2017 Vol. 36, No. 5

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A fresh look at the deepwater overflow in the Luzon Strait
ZHU Yaohua, SUN Junchuan, WEI Zexun, WANG Yonggang, FANG Guohong, QU Tangdong
2017, 36(5): 1-8. doi: 10.1007/s13131-017-1057-4
On the basis of the latest version of a U.S. Navy generalized digital environment model (GDEM-V3.0) and World Ocean Atlas (WOA13), the hydraulic theory is revisited and applied to the Luzon Strait, providing a fresh look at the deepwater overflow there. The result reveals that:(1) the persistent density difference between two sides of the Luzon Strait sustains an all year round deepwater overflow from the western Pacific to the South China Sea (SCS); (2) the seasonal variability of the deepwater overflow is influenced not only by changes in the density difference between two sides of the Luzon Strait, but also by changes in its upstream layer thickness; (3) the deepwater overflow in the Luzon Strait shows a weak semiannual variability; (4) the seasonal mean circulation pattern in the SCS deep basin does not synchronously respond to the seasonality of the deepwater overflow in the Luzon Strait. Moreover, the deepwater overflow reaches its seasonal maximum in December (based on GDEM-V3.0) or in fall (October-December, based on the WOA13), accompanied by the lowest temperature of the year on the Pacific side of the Luzon Strait. The seasonal variability of the deepwater overflow is consistent with the existing longest (3.5 a) continuous observation along the major deepwater passage of the Luzon Strait.
Meso-scale eddy in the South China Sea simulated by an eddy-resolving ocean model
FENG Baoxin, LIU Hailong, LIN Pengfei, WANG Qi
2017, 36(5): 9-25. doi: 10.1007/s13131-017-1058-3
Mesoscale eddies (MEs) in the South China Sea (SCS) simulated by a quasi-global eddy-resolving ocean general circulation model are evaluated against satellite data during 1993-2007. The modeled ocean data show more activity than shown by the satellite data and reproduces more eddies in the SCS. A total of 345 (428) cyclonic eddies (CEs) and 330 (371) anti-cyclonic eddies (AEs) generated for satellite (model) data are identified during the study period, showing increase of~24% and~12% for the model data, respectively. Compared with eddies in satellite, the simulated eddies tend to have smaller radii, larger amplitudes, a slightly longer lifetime, faster movement and rotation speed, a slightly larger nonlinear properties (U/c) in the model. However, the spatial distribution of generated eddies appears to be inhomogeneous, with more CEs in the northern part of SCS and fewer AEs in the southern part. This is attributed to the exaggerated Kuroshio intrusion in the model because the small islands in the Luzon Strait are still not well resolved although the horizontal resolution reaches (1/10)°. The seasonal variability in the number and the amplitude of eddies generated is also investigated.
The variation of turbulent diapycnal mixing at 18°N in the South China Sea stirred by wind stress
LIU Yongzheng, JING Zhao, WU Lixin
2017, 36(5): 26-30. doi: 10.1007/s13131-017-1067-2
The spatial and temporal variations of turbulent diapycnal mixing along 18°N in the South China Sea (SCS) are estimated by a fine-scale parameterization method based on strain, which is obtained from CTD measurements in yearly September from 2004 to 2010. The section mean diffusivity can reach~10-4 m2/s, which is an order of magnitude larger than the value in the open ocean. Both internal tides and wind-generated near-inertial internal waves play an important role in furnishing the diapycnal mixing here. The former dominates the diapycnal mixing in the deep ocean and makes nonnegligible contribution in the upper ocean, leading to enhanced diapycnal mixing throughout the water column over rough topography. In contrast, the influence of the wind-induced near-inertial internal wave is mainly confined to the upper ocean. Over both flat and rough bathymetries, the diapycnal diffusivity has a growth trend from 2005 to 2010 in the upper 700 m, which results from the increase of wind work on the near-inertial motions.
