2014 Vol. 33, No. 11

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An estimation of nutrient fluxes to the East China Sea continental shelf from the Taiwan Strait and Kuroshio subsurface waters in summer
LI Hongmei, SHI Xiaoyong, WANG Hao, HAN Xiurong
2014, 33(11): 1-10. doi: 10.1007/s13131-014-0550-2
According to historical mean ocean current data through the field observations of the Taiwan Ocean Research Institute during 1991-2005 and survey data of nutrients on the continental shelf of the East China Sea (ECS) in the summer of 2006, nutrient fluxes from the Taiwan Strait and Kuroshio subsurface waters are estimated using a grid interpolation method, which both are the sources of the Taiwan Warm Current. The nutrient fluxes of the two water masses are also compared. The results show that phosphate (PO4-P), silicate (SiO3-Si) and nitrate (NO3-N) fluxes to the ECS continental shelf from the Kuroshio upwelling water are slightly higher than those from the Taiwan Strait water in the summer of 2006. In contrast, owing to its lower velocity, the nutrient flux density (i.e., nutrient fluxes divided by the area of the specific section) of the Kuroshio subsurface water is lower than that of the Taiwan Strait water. In addition, the Taiwan Warm Current deep water, which is mainly constituted by the Kuroshio subsurface water, might directly reach the areas of high-frequency harmful alga blooms in the ECS.
A two-time-level split-explicit ocean circulation model (MASNUM) and its validation
2014, 33(11): 11-35. doi: 10.1007/s13131-014-0553-z
A two-time-level, three-dimensional numerical ocean circulation model (named MASNUM) was established with a two-level, single-step Eulerian forward-backward time-differencing scheme. A mathematical model of large-scale oceanic motions was based on the terrain-following coordinated, Boussinesq, Reynolds-averaged primitive equations of ocean dynamics. A simple but very practical Eulerian forward-backward method was adopted to replace the most preferred leapfrog scheme as the time-differencing method for both barotropic and baroclinic modes. The forward-backward method is of second-order of accuracy, computationally efficient by requiring only one function evaluation per time step, and free of the computational mode inherent in the three-level schemes. This method is superior to the leapfrog scheme in that the maximum time step of stability is twice as large as that of the leapfrog scheme in staggered meshes thus the computational efficiency could be doubled. A spatial smoothing method was introduced to control the nonlinear instability in the numerical integration. An ideal numerical experiment simulating the propagation of the equatorial Rossby soliton was performed to test the amplitude and phase error of this new model. The performance of this circulation model was further verified with a regional (northwest Pacific) and a quasi-global (global ocean simulation with the Arctic Ocean excluded) simulation experiments. These two numerical experiments show fairly good agreement with the observations. The maximum time step of stability in these two experiments were also investigated and compared between this model and that model which adopts the leapfrog scheme.
Mesoscale oceanic eddies in the South China Sea from 1992 to 2012: evolution processes and statistical analysis
DU Yunyan, YI Jiawei, WU Di, HE Zhigang, WANG Dongxiao, LIANG Fuyuan
2014, 33(11): 36-47. doi: 10.1007/s13131-014-0530-6
Automated identification and tracking of mesoscale ocean eddies has recently become one research hotspot in physical oceanography. Several methods have been developed and applied to survey the general kinetic and geometric characteristics of the ocean eddies in the South China Sea (SCS). However, very few studies attempt to examine eddies' internal evolution processes. In this study, we reported a hybrid method to trace eddies' propagation in the SCS based on their internal structures, which are characterized by eddy centers, footprint borders, and composite borders. Eddy identification and tracking results were represented by a GIS-based spatiotemporal model. Information on instant states, dynamic evolution processes, and events of disappearance, reappearance, split, and mergence is stored in a GIS database. Results were validated by comparing against the ten Dongsha Cyclonic Eddies (DCEs) and the three long-lived anticyclonic eddies (ACEs) in the northern SCS, which were reported in previous literature. Our study confirmed the development of these eddies. Furthermore, we found more DCE-like and ACE-like eddies in these areas from 2005 to 2012 in our database. Spatial distribution analysis of disappearing, reappearing, splitting, and merging activities shows that eddies in the SCS tend to cluster to the northwest of Luzon Island, southwest of Luzon Strait, and around the marginal sea of Vietnam. Kuroshio intrusions and the complex sea floor topography in these areas are the possible factors that lead to these spatial clusters.
