2019 Vol. 38, No. 11

2019, Vol. 38, No. 11 Content
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Physical Oceanography, Marine Meteorology and Marine Physics
Origin and formation of the Ryukyu Current revealed by HYCOM reanalysis
Wang Min, Liu Zhaojun, Zhu Xiaohua, Yan Xiaomei, Zhang Zhongzhe, Zhao Ruixiang
2019, 38(11): 1-10. doi: 10.1007/s13131-018-1329-7
The origin of the Ryukyu Current (RC) and the formation of its subsurface velocity core were investigated using a 23-year (1993–2015) global Hybrid Coordinate Ocean Model (HYCOM) dataset. The volume transport of the RC comes from the Kuroshio eastward branch (KEB) east of Taiwan and part of the North Pacific Subtropical Gyre (p-NPSG). From the surface to 2 000 m depth, the KEB (p-NPSG) transport contributes 41.5% (58.5%) to the mean total RC transport. The KEB originally forms the subsurface velocity core of the RC east of Taiwan due to blockage of the subsurface Kuroshio by the Ilan Ridge (sill depth: 700 m). Above 700 m, the Kuroshio can enter the East China Sea (ECS) over the Ilan Ridge, meanwhile, the blocked Kuroshio below 700 m turns to the right and flows along the Ryukyu Islands. With the RC flowing northeastward, the p-NPSG contribution strengthens the subsurface maximum structure of the RC owing to the blockage of the Ryukyu Ridge. In the surface layer, the p-NPSG cannot form a stable northeastward current due to frequent disturbance by mesoscale eddies and water exchange through the gaps (with net volume transport into ECS) between the Ryukyu Islands.
Upwelling off the west coast of Hainan Island: sensitivity to wave-mixing
Bai Peng, Yang Jingling, Zhang Shuwen, Xie Lingling, Wu Junshan
2019, 38(11): 11-19. doi: 10.1007/s13131-019-1494-3
The coupled ocean–atmosphere–wave–sediment transport (COAWST) modeling system is employed to investigate the role of wave-mixing playing in the upwelling off the west coast of Hainan Island (WHU). Waves, tides and sea surface temperature (SST) are reproduced reasonably well by the model when validated by observations. Model results suggest the WHU is tidally driven. Further investigations indicate that inclusion of wave-mixing promotes the intensity of the WHU, making the simulated SST become more consistent with remote-sensed ones. Dynamically, wave-mixing facilitates the “outcrop” of more upwelled cold water, triggering stronger WHU and leading to a three-dimensional dynamical adjustment. From the perspective of time, wave-mixing contributes to establishing an earlier tidal mixing front strong enough to generate WHU and that is, WHU may occur earlier when taking wave-mixing into consideration.
A numerical study of generation and propagation of type-a and type-b internal solitary waves in the northern South China Sea
Zeng Zhi, Chen Xueen, Yuan Chunxin, Tang Shengquan, Chi Lequan
2019, 38(11): 20-30. doi: 10.1007/s13131-019-1495-2
Numerical simulations based on a high-resolution three-dimensional MIT general circulation model (MITgcm) using realistic topography and tidal forcing are conducted to investigate the generation and propagation of the so-called type-a waves (large-amplitude rank-ordered wave packets) and type-b waves (isolated wave packets) in the northern South China Sea. At first, we summarized and analyzed the generation and propagation characteristics of these waves. Then, energy budget at the Luzon Strait is calculated. Energy generation has three local maxima every day, of which the largest one corresponds to the emergence of the type-a wave. Energy flux at the west boundary of the Luzon Strait shows two local maxima each day. The larger one is consistent with the generation of the type-a wave and the smaller one is in correspondence with the generation of the type-b wave. Sensitivity experiments are designed to explore the role of the east and west ridge of the Luzon Strait on the generation and propagation of the type-a and type-b waves. It is found that the east ridge is indispensable on the generation of the type-a wave while the west ridge has little contribution. The west ridge diminishes the type-a waves' amplitude but hardly changes their propagation speed. The type-b waves also come from perturbation signals which originate from the east ridge and are enhanced in amplitude and reduced in propagation speed by the west ridge.
