2015 Vol. 34, No. 7

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2015, 34(7): .
2015, 34(7): .
2015, 34(7): .
Comparative simulation study of effects of eddy-topography interaction in the East/Japan Sea deep circulation
CHOI Youngjin
2015, 34(7): 1-18. doi: 10.1007/s13131-015-0693-1
In this study the structure and seasonal variations of deep mean circulation in the East/Japan Sea (EJS) were numerically simulated using a mid-resolution ocean general circulation model with two different parameterizations for the eddy-topography interaction (ETI). The strong deep mean circulations observed in the EJS are well reproduced when using the ETI parameterizations. The seasonal variability in the EJS deep layer is shown by using ETI parameterization based on the potential vorticity approach, while it is not shown in the statistical dynamical parameterization. The driving mechanism of the strong deep mean currents in the EJS are discussed by investigating the effects of model grids and parameterizations. The deep mean circulation is more closely related to the baroclinic process and potential vorticity than it is to the wind driven circulation.
The Beaufort Gyre variation and its impacts on the Canada Basin in 2003-2012
ZHONG Wenli, ZHAO Jinping, SHI Jiuxin, CAO Yong
2015, 34(7): 19-31. doi: 10.1007/s13131-015-0657-0
The Beaufort Gyre (BG) was spun up in the last decade which is an important factor in regulating the variation of the upper ocean. The heat content and freshwater content of the upper ocean increased gradually in the Canada Basin, as did momentum input. Both the geostrophic wind curl and freshwater content could contribute to the spin-up of BG. However, even though there is no change of the wind field the increasing freshwater alone could result in the spin-up of BG. In this study we show that the Pacific Water is difficult to flow into the central basin as the BG spins up and the maximum temperature of the Pacific Summer Water (PSW) experienced a dramatic decrease inside the BG in 2005 and 2009 due to a change of flow pathway of PSW. The enhancement of Ekman Pumping (EP) contributed to the deepening of the Pacific Winter Water by piling up more freshwater. This change of water column dynamics has also contributed to the deepening ofthe Atlantic Water core after 2007. The EP decreased significantly in 2012 (indicating a spin down of BG) and the direction of Ekman transport turned to the north, which favoured the release of freshwater that had resided in the basin for years.
Comparing the steric height in the Nordic Seas with satellite altimeter sea surface height
SHAO Qiuli, ZHAO Jinping
2015, 34(7): 32-37. doi: 10.1007/s13131-015-0658-z
In this study the steric height anomaly which is calculated from the hydrological data (EN3) is compared with the sea level anomaly derived from satellite altimetry in the Nordic Seas. The overall pattern of steric height is that it is higher in the margin area and lower in the middle area. The extreme values of steric height linear change from 1993 to 2010 occur in the Lofoten Basin and off the Norwegian coast, respectively. Such a distribution may be partly attributed to the freshening trend of the Nordic Seas. The correlation between SLA (sea level anomaly) and SHA (steric height anomaly) is not uniform over the Nordic Seas. The time series of SLA and SHA agree well in the Lofoten Basin and northern Norwegian Basin, and worse in the northern Norwegian Sea, implying that the baroclinic effect plays a dominant role in most areas in the Norwegian Sea and the barotropic effect plays a dominant role in the northern Norwegian Sea. The weaker correlations between SLA and SHA in the Greenland and Iceland Seas lead a conclusion that the barotropic contribution is significant in these areas. The area-mean SHA over the entire Nordic Seas has similar amplitudes compared with the SLA during 1996-2002, but SHA has become lower than SLA, being less than half of SLA since 2006.