A global study of temporal and spatial variation of SWH and wind speed and their correlation
HAN Shuzong, ZHANG Huirong, ZHENG Yunxia
2014, 33(11): 48-54. doi: 10.1007/s13131-014-0554-y
The climatology of significant wave height (SWH) and sea surface wind speed are matters of concern in the fields of both meteorology and oceanography because they are very important parameters for planning offshore structures and ship routings. The TOPEX/Poseidon altimeter, which collected data for about 13 years from September 1992 to October 2005, has measured SWHs and surface wind speeds over most of the world's oceans. In this paper, a study of the global spatiotemporal distributions and variations of SWH and sea surface wind speed was conducted using the TOPEX/Poseidon altimeter data set. The range and characteristics of the variations were analyzed quantitatively for the Pacific, Atlantic, and Indian oceans. Areas of rough waves and strong sea surface winds were localized precisely, and the correlation between SWH and sea surface wind speed analyzed.
Warm water wake off northeast Vietnam in the South China Sea
YAN Yunwei, CHEN Changlin, LING Zheng
2014, 33(11): 55-63. doi: 10.1007/s13131-014-0555-x
Due to orographic blockage, a weak wind wake occurs in summer off northeast Vietnam in the South China Sea. Under the wind wake, warm water is observed from both high-resolution satellite data and hydrographic observations. The wake of warm water forms in June, continues to mature in July and August, starts to decay in September, and disappears in October. The warm water wake also shows robust diurnal variation-it intensifies during the day and weakens in the night. Warm water wakes can be generated through wind-induced mixing and thermal (latent heat flux) processes. In this paper, a mixed layer model is used to evaluate the relative importance of the two processes on seasonal and diurnal timescales, respectively. The results demonstrate that thermal processes make a greater contribution to the wake than wind-induced mixing processes on a seasonal timescale, while the warm water wake is dominated by wind-induced mixing processes on a diurnal timescale.
The study on seasonal characteristics of water masses in the western East China Sea shelf area
ZHANG Qilong, LIU Hongwei, QIN Sisi, YANG Dezhou, LIU Zhiliang
2014, 33(11): 64-74. doi: 10.1007/s13131-014-0556-9
On the basis of the CTD data and the modeling results in the winter and summer of 2009, the seasonal characteristics of the water masses in the western East China Sea shelf area were analyzed using a cluster analysis method. The results show that the distributions and temperature-salinity characteristics of the water masses in the study area are of distinct seasonal difference. In the western East China Sea shelf area, there are three water masses during winter, i.e., continental coastal water (CCW), Taiwan Warm Current surface water (TWCSW) and Yellow Sea mixing water (YSMW), but four ones during summer, i.e., the CCW, the TWCSW, Taiwan Warm Current deep water (TWCDW) and the YSMW. Of all, the CCW, the TWCSW and the TWCDW are all dominant water masses. The CCW, primarily characterized by a low salinity, has lower temperature, higher salinity and smaller spatial extent in winter than in summer. The TWCSW is warmer, fresher and smaller in summer than in winter, and it originates mostly from the Kuroshio surface water (KSW) northeast of Taiwan, China and less from the Taiwan Strait water during winter, but it consists of the strait water and the KSW during summer. The TWCDW is characterized by a low temperature and a high salinity, and originates completely in the Kuroshio subsurface water northeast of Taiwan.
A comparison of the CMIP5 models on the historical simulation of the upper ocean heat content in the South China Sea
WANG Gang, LIN Min
2014, 33(11): 75-84. doi: 10.1007/s13131-014-0557-8
Seventeen models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5) activity are compared on their historical simulation of the South China Sea (SCS) ocean heat content (OHC) in the upper 300 m. Ishii's temperature data, based on the World Ocean Database 2005 (WOD05) and World Ocean Atlas 2005 (WOA05), is used to assess the model performance by comparing the spatial patterns of seasonal OHC anomaly (OHCa) climatology, OHC climatology, monthly OHCa climatology, and interannual variability of OHCa. The spatial patterns in Ishii's data set show that the seasonal SCS OHCa climatology, both in winter and summer, is strongly affected by the wind stress and the current circulations in the SCS and its neighboring areas. However, the CMIP5 models present rather different spatial patterns and only a few models properly capture the dominant features in Ishii's pattern. Among them, GFDL-ESM2G is of the best performance. The SCS OHC climatology in the upper 300 m varies greatly in different models. Most of them are much greater than those calculated from Ishii's data. However, the monthly OHCa climatology in each of the 17 CMIP5 models yields similar variation and magnitude as that in Ishii's. As for the interannual variability, the standard deviations of the OHCa time series in most of the models are somewhat larger than those in Ishii's. The correlation between the interannual time series of Ishii's OHCa and that from each of the 17 models is not satisfactory. Among them, BCC-CSM1.1 has the highest correlation to Ishii's, with a coefficient of about 0.6.
