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A harmonic analyzed parameterization of tide-induced mixing for ocean models
Zexun WEI, Junchuan SUN, Fei TENG, Tengfei XU, Yonggang WANG, Xiaoqing XU, Guohong FANG
2018, 37(7): 1-7. doi: 10.1007/s13131-018-1239-8
Abstract(32) FullText HTML(5) FullText PDF(1)
Abstract:
The tide-induced mixing plays an important role in the regulation of ocean circulation. Numerical simulation of continental shelf circulation is found to exhibit an unreasonable vertical thermohaline structure without consideration of tide effects. In this study, we establish a harmonic analyzed parameterization of tide-induced (HAT) mixing, by which means to derive time-depended function of mixing coefficient based on harmonic analysis of the vertical mixing coefficient. By employing HAT mixing parameterization scheme, a series of numerical experiments are conducted for the Yellow Sea. Numerical results show that an ocean circulation model with the HAT mixing involved is capable of reproducing the reasonable thermohaline structure of the Yellow Sea Cold Water Mass, similar to structures produced by explicit tidal forcing on the open boundary. The advantage of the HAT method is its faster computation time, compared with models that directly resolve explicit tidal motion. The HAT parameterization for the tide-induced mixing has potential to improve both the accuracy and efficiency of ocean circulation and climate models.
Relationships between intensity of the Kuroshio current in the East China Sea and the East Asian winter monsoon
Ming YIN, Xin LI, Ziniu XIAO, Chongyin LI
2018, 37(7): 8-19. doi: 10.1007/s13131-018-1240-2
Abstract(20) FullText HTML(5) FullText PDF(1)
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Based on satellite altimeter and reanalysis data, this paper studies the relationships between the intensity of the Kuroshio current in the East China Sea (ECS) and the East Asian winter monsoon (EAWM). The mechanisms of their possible interaction are also discussed. Results indicate that adjacent transects show consistent variations, and on an interannual timescale, when the EAWM is anomalously strong (weak), the downstream Kuroshio in the ECS is suppressed (enhanced) in the following year from February to April. This phenomenon can be attributed to both the dynamic effect (i.e., Ekman transport) and the thermal effect of the EAWM. When the EAWM strengthens (weakens), the midstream and downstream Kuroshio in the ECS are also suppressed (intensified) during the following year from October to December. The mechanisms vary for these effects. The EAWM exerts its influence on the Kuroshio's intensity in the following year through the tropospheric biennial oscillation (TBO), and oceanic forcing is dominant during this time. The air-sea interaction is modulated by the relative strength of the EAWM and the Kuroshio in the ECS. The non-equivalence of spatial scales between the monsoon and the Kuroshio determines that their interactions are aided by processes with a smaller spatial scale-i.e., local wind stress and heating at the sea surface.
Methane seepage intensities traced by sulfur isotopes of pyrite and gypsum in sediment from the Shenhu area, South China Sea
Mei ZHANG, Hongfeng LU, Hongxiang GUAN, Lihua LIU, Daidai WU, Nengyou WU
2018, 37(7): 20-27. doi: 10.1007/s13131-018-1241-1
Abstract(15) FullText HTML(4) FullText PDF(0)
Abstract:
The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments. Anaerobic oxidation of methane (AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment, including pyrite and gypsum. Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor (cmbsf) in the entire core sediment of HS328 from the northern South China Sea. Most pyrites are tubes consisting of framboidal cores and outer crusts. Gypsum aggregates occur as rosettes and spheroids consisting of plates. Some of them grow over pyrite, indicating that gypsum precipitation postdates pyrite formation. The sulfur isotopic values (δ34S) of pyrite vary greatly (from –46.6‰ to –12.3‰ V-CDT) and increase with depth. Thus, the pyrite in the shallow sediments resulted from organoclastic sulfate reduction (OSR) and is influenced by AOM with depth. The relative high abundance and δ34S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone (SMTZ). The sulfur isotopic composition of gypsum (from –25‰ to –20.7‰) is much lower than that of the seawater sulfate, indicating the existence of a 34S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR. Pyrite oxidation is controlled by ambient electron acceptors such as MnO2, iron (Ⅲ) and oxygen driven by the SMTZ location shift to great depths. The δ34S values of gypsum at greater depth are lower than those of the associated pyrite, revealing downward diffusion of 34S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate. These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral, which will be favor to the formation of gypsum. Overall, the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities, and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.