Sea surface temperature data from coastal observation stations: quality control and semidiurnal characteristics
Yang Hua, Gao Qingqing, Ji Huifeng, He Peidong, Zhu Tianmao
2019, 38(11): 31-39. doi: 10.1007/s13131-019-1496-1
Sea surface temperature (SST) data obtained from coastal stations in Jiangsu, China during 2010–2014 are quality controlled before analysis of their characteristic semidiurnal and seasonal cycles, including the correlation with the variation of the tide. Quality control of data includes the validation of extreme values and checking of hourly values based on temporally adjacent data points, with 0.15℃/h considered a suitable threshold for detecting abnormal values. The diurnal variation amplitude of the SST data is greater in spring and summer than in autumn and winter. The diurnal variation of SST has bimodal structure on most days, i.e., SST has a significant semidiurnal cycle. Moreover, the semidiurnal cycle of SST is negatively correlated with the tidal data from March to August, but positively correlated with the tidal data from October to January. Little correlation is detected in the remaining months because of the weak coastal–offshore SST gradients. The quality control and understanding of coastal SST data are particularly relevant with regard to the validation of indirect measurements such as satellite-derived data.
A numerical study on salinity stratification at the Oujiang River Estuary, China
Li Yichun, Liu Jingui
2019, 38(11): 40-50. doi: 10.1007/s13131-019-1497-0
The Oujiang River Estuary (ORE) is a macrotidal estuary with drastic variation of river discharge and large tidal range. Numerical simulations based on the unstructured grid, Finite-Volume, primitive equation Community Ocean Model (FVCOM) are conducted to investigate the intratidal and intertidal variations of salinity with an extremely upstream river boundary and large computational domain. The dynamic equation of potential energy anomaly is adopted to evaluate the stratification and mixing processes from model results. Meanwhile, the stability of estuarine stratification on different timescales and its spatial variation are studied using estuarine Richardson number and stratification parameter. The critical values of tidal range and river discharge that determine the stratification state are obtained. The critical values exhibit distinct spatial difference. The north branch of the ORE exhibits well-mixed conditions when the tidal range exceeds 3.8, 4.0 and 4.6 m at upper inlet, middle segment and the river mouth, respectively. When river discharge is below 280 m3/s or exceeds 510 m3/s, the upper part of the north branch is well-mixed sustainably. Near the river mouth, river discharge of 280 m3/s is a rough critical value that separates well-mixed and stratified states. It is also concluded that periodic stratification exists in the North Channel. The lower estuary appears to be partially stratified at early ebb or early flood tide, and well-mixed in other tidal stages. The stratification only develops during early ebb in the upper segment. The enhancement of stratification is mainly caused by longitudinal advection and lateral velocity shear, while turbulent mixing and longitudinal tidal strain are the main factors of stratification attenuation.
Significant salinity increase in subsurface waters of the South China Sea during 2016–2017
Chen Xingrong, Liu Zenghong, Wang Haiyan, Xu Dongfeng, Wang Lei
2019, 38(11): 51-61. doi: 10.1007/s13131-019-1498-z
The South China Sea (SCS) is the largest semi-enclosed marginal sea in the North Pacific. Salinity changes in the SCS play an important role in regional and global ocean circulation and the hydrological cycle. However, there are few studies on salinity changes over the SCS due to lack of high-quality and long-term observations. In the past decade, the deployment of floats from the Argo program in the SCS and their accumulated temperature and salinity profiles have made it possible for us to examine salinity changes over the entire basin. In this study, salinity changes were investigated with Argo and underwater glider temperature and salinity observations and gridded temperature–salinity objective analyses (UK Met Office Hadley Centre EN4.2.1 objective analysis and China Argo Real-time Data Center BOA_Argo). The results indicated that the subsurface water in the entire SCS became significantly saltier during 2016–2017. The most significant salinity increase was found during 2016 in the northeastern SCS. The subsurface water in the northeastern SCS exhibited a salinity maximum above 35, which was recorded by three Argo floats during 2015–2016. Such high salinity water was rarely observed and reported prior to the Argo era. Average salinity of 2016–2017 along the 25.5σθ–23.5σθ isopycnal surfaces in the whole SCS is 0.014-0.130 higher than the climatology. Increases in subsurface salinity started from the northeastern SCS and extended southwestward gradually. Moreover, the subsurface salinity changes, especially in the northern SCS, exhibited a semiannual lead behind the subsurface Luzon Strait transport. Further analysis indicated that the predominance of advection, driven by subsurface Luzon Strait transport, led to salinification along the western boundary of the SCS. In other parts of the SCS, negative wind stress curl trends tended to preserve the high salinity characteristics of the subsurface water.
