Intercomparison of three South China Sea circulation models

Three numerical oceanic circulation models: POM(Princeton ocean model), MICOM(Miami isopycnal coordinates ocean model) and GFDL model, which adopt sigma coordinate, isopycnal coordinate and depth coordinate respectively, are used in the South China Sea(SCS) circulation modeling. Model domain has the same topography, grid resolution, initial conditions and surface boundary conditions. The maximum ocean depth is set as 1 000 m. Grid resolution is 0.5o×0.5o.Initial conditions are supplied by climatological temperature and salinity data in January. Climatological wind stress, surface temperature and salinity are used as surface forcing. Lateral boundaries take enclosed boundary conditions artificially. Focusing on the common point of different ocean circulation models, the circulation pattern in winter and summer, sea surface height in the northern SCS, seasonal cycle of the mixed layer thickness in the southern SCS, barotropic stream function in winter are selected to carry out intercomparison. In winter, a strong cyclonic gyre occupies the whole SCS. In summer, a strong anticyclonic gyre occupies the southern SCS and a weak cyclonic gyre occupies the northern SCS. The thickness of the mixed layer shows bimodal features in the southern SCS. Sea surface height anomaly(SSHA) in the northern SCS has an eastward propagating feature, in agreement with the remote sensing observation. Barotropic stream functions indicate that the circulation of the upper ocean is mainly forced by inputting of wind stress curl under closed boundary conditions. In addition, three models also show distinct differences. The basin-scale circulation from MICOM is distinct. Output of POM has more mesoscale eddies than others. GFDL model seems good at simulating westward intensification.

The calculation of the circulation in South China Sea by a diagnostic model

A high resolved two-dimensional linear global diagnostic model combining with the dynamical calculation is used to calculate velocity field in the South China Sea(SCS). The study of model results shows that eddy diffusion does not change basic structure of circulation in the SCS and does not change the direction of invasive water, but changes the value of transport considerably especially in straits. The velocity field is not changed whether the wind stress is considered or not. This result shows the circulation is largely determined by a density field which well records most of the important contribution of the wind stress effect. Potential vorticity is calculated to testify the dynamics of the model results. The result shows that a good conservation of the nonlinear PV. This indicates most effects of the important nonlinear processes are well recorded in density and the nonlinear term is negligible so that the simplified model is reliable. The model results show the water exchanges between the SCS and open ocean or surrounding seas. Cold deep water invades through Luzon Strait and Warm shallow water is pushed out mainly through Karimata Straits. The model results also reveal the structure of the circulation in the SCS basin. In two circulations of upper and middle layers, a cyclonic one in the north and an anti-cyclonic one in the south, reflect the climatologic average of the circulation driven by monsoon. In the deep or bottom layer, these two circulations reflect the topography of the basin. Above the middle layer, invasive water enters westward in the north but the way of invasion of Kuroshio is not clear. Below the deep layer, a current goes down south near the east basin ,and invasive water enters in the basin from the west Pacific.

Case study on the three-dimensional structure of meso-scale eddy in the South China Sea based on a high-resolution model

Meso-scale eddies are important features in the South China Sea（SCS）. The eddies with diameters of 50–200 km can greatly impact the transport of heat, momentum, and tracers. A high-resolution wave-tide-circulation coupled model was developed to simulate the meso-scale eddy in the SCS in this study. The aim of this study is to examine the model ability to simulate the meso-scale eddy in the SCS without data assimilations The simulated Sea Surface Height（SSH） anomalies agree with the observed the AVISO SSH anomalies well. The simulated subsurface temperature profiles agree with the CTD observation data from the ROSE（Responses of Marine Hazards to climate change in the Western Pacific） project. The simulated upper-ocean currents also agree with the main circulation based on observations. A warm eddy is identified in winter in the northern SCS. The position and domain of the simulated eddy are confirmed by the observed sea surface height data from the AVISO. The result shows that the model has the ability to simulate the meso-scale eddy in the SCS without data assimilation.The three-dimensional structure of the meso-scale eddy in the SCS is analyzed using the model result. It is found that the eddy center is tilted vertically, which agrees with the observation. It is also found that the velocity center of the eddy does not coincide with the temperature center of the eddy. The result shows that the model has the ability to simulate the meso-scale eddy in the SCS without data assimilations. Further study on the forming mechanism and the three-dimensional structure of the meso-scale eddies will be carried out using the model result and cruise observation data in the near future.

