2016 Vol. 35, No. 9

2016, Vol. 35, No. 9 Content
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Physical Oceanography, Marine Meteorology and Marine Physics
Numerical simulation of sediment transport in coastal waves and wave-induced currents
LIU Tongya, XU Jiexin, HE Yinghui, Lü Haibin, YAO Yuan, CAI Shuqun
2016, 35(9): 1-12. doi: 10.1007/s13131-016-0930-x
Owing to lack of observational data and accurate definition, it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea (SCS). By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm, the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated. Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions. In winter, the intruding Kuroshio water reaches the farthest, almost occupying the area from 18°N to 23°N and 114°E to 121°E, with a small branch flowing towards the Taiwan Strait. The intrusion region of the Kuroshio water decreases with depth gradually. However, in summer, the Kuroshio water is confined to the east of 118°E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m, then it decreases with depth. The estimated annual mean of Kuroshio Intrusion Transport (KIT) via the Luzon Strait is westward to the SCS in an amount of -3.86×106 m3/s, which is larger than the annual mean of Luzon Strait Transport (LST) of -3.15×106 m3/s. The KIT above 250 m accounts for 60%-80% of the LST throughout the entire water column. By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012, we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Ni.a years than those in El Niño and normal years, which may be attributed to the wind stress curl off the southeast China then. Furthermore, the KIT correlates the Niño 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41, which is lower than that of the LST with the Niño 3.4 index, i.e., 0.78.
Speckle suppression in synthetic aperture radar ocean internal solitary wave images with curvelet transform
ZHA Guozhen, HE Qingyou, GUAN Changlong, SUN Jian, HE Mingxia
2016, 35(9): 13-21. doi: 10.1007/s13131-016-0929-3
This paper proposes a speckle-suppression method for ocean internal solitary wave (ISW) synthetic aperture radar (SAR) images by using the curvelet transform. The band-shaped signatures of ocean ISWs in SAR images show obvious scale and directional characteristics. The curvelet transform possesses a very high scale and directional sensitivity. Therefore, the curvelet transform is very efficient in analyzing wave signals in SAR images. A noisy ocean ISW SAR image can be decomposed at different scales, directions, and positions using the curvelet transform. The information of the ISWs is centralized in the curvelet coefficients of certain directions under certain scales, whereas the speckle noise is distributed in every scale and direction. By manipulating the curvelet coefficients, the signals of the ISWs can be extracted from the noisy SAR image. Finally, the speckle noise is suppressed and the ISW feature is enhanced by adding the signals of the ISWs back to the original SAR image. Experiments demonstrate the effectiveness of this method.
A study of sea level variability and its long-term trend in the South China Sea
XU Ying, LIN Mingsen, ZHENG Quan'an, SONG Qingtao, YE Xiaomin
2016, 35(9): 22-33. doi: 10.1007/s13131-016-0788-3
On the basis of the satellite maps of sea level anomaly (MSLA) data and in situ tidal gauge sea level data, correlation analysis and empirical mode decomposition (EMD) are employed to investigate the applicability of MSLA data, sea level correlation, long-term sea level variability (SLV) trend, sea level rise (SLR) rate and its geographic distribution in the South China Sea (SCS). The findings show that for Dongfang Station, Haikou Station, Shanwei Station and Zhapo Station, the minimum correlation coefficient between the closest MSLA grid point and tidal station is 0.61. This suggests that the satellite altimeter MSLA data are effective to observe the coastal SLV in the SCS. On the monthly scale, coastal SLV in the western and northern part of SCS are highly associated with coastal currents. On the seasonal scale, SLV of the coastal area in the western part of the SCS is still strongly influenced by the coastal current system in summer and winter. The Pacific change can affect the SCS mainly in winter rather than summer and the affected area mostly concentrated in the northeastern and eastern parts of the SCS. Overall, the average SLR in the SCS is 90.8 mm with a rising rate of (5.0±0.4) mm/a during 1993-2010. The SLR rate from the southern Luzon Strait through the Huangyan Seamount area to the Xisha Islands area is higher than that of other areas of the SCS.