Experimental analysis of infragravity waves in two eroded microtidal beaches
MARIO Conde-Frias, LUIS Otero, JUAN Camilo Restrepo, JUAN Carlos Ortíz
2017, 36(5): 31-43. doi: 10.1007/s13131-017-1054-7
This work aims to contribute to the characterization and understanding of infragravity waves on two beaches with erosion problems. For this reason, we have used an array of ADCP and a pressure sensor to measure wave parameters and pressure inside and outside of the surf zone during the dry and rainy period in the beaches of Galerazamba and Manzanillo del Mar (both dissipative and eroded beaches) located in the Colombian Caribbean coast. Based on these measurements, we have carried out a spectral analysis in order to identify the frequency components that characterize the wave and its energy; thus, we identified the characteristic frequencies of infragravity waves to finally filter the infragravity signal on each beach in different seasonal periods. Among the results of the Welch spectrum applied to surface elevation time series, we found that, the frequencies' energy of the sea-swell band decreases due to bottom friction and wave breaking as the wave approaches the shore, while the frequencies' energy of the infragravity band increases significantly. In addition, for the wavelet analysis, we could observe how the energy of the infragravity band, especially the lowest frequencies gain energy as the waves approaches the coast. Furthermore, based on the infragravity wave obtained from the extreme wave event registered during the field campaign we can conclude that the contribution of this signal is important in the erosion problems presented in the beaches of Galerazamba and Manzanillo del Mar. Finally, these results show the need to realize other studies that allow us to understand deeply, the role of infragravity waves on the morphological changes that occurs in these beaches.
A study on the relationships between the wave height and the El Niño in the north area of the South China Sea
HAN Shuzong, FAN Yongbin, DONG Yangyang, WU Shuangquan
2017, 36(5): 44-50. doi: 10.1007/s13131-017-1059-2
On the basis of the European Centre for Medium-range Weather Forecasts (ECMwF) 20 a wind field reanalysis data, the wave field of the north area of the South China Sea is calculated with the combination of the HIRHAM wind field model and the SWAN wave model. Then a significant wave height compared with the El Niño index to study the relationships between these variables. The following conclusions are drawn:(1) the wave height of the South China Sea has a strong seasonal variation, the wave height is much larger in winter than in summer; (2) in the South China Sea, the monthly average wave height of the north area has a negative correlation with the Niño3.4 index, most area of the South China Sea has a moderate correlation and the area between Taiwan Province of China and Philippines is highly correlated; and (3) in the strong El Niño years, the significant wave height in the north area of the South China Sea is significantly smaller than in other years; if the El Niño index variability is greater, the wave height decreases. In contrast, the significant wave height in the north area of the South China Sea is larger in the strong La Niña years.
Dramatic weakening of the ear-shaped thermal front in the Yellow Sea during 1950s-1990s
Tana, FANG Yue, LIU Baochao, SUN Shuangwen, WANG Huiwu
2017, 36(5): 51-56. doi: 10.1007/s13131-016-0885-y
The ear-shaped thermal front (ESTF), formed by the convergence of the Yellow Sea Warm Current (YSWC) and the Shandong Coastal Current (SCC), is a very important oceanic phenomenon in the Yellow Sea (YS) in winter. In situ measurements and reanalysis datasets all demonstrate that the ESTF has been weakening during 1950s-1990s, and a similar weakening trend is also found in winter monsoon over the YS. Numerical experiments show that the weakening of winter monsoon can induce an anomalous circulation in the YS on multi-decadal timescale with northward anomalous currents along China's coast and southward anomalous currents in the central YS—generally opposite to seasonal mean circulation. The anomalous circulation causes slowdown of the YSWC and the SCC, and thus weakens the ESTF. Since the ESTF plays important roles in regional ocean dynamics and air-sea interactions, its weakening has important implications for regional climate in the YS in winter.
Prediction of salinity intrusion in the sheltered estuary of Terengganu River in Malaysia using 1-D empirical intrusion model
LEE Hin Lee, TANGANG Fredolin, GISEN Jacqueline Isabella, SURATMAN Saim
2017, 36(5): 57-66. doi: 10.1007/s13131-017-1060-9
Generally one dimensional (1-D) empirical salinity intrusion model is limited to natural alluvial estuary. However, this study attempts to investigate its ability to model a sheltered alluvial estuary of the Terengganu River in Malaysia. The constructed breakwater at the mouth of the river shelters the estuary from direct influence of the open sea. The salinity density along the estuary was collected during the wet and dry seasons for scenarios before and after the constructed breakwater. Moreover, the freshwater discharges, tidal elevations and bathymetry data were also measured as model inputs. A good fit was demonstrated between simulated and observed variables, namely salinity distribution and intrusion length for both scenarios. Thus, the results show that 1-D empirical salinity model can be utilized for sheltered estuarine condition at the Terengganu Estuary, but with an appropriate determination of an initial point. Furthermore, it was observed that the salinity intrusion in the study area is largely dependent on the freshwater discharge rather than tidal elevation fluctuations. The scale of the salinity intrusion length in the study area is proportional to the river discharge of the -1/2 power. It was appeared that the two lines of the 1-D empirical salinity model and discharge power based equation fitted well to each other, with the average predicted minimum freshwater discharge of 150 m3/s is going to be required to maintain acceptable salinity levels during high water slack (HWS) near the water intake station, which is located at 10.63 km from river mouth.