A numerical study of the generation mechanism of internal solitary waves in the Luzon Strait
WANG Jing, SUN Meiling, ZHANG Xudong, SUN Lina, MENG Junmin
2015, 34(7): 38-43. doi: 10.1007/s13131-015-0695-7
The South China Sea (SCS) is a hot spot for oceanic internal solitary waves due to many factors, such as the complexity of the terrain environment. The internal solitary waves in the northern SCS mainly originate in the Luzon Strait. The generation mechanism of the internal solitary waves in the Luzon Strait is discussed using a modulation instability. The energy gain of the modulation instability is derived based on the fully nonlinear Schrödinger equation. The peak value of the gain is calculated under different conditions of stratification, wavelength and the initial amplitude of an internal tidal wave. The characteristics of the modulation instability in the Luzon Strait are investigated. The conditions that make the internal tidal wave evolve into an internal solitary wave in the Luzon strait are also obtained. The results show that the internal tide waves can generate the modulation instability in the Luzon Strait and that the maximum gain occur at the eastern sill of the Luzon Strait, where the internal tide waves start to break up into internal solitary trains. The magnitude and the scope of the peak gain are relevant to the stratification and the initial conditions of the internal tide waves. The numerical simulation results are consistent with the in-situ data.
Seasonal variability in the thermohaline structure of the Western Pacific Warm Pool
QIN Sisi, ZHANG Qilong, YIN Baoshu
2015, 34(7): 44-53. doi: 10.1007/s13131-015-0696-6
Using the 28℃ isotherm to define the Western Pacific Warm Pool (WPWP), this study analyzes the seasonal variability of the WPWP thermohaline structure on the basis of the monthly-averaged sea temperature and salinity data from 1950 to 2011, and the dynamic and thermodynamic mechanisms based on the monthly-averaged wind, precipitation, net heat fluxes and current velocity data. A △T=-0.4℃ is more suitable than other temperature criterion for determining the mixed layer (ML) and barrier layer (BL) over the WPWP using monthly-averaged temperature and salinity data. The WPWP has a particular thermohaline structure and can be vertically divided into three layers, i.e., the ML, BL, and deep layer (DL). The BL thickness (BLT) is the thickest, while the ML thickness (MLT) is the thinnest. The MLT has a similar seasonal variation to the DL thickness (DLT) and BLT. They are all thicker in spring and fall but thinner in summer. The temperatures of the ML and BL are both higher in spring and autumn but lower in winter and summer with an annual amplitude of 0.15℃, while the temperature of the DL is higher in May and lower in August. The averaged salinities at these three layers are all higher in March but lower in September, with annual ranges of 0.41-0.45. Zonal currents, i.e., the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), and winds may be the main dynamic factors driving the seasonal variability in the WPWP thermohaline structure, while precipitation and net heat fluxes are both important thermodynamic factors. Higher (lower) winds cause both the MLT and BLT to thicken (thin), a stronger (weaker) NECC induces MLT, BLT, and DLT to thin (thicken), and a stronger (weaker) SEC causes both the MLT and BLT to thicken (thin) and the DLT to thin (thicken). An increase (decrease) in the net heat fluxes causes the MLT and BLT to thicken (thin) but the DLT to thin (thicken), while a stronger (weaker) precipitation favors thinner (thicker) MLT but thicker (thinner) BLT and DLT. In addition, a stronger (weaker) NECC and SEC cause the temperature of the three layers to decrease (increase), while the seasonal variability in salinity at the ML, BL, and DL might be controlled by the subtropical cell (STC).
Assimilating operational SST and sea ice analysis data into an operational circulation model for the coastal seas of China
JI Qiyan, ZHU Xueming, WANG Hui, LIU Guimei, GAO Shan, JI Xuanliang, XU Qing
2015, 34(7): 54-64. doi: 10.1007/s13131-015-0691-y
The prediction of sea surface temperature (SST) is an essential task for an operational ocean circulation model. A sea surface heat flux, an initial temperature field, and boundary conditions directly affect the accuracy of a SST simulation. Here two quick and convenient data assimilation methods are employed to improve the SST simulation in the domain of the Bohai Sea, the Yellow Sea and the East China Sea (BYECS). One is based on a surface net heat flux correction, named as Qcorrection (QC), which nudges the flux correction to the model equation; the other is ensemble optimal interpolation (EnOI), which optimizes the model initial field. Based on such two methods, the SST data obtained from the operational SST and sea ice analysis (OSTIA) system are assimilated into an operational circulation model for the coastal seas of China. The results of the simulated SST based on four experiments, in 2011, have been analyzed. By comparing with the OSTIA SST, the domain averaged root mean square error (RMSE) of the four experiments is 1.74, 1.16, 1.30 and 0.91℃, respectively; the improvements of assimilation experiments Exps 2, 3 and 4 are about 33.3%, 25.3%, and 47.7%, respectively. Although both two methods are effective in assimilating the SST, the EnOI shows more advantages than the QC, and the best result is achieved when the two methods are combined. Comparing with the observational data from coastal buoy stations, show that assimilating the high-resolution satellite SST products can effectively improve the SST prediction skill in coastal regions.