Impact of the central Pacific zonal wind divergence and convergence on the central Pacific El Niño event
ZUO Tao, CHEN Jinnian, WANG Hongna
2014, 33(11): 85-89. doi: 10.1007/s13131-014-0497-3
The central Pacific (CP) zonal wind divergence and convergence indices are defined, and the forming mechanism of CP El Niño (La Niña) events is discussed preliminarily. The results show that the divergence and convergence of the zonal wind anomaly (ZWA) are the key process in the forming of CP El Niño (La Niña) events. A correlation analysis between the central Pacific zonal wind divergence and convergence indices and central Pacific El Niño indices indicates that there is a remarkable lag correlation between them. The central Pacific zonal wind divergence and convergence indices can be used to predict the CP events. Based on these results, a linear regression equation is obtained to predict the CP El Niño (La Niña) events 5 months ahead.
An upper ocean response to Typhoon Bolaven analyzed with Argo profiling floats
LIU Zenghong, XU Jianping, SUN Chaohui, WU Xiaofen
2014, 33(11): 90-101. doi: 10.1007/s13131-014-0558-7
In situ observations from Argo profiling floats combined with satellite retrieved SST and rain rate are used to investigate an upper ocean response to Typhoon Bolaven from 20 through 29 August 2012. After the passage of Typhoon Bolaven, the deepening of mixed layer depth (MLD), and the cooling of mixed layer temperature (MLT) were observed. The changes in mixed layer salinity (MLS) showed an equivalent number of increasing and decreasing because the typhoon-induced salinity changes in the mixed layer were influenced by precipitation, evaporation, turbulent mixing and upwelling of thermocline water. The deepening of the MLD and the cooling of the MLT indicated a significant rightward bias, whereas the MLS was freshened to the left side of the typhoon track and increased on the other side. Intensive temperature and salinity profiles observed by Iridium floats make it possible to view response processes in the upper ocean after the passage of a typhoon. The cooling in the near-surface and the warming in the subsurface were observed by two Iridium floats located to the left side of the cyclonic track during the development stage of the storm, beyond the radius of maximum winds relative to the typhoon center. Water salinity increases at the base of the mixed layer and the top of the thermocline were the most obvious change observed by those two floats. On the right side of the track and near the typhoon center when the typhoon was intensified, the significant cooling from sea surface to a depth of 200×104 Pa, with the exception of the water at the top of the thermocline, was observed by the other Iridium float. Owing to the enhanced upwelling near the typhoon center, the water salinity in the near-surface increased noticeably. The heat pumping from the mixed layer into the thermocline induced by downwelling and the upwelling induced by the positive wind stress curl are the main causes for the different temperature and salinity variations on the different sides of the track. It seems that more time is required for the anomalies in the subsurface to be restored to pretyphoon conditions than for the anomalies in the mixed layer.
Near-inertial waves in the wake of 2011 Typhoon Nesat in the northern South China Sea
YANG Bing, HOU Yijun
2014, 33(11): 102-111. doi: 10.1007/s13131-014-0559-6
In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea (SCS). A wake of baroclinic near-inertial waves (NIWs) commenced after Nesat passed the array. The associated near-inertial currents are surface-intensified and clockwise-polarized. The vertical range of NIWs reached 300 m, where the vertical range is defined as the maximum depth of the horizontal near-inertial velocity 5 cm/s. The current oscillations have a frequency of 0.709 9 cycles per day (cpd), which is 0.025f higher than the local inertial frequency. The NIWs have an e-folding time-scale of 10 d based on the evolution of the near-inertial kinetic energy. The depth-leading phase of near-inertial currents indicates downward group velocity and energy flux. The estimated vertical phase velocity and group velocity are 0.27 and 0.08 cm/s respectively, corresponding to a vertical wavelength of 329 m. A spectral analysis reveals that NIWs act as a crucial process to redistribute the energy injected by Typhoon Nesat. A normal mode and an empirical orthogonal function analysis indicate that the second mode has a dominant variance contribution of 81%, and the corresponding horizontal phase velocity and wavelength are 3.50 m/s and 420 km respectively. The remarkable large horizontal phase velocity is relevant to the rotation of the earth, and a quantitative analysis suggests that the phase velocity of the NIWs with a blue-shift of 0.025f overwhelms that of internal gravity waves by a factor of 4.6.