The architecture of the lower parts of submarine canyons on the western Nigerian continental margin
Rasheed Olayinka JIMOH, Yong TANG, Jiabiao LI, Larry Folajimi AWOSIKA, He LI, Edward Akintoye AKINNIGBAGE, Adedayo Oluwaseun ADELEYE
2018, 37(7): 28-40. doi: 10.1007/s13131-018-1242-0
Abstract(27) FullText HTML(5) FullText PDF(1)
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Multi-beam, sub-bottom and multichannel seismic data acquired from the western Nigerian continental margin are analysed and interpreted to examine the architectural characteristics of the lower parts of the submarine canyons on the margin. The presence of four canyons: Avon, Mahin, Benin, and Escravos, are confirmed from the multi-beam data map and identified as cutting across the shelf and slope areas, with morphological features ranging from axial channels, moderate to high sinuosity indices, scarps, terraces and nickpoints which are interpreted as resulting from erosional and depositional activities within and around the canyons. The Avon Canyon, in particular, is characterised by various branches and sub-branches with complex morphologies. The canyons are mostly U-shaped in these lower parts with occasional V-shapes down their courses. Their typical orientation is NE–SW. Sedimentary processes are proposed as being a major controlling factor in these canyons. Sediments appear to have been discharged directly into the canyons by rivers during the late Quaternary low sea level which allows river mouths to extend as far as the shelf edge. The current sediment supply is still primarily sourced from these rivers in the case of the Benin and Escravos Canyons, but indirectly in the case of the Avon and Mahin Canyons where the rivers discharge sediments into the lagoons and the lagoons bring the sediments on to the continental shelf before they are dispersed into the canyon heads. Ancient canyons that have long been buried underneath the Avon Canyon are identified in the multichannel seismic profile across the head of the Avon Canyon, while a number of normal faults around the walls of the Avon and Mahin Canyons are observed in the selected sub-bottom profiles. The occurrence of these faults, especially in the irregular portions of the canyon walls, suggests that they also have some effect on the canyon architecture. The formation of the canyons is attributed to the exposure of the upper marginal area to incisions from erosion during the sea level lowstand during the glacial period. The incisions are widened and lengthened by contouric currents, turbidity currents and slope failures resulting in the canyons.
The dispersion of surface contaminants by Stokes drift in random waves
Guoxing HUANG, Wing-Keung Adrian LAW
2018, 37(7): 41-47. doi: 10.1007/s13131-018-1243-z
Abstract(19) FullText HTML(4) FullText PDF(0)
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Contaminants that are floating on the surface of the ocean are subjected to the action of random waves. In the literature, it has been asserted by researchers that the random wave action will lead to a dispersion mechanism through the induced Stokes drift, and that this dispersion mechanism may have the same order of significance comparable with the others means due to tidal currents and wind. It is investigated whether or not surface floating substances will disperse in the random wave environment due to the induced Stokes drift. An analytical derivation is first performed to obtain the drift velocity under the random waves. From the analysis, it is shown that the drift velocity is a time-independent value that does not possess any fluctuation given a specific wave energy spectrum. Thus, the random wave drift by itself should not have a dispersive effect on the surface floating substances. Experiments were then conducted with small floating objects subjected to P-M spectral waves in a laboratory wave flume, and the experimental results reinforced the conclusion drawn.
Numerical modeling investigation on turbulent oscillatory flow over a plane rough bed composed by randomly arrayed particles
Zhibo ZHOU, Jinfeng ZHANG, Qinghe ZHANG, Run LIU
2018, 37(7): 48-54. doi: 10.1007/s13131-018-1244-y
Abstract(62) FullText HTML(4) FullText PDF(0)
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A three-dimensional numerical model is established to simulate the turbulent oscillatory boundary layer over a fixed and rough bed composed by randomly arrayed solid spheres based on the lattice Boltzmann method and the large eddy simulation model. The equivalent roughness height, the location of the theoretical bed and the time variation of the friction velocity are investigated using the log-fit method. The time series of turbulent intensity and Reynolds stress are also investigated. The equivalent roughness height of cases with Reynolds numbers of 1×104–6×104 is approximately 2.81 d (grain size). The time variation of the friction velocity in an oscillatory cycle exhibits sinusoidal-like behavior. The friction factor depends on the relative roughness in the rough turbulent regime, and the pattern of solid particles arrayed as the rough bed in the numerical simulations has no obvious effect on the friction factor.