A high wind geophysical model fuction for QuikSCAT wind retrievals and application to Typhoon IOKE
ZOU Juhong, ZENG Tao, CUI Songxue
2015, 34(7): 65-73. doi: 10.1007/s13131-015-0698-4
The geophysical model function (GMF) describes the relationship between a backscattering and a sea surface wind, and enables a wind vector retrieval from backscattering measurements. It is clear that the GMF plays an important role in an ocean wind vector retrieval. The performance of the existing Ku-band model function QSCAT-1 is considered to be effective at low and moderate wind speed ranges. However, in the conditions of higher wind speeds, the existing algorithms diverge alarmingly. owing to the lack of in situ data required for developing the GMF for the high wind conditions, the QSCAT-1 appears to overestimate the σ0, which results in underestimating the wind speeds. Several match-up QuikSCAT and special sensor microwave/imager (SSM/I) wind speed measurements of the typhoons occurring in the west Pacific Ocean are analyzed. The results show that the SSM/I wind exhibits better agreement with the "best track" analysis wind speed than the QuikSCAT wind retrieved using QSCAT-1. On the basis of this evaluation, a correction of the QSCAT-1 model function for wind speed above 16 m/s is proposed, which uses the collocated SSM/I and QuikSCAT measurements as a training set, and a neural network approach as a multiple nonlinear regression technologytechnology.In order to validate the revised GMF for high winds, the modified GMF was applied to the QuikSCAT observations of Hurricane IOKE. The wind estimated by the QuikSCAT for Typhoon IOKE in 2006 was improved with the maximum wind speed reaching 55 m/s. An error analysis was performed using the wind fields from the Holland model as the surface truth. The results show an improved agreement with the Holland model wind when compared with the wind estimated using the QSCAT-1. However, large bias still existed, indicating that the effects of rain must be considered for further improvement.
Generation of high resolution sea surface temperature using multi-satellite data for operational oceanography
YANG Chan-Su, KIM Sun-Hwa, OUCHI Kazuo, BACK Ji-Hun
2015, 34(7): 74-88. doi: 10.1007/s13131-015-0694-8
In the present article, we introduce a high resolution sea surface temperature (SST) product generated daily by Korea Institute of Ocean Science and Technology (KIOST). The SST product is comprised of four sets of data including eight-hour and daily average SST data of 1 km resolution, and is based on the four infrared (IR) satellite SST data acquired by advanced very high resolution radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), Multifunctional Transport Satellites-2 (MTSAT-2) Imager and Meteorological Imager (MI), two microwave radiometer SSTs acquired by Advanced Microwave Scanning Radiometer 2 (AMSR2), and WindSAT with in-situ temperature data. These input satellite and in-situ SST data are merged by using the optimal interpolation (OI) algorithm. The root-mean-square-errors (RMSEs) of satellite and in-situ data are used as a weighting value in the OI algorithm. As a pilot product, four SST data sets were generated daily from January to December 2013. In the comparison between the SSTs measured by moored buoys and the daily mean KIOST SSTs, the estimated RMSE was 0.71℃ and the bias value was -0.08℃. The largest RMSE and bias were 0.86 and -0.26℃ respectively, observed at a buoy site in the boundary region of warm and cold waters with increased physical variability in the Sea of Japan/East Sea. Other site near the coasts shows a lower RMSE value of 0.60℃ than those at the open waters. To investigate the spatial distributions of SST, the Group for High Resolution Sea Surface Temperature (GHRSST) product was used in the comparison of temperature gradients, and it was shown that the KIOST SST product represents well the water mass structures around the Korean Peninsula. The KIOST SST product generated from both satellite and buoy data is expected to make substantial contribution to the Korea Operational Oceanographic System (KOOS) as an input parameter for data assimilation.