The observed analysis on the wave spectra of Hurricane Juan (2003)
XU Fumin, BUI THI Thuy Duyen, PERRIE Will
2014, 33(11): 112-122. doi: 10.1007/s13131-014-0560-0
Hurricane Juan provides an excellent opportunity to probe into the detailed wave spectral patterns and spectral parameters of a hurricane system, with enough wave spectral observations around Juan's track in the deep ocean and shallow coastal water. In this study, Hurricane Juan and wave observation stations around Juan's track are introduced. Variations of wave composition are discussed and analyzed based on time series of one-dimensional frequency spectra, as well as wave steepness around Juan's track: before, during, and after Juan's passing. Wave spectral involvement is studied based on the observed one-dimensional spectra and two-dimensional spectra during the hurricane. The standardization method of the observed wave spectra during Hurricane Juan is discussed, and the standardized spectra show relatively conservative behavior, in spite of the huge variation in wave spectral energy, spectral peak, and peak frequency during this hurricane. Spectral widths' variation during Hurricane Juan are calculated and analyzed. A two-layer nesting WW3 model simulation is applied to simulate the one-dimensional and two-dimensional wave spectra, in order to examine WW3's ability in simulating detailed wave structure during Hurricane Juan.
Simulation of Typhoon Muifa using a mesoscale coupled atmosphere-ocean model
SUN Minghua, DUAN Yihong, ZHU Jianrong, WU Hui, ZHANG Jin, HUANG Wei
2014, 33(11): 123-133. doi: 10.1007/s13131-014-0561-z
A mesoscale coupled atmosphere-ocean model has been developed based on the GRAPES (Global and Regional Assimilation and Prediction System) regional typhoon model (GRAPES_TYM) and ECOM-si (estuary, coast and ocean model (semi-implicit)). Coupling between the typhoon and ocean models was conducted by exchanging wind stress, heat, moisture fluxes, and sea surface temperatures (SSTs) using the coupler OASIS3.0. Numerical prediction experiments were run with and without coupling for the case of Typhoon Muifa in the western North Pacific. To investigate the impact of using more accurate SST information on the simulation of the track and the intensity of Typhoon Muifa, experiments were also conducted using increased SST resolution in the initial condition field of the control test. The results indicate that increasing SST resolution in the initial condition field somewhat improved the intensity forecast, and use of the coupled model improved the intensity forecast significantly, with mean absolute errors in maximum wind speed within 48 and 72 h reduced by 32% and 20%, respectively. Use of the coupled model also resulted in less pronounced over-prediction of the intensity of Typhoon Muifa by the GRAPES_TYM. Moreover, the effects of using the coupled model on the intensity varied throughout the different stages of the development of Muifa owing to changes in the oceanic mixed layer depth. The coupled model had pronounced effects during the later stage of Muifa but had no obvious effects during the earlier stage. The SSTs predicted by the coupled model decreased by about 5-6℃ at most after the typhoon passed, in agreement with satellite data. Furthermore, based on analysis on the sea surface heat flux, wet static energy of the boundary layer, atmospheric temperature, and precipitation forecasted by the coupled model and the control test, the simulation results of this coupled atmosphere-ocean model can be considered to reasonably reflect the primary mechanisms underlying the interactions between tropical cyclones and oceans.
An Arctic sea ice thickness variability revealed from satellite altimetric measurements
BI Haibo, HUANG Haijun, SU Qiao, YAN Liwen, LIU Yanxia, XU Xiuli
2014, 33(11): 134-140. doi: 10.1007/s13131-014-0562-y
A modified algorithm taking into account the first year (FY) and multiyear (MY) ice densities is used to derive a sea ice thickness from freeboard measurements acquired by satellite altimetry ICESat (2003-2008). Estimates agree with various independent in situ measurements within 0.21 m. Both the fall and winter campaigns see a dramatic extent retreat of thicker MY ice that survives at least one summer melting season. There were strong seasonal and interannual variabilities with regard to the mean thickness. Seasonal increases of 0.53 m for FY the ice and 0.29 m for the MY ice between the autumn and the winter ICESat campaigns, roughly 4-5 month separation, were found. Interannually, the significant MY ice thickness declines over the consecutive four ICESat winter campaigns (2005-2008) leads to a pronounced thickness drop of 0.8 m in MY sea ice zones. No clear trend was identified from the averaged thickness of thinner, FY ice that emerges in autumn and winter and melts in summer. Uncertainty estimates for our calculated thickness, caused by the standard deviations of multiple input parameters including freeboard, ice density, snow density, snow depth, show large errors more than 0.5 m in thicker MY ice zones and relatively small standard deviations under 0.5 m elsewhere. Moreover, a sensitivity analysis is implemented to determine the separate impact on the thickness estimate in the dependence of an individual input variable as mentioned above. The results show systematic bias of the estimated ice thickness appears to be mainly caused by the variations of freeboard as well as the ice density whereas the snow density and depth brings about relatively insignificant errors.