A model for calculating the erosion distance of soft sea cliff under wave loading
Fangqiang CHANG, Zhonglei SHU
2018, 37(7): 55-63. doi: 10.1007/s13131-018-1245-x
Abstract(20) FullText HTML(4) FullText PDF(0)
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A model for calculating the erosion distance of soft sea cliff under wave loading is established based on the erosion mechanism of soft sea cliff under wave loading and for considering wave hydrodynamic and sea cliff material parameters. The model is verified, and the parameters are regressed using an indoor flume experiment. The erosion distances of the sea cliff in the northeast of the Pingtan Island are calculated by the model, and the results are compared with the measured data. The maximum erosion occurs in static water level, the location of the maximum erosion moves up as the wave continues, and the erosion stops when the wave lasts for a period of time. The erosion does not occur until the wave height exceeds a critical value; however, the contribution of large waves to the erosion is not relatively substantial. The calculated erosion distances at two places in the northeast of Pingtan Island are 0.32 m and 0.26 m.
High-resolution bottom detection algorithm for a multibeam echo-sounder system with a U-shaped array
Ying JIANG, Zhiguo YANG, Zongwei LIU, Chunmei YANG
2018, 37(7): 64-70. doi: 10.1007/s13131-017-1246-9
Abstract(22) FullText HTML(4) FullText PDF(1)
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High-resolution approaches such as multiple signal classification and estimation of signal parameters via rotational invariance techniques (ESPRIT) are currently employed widely in multibeam echo-sounder (MBES) systems for sea floor bathymetry, where a uniform line array is also required. However, due to the requirements in terms of the system coverage/resolution and installation space constraints, an MBES system usually employs a receiving array with a special shape, which means that high-resolution algorithms cannot be applied directly. In addition, the short-term stationary echo signals make it difficult to estimate the covariance matrix required by the high-resolution approaches, which further increases the complexity when applying the high-resolution algorithms in the MBES systems. The ESPRIT with multiple-angle subarray beamforming is employed to reduce the requirements in terms of the signal-to-noise ratio, number of snapshots, and computational effort. The simulations show that the new processing method can provide better fine-structure resolution. Then a high-resolution bottom detection (HRBD) algorithm is developed by combining the new processing method with virtual array transformation. The application of the HRBD algorithm to a U-shaped array is also discuss. The computer simulations and experimental data processing results verify the effectiveness of the proposed algorithm.
An analytical approximation of focusing-wave-induced load on a semi-submerged cylinder
Zhe HU, Xiaoying ZHANG, Yan LI, Xiaowen LI
2018, 37(7): 71-90. doi: 10.1007/s13131-018-1247-8
Abstract(16) FullText HTML(3) FullText PDF(0)
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Owing to the complexity of the physical mechanisms of rouge waves, the theoretical study of the rogue-wave-structure interaction problems still makes little progress. However, for regular-shaped structures, it is possible to give a theoretical analysis, if a relatively simple model of the rogue waves is used. The wave load, induced by a focusing wave which is known as an intuitive basic model of the rouge waves, upon a semi-submerged cylinder is studied analytically. The focusing wave is approximate by the Gauss envelope wave, an ideal model which contains most features of the rogue wave. The diffraction velocity potential is derived through the separation of flow field, and the formulas of the horizontal force and bending moment are proposed. The derived formulas are simplified appropriately, and validated through comparison against numerical results. In addition, the influence of parameters, such as the focusing degree, the submerging depth and the wave focusing position, is thoroughly investigated.
The dynamic observation of dissolved organic matter in the Zhujiang (Pearl River) Estuary in China from space
Dong LIU, Yan BAI, Xianqiang HE, Delu PAN, Difeng WANG, Ji'an WEI, Lin ZHANG
2018, 37(7): 91-103. doi: 10.1007/s13131-017-1248-7
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The distributions of estuarine colored dissolved organic matter (CDOM) are the combined results of physical-biogeochemical processes. Remote sensing is needed to monitor highly dynamically estuarine CDOM. Using in situ data from four seasonal cruises, an algorithm is developed to estimate CDOM absorption coefficient at 400 nm (aCDOM(400)) in the Zhujiang (Pearl River) Estuary (ZJE). The algorithm uses band ratios of Rrs(667)/Rrs(443) and Rrs(748)/Rrs(412). By applying it to moderate resolution imaging spectroradiometer onboard Aqua satellite (MODIS/Aqua) data from 2002 to 2014, seasonal climatology aCDOM(400) in the ZJE is calculated. CDOM distributions are majorly influenced by water discharge from the Zhujiang River and underwater topography. Along the section vertical to a water depth gradient, the seasonal aCDOM(400) exponentially decreased (y=aebx, b<0), but with great differences among seasons. Riverine fresh water is the primary source of CDOM in the ZJE. Fulvic acid fraction decreases with increasing salinity. Using developed algorithms, conservative CDOM mixing equation, and river discharge, effective riverine end-member concentration and flux of dissolved organic carbon (DOC) in summer and winter from 2002 to 2014 are first estimated from the MODIS/Aqua data. Both effective riverine end-member DOC concentration and flux are positively related to the river discharge, significantly in summer withR2 of 0.698 for concentration and 0.965 7 for flux.