Characteristics and possible causes of sea level anomalies in the Xisha sea area
WANG Hui, HAN Shuzong, FAN Wenjing, WANG Guosong, LIU Kexiu, ZHANG Zengjian
2016, 35(9): 34-41. doi: 10.1007/s13131-016-0938-2
Based on the analysis of wind, ocean currents, sea surface temperature (SST) and remote sensing satellite altimeter data, the characteristics and possible causes of sea level anomalies in the Xisha sea area are investigated. The main results are shown as follows:(1) Since 1993, the sea level in the Xisha sea area was obviously higher than normal in 1998, 2001, 2008, 2010 and 2013. Especially, the sea level in 1998 and 2010 was abnormally high, and the sea level in 2010 was 13.2 cm higher than the muti-year mean, which was the highest in the history. In 2010, the sea level in the Xisha sea area had risen 43 cm from June to August, with the strength twice the annual variation range. (2) The sea level in the Xisha sea area was not only affected by the tidal force of the celestial bodies, but also closely related to the quasi 2 a periodic oscillation of tropical western Pacific monsoon and ENSO events. (3) There was a significant negative correlation between sea level in the Xisha sea area and ENSO events. The high sea level anomaly all happened during the developing phase of La Ni.a. They also show significant negative correlations with Niño 4 and Niño 3.4 indices, and the lag correlation coefficients for 2 months and 3 months are -0.46 and -0.45, respectively. (4) During the early La Ni.a event form June to November in 2010, the anomalous wind field was cyclonic. A strong clockwise vortex was formed for the current in 25 m layer in the Xisha sea area, and the velocity of the current is close to the speed of the Kuroshio near the Luzon Strait. In normal years, there is a "cool eddy". While in 2010, from July to August, the SST in the area was 2-3℃ higher than that of the same period in the history.
The influence of explicit tidal forcing in a climate ocean circulation model
YU Yi, LIU Hailong, LAN Jian
2016, 35(9): 42-50. doi: 10.1007/s13131-016-0931-9
The eight main tidal constituents have been implemented in the global ocean general circulation model with approximate 1° horizontal resolution. Compared with the observation data, the patterns of the tidal amplitudes and phases had been simulated fairly well. The responses of mean circulation, temperature and salinity are further investigated in the global sense. When implementing the tidal forcing, wind-driven circulations are reduced, especially those in coastal regions. It is also found that the upper cell transport of the Atlantic meridional overturning circulation (AMOC) reduces significantly, while its deep cell transport is slightly enhanced from 9×106 m3/s to 10×106 m3/s. The changes of circulations are all related to the increase of a bottom friction and a vertical viscosity due to the tidal forcing. The temperature and salinity of the model are also significantly affected by the tidal forcing through the enhanced bottom friction, mixing and the changes in mean circulation. The largest changes occur in the coastal regions, where the water is cooled and freshened. In the open ocean, the changes are divided into three layers:cooled and freshened on the surface and below 3 000 m, and warmed and salted in the middle in the open ocean. In the upper two layers, the changes are mainly caused by the enhanced mixing, as warm and salty water sinks and cold and fresh water rises; whereas in the deep layer, the enhancement of the deep overturning circulation accounts for the cold and fresh changes in the deep ocean.
The characteristics of warm water inflowing and its temporal and spatial variation on the Prydz Bay continental shelf, Antarctic
LIN Lina, CHEN Hongxia, LIU Na
2016, 35(9): 51-57. doi: 10.1007/s13131-016-0934-6
Hydrographic observations collected by conductivity-temperature-depth (CTD) and instrumented elephant seals on the Prydz Bay continental shelf during 2012 and 2013 are used to characterize the intrusion of modified circumpolar deep water. As a regular occurrence, modified circumpolar deep water (MCDW) intrudes onto the shelf mainly between 150-300 m layer of 73°-75°E and then turns southeast affected by the cyclonic gyre of the Prydz Bay. The southernmost point of the warm water signal is captured on the east front of Amery Ice Shelf during March 2012. In terms of vertical distribution, MCDW occupies the central layer of 200 m with about 100 m thickness in the austral summer, but when to winter transition, the layer of MCDW deepens with time on the central shelf.