Sea level anomalies in the northwestern Pacific during 2011 associated with La Niña and negative Indian Ocean Dipole
Qiu Fuwen, Qiu Yun, Pan Aijun, Cha Jing, Zhang Shanwu
2019, 38(11): 62-69. doi: 10.1007/s13131-019-1499-y
The sea level anomalies (SLAs) pattern in the northwestern Pacific delineated significant differences between La Niña events occurring with and without negative Indian Ocean Dipole (IOD) events. During the pure La Niña events, positive the sea surface level anomalies (SLAs) appear in the northwestern Pacific, but SLAs are weakened and negative SLAs appear in the northwestern Pacific under the contribution of the negative IOD events in 2010/2011. The negative IOD events can trigger significant westerly wind anomalies in the western tropical Pacific, which lead to the breakdown of the pronounced positive SLAs in the northwestern Pacific. Meanwhile, negative SLAs excited by the positive wind stress curl near the dateline propagated westward in the form of Rossby waves until it approached the western Pacific boundary in mid-2011, which maintained and enhanced the negative phase of SLAs in the northwestern Pacific and eventually, it could significantly influence the bifurcation and transport of the North Equatorial Current (NEC).
Bimodality and growth of the spectra of typhoon-generated waves in northern South China Sea
Mo Dongxue, Liu Yahao, Hou Yijun, Liu Ze
2019, 38(11): 70-80. doi: 10.1007/s13131-019-1500-9
Buoy-based observations of wave spectra during the passage of three typhoons in the northern South China Sea are examined. Though most spectra of mature typhoon-generated waves are unimodal, double-peaked spectra account for a significant proportion during the growing and decaying stages. This is due either to the superposition of swells on local wind waves or to the mechanism of nonlinear interaction between different wave components. The growth rate of energy density is an effective way to predict spectrum variation. The dominant wave direction depends on the location of the typhoon center to the site, but the direction spread shows no regularity in distant regions. In this study, a new six-parameter spectral formula is proposed to represent double-peaked spectra and is shown to provide a better fit than previous models. The theoretical relationship between shape parameter and spectral width is still applicable to each peak. The characteristics of the variations of spectral parameters are analyzed. It is demonstrated that the spectral parameters are not only related to the typhoon intensity and typhoon track, but also have strong intercorrelations. Moreover, the growth relation between significant wave height and significant wave period is obtained to fit the typhoon-generated waves.