A new merged dataset of global ocean chlorophyll a concentration with higher spatial and temporal coverage
Yanfang XIAO, Jie ZHANG, Tingwei CUI, Ling SUN
2018, 37(7): 104-116. doi: 10.1007/s13131-018-1249-6
Abstract(22) FullText HTML(3) FullText PDF(0)
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Understanding the ocean’s role in the global carbon cycle and its response to environmental change requires a high spatio-temporal resolution of observation. Merging ocean color data from multiple sources is an effective way to alleviate the limitation of individual ocean color sensors (e.g., swath width and gaps, cloudy or rainy weather, and sun glint) and to improve the temporal and spatial coverage. Since the missions of Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) and Medium-spectral Resolution Imaging Spectrometer (MERIS) ended on December 11, 2010 and May 9, 2012, respectively, the number of available ocean color sensors has declined, reducing the benefits of the merged ocean color data with respect to the spatial and temporal coverage. In present work, Medium Resolution Spectral Imager (MERSI)/FY-3 of China is added in merged processing and a new dataset of global ocean chlorophyll a (Chl a) concentration (2000–2015) is generated from the remote sensing reflectance (Rrs (λ)) observations of MERIS, Moderate-resolution imaging spectra-radiometer (MODIS)-AQUA, Visible infrared Imaging Radiometer (VIIRS) and MERSI. These data resources are first merged into unified remote sensing reflectance data, and then Chl a concentration data are inversed using the combined Chl a algorithm of color index-based algorithm (CIA) and OC3. The merged data products show major improvements in spatial and temporal coverage from the addition of MERSI. The average daily coverage of merged products is approximately 24% of the global ocean and increases by approximately 9% when MERSI data are added in the merging process. Sampling frequency (temporal coverage) is greatly improved by combining MERSI data, with the median sampling frequency increasing from 15.6% (57 d/a) to 29.9% (109 d/a). The merged Chl a products herein were validated by in situ measurements and comparing them with the merged products using the same approach except for omitting MERSI and GlobColour and MEaSUREs merged data. Correlation and relative error between the new merged Chl a products and in situ observation are stable relative to the results of the merged products without the addition of MERSI. Time series of the Chl a concentration anomalies are similar to the merged products without adding MERSI and single sensors. The new merged products agree within approximately 10% of the merged Chl a product from GlobColour and MEaSUREs.
Vessel fusion tracking with a dual-frequency high-frequency surface wave radar and calibrated by an automatic identification system
Hui ZHANG, Yongxin LIU, Yonggang JI, Linglin WANG
2018, 37(7): 117-126. doi: 10.1007/s13131-018-1250-0
Abstract(14) FullText HTML(4) FullText PDF(0)
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High-frequency surface wave radar (HFSWR) and automatic identification system (AIS) are the two most important sensors used for vessel tracking. The HFSWR can be applied to tracking all vessels in a detection area, while the AIS is usually used to verify the information of cooperative vessels. Because of interference from sea clutter, employing single-frequency HFSWR for vessel tracking may obscure vessels located in the blind zones of Bragg peaks. Analyzing changes in the detection frequencies constitutes an effective method for addressing this deficiency. A solution consisting of vessel fusion tracking is proposed using dual-frequency HFSWR data calibrated by the AIS. Since different systematic biases exist between HFSWR frequency measurements and AIS measurements, AIS information is used to estimate and correct the HFSWR systematic biases at each frequency. First, AIS point measurements for cooperative vessels are associated with the HFSWR measurements using a JVC assignment algorithm. From the association results of the cooperative vessels, the systematic biases in the dual-frequency HFSWR data are estimated and corrected. Then, based on the corrected dual-frequency HFSWR data, the vessels are tracked using a dual-frequency fusion joint probabilistic data association (JPDA)-unscented Kalman filter (UKF) algorithm. Experimental results using real-life detection data show that the proposed method is efficient at tracking vessels in real time and can improve the tracking capability and accuracy compared with tracking processes involving single-frequency data.