Study of relationship between wave transport and sea surface temperature anomaly (SSTA) in the tropical Pacific
SHI Yongfang, WU Kejian, ZHU Xianye, YANG Fan, ZHANG Yuming
2016, 35(9): 58-66. doi: 10.1007/s13131-016-0928-4
Large-scale water transport is one of the key factors that affect sea surface temperature anomaly (SSTA) in the eastern equatorial Pacific (EEP). The relationship between the wave transport in the tropical Pacific and the SSTA in the EEP is examined by different methods, including band-pass filtering, period analysis, correlation analysis, significant analysis, and empirical orthogonal function (EOF) analysis. We have found that the eastward shift of the wave transport anomaly in the tropical Pacific, with a period of 2 a and enhancing the transport of warm waters from the western Pacific warm pool, precedes the increase of sea surface temperature (SST) in the EEP. The wave transport and the SSTA in the EEP have a maximum correlation of 0.65 with a time-lag of 6 months (transport variation precedes the temperature). The major periods (3.7 a and 2.45 a) of the wave transport variability, as revealed by the EOF analysis, appear to be consistent with the SSTA oscillation cycle in the EEP. Based on the first occurrence of a significant SSTA in the Niño 3 region (5°S-5°N, 90°-150°W), two types of warm events are defined. The wave transport anomalies in two types present predominantly the west anomaly in the tropical Pacific, it is that the wave transport continues transport warm water from west to east before the onset of the warm event. The impact of wave-induced water transport on the SSTA in the EEP is confirmed by the heat flux of the wave transport. The wave transport exerts significant effect on the SSTA variability in the EEP and thus is not neglectable in the further studies.
The influences of winds on the salt balance of the Tampa Bay during the fall of 2001
HAN Shuzong, ZHU Jun, FAN Yongbin, QI Hongshuai, WANG Xingchi
2016, 35(9): 67-73. doi: 10.1007/s13131-016-0937-3
A three-dimensional, high resolution, Finite Volume Coastal Ocean Model (FVCOM) is used to diagnose the influences of winds during the fall of 2001 on the salt balance of the Tampa Bay. To distinguish the wind effecting, two experiments are designed:one is driven by tides and rivers; the other is driven by tides, rivers and winds. First, the salinity change induced by wind is provided. Wind forcing can significantly increase the salinity inside the Tampa Bay, along with decreases of horizontal and vertical salt gradients. Subsequently, the salt balance principles are detailed addressed. The primary salt balance is between the total (horizontal plus vertical) advective salt flux divergence and vertical diffusive salt flux divergence except at the channel bottom where horizontal diffusive salt flux divergence comes into play. At last, the salt balance variation induced by winds is further addressed. Wind forcing does not change the relative importance of salt balance terms. The variations of the salt balance terms influenced by winds are highly dependent on the specific locations. Besides, the variations of the total advective salt flux divergence and the vertical diffusive salt flux divergence are nearly couterbalance, and both of two terms are much greater than that of the horizontal diffusive salt flux divergence.
The study on an Antarctic sea ice identification algorithm of the HY-2A microwave scatterometer data
ZOU Juhong, ZENG Tao, GUO Maohua, CUI Songxue
2016, 35(9): 74-79. doi: 10.1007/s13131-016-0927-5
An Antarctic sea ice identification algorithm on the HY-2A scatterometer (HSCAT) employs backscattering coefficient (σ0) and active polarization ratio (APR) for a preliminary sea ice identification. Then standard deviation (STD) filtering and space filtering are carried out. Finally, it is used to identify sea ice. A process uses a σ0, STD threshold and an APR as sea ice indicators. The sea ice identification results are verified using the sea ice distribution data of the ASMR2 released by the National Snow and Ice Data Center as a reference. The results show very good consistence of sea ice development trends, seasonal changes, area distribution, and sea ice edge distribution of the sea ice identification results obtained by this algorithm relative to the ASMR2 sea ice results. The accuracy of a sea ice coverage is 90.8% versus the ASMR2 sea ice results. This indicates that this algorithm is reliable.
