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The winter western boundary current of the South China Sea: physical structure and volume transport in December 1998
Li LI, Xiaogang GUO, Risheng WU
2018, 37(3): 1-7. doi: 10.1007/s13131-018-1195-3
Abstract(21) FullText HTML(4) FullText PDF(1)
Abstract:
The unique survey in December 1998 mapped the entire western boundary area of the South China Sea (SCS), which reveals the three-dimensional structure and huge volume transport of the swift and narrow winter western boundary current of the SCS (SCSwwbc) in full scale. The current is found to flow all the way from the shelf edge off Hong Kong to the Sunda Shelf with a width around 100 km and a vertical scale of about 400 m. It appears to be the strongest off the Indo-China Peninsula, where its volume transport reached over 20×106 m3/s. The current is weaker upstream in the northern SCS to the west of Hong Kong. A Kuroshio loop or detached eddy intruded through the Luzon Strait is observed farther east where the SCSwwbc no more exists. The results suggest that during the survey the SCSwwbc was fed primarily by the interior recirculation of the SCS rather than by the " branching” of the Kuroshio from the Luzon Strait as indicated by surface drifters, which is likely a near-surface phenomenon and only contributes a minor part to the total transport of the SCSwwbc. Several topics related to the SCSwwbc are also discussed.
Evaluation on data assimilation of a global high resolution wave-tide-circulation coupled model using the tropical Pacific TAO buoy observations
Junqiang SHI, Xunqiang YIN, Qi SHU, Bin XIAO, Fangli QIAO
2018, 37(3): 8-20. doi: 10.1007/s13131-018-1196-2
Abstract(19) FullText HTML(5) FullText PDF(2)
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In order to evaluate the assimilation results from a global high resolution ocean model, the buoy observations from tropical atmosphere ocean (TAO) during August 2014 to July 2015 are employed. The horizontal resolution of wave-tide-circulation coupled ocean model developed by The First Institute of Oceanography (FIOCOM model) is 0.1°×0.1°, and ensemble adjustment Kalman filter is used to assimilate the sea surface temperature (SST), sea level anomaly (SLA) and Argo temperature/salinity profiles. The simulation results with and without data assimilation are examined. First, the overall statistic errors of model results are analyzed. The scatter diagrams of model simulations versus observations and corresponding error probability density distribution show that the errors of all the observed variables, including the temperature, isotherm depth of 20°C (D20), salinity and two horizontal component of velocity are reduced to some extent with a maximum improvement of 54% after assimilation. Second, time-averaged variables are used to investigate the horizontal and vertical structures of the model results. Owing to the data assimilation, the biases of the time-averaged distribution are reduced more than 70% for the temperature and D20 especially in the eastern Pacific. The obvious improvement of D20 which represents the upper mixed layer depth indicates that the structure of the temperature after the data assimilation becomes more close to the reality and the vertical structure of the upper ocean becomes more reasonable. At last, the physical processes of time series are compared with observations. The time evolution processes of all variables after the data assimilation are more consistent with the observations. The temperature bias and RMSE of D20 are reduced by 76% and 56% respectively with the data assimilation. More events during this period are also reproduced after the data assimilation. Under the condition of strong 2014/2016 El Niño, the Equatorial Undercurrent (EUC) from the TAO is gradually increased during August to November in 2014, and followed by a decreasing process. Since the improvement of the structure in the upper ocean, these events of the EUC can be clearly found in the assimilation results. In conclusion, the data assimilation in this global high resolution model has successfully reduced the model biases and improved the structures of the upper ocean, and the physical processes in reality can be well produced.
Numerical simulations of rip currents off arc-shaped coastlines
Hong WANG, Shouxian ZHU, Xunqiang LI, Wenjing ZHANG, Yu NIE
2018, 37(3): 21-30. doi: 10.1007/s13131-018-1197-1
Abstract(17) FullText HTML(4) FullText PDF(0)
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The rip currents induced by waves off arc-shaped coastlines are seriously harmful to humans, but understanding of their characteristics is lacking. In this study, the FUNWAVE model was used to calculate the wave-induced currents in the Haller experiment and the ideal arc-shaped coast similar to Sanya Dadonghai, Hainan Province, China. The results showed that the FUNWAVE model has considerable ability to simulate the rip currents, and it was used to further simulate rip currents off arc-shaped coastlines to investigate their characteristics. The rip currents were found to be stronger as the curvature of arc-shaped coastline increased. Coastal beach slope exerts a significant influence on rip currents; in particular, an overly steep or overly mild slope is not conducive to creating rip currents. Furthermore, the rip currents were found to become weaker as the size of arc-shaped coast decreased. When the height and period of waves increase, the strength of rip currents also increases, and, in some cases, wave heights of 0.4 m may produce dangerous rip currents.
