

The seasonal cycle of phosphate in the world ocean is described using all historical data (over 170,000 profiles) held at the U.S. National Oceanographic Data Center and World Data Center-A for Oceanography. Generally, phosphate concentrations are depressed in the season of highest primary production, in conformance with phosphate's role as a major nutrient. Mean phosphate concentration in the North Pacific are twice those in the North Atlantic. The largest seasonal differences occur in the sub-polar North Atlantic and Pacific, where changes in concentration are as large as a factor of two. Temperate and equatorial regions exhibit less seasonal variability. High latitudes, upwelling areas, and river mouths exhibit a notable seasonal signal in phosphate. Enrichment of phosphate from the Amazon and Orinoco rivers appears to dominate the seasonal signal in the tropical Atlantic. In fact, the extent of the rivers’ effects extend so far north into the North Atlantic gyre that it obscures the normal pattern of summer depletion occurring elsewhere in the basin. The seasonal signal in the tropical Pacific Ocean is a function of seasonal variability in the winds, which affect the strength of coastal upwelling.     

On the parameterization of drag coefficient over sea surface

Six parameterization schemes of roughness or drag coefficient are evaluated on the basis of the data from six experiments. They present great consistency with measurement when friction velocity u*<0.5 m/s (approximately corresponding to 10 m wind speed U10 <12 m/s) and large deviation from measurement when u*≥0.5 m/s (approximately U10≥12 m/s). In order to improve the deviation, a new parameterization of drag coefficient is derived on the basis of the similarity theory, Charnock relationship and Toba 3/2 power law. Wave steepness and wind-sea Reynolds number are considered in the new parameterization. Then it is tested on the basis of the measurements and shows significant improvement when u*≥0.5 m/s. Its standard errors are much smaller than the ones of the other six parameterizations. However, the new parameterization still needs more tests especially for high winds.     

Simultaneous acoustic observations of turbulence and zooplankton in the ocean

Models and laboratory experiments show that zooplankton may locate food more easily in turbulent waters, but whether plankton seek or avoid turbulence in the ocean is an open question. It is difficult to measure turbulence and plankton simultaneously and with the necessary spatial resolution using traditional methods (nets and airfoil shear sensors). Acoustics is commonly used to survey zooplankton abundance and recent studies have shown that stratified turbulence can also be a significant source of sound scatter. This may seem like more of a complication than a boon for those aiming to use acoustics to observe plankton in turbulence. We present acoustic data, however, that show that zooplankton and turbulence can be observed simultaneously with a single 307 kHz sounder. The different natures of the two targets (discrete targets versus a volume effect) allow them to be distinguished. The key is sampling the same targets at multiple ranges. The volume scattering strength of a discrete target will increase as the target nears the sounder, because the volume sampled decreases. Turbulence, as a volume scattering effect, has little range dependence to its volume scattering strength.     

引入拖曳系数参数化的海冰自由漂流模拟研究

Many interesting characteristics of sea ice drift depend on the atmospheric drag coefficient(C_a) and oceanic drag coefficient(C_w).Parameterizations of drag coefficients rather than constant values provide us a way to look insight into the dependence of these characteristics on sea ice conditions.In the present study,the parameterized ice drag coefficients are included into a free-drift sea ice dynamic model,and the wind factor α and the deflection angle θ between sea ice drift and wind velocity as well as the ratio of C_a to C_w are studied to investigate their dependence on the impact factors such as local drag coefficients,floe and ridge geometry.The results reveal that in an idealized steady ocean,C_a/C_w increases obviously with the increasing ice concentration for small ice floes in the marginal ice zone,while it remains at a steady level(0.2-0.25) for large floes in the central ice zone.The wind factor α increases rapidly at first and approaches a steady level of 0.018 when A is greater than 20%.And the deflection angle θ drops rapidly from an initial value of approximate 80° and decreases slowly as A is greater than20%without a steady level like α.The values of these parameters agree well with the previously reported observations in Arctic.The ridging intensity is an important parameter to determine the dominant contribution of the ratio of skin friction drag coefficient(C_s' /C_s) and the ratio of ridge form drag coefficient(C_r'/C_r) to the value of C_a/C_w,α,and θ,because of the dominance of ridge form drag for large ridging intensity and skin friction for small ridging intensity among the total drag forces.Parameterization of sea ice drag coefficients has the potential to be embedded into ice dynamic models to better account for the variability of sea ice in the transient Arctic Ocean.     