Satellite SAR observation of the sea surface wind field caused by rain cells
YE Xiaomin, LIN Mingsen, YUAN Xinzhe, DING Jing, XIE Xuetong, ZHANG Yi, XU Ying
2016, 35(9): 80-85. doi: 10.1007/s13131-016-0936-4
Rain cells or convective rain, the dominant form of rain in the tropics and subtropics, can be easy detected by satellite Synthetic Aperture Radar (SAR) images with high horizontal resolution. The footprints of rain cells on SAR images are caused by the scattering and attenuation of the rain drops, as well as the downward airflow. In this study, we extract sea surface wind field and its structure caused by rain cells by using a RADARSAT-2 SAR image with a spatial resolution of 100 m for case study. We extract the sea surface wind speeds from SAR image by using CMOD4 geophysical model function with outside wind directions of NCEP final operational global analysis data, Advance Scatterometer (ASCAT) onboard European MetOp-A satellite and microwave scatterometer onboard Chinese HY-2 satellite, respectively. The root-mean-square errors (RMSE) of these SAR wind speeds, validated against NCEP, ASCAT and HY-2, are 1.48 m/s, 1.64 m/s and 2.14 m/s, respectively. Circular signature patterns with brighter on one side and darker on the opposite side on SAR image are interpreted as the sea surface wind speed (or sea surface roughness) variety caused by downdraft associated with rain cells. The wind speeds taken from the transect profile which superposes to the wind ambient vectors and goes through the center of the circular footprint of rain cell can be fitted as a cosine or sine curve in high linear correlation with the values of no less than 0.80. The background wind speed, the wind speed caused by rain cell and the diameter of footprint of the rain cell with kilometers or tens of kilometers can be acquired by fitting curve. Eight cases interpreted and analyzed in this study all show the same conclusion.
Oil spill detection by a support vector machine based on polarization decomposition characteristics
ZOU Yarong, SHI Lijian, ZHANG Shengli, LIANG Chao, ZENG Tao
2016, 35(9): 86-90. doi: 10.1007/s13131-016-0935-5
Marine oil spills have caused major threats to marine environment over the past few years. The early detection of the oil spill is of great significance for the prevention and control of marine disasters. At present, remote sensing is one of the major approaches for monitoring the oil spill. Full polarization synthetic aperture radarc SAR data are employed to extract polarization decomposition parameters including entropy (H) and reflection entropy (A). The characteristic spectrum of the entropy and reflection entropy combination has analyzed and the polarization characteristic spectrum of the oil spill has developed to support remote sensing of the oil spill. The findings show that the information extracted from (1-A)×(1-H) and (1-H)×A parameters is relatively evident effects. The results of extraction of the oil spill information based on H×A parameter are relatively not good. The combination of the two has something to do with H and A values. In general, when H>0.7, A value is relatively small. Here, the extraction of the oil spill information using (1-A)×(1-H) and (1-HA parameters obtains evident effects. Whichever combined parameter is adopted, oil well data would cause certain false alarm to the extraction of the oil spill information. In particular the false alarm of the extracted oil spill information based on (1-A)×(1-H) is relatively high, while the false alarm based on (1-AH and (1-HA parameters is relatively small, but an image noise is relatively big. The oil spill detection employing polarization characteristic spectrum support vector machine can effectively identify the oil spill information with more accuracy than that of the detection method based on single polarization feature.
Retrieval of sea surface winds under hurricane conditions from GNSS-R observations
JING Cheng, YANG Xiaofeng, MA Wentao, YU Yang, DONG Di, LI Ziwei, XU Cong
2016, 35(9): 91-97. doi: 10.1007/s13131-016-0933-7
Reflected signals from global navigation satellite systems (GNSSs) have been widely acknowledged as an important remote sensing tool for retrieving sea surface wind speeds. The power of GNSS reflectometry (GNSS-R) signals can be mapped in delay chips and Doppler frequency space to generate delay Doppler power maps (DDMs), whose characteristics are related to sea surface roughness and can be used to retrieve wind speeds. However, the bistatic radar cross section (BRCS), which is strongly related to the sea surface roughness, is extensively used in radar. Therefore, a bistatic radar cross section (BRCS) map with a modified BRCS equation in a GNSS-R application is introduced. On the BRCS map, three observables are proposed to represent the sea surface roughness to establish a relationship with the sea surface wind speed. Airborne Hurricane Dennis (2005) GNSS-R data are then used. More than 16 000 BRCS maps are generated to establish GMFs of the three observables. Finally, the proposed model and classic one-dimensional delay waveform (DW) matching methods are compared, and the proposed model demonstrates a better performance for the high wind speed retrievals.