Marine Geology
Intra-trench variations in flexural bending of the subducting Pacific Plate along the Tonga-Kermadec Trench
Zhang Fan, Lin Jian, Zhou Zhiyuan
2019, 38(11): 81-90. doi: 10.1007/s13131-019-1493-4
We conducted a detailed analysis of along-trench variations in the flexural bending of the subducting Pacific Plate at the Tonga-Kermadec Trench. Inversions were conducted to obtain best-fitting solutions of trench-axis loadings and variations in the effective elastic plate thickness for the analyzed flexural bending profiles. Results of the analyses revealed significant along-trench variations in plate flexural bending: the trench relief (W0) of 1.9 to 5.1 km; trench-axis vertical loading (V0) of –0.5×1012 to 2.2×1012 N/m; axial bending moment (M0) of 0.1×1017 to 2.2×1017 N; effective elastic plate thickness seaward of the outer-rise region (TeM) of 20 to 65 km, trench-ward of the outer-rise (Tem) of 11 to 33 km, and the transition distance (Xr) of 20 to 95 km. The Horizon Deep, the second greatest trench depth in the world, has the greatest trench relief (W0 of 5.1 km) and trench-axis loading (V0 of 2.2×1012 N/m); these values are only slightly smaller than that of the Challenger Deep (W0 of 5.7 km and V0 of 2.9×1012 N/m) and similar to that of the Sirena Deep (W0 of 5.2 km and V0 of 2.0×1012 N/m) of the Mariana Trench, suggesting that these deeps are linked to great flexural bending of the subducting plates. Analyses using three independent methods, i.e., the TeM/Tem inversion, the flexural curvature/yield strength envelope analysis, and the elasto-plastic bending model with normal faults, all yielded similar average Te reduction of 28%–36% and average Te reduction area STe of 1 195–1 402 km2 near the trench axis. The calculated brittle yield zone depth from the flexural curvature/yield strength envelope analysis is also consistent with the distribution of the observed normal faulting earthquakes. Comparisons of the Manila, Philippine, Tonga-Kermadec, Japan, and Mariana Trenches revealed that the average values of TeM and Tem both in general increase with the subducting plate age.
Geotechnical properties and stability of the submarine canyon in the northern South China Sea
Liu Jie, Liu Lejun, Li Ping, Gao Shan, Gao Wei, Xu Yuanqin
2019, 38(11): 91-98. doi: 10.1007/s13131-019-1501-8
The upper part of the continental slope in the northern South China Sea is prone to submarine landslide disasters, especially in submarine canyons. This work studies borehole sediments, discusses geotechnical properties of sediments, and evaluates sediment stability in the study area. The results show that sediment shear strength increases with increasing depth, with good linear correlation. Variations in shear strength of sediments with burial depth have a significantly greater rate of change in the canyon head and middle part than those in the canyon bottom. For sediments at the same burial depth, shear strength gradually increased and then decreased from the head to the bottom of the canyon, and has no obvious correlation with the slope angle of the sampling site. Under static conditions, the critical equilibrium slope angle of the sediments in the middle part of the canyon is 10° to 12°, and the critical slope angle in the head and the bottom of the canyon is 7°. The results indicate that potential landslide hazard areas are mainly distributed in distinct spots or narrow strips on the canyon walls where there are high slope angles.
Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region
Wu Tuoyu, Tian Liyan, Gao Jinwei, Dong Yanhui
2019, 38(11): 99-110. doi: 10.1007/s13131-019-1469-x
Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich (~42 wt%), but have low contents in Al2O3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount (mostly >25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements (Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.
Late Holocene mangrove development and response to sea level change in the northwestern South China Sea
Xia Peng, Meng Xianwei, Li Zhen, Zhi Pengyao, Zhao Mengwei, Wang Enkang
2019, 38(11): 111-120. doi: 10.1007/s13131-019-1359-9
Mangroves, widely distributed along the coasts of tropical China, are influenced by Asia monsoon, relative sea level change and enhanced human activity. To predict the impacts of future climate change on mangrove ecosystems, it can be understood by reconstructing past mangrove dynamics using proxies preserved in coastal sediments. In this study, we quantitatively partitioned buried organic matter (OM) sources, collected from a vulnerable mangrove swamp in the Qinzhou Bay of northwestern South China Sea, using a ternary end-member mixing model of δ13C and C:N values. Mangrove-derived OM (MOM) contribution was used as a tracer for mangrove development since 2.34 cal ka BP. This information, together with paleoclimate records (i.e., speleothem δ18O values, sea level change, grain size parameters) and human activity, was used to divide mangrove development into three stages during the late Holocene: relative flourish (2.34–1.13 cal ka BP), relative degradation (1.13–0.15 cal ka BP) and further degradation (0.15–0 cal ka BP). Before 1.13 cal ka BP, mangroves flourished with a high MOM contribution ((88.9±10.6)%), corresponding to stable and high sea level under a warm and humid climate. After 1.13 cal ka BP, rapid fall in relative sea level coupled with the strengthening of the Asian winter monsoon, resulted in mangrove degradation and MOM reduction ((62.4±18.9)%). Compared with air temperature and precipitation, the relative sea level fall was the main controlling factor in mangrove development before entering the Anthropocene (the time of the Industrial Revolution). After ~150 cal a BP, reclamation of mangrove swamps to shrimp ponds is the main factor causing mangrove degradation and MOM reduction.