Turbulent mixing above the Atlantic Water around the Chukchi Borderland in 2014
Wenli ZHONG, Guijun GUO, Jinping ZHAO, Tao LI, Xiaoyu WANG, Longjiang MU
2018, 37(3): 31-41. doi: 10.1007/s13131-018-1198-0
Abstract(12) FullText HTML(4) FullText PDF(0)
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This study presents an analysis of the CTD data and the turbulent microstructure data collected in 2014, the turbulent mixing environment above the Atlantic Water (AW) around the Chukchi Borderland region is studied. Surface wind becomes more efficient in driving the upper ocean movement along with the rapid decline of sea ice, thus results in a more restless interior of the Arctic Ocean. The turbulent dissipation rate is in the range of 4.60×10–10–3.31×10–9 W/kg with a mean value of 1.33×10–9 W/kg, while the diapycnal diffusivity is in the range of 1.45×10–6–1.46×10–5 m2/s with a mean value of 4.84×10–6 m2/s in 200–300 m (above the AW). After investigating on the traditional factors (i.e., wind, topography and tides) that may contribute to the turbulent dissipation rate, the results show that the tidal kinetic energy plays a dominating role in the vertical mixing above the AW. Besides, the swing of the Beaufort Gyre (BG) has an impact on the vertical shear of the geostrophic current and may contribute to the regional difference of turbulent mixing. The parameterized method for the double-diffusive convection flux above the AW is validated by the direct turbulent microstructure results.
Impact of polar lows on synoptic scale variability of Atlantic inflow in the Fram Strait
Ruili SUN, Guoping GAO
2018, 37(3): 42-50. doi: 10.1007/s13131-018-1199-z
Abstract(15) FullText HTML(3) FullText PDF(0)
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The Atlantic inflow in the Fram Strait (78°50′N) has synoptic scale variability based on an array of moorings over the period of 1998–2010. The synoptic scale variability of Atlantic inflow, whose significant cycle is 3–16 d, occurs mainly in winter and spring (from January to April) and is related with polar lows in the Barents Sea. On the synoptic scale, the enhancement (weakening) of Atlantic inflow in the Fram Strait is accompanied by less (more) polar lows in the Barents Sea. Wind stress curl induced by polar lows in the Barents Sea causes Ekman-transport, leads to decrease of sea surface height in the Barents Sea, due to geostrophic adjustment, further induces a cyclonic circulation anomaly around the Barents Sea, and causes the weakening of the Atlantic inflow in the Fram Strait. Our results highlight the importance of polar lows in forcing the Atlantic inflow in the Fram Strait and can help us to further understand the effect of Atlantic warm water on the change of the Arctic Ocean.
On the subtropical Northeast Pacific mixed layer depth and its influence on the subduction
Ruibin XIA, Chengyan LIU, Chen CHENG
2018, 37(3): 51-62. doi: 10.1007/s13131-017-1102-3
Abstract(33) FullText HTML(7) FullText PDF(1)
Abstract:
The present climate simulations of the mixed layer depth (MLD) and the subduction rate in the subtropical Northeast Pacific are investigated based on nine of the CMIP5 models. Compared with the observation data, spatial patterns of the MLD and the subduction rate are well simulated in these models. The spatial pattern of the MLD is nonuniform, with a local maximum MLD (>140 m) region centered at (28°N, 135°W) in late winter. The nonuniform MLD pattern causes a strong MLD front on the south of the MLD maximum region, controls the lateral induction rate pattern, and then decides the nonuniform distribution of the subduction rate. Due to the inter-regional difference of the MLD, we divide this area into two regions. The relatively uniform Ekman pumping has little effect on the nonuniform subduction spatial pattern, though it is nearly equal to the lateral induction in values. In the south region, the northward warm Ekman advection (–1.75×10–7 K/s) controls the ocean horizontal temperature advection (–0.85×10–7 K/s), and prevents the deepening of the MLD. In the ensemble mean, the contribution of the ocean advection to the MLD is about –29.0 m/month, offsetting the sea surface net heat flux contribution (33.9 m/month). While in the north region, the southward cold advection deepens the MLD (21.4 m/month) as similar as the heat flux (30.4 m/month). In conclusion, the nonuniform MLD pattern is dominated by the nonuniform ocean horizontal temperature advection. This new finding indicates that the upper ocean current play an important role in the variability of the winter MLD and the subduction rate.