Role of surface warming in the northward shift of tropical cyclone tracks over the South China Sea in November
SUN Jia, WANG Guihua, ZUO Juncheng, LING Zheng, LIU Dahai
2017, 36(5): 67-72. doi: 10.1007/s13131-017-1061-8
Tropical cyclones (TCs) formed in the Northwest Pacific Ocean (NWP) can cross the South China Sea (SCS) sometimes. It is found that the TC tracks in the SCS in November are shifted to the north after 1980 compared with those before 1980. Both data analyses and numerical simulations show that the surface warming in the SCS may contribute to this more northward shift. The warming produces a cyclonic atmosphere circulation anomaly in the northwestern SCS and an associated southerly in the central SCS steering the TCs to the north.
The September 16, 2015 Mw 8.3 Illapel, Chile Earthquake: characteristics of tsunami wave from near-field to far-field
REN Zhiyuan, YUAN Ye, WANG Peitao, FAN Tingting, WANG Juncheng, HOU Jingming
2017, 36(5): 73-82. doi: 10.1007/s13131-017-1005-3
On September 16, 2015, an earthquake with magnitude of Mw 8.3 occurred 46 km offshore from Illapel, Chile, generating a 4.4-m local tsunami measured at Coquimbo. In this study, the characteristics of tsunami are presented by a combination of analysis of observations and numerical simulation based on sources of USGS and NOAA. The records of 16 DART buoys in deep water, ten tidal gauges along coasts of near-field, and ten coastal gauges in the far-field are studied by applying Fourier analyses. The numerical simulation based on nonlinear shallow water equations and nested grids is carried out to provide overall tsunami propagation scenarios, and the results match well with the observations in deep water and but not well in coasts closed to the epicenter. Due to the short distance to the epicenter and the shelf resonance of southern Peru and Chile, the maximum amplitude ranged from 0.1 m to 2 m, except for Coquimbo. In deep water, the maximum amplitude of buoys decayed from 9.8 cm to 0.8 cm, suggesting a centimeter-scale Pacific-wide tsunami, while the governing period was 13-17 min and 32 min. Whereas in the far-field coastal region, the tsunami wave amplified to be around 0.2 m to 0.8 m, mostly as a result of run-up effect and resonance from coast reflection. Although the tsunami was relatively moderate in deep water, it still produced non-negligible tsunami hazards in local region and the coasts of far-field.
An advanced wind vector retrieval algorithm for the rotating fan-beam scatterometer
XIE Xuetong, WEN Ya, HUANG Zhou
2017, 36(5): 83-89. doi: 10.1007/s13131-017-1062-7
The rotating fan-beam scatterometer (RFSCAT) is a new type of satellite scatterometer that is proposed approximately 10 a ago. However, similar to other rotating scatterometers, relatively larger wind retrieval errors occur in the nadir and outer regions compared with the middle regions of the swath. For the RFSCAT with the given parameters, a wind direction retrieval accuracy decreases by approximately 9 in the outer regions compared with the middle region. To address this problem, an advanced wind vector retrieval algorithm for the RFSCAT is presented. The new algorithm features an adaptive extension of the range of wind direction for each wind vector cell position across the whole swath according to the distribution histogram of a retrieved wind direction bias. One hundred orbits of Level 2A data are simulated to validate and evaluate the new algorithm. Retrieval experiments demonstrate that the new advanced algorithm can effectively improve the wind direction retrieval accuracy in the nadir and outer regions of the RFSCAT swath. Approximately 1.6 and 9 improvements in the wind direction retrieval are achieved for the wind vector cells located at the nadir and the edge point of the swath, respectively.