Impact of rain-induced sea surface roughness variations on salinity retrieval from the Aquarius/SAC-D satellite
MA Wentao, YANG Xiaofeng, YU Yang, LIU Guihong, LI Ziwei, JING Cheng
2015, 34(7): 89-96. doi: 10.1007/s13131-015-0660-5
Rainfall has two significant effects on the sea surface, including salinity decreasing and surface becoming rougher, which have further influence on L-band sea surface emissivity. Investigations using the Aquarius and TRMM 3B42 matchup dataset indicate that the retrieved sea surface salinity (SSS) is underestimated by the present Aquarius algorithm compared to numerical model outputs, especially in cases of a high rain rate. For example, the bias between satellite-observed SSS and numerical model SSS is approximately 2 when the rain rate is 25 mm/h. The bias can be eliminated by accounting for rain-induced roughness, which is usually modeled by rain-generated ring-wave spectrum. The rain spectrum will be input into the Small Slope Approximation (SSA) model for the simulation of sea surface emissivity influenced by rain. The comparison with theoretical model indicated that the empirical model of rain spectrumis more suitable to be used in the simulation. Further, the coefficients of the rain spectrum are modified by fitting the simulations with the observations of the 2-year Aquarius and TRMM matchup dataset. The calculations confirm that the sea surface emissivity increases with the wind speed and rain rate. The increase induced by the rain rate is rapid in the case of low rain rate and low wind speed. Finally, a modified model of sea surface emissivity including the rain spectrum is proposed and validated by using the matchup dataset in May 2014. Compared with observations, the bias of the rain-induced sea surface emissivity simulated by the modified modelis approximately 1e-4, and the RMSE is slightly larger than 1e-3. With using more matchup data, thebias between model retrieved sea surface salinities and observationsmay be further corrected, and the RMSE may be reduced to less than 1 in the cases of low rain rate and low wind speed.
Features of the physical environment associated with green tide in the southwestern Yellow Sea during spring
BAO Min, GUAN Weibing, WANG Zongling, WANG Difeng, CAO Zhenyi, CHEN Qi
2015, 34(7): 97-104. doi: 10.1007/s13131-015-0692-x
Massive green tides caused by Ulva prolifera in the Yellow Sea have occurred every summer since 2007 and have caused huge economic losses for local governments. The Subei (North Jiangsu Province, China) Shoal, with its large-scale Porphyra aquaculture, has been regarded as the most important source of U. prolifera for green tides. To reveal the physical mechanisms of floating and drifting algae in this area, the characteristics of the current, the temperature, the salinity and suspended particulate matter (SPM) in the southwestern Yellow Sea, especially in the Subei Shoal, were studied. The topography of the radial sand ridges in the Subei Shoal constrains the features of the currents and causes net longitudinal and latitudinal movements. The longitudinal net movement is a dominant dynamic factor that can bring U. prolifera into offshore waters. The amount of gas that is produced by algae during photosynthesis determines whether U. prolifera can float well on the sea surface after it is disposed into the water from Porphyra aquacultural apparatus. The Subei Shoal is characterized by a high turbidity, which can result in significant light attenuation and affect the photosynthesis together with the buoyancy of a U. prolifera in the water. According to satellite remote sensing data from 2012, the three-month-averaged surface SPM (April, May and June) in the Subei Shoal was 140 mg/dm3, and the north of the Subei Shoal (the north of 34.5°N), it was 11 mg/dm3. According to the monthly averaged surface SPM in April, the transparency in the Subei Shoal was only 0.1 m, but it often exceeded 2.0 m outside of the Subei Shoal. The results explain why the floating ability of U. prolifera increases significantly once the green algae drifted outside the Subei Shoal.