Potential effects of subduction rate in the key ocean on global warming hiatus
Xingrong CHEN, Shan LIU, Yi CAI, Shouwen ZHANG
2018, 37(3): 63-68. doi: 10.1007/s13131-017-1130-z
Abstract(14) FullText HTML(3) FullText PDF(0)
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In this study, the possible effects of subduction rate on global warming hiatus were investigated using Simple Ocean Data Assimilation (SODA) data. This study first analyzed the characteristics of the temporal and spatial distribution of global subduction rate, which revealed that the North Atlantic meridional overturning circulation region and the Antarctic Circumpolar Current region are the two main sea areas with great subduction variations. On this basis, four key areas were selected to explore the relationship between the local subduction rate and the global mean sea surface temperature. In addition, the reason for the variations in subduction rate was preliminarily explored. The results show good correspondence of the subduction of the key areas in the North Atlantic meridional overturning the circulation region and the Antarctic Circumpolar Current region to the global warming hiatus, with the former leading by about 10 years. The subduction process may be a physical mechanism by which the North Atlantic overturning circulation and the Antarctic Circumpolar Current act on the stagnation of global warming. Advection effect plays an important role in the variations in subduction in the key regions. In the Antarctic Circumpolar Current region, the magnitude of sea surface wind stress is closely related to the local changes in subduction.
The long-term prediction of the oil-contaminated water from the Sanchi collision in the East China Sea
Liping YIN, Min ZHANG, Yuanling ZHANG, Fangli QIAO
2018, 37(3): 69-72. doi: 10.1007/s13131-018-1193-5
Abstract(16) FullText HTML(4) FullText PDF(0)
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The condensate and bunker oil leaked from the Sanchi collision would cause a persistent impact on marine ecosystems in the surrounding areas. The long-term prediction for the distribution of the oil-polluted water and the information for the most affected regions would provide valuable information for the oceanic environment protection and pollution assessment. Based on the operational forecast system developed by the First Institute of Oceanography, State Oceanic Administration, we precisely predicted the drifting path of the oil tanker Sanchi after its collision. Trajectories of virtual oil particles show that the oil leaked from the Sanchi after it sank is mainly transported to the northeastern part of the sink location, and quickly goes to the open ocean along with the Kuroshio. Risk probability analysis based on the outcomes from the operational forecast system for years 2009 to 2017 shows that the most affected area is at the northeast of the sink location.
The deep structure of the Duanqiao hydrothermal field at the Southwest Indian Ridge
Chaofeng SUN, Zhaocai WU, Chunhui TAO, Aiguo RUAN, Guoyin ZHANG, Zhikui GUO, Enxian HUANG
2018, 37(3): 73-79. doi: 10.1007/s13131-017-0986-2
Abstract(14) FullText HTML(4) FullText PDF(0)
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Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association (COMRA ) cruises (CCCs) along the Southwest Indian Ridge (SWIR). In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs 21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field (37°39′S, 50°24′E). The " subarea calibration” terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer (OBS) profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated. The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex (OCC).
Wave flume experiments on the contribution of seabed fluidization to sediment resuspension
Shaotong ZHANG, Yonggang JIA, Zhenhao WANG, Mingzheng WEN, Fang LU, Yaqi ZHANG, Xiaolei LIU, Hongxian SHAN
2018, 37(3): 80-87. doi: 10.1007/s13131-018-1143-2
Abstract(17) FullText HTML(3) FullText PDF(0)
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Sediment resuspension is commonly assumed to be eroded from the seabed surface by an excess bottom shear stress and evolves in layers from the top down. Although considerable investigations have argued the importance of wave-induced seabed fluidization in affecting the sediment resuspension, few studies have been able to reliably evaluate its quantitative contribution till now. Attempt is made to preliminarily quantify the contribution of fluidization to resuspension using a series of large-scale wave flume experiments. The experimental results indicated that fluidization of the sandy silts of the Huanghe Delta account for 52.5% and 66.8% of the total resuspension under model scales of 4/20 and 6/20 (i.e., relative water depth: the ratio of wave height to water depth), respectively. Some previously reported results obtained using the same flume and sediments are also summarized for a contrastive analysis, through which not only the positive correlation is confirmed, but also a parametric equation for depicting the relationship between the contribution of fluidization and the model scale is established. Finally, the contribution of fluidization is attributed to two physical mechanisms: (1) an attenuation of the erosion resistance of fluidized sediments in surface layers due to the disappearing of original cohesion and the uplifting effect resulting from upward seepage flows, and (2) seepage pumping of fines from the interior to the surface of fluidized seabed.