利用卫星观测海面信息反演三维温度场

基于历史观测的温盐剖面资料,采用回归分析方法统计出海面温度异常、海面动力高度异常与温度剖面异常之间的相关关系;然后利用高分辨率的卫星遥感海表面温度(SST)和卫星观测海面高度(SSH)信息重构了三维海洋温度场。在台湾岛周边海域建立了时间分辨率为天、空间分辨率为0.25°×0.25°的三维温度分析场。通过与实测资料的比较分析,文章所构建的分析场能够较好地描述海洋三维温度场的结构特征,能够较为真实地反映海洋的中尺度变化过程。该分析场可以作为海洋数值模式的初始场,也可以作为伪观测同化到海洋数值再分析和预报系统中,进而改善三维温、盐、流的数值再分析和预报。     

SWAN模型中不同风拖曳力系数对风浪模拟的影响

本文以荷兰哈灵水道海域为实验区域,通过敏感性实验,研究了在14 m/s、31.5 m/s和50 m/s（分别代表一般大风、强热带风暴和强台风的极端条件）定常风速下SWAN模型中不同风拖曳力系数对风浪模拟的影响程度。结果表明,对于近岸浅水区域（水深小于20 m）,风拖曳力系数计算方案的选择对有效波高影响较小,而且当风速增加到一定程度后,波浪破碎成为影响波高值的主要因素;对于深水区域（水深大于30 m）,一般大风条件下风拖曳力系数计算方案的选择对有效波高影响仍然较小,随着风速的继续增大,风拖曳力系数计算方案的选择对有效波高的影响逐渐显著。对于平均周期,风拖曳力系数计算方案的选择和风速的改变对其影响均较小,而由水深变浅导致的波浪破碎对其影响较为显著。根据敏感性实验结果,本文对SWAN模型中风拖曳力系数计算方案的选择做出如下建议:计算近岸浅水区域风浪场或深水区域一般大风条件风浪场时,其风拖曳力系数可以直接采用模型默认选项;而对于深水区域更大风速条件,可首先采用模型默认选项试算,然后结合当地海域实测波浪资料进行修正。     

Twenty years of ocean observations with China Argo

The international Argo program, a global observational array of nearly 4 000 autonomous profiling floats initiated in the late 1990s, which measures the water temperature and salinity of the upper 2 000 m of the global ocean, has revolutionized oceanography. It has been recognized one of the most successful ocean observation systems in the world. Today, the proposed decade action “OneArgo” for building an integrated global, full-depth, and multidisciplinary ocean observing array for beyond 2020 ...     

Measurements of air-ice drag coefficient over the ice-covered Sea of Okhotsk

The air-ice drag coefficient under neutral stratification C _DN was measured with the eddy correlation method in the southern Sea of Okhotsk. The disturbance of the wind field caused by the ship’s structure was evaluated by computational fluid dynamics (CFD), and two types of correction methods were applied to estimate the error span of C _DN : one is based on the results of CFD, and the other is based on the parameterization of C _DN over open water suggested by Taylor and Yelland (2001). The C _DN × 10³ values finally obtained ranged from 1.9 to 5.4 with a mean value of 2.7 by the CFD correction and from 1.5 to 5.0 with a mean value of 3.1 by the other method. This is somewhat larger than the value of 2.5 suggested by Shirasawa (1981), and in the same range as 2.2–4.0 over rough ice and 3.1–5.0 over very rough ice, values which were complied by Guest and Davidson (1991) for first year ice. Most of the ice conditions were characterized by broken floes with a diameter less than 100 m and raised rims, which made the surface rougher than flat, level ice. The relation between C _DN and ice concentration was not clear, mainly because the contribution of the form drag caused at the freeboard was undetectable due to the great variation of ice surface condition. The roughness length z _M was also evaluated using the model developed for snow covered ice in a previous study.