Evaluation of numerical wave model for typhoon wave simulation in South China Sea

The simulating waves nearshore(SWAN) model has typically been designed for wave simulations in near-shore regions. In this study, the model's applicability to the simulation of typhoon waves in the South China Sea(SCS) was evaluated. A blended wind field, consisting of an interior domain based on Fujita's model and an exterior domain based on Takahashi's model, was used as the driving wind field. The waves driven by Typhoon Kai-tak over the SCS that occurred in 2012 were selected for the numerical simulation research. Sensitivity analyses of time step, grid resolution, and angle resolution were performed in order to obtain optimal model settings. Through sensitivity analyses, it can be found that the time step has a large influence on the results, while grid resolution and angle resolution have a little effect on the results.

Meso-scale eddy in the South China Sea simulated by an eddy-resolving ocean model

Mesoscale eddies （MEs） in the South China Sea （SCS） simulated by a quasi-global eddy-resolving ocean general circulation model are evaluated against satellite data during 1993-2007. The modeled ocean data show more activity than shown by the satellite data and reproduces more eddies in the SCS. A total of 345 （428） cyclonic eddies （CEs） and 330 （371） anti-cyclonic eddies （AEs） generated for satellite （model） data are identified during the study period, showing increase of -24% and -12% for the model data, respectively. Compared with eddies in satellite, the simulated eddies tend to have smaller radii, larger amplitudes, a slightly longer lifetime, faster movement and rotation speed, a slightly larger nonlinear properties （U/c） in the model. However, the spatial distribution of generated eddies appears to be inhomogeneous, with more CEs in the northern part of SCS and fewer AEs in the southern part. This is attributed to the exaggerated Kuroshio intrusion in the model because the small islands in the Luzon Strait are still not well resolved although the horizontal resolution reaches （1/10）°. The seasonal variability in the number and the amplitude of eddies generated is also investigated.

Development of a fine-resolution atmosphere-wave-ocean coupled forecasting model for the South China Sea and its adjacent seas

A 72-h fine-resolution atmosphere-wave-ocean coupled forecasting system was developed for the South China Sea and its adjacent seas. The forecasting model domain covers from from 15°S to 45°N in latitude and 99°E to135°E in longitude including the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the Indonesian seas. To get precise initial conditions for the coupled forecasting model, the forecasting system conducts a 24-h hindcast simulation with data assimilation before forecasting. The Ensemble Adjustment Kalman Filter(EAKF) data assimilation method was adopted for the wave model MASNUM with assimilating Jason-2 significant wave height(SWH) data. The EAKF data assimilation method was also introduced to the ROMS model with assimilating sea surface temperature(SST), mean absolute dynamic topography(MADT) and Argo profiles data. To improve simulation of the structure of temperature and salinity, the vertical mixing scheme of the ocean model was improved by considering the surface wave induced vertical mixing and internal wave induced vertical mixing. The wave and current models were integrated from January 2014 to October 2015 driven by the ECMWF reanalysis 6 hourly mean dataset with data assimilation. Then the coupled atmosphere-wave-ocean forecasting system was carried out 14 months operational running since November 2015. The forecasting outputs include atmospheric forecast products, wave forecast products and ocean forecast products. A series of observation data are used to evaluate the coupled forecasting results, including the wind, SHW, ocean temperature and velocity.The forecasting results are in good agreement with observation data. The prediction practice for more than one year indicates that the coupled forecasting system performs stably and predict relatively accurate, which can support the shipping safety, the fisheries and the oil exploitation.

Stress field modeling of northwestern South China Sea since 5.3 Ma and its tectonic significance

Tectonically, the northwestern South China Sea （SCS） is located at the junction between three micro-plates, i.e., the Indochina, South China and Zhongsha-Xisha micro-plates, and involves three basins, i.e., the Yinggehai Basin, the Qiongdongnan Basin and Xisha Trough in the east, and the Zhongjiannan Basin in the south. Since the Pliocene （5.3 Ma）, the Yinggehai Basin has experienced repeated accelerating subsidence, high thermal fluid, and widely developing mud-rich overpressure chambers, abundant mud diapers and crust-mantle mixed CO2. While a large central canyon was developed in the Qiongdongnan Basin, new rift occurred in the Xisha ~rough. These characteristics demonstrate a single tectonic unit for the northwestern SCS, for which we have undertaken stress field modeling to understand its plate deformations and sedimen- tary responses. Our results demonstrate that an extension tectonic event occurred after 5.3 Ma in theYingge- hal-Qiongdongnan-Xisha trough area, which is characterized by thinner crust C〈16000 m）, half-graben or graben structural style and thicker sedimentary sequences （〉3 500 m）. A new rift system subsequently was developed in this area; this event was mainly driven by the combined effects of different movement veloc- ity and direction of the three micro-plates, and the far-field effect of the continental collision between the Indian Plate and the Tibetan Plateau, and subduction of the Pacific Plate underneath the Eurasian Plate.