The temporal-spatial distribution and changes of dissolved oxygen in the Changjiang Estuary and its adjacent waters for the last 50 a
LU Wenhai, XIANG Xianquan, YANG Lu, XU Yan, LI Xiao, LIU Shuming
2017, 36(5): 90-98. doi: 10.1007/s13131-017-1063-6
The Changjiang Estuary and its adjacent waters form one of the most important estuarine and coastal areas in China. Multi source and long-term data are assembled to examine the temporal-spatial distribution features of dissolved oxygen (DO) in the Changjiang Estuary and its adjacent waters for the past 50 a. The results show that the DO concentration in the surface of different seasons generally stays stable, while the DO concentration in winter displays a slight increase for the last 50 a. The DO average concentration in winter and spring varies from 7 to 11 mg/L, and in summer and autumn from 6 to 8 mg/L. Hypoxic values first appear in May, and low DO value plume can be observed on the bottom in spring along coastal areas of Zhejiang and Fujian Provinces, China. In summer, the plume advances northward, and the hypoxic intensity of northern transects is much higher than southern transects. Until autumn, hypoxia areas fade away little by little, and completely disappear in winter. Within last 50 a, hypoxia in the Changjiang Estuary and its adjacent waters starts to appear in the 1980s. Since 2000, the degree of hypoxia has increased seriously and the distribution depth has become smaller. It is performed based on a large amount of historical data, and the research results will be of great significance to further study on the dynamic development of hypoxia around the Changjiang Estuary.
A new electroplated Ir/Ir(OH)x pH electrode and its application in the coastal areas of Newport Harbor, California
ZHANG Xiao, YE Ying, KAN Yating, HUANG Yuanfeng, JIA Jianjun, ZHAO Yue, CHEN Chen-Tung Arthur, QIN Huawei
2017, 36(5): 99-104. doi: 10.1007/s13131-017-1064-5
Resulting from the rising levels of atmospheric carbon, ocean acidification has become a global problem. It has significant impacts on the development, survival, growth and physiology of marine organisms. Therefore, a high-precision sensor is urgently needed to measure the pH of sea-water. Iridium wire with a diameter of 0.25 mm is used as the substrate, and an Ir/Ir(OH)x pH electrode is prepared by a one-step electrochemical method in a LiOH solution at the room temperature. A scanning electron microscope (SEM) observation reveals that it is coated with nanoscale particles. In laboratory tests, the electrode exhibits a very promising pH response, with an ideal Nernst slope (56.14-59.52), fast response, good stability and long life-span in tested pH buffer solutions. For a sea trial, four pH electrodes and one Ag/AgCl reference electrode are integrated with a self-made chemical sensor, and a profile detection of nearly 70 m is implemented near Newport Harbor, California on August 3, 2015. The results reflect that the pH value measured by the sensor is very close to the data given by Sea-Bird 911 plus CTD, with a difference value ranging from 0.000 075 to 0.064 719. And the sensor shows a better data matching degree in 0-40 m water depth. In addition, the high precision and accuracy of the sensor make it possible to use in the ocean observation field.
The polarimetric features of oil spills in full polarimetric synthetic aperture radar images
ZHENG Honglei, ZHANG Yanmin, WANG Yunhua, ZHANG Xi, MENG Junmin
2017, 36(5): 105-114. doi: 10.1007/s13131-017-1065-4
Compared with single-polarized synthetic aperture radar (SAR) images, full polarimetric SAR images contain not only geometrical and backward scattering characteristics, but also the polarization features of the scattering targets. Therefore, the polarimetric SAR has more advantages for oil spill detection on the sea surface. As a crucial step in the oil spill detection, a feature extraction directly influences the accuracy of oil spill discrimination. The polarimetric features of sea oil spills, such as polarimetric entropy, average scatter angle, in the full polarimetric SAR images are analyzed firstly. And a new polarimetric parameter P which reflects the proportion between Bragg and specular scattering signals is proposed. In order to investigate the capability of the polarimetric features for observing an oil spill, systematic comparisons and analyses of the multipolarization features are provided on the basis of the full polarimetric SAR images acquired by SIR-C/X-SAR and Radarsat-2. The experiment results show that in C-band SAR images the oil spills can be detected more easily than in L-band SAR images under low to moderate wind speed conditions. Moreover, it also finds that the new polarimetric parameter is sensitive to the sea surface scattering mechanisms. And the experiment results demonstrate that the new polarimetric parameter and pedestal height perform better than other polarimetric parameters for the oil spill detection in the C-band SAR images.