Marine Technology
The impact of rain to observed signal from Chinese Gaofen-3 synthetic aperture radar in typhoons
Shi Jian, Hu Jiachen, Shao Weizeng, Wang Xiaoqing, Yuan Xinzhe, Zhao Liangbo, Li Xiaofeng
2019, 38(11): 121-133. doi: 10.1007/s13131-019-1502-7
Gaofen-3 (GF-3), a Chinese civil synthetic aperture radar (SAR) at C-band, has operated since August 2016. Remarkably, several typhoons have been captured by GF-3 around the China Seas over its last two-year mission. In this study, six images acquired in Global Observation (GLO) and Wide ScanSAR (WSC) modes at vertical-vertical (VV) polarization channel are discussed. This work focuses on investigating the observation of rainfall using GF-3 SAR. These images were collocated with winds from the European Centre for Medium-Range Weather Forecasts (ECMWF), significant wave height simulated from the WAVEWATCH-III (WW3) model, sea surface currents from climate forecast system version 2 (CFSv2) of the National Centers for Environmental Prediction (NCEP) and rain rate data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Sea surface roughness, was compared with the normalized radar cross section (NRCS) from SAR observations, and indicated a 0.8 correlation (COR). We analyzed the dependences of the difference between model-simulated NRCS and SAR-measured NRCS on the TRMM rain rate and WW3-simulated significant wave height. It was found that the effects of rain on SAR damps the radar signal at incidence angles ranging from 15° to 30°, while it enhances the radar signal at incidence angles ranging from 30° to 45° and incidence angles smaller than 10°. This behavior is consistent with previous studies and an algorithm for rain rate retrieval is anticipated for GF-3 SAR.
Marine Information Science
Measurement analyses and evaluations of sea-level heights using the HY-2A satellite’s radar altimeter
Jiang Xingwei, Jia Yongjun, Zhang Youguang
2019, 38(11): 134-139. doi: 10.1007/s13131-019-1503-6
The HY-2A satellite is China’s first independent oceanic dynamic environmental satellite, and has been operating continuously for more than six years. The satellite’s radar altimeter, which is one of the main loads on the satellite, has the ability to realize all-weather and all-day observations of global sea-surface heights, as well as significant wave heights and sea-surface wind speeds. These observed data have been widely used in marine disaster prevention and reduction, along with resource development, maritime security and other fields. In order to achieve a comprehensive understanding of the multi-year overall observational performances of the HY-2A satellite’s radar altimeter, all of the observational data of the IGDR product from October 26, 2012 to August 27, 2017 were selected in this study for a comprehensive evaluation. The height measurement capability of the HY-2A satellite’s radar altimeter was evaluated using self-crossover and Jason-2 crossover methods. The height discrepancies at the self-crossover point of the HY-2A satellite’s ascending and descending orbits were also calculated. It was found that for the HY-2A satellite’s radar altimeter in global waters under the restriction conditions of ascending and descending orbits, the height anomaly differences were within a range of less than 30 cm. The absolute mean error was determined to be 5.81 cm, and the height anomaly standard deviation was 7.76 cm. Under the conditions of the observational areas being limited within a scope of 60° from the Equator, it was determined that the sea-level height anomaly differences were less than 10 cm at the junction of the ascending and descending orbits, the absolute mean error was 3.95 cm. In addition, the sea-level height anomaly standard deviation was observed to be 4.76 cm. Using a mutual cross method with the Jason-2 satellite, it was found that under the conditions of the observational area being within the scope of 66° from the equator, the height anomaly differences at the junction were less than 30 cm, and the absolute mean error of HY-2A and Jason-2 sea level height anomaly was 5.86 cm, with a standard deviation of 7.52 cm. It was observed that, if within the sea area the sea level height anomaly difference was limited to within 10 cm, then the absolute mean error and standard deviation could reach 4.19 cm and 4.98 cm, respectively. It was confirmed that the HY-2A satellite’s radar altimeter had successfully reached the height measurement level of similar international altimeters. Therefore, it had the ability to meet the needs of marine scientific research and ocean circulation inversions.