Marine Chemistry
Heavy metals in surficial sediments of the central Bohai Sea: their distribution, speciation and sources
LIU Ming, FAN Dejiang, LIAO Yongjie, CHEN Bin, YANG Zuosheng
2016, 35(9): 98-110. doi: 10.1007/s13131-016-0926-6
The semi-enclosed Bohai Sea has received large amount of pollutants from surrounding rivers and sewage channels along the densely populated and industrializing Bohai coasts, as well as the offshore oil exploration in the sea. The concentrations of heavy metals copper, cobalt, nickel, zinc, lead, chromium and cadmium in 25 surface sediment samples from the central Bohai Sea are obtained by ICP-MS analysis. The speciation of these heavy metals is extracted and their distribution, fractionation, pollution status and sources are presented. High concentrations of copper, cobalt, nickel, zinc and chromium are found in the sediments off the coastal area of the Bohai Bay and the central Bohai Sea mud, while high concentrations of cadmium and lead are found in the sediments in the whole study area. The residual fraction is the dominant form of copper, cobalt, nickel, zinc and chromium in the surface sediments, while cadmium and lead have large proportions in the nonresidual fractions. The ecological risk assessment shows that the sediments in the study area are unpolluted with respect to the heavy metals of cobalt, nickel and chromium and unpolluted to moderately polluted with respect to copper, zinc, cadmium and lead. Cobalt, nickel and chromium mainly have natural origin and their concentrations significantly vary with the composition of the sediments. The contents of copper, zinc, lead and especially cadmium in sediments are suggested to be influenced by pollutants of human activities. The heavy metals in the surface sediments at most sampling stations are mainly come from the Huanghe River; the heavy metals in the sediments in the northernmost part of the study area are significantly affected by the sediment from the Luanhe River; while in the Bohai Bay and the central region they were affected by the sediment from the Haihe River and aerosol deposition.
Ocean Engineering
Numerical simulation of the Kuroshio intrusion into the South China Sea by a passive tracer
TANG Jun, LYU Yigang, SHEN Yongming
2016, 35(9): 111-116. doi: 10.1007/s13131-016-0932-8
Prediction of coastal sediment transport is of particularly importance for analyzing coast erosion accurately and solving the corresponding coast protection engineering problems. The present study provided a numerical scheme for sediment transport in coastal waves and wave-induced currents. In the scheme, the sand transport model was implemented with wave refraction-diffraction model and near-shore current model. Coastal water wave was simulated by using the parabolic mild-slope equation in which wave refraction, diffraction and breaking effects are considered. Wave-induced current was simulated by using the nonlinear shallow water equations in which wave provides radiation stresses for driving current. Then, sediment transport in waves and wave-induced currents was simulated by using the two-dimensional suspended sediment transport equations for suspended sediment and the bed-load transport equation for bed load. The numerical scheme was validated by experiment results from the Large-scale Sediment Transport Facility at the US Army Corps of Engineer Research and Development Center in Vicksburg. The numerical results showed that the present scheme is an effective tool for modeling coastal sediment transport in waves and near-shore currents.
A synthetic aperture radar sea surface distribution estimation by n-order Bézier curve and its application in ship detection
LANG Haitao, ZHANG Jie, WANG Yiduo, ZHANG Xi, MENG Junmin
2016, 35(9): 117-125. doi: 10.1007/s13131-016-0924-8
To dates, most ship detection approaches for single-pol synthetic aperture radar (SAR) imagery try to ensure a constant false-alarm rate (CFAR). A high performance ship detector relies on two key components:an accurate estimation to a sea surface distribution and a fine designed CFAR algorithm. First, a novel nonparametric sea surface distribution estimation method is developed based on n-order Bézier curve. To estimate the sea surface distribution using n-order Bézier curve, an explicit analytical solution is derived based on a least square optimization, and the optimal selection also is presented to two essential parameters, the order n of Bézier curve and the number m of sample points. Next, to validate the ship detection performance of the estimated sea surface distribution, the estimated sea surface distribution by n-order Bézier curve is combined with a cell averaging CFAR (CA-CFAR). To eliminate the possible interfering ship targets in background window, an improved automatic censoring method is applied. Comprehensive experiments prove that in terms of sea surface estimation performance, the proposed method is as good as a traditional nonparametric Parzen window kernel method, and in most cases, outperforms two widely used parametric methods, K and G0 models. In terms of computation speed, a major advantage of the proposed estimation method is the time consuming only depended on the number m of sample points while independent of imagery size, which makes it can achieve a significant speed improvement to the Parzen window kernel method, and in some cases, it is even faster than two parametric methods. In terms of ship detection performance, the experiments show that the ship detector which constructed by the proposed sea surface distribution model and the given CA-CFAR algorithm has wide adaptability to different SAR sensors, resolutions and sea surface homogeneities and obtains a leading performance on the test dataset.