A novel algorithm for ocean wave direction inversion from X-band radar images based on optical flow method
Li WANG, Yunfei CHENG, Lijuan HONG, Xinyu LIU
2018, 37(3): 88-93. doi: 10.1007/s13131-018-1201-9
Abstract(17) FullText HTML(4) FullText PDF(0)
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As one of the important sea state parameters for navigation safety and coastal resource management, the ocean wave direction represents the propagation direction of the wave. A novel algorithm based on an optical flow method is developed for the ocean wave direction inversion of the ocean wave fields imaged by the X-band radar continuously. The proposed algorithm utilizes the echo images received by the X-band wave monitoring radar to estimate the optical flow motion, and then the actual wave propagation direction can be obtained by taking a weighted average of the motion vector for each pixel. Compared with the traditional ocean wave direction inversion method based on frequency-domain, the novel algorithm is fully using a time-domain signal processing method without determination of a current velocity and a modulation transfer function (MTF). In the meantime, the novel algorithm is simple, efficient and there is no need to do something more complicated here. Compared with traditional ocean wave direction inversion method, the ocean wave direction of derived by using this proposed method matches well with that measured by an in situ buoy nearby and the simulation data. These promising results demonstrate the efficiency and accuracy of the algorithm proposed in the paper.
A statistical distribution of quad-pol X-band sea clutter time series acquired at a grazing angle
Yunhua WANG, Qun LI, Yanmin ZHANG
2018, 37(3): 94-102. doi: 10.1007/s13131-018-1202-8
Abstract(22) FullText HTML(4) FullText PDF(0)
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Although the complex Wishart distribution has been widely used to analyze the statistic properties of quad-pol SAR spatial data, the applicability of this complex distribution to the time series of sea clutter is rarely discussed. The measured data of the quad-pol X-band marine radar demonstrate that the time series of the sea echoes are also satisfied the circular Gaussian distributions if the low intensity signals, which are mainly dominated by a radar noise, in the shadow regions of the large-scale waves are removed. On the basis of this fact, the probability density functions (PDFs) of the intensity as well as the phase, the real and the imaginary parts of the sea echoes obtained by the marine radar have been derived, and the theoretical models are all expressed in closed forms. In order to validate the theoretical results, the PDFs are compared with the experimental data collected by the McMaster IPIX radar. And the comparisons show that the PDF models are in good agreement with the experimental data.
Estimating significant wave height from SAR imagery based on an SVM regression model
Dong GAO, Yongxin LIU, Junmin MENG, Yongjun JIA, Chenqing FAN
2018, 37(3): 103-110. doi: 10.1007/s13131-018-1203-7
Abstract(18) FullText HTML(3) FullText PDF(0)
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A new method for estimating significant wave height (SWH) from advanced synthetic aperture radar (ASAR) wave mode data based on a support vector machine (SVM) regression model is presented. The model is established based on a nonlinear relationship between σ0, the variance of the normalized SAR image, SAR image spectrum spectral decomposition parameters and ocean wave SWH. The feature parameters of the SAR images are the input parameters of the SVM regression model, and the SWH provided by the European Centre for Medium-range Weather Forecasts (ECMWF) is the output parameter. On the basis of ASAR matching data set, a particle swarm optimization (PSO) algorithm is used to optimize the input kernel parameters of the SVM regression model and to establish the SVM model. The SWH estimation results yielded by this model are compared with the ECMWF reanalysis data and the buoy data. The RMSE values of the SWH are 0.34 and 0.48 m, and the correlation coefficient is 0.94 and 0.81, respectively. The results show that the SVM regression model is an effective method for estimating the SWH from the SAR data. The advantage of this model is that SAR data may serve as an independent data source for retrieving the SWH, which can avoid the complicated solution process associated with wave spectra.
A routine operational backscattering coefficient regrouping algorithm for a HY-2A scatterometer
Juhong ZOU, Mingsen LIN, Bin ZOU, Maohua GUO, Yi ZHANG
2018, 37(3): 111-116. doi: 10.1007/s13131-018-1204-6
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The routine operational sigma0 regrouping method is proposed for a HY-2A scatterometer (HSCAT) that maps time-ordered sigma0s and related parameters into a subtrack aligned grid of wind vector cells (WVCs). The regrouping method consists of two critical steps: ground grid generation and sigma0 resampling. The HSCAT uses subtrack swath coordinates, in which the nadir track of the satellite represents the center and the designated positions are specified in terms of a pair of along-track and cross-track coordinates. To calculate the subtrack coordinates for each sigma0, a " triangle marking” resampling method is developed. Three points, including the point of intersection, the center of a pulse footprint, and the origin of the subtrack coordinate system, form a right triangle; the length of the two right-angled sides is used to represent the cross-track and the along-track coordinates in the subtrack coordinate system. In addition, a nadir point interpolation correction is used to ensure the operation of the regrouping algorithm when the nadir point positional information is missing. To illustrate the ability of the proposed regrouping algorithm, the distribution of the WVC positions and wind vector retrieval results are analyzed, which show that the proposed regrouping algorithm meets the requirements for high-quality sea surface wind field retrieval.