Ocean Engineering
Calibration and validation of a sand model considering the effects of wave-induced principal stress axes rotation
LIU Peng, WANG Zhongtao, LI Xinzhong, CHAN Andrew
2015, 34(7): 105-115. doi: 10.1007/s13131-015-0655-2
Principal stress axes rotation influences the stress-strain behavior of sand under wave loading. A constitutive model for sand, which considers principal stress orientation and is based on generalized plasticity theory, is proposed. The new model, which employs stress invariants and a discrete memory factor during reloading, is original because it quantifies model parameters using experimental data. Four sets of hollow torsion experiments were conducted to calibrate the parameters and predict the capability of the proposed model, which describes the effects of principal stress orientation on the behavior of sand. The results prove the effectiveness of the proposed calibration method.
Runout prediction and dynamic characteristic analysis of a potential submarine landslide in Liwan 3-1 gas field
XIU Zongxiang, LIU Lejun, XIE Qiuhong, LI Jiagang, HU Guanghai, YANG Jianghui
2015, 34(7): 116-122. doi: 10.1007/s13131-015-0697-2
A large number of submarine landslides with different scales have been identified in the canyon area of the submarine pipeline route of Liwan 3-1 gas field. There is still much chance that submarine slope failures would happen, and the following mass movement would present great risk to the submarine pipeline. In view of this, a numerical prediction method based on Eulerian-Eulerian two-phase flow model is introduced to simulate the mass movement of potential submarine landslides. The sliding soil and ambient water are respectively simulated by Herschel-Bulkley rheology model and Newtonian fluid model. The turbulence is simulated using the k-ε model. Compared with both the experiment data and Bing result, the two-phase flow model shows a good accuracy, and its result is more close to the actual situation; the dynamic coupling between soil and ambient water can be effectively simulated and the phenomena of hydroplaning and head detachment can be obtained. Finally, the soil movement of a potential submarine landslide is simulated as an example, according to the seismic profile in the canyon area. The result shows that the hydroplaning occurs during the movement process. The runout distance calculated by the two-phase flow model is 877 m, which is 27.1% larger than the Bing result. However, the peak front velocity of soil is relative small, with a maximum value of 8.32 m/s. The Bing program with a simple and rapid process can be used for a preliminary evaluation, while the two-phase flow model is more appropriate for an accurate assessment.
The self-regulation process and its mechanism of channels’ bed changes in the Changjiang (Yangtze) Estuary in China
ZHANG Xiaohe, LI Jiufa, ZHU Wenwu, CHENG Heqin, CHEN Wei
2015, 34(7): 123-130. doi: 10.1007/s13131-015-0699-3
Recent bathymetric changes in the Changjiang Estuary under the influence of artificial regulation engineerings and basin reservoirs have been analyzed based on the maritime charts since 1997 and recent fieldworks. The results indicate a slight erosion of the channels in the upper and middle estuary, continuing deposition and seaward move of the mouth bar crest and intensifying erosion at the nearshore seabed. It is noteworthy that the morphological evolution caused by intensive human activities dominates over the changes from nature process. First, the riverbes are eroded overall in the South Branch (SB), the South Channel (SC) and the upper and middle reaches of the North Channel (NC). The nearshore seabed outside the river mouth is being eroded slightly, which is attributed to the declining sediment supply from the Changjiang Basin due to the construction of the Three Gorges Dam upstream. The sediment above the seabed is very active and coarsened, meanwhile, sand waves are becoming more distinct. Second, a deposition occurs in the North Brach (NB), the mouth of the NC, the mouth bars of the North Passage (NP) and the South Passage (SP) and especially the main channel of the NP, where it shows a massive siltation after the deep waterway project. The reasons for the recent changes are not only the dynamic structure in estuarine mouth bars, but also the supply of sediment resuspension in a local and offshore area. Meanwhile, the severe erosion and siltation in some reaches is related to the construction of estuarine engineerings. It is indicated that the Changjiang Estuary is gradually self-adjusted and adapting to the varying natural factors and intensive human activities. The study on the mechanism of self-regulation of the recent bathymetric changes in the Changjiang Estuary has important and practical significance.