An accelerated nonlocal means algorithm for synthetic aperture radar ocean image despeckling
Zha Guozhen, Xu Dewei, Yang Yanming, Song Xin'gai, Zhong Fuhuang
2019, 38(11): 140-148. doi: 10.1007/s13131-019-1504-5
Synthetic aperture radar (SAR) images play an increasingly important role in ocean environmental monitoring and research. However, SAR images are inherently corrupted by speckle noise. SAR ocean images have some unique characteristics. The signatures of ocean phenomena in SAR images mainly exhibit as stripe or plaque shaped features. These features typically have a high degree of self-similarity or redundancy. The nonlocal means (NLM) method can measure the structural similarity between different image patches and take advantage of redundant information in images. Considering that the NLM algorithm is computationally intensive and time-consuming, an accelerated NLM algorithm for SAR ocean image despeckling is proposed in this paper. A method is used to discriminate between texture and flat pixels in SAR images. Large similarity and search windows are used on texture pixels, whereas small similarity and search windows are used on flat pixels. Furthermore, the improved NLM algorithm is accelerated by a graphic processing unit (GPU) based on the compute unified device architecture (CUDA) parallel computation framework. The computational efficiency is improved by approximately 200 times.
Acoustic sound speed profile inversion based on orthogonal matching pursuit
Li Qianqian, Shi Juan, Li Zhenglin, Luo Yu, Yang Fanlin, Zhang Kai
2019, 38(11): 149-157. doi: 10.1007/s13131-019-1505-4
The estimation of ocean sound speed profiles (SSPs) requires the inversion of an acoustic field using limited observations. Such inverse problems are underdetermined, and require regularization to ensure physically realistic solutions. The empirical orthonormal function (EOF) is capable of a very large compression of the data set. In this paper, the non-linear response of the sound pressure to SSP is linearized using a first order Taylor expansion, and the pressure is expanded in a sparse domain using EOFs. Since the parameters of the inverse model are sparse, compressive sensing (CS) can help solve such underdetermined problems accurately, efficiently, and with enhanced resolution. Here, the orthogonal matching pursuit (OMP) is used to estimate range-independent acoustic SSPs using the simulated acoustic field. The superior resolution of OMP is demonstrated with the SSP data from the South China Sea experiment. By shortening the duration of the training set, the temporal correlation between EOF and test sets is enhanced, and the accuracy of sound velocity inversion is improved. The SSP estimation error versus depth is calculated, and the 99% confidence interval of error is within ±0.6 m/s. The 82% of mean absolute error (MAE) is less than 1 m/s. It is shown that SSPs can be well estimated using OMP.
News and Views
China-France Oceanography Satellite (CFOSAT) simultaneously observes the typhoon-induced wind and wave fields
Xu Ying, Liu Jianqiang, Xie Lingling, Sun Congrong, Liu Jinpu, Li Junyi
2019, 38(11): 158-161. doi: 10.1007/s13131-019-1506-3
Bedrock samples from the Chukchi Borderland, Arctic Ocean—First Chinese dredge in the polar regions
Zhang Tao, Dong Yanhui, Yang Chunguo, Guan Qingsheng, Gao Jinyao
2019, 38(11): 162-164. doi: 10.1007/s13131-019-1507-2