Mapping sea surface velocities in the Changjiang coastal zone with advanced synthetic aperture radar
WANG Lihua, ZHOU Yunxuan, GE Jianzhong, JOHANNESSEN Johnny A., SHEN Fang
2014, 33(11): 141-149. doi: 10.1007/s13131-014-0563-x
Range Doppler velocities derived from the Envisat advanced synthetic aperture radar (ASAR) wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model (FVCOM) for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency (ESA) Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells.The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.
Component characteristics of polycyclic aromatic compounds in sediments of the South Mid-Atlantic Ridge
HUANG Xin, ZENG Zhigang, CHEN Shuai, YIN Xuebo, WANG Xiaoyuan, MA Yao, YANG Baoju, RONG Kunbo, SHU Yunchao, JIANG Tao
2014, 33(11): 150-154. doi: 10.1007/s13131-014-0564-9
10 samples of sediments obtained from the South Mid-Atlantic Ridge were measured for the abundances and distributions of polycyclic aromatic compounds (PAHs). The total concentrations of PAHs (∑PAHs) ranged from 2.768 to 9.826 μg/g dry sediment. The ∑PAHs was higher in sample 22V-TVG10 and sample 26V-TVG05 which were close to hydrothermal fields, with the lowest value in sample 22V-TVG14 which was farthest from hydrothermal fields, suggesting a probable hydrothermal origin of ∑PAHs of samples. Approximately nine kinds of PAHs were identified, and low molecular mass tricyclic and tetracyclic aromatic compounds were predominant in the samples. The concentrations of fluoranthene which were typical as hydrothermal alteration compounds were the highest among PAHs with dry weight between 0.913-3.157 μg/g. The phenanthrene homologue was most abundant in the samples, and the ratios between parent phenanthrene and methylphenanthrene which probably reflected the degree of hydrothermal alteration ranged from 0.097 to 1.602. The sample 22V-TVG10 possessing a maximum ratio value showed the intense influence of the hydrothermal alteration on this sample, which might further imply that PAHs in sediments were mainly derived from the hydrothermal alteration.
Research Notes
A continental slope stability evaluation in the Zhujiang River Mouth Basin in the South China Sea
LIU Ke, WANG Jianhua
2014, 33(11): 155-160. doi: 10.1007/s13131-014-0565-8
In nature, a slope stability is determined by the ratio of a sliding resistance to a slide force. The slide force of a marine deep-water continental slope is mainly affected by sediment mechanics properties, a topography, and a marine seismic. However, the sliding resistance is mainly affected by sedimentary patterns and a sedimentary stress history. Both of these are different from case to case, and their impact can be addressed when the data are organized in a geographic information system (GIS). The study area on the continental slope in Zhujiang River Mouth Basin in South China Sea provides an excellent opportunity to apply GIS spatial analysis technology for the evaluation of the slope stability. In this area, a continental slope topography and a three-dimension (3-D) topography mapping show a sea-floor morphology and the distribution of a slope steepness in good detail, and the sediment analysis of seabed samples and an indoor appraisal reveals the variability of a sediment density near the sea-floor surface. On the basis of the results of nine geotechnical studies of submarine study areas, it has worked out that an equivalent cyclic shear stress ratio is roughly between 0.158 and 0.933, which is mainly depending on the initial water content of sediment. A regional density, slope and level of anticipated seismic shaking information are combined in a GIS framework to yield a map that illustrates a continental slope stability zoning under the influencing factors in Zhujiang River Mouth Basin in the South China Sea. The continental slope stability evaluation can contribute to north resources development in the South China Sea, the marine functional zoning, the marine engineering construction and adjust measures to local conditions, at the same time also can provide references for other deep-water slope stability analysis.