Model-Simulated Coastal Trapped Waves Stimulated by Typhoon in Northwestern South China Sea

In this paper, we apply an unstructured grid coastal ocean model to simulate variations in the sea level and currents forced by two typhoons in the northwestern South China Sea(SCS). The model simulations show distinct differences for the two cases in which the typhoon paths were north and south of the Qiongzhou(QZ) Strait. In both cases, coastal trapped waves(CTWs) are stimulated but their propagation behaviors differ. Model sensitivity simulations suggest the dominant role played by alongshore wind in the eastern SCS(near Shanwei) and southeast of Hainan Island. We also examine the influence of the Leizhou Peninsula by changing the coastline in simulation experiments. Based on our results, we can draw the following conclusions: 1) The CTWs stimulated by the northern typhoon are stronger than the southern CTW. 2) In the two cases, the directions of the current structures of the QZ cross-transect are reversed. The strongest flow cores are both located in the middle-upper area of the strait and the results of our empirical orthogonal function analysis show that the vertical structure is highly barotropic. 3) The simulated CTWs divide into two branches in the QZ Strait for the northern typhoon, and an island trapped wave(ITW) around Hainan Island for the southern typhoon. 4) The Leizhou Peninsula plays a significant role in the distribution of the kinetic energy flux between the two CTW branches. In the presence of the Leizhou Peninsula, the QZ branch has only 39.7 percent of the total energy, whereas that ratio increases to 72.2 percent in its absence.

Relationship between South China Sea Precipitation Variability and Tropical Indo-Pacific SST Anomalies in IPCC CMIP5 Models during Spring-to-Summer Transition

The present study evaluates the precipitation variability over the South China Sea（SCS） and its relationship to tropical Indo-Pacific SST anomalies during spring-to-summer transition（April–May–June,AMJ） simulated by 23 Intergovernmental Panel on Climate Change Coupled Model Intercomparison Project Phase 5 coupled models.Most of the models have the capacity to capture the AMJ precipitation variability in the SCS.The precipitation and SST anomaly（SSTA） distribution in the SCS,tropical Pacific Ocean（TPO）,and tropical Indian Ocean（TIO） domains is evaluated based on the pattern correlation coefficients between model simulations and observations.The analysis leads to several points of note.First,the performance of the SCS precipitation anomaly pattern in AMJ is model dependent.Second,the SSTA pattern in the TPO and TIO is important for capturing the AMJ SCS precipitation variability.Third,a realistic simulation of the western equatorial Pacific（WEP） and local SST impacts is necessary for reproducing the AMJ SCS precipitation variability in some models.Fourth,the overly strong WEP SST impacts may disrupt the relationship between the SCS precipitation and the TPO–TIO SST.Further work remains to be conducted to unravel the specific reasons for the discrepancies between models and observations in various aspects.

Comparison of air-sea CO2 flux and biological productivity in the South China Sea,East China Sea,and Yellow Sea:a three-dimensional physical-biogeochemical modeling study

Marginal seas play important roles in regulating the global carbon budget, but there are great uncertainties in estimating carbon sources and sinks in the continental margins. A Pacific basin-wide physical-biogeochemical model is used to estimate primary productivity and air-sea CO_2 flux in the South China Sea（SCS）, the East China Sea（ECS）, and the Yellow Sea（YS）. The model is forced with daily air-sea fluxes which are derived from the NCEP2 reanalysis from 1982 to 2005. During the period of time, the modeled monthly-mean air-sea CO_2 fluxes in these three marginal seas altered from an atmospheric carbon sink in winter to a source in summer. On annualmean basis, the SCS acts as a source of carbon to the atmosphere（16 Tg/a, calculated by carbon, released to the atmosphere）, and the ECS and the YS are sinks for atmospheric carbon（–6.73 Tg/a and –5.23 Tg/a, respectively,absorbed by the ocean）. The model results suggest that the sea surface temperature（SST） controls the spatial and temporal variations of the oceanic pCO_2 in the SCS and ECS, and biological removal of carbon plays a compensating role in modulating the variability of the oceanic pCO_2 and determining its strength in each sea,especially in the ECS and the SCS. However, the biological activity is the dominating factor for controlling the oceanic pCO_2 in the YS. The modeled depth-integrated primary production（IPP） over the euphotic zone shows seasonal variation features with annual-mean values of 293, 297, and 315 mg/（m^2·d） in the SCS, the ECS, and the YS, respectively. The model-integrated annual-mean new production（uptake of nitrate） values, as in carbon units, are 103, 109, and 139 mg/（m^2·d）, which yield the f-ratios of 0.35, 0.37, and 0.45 for the SCS, the ECS, and the YS, respectively. Compared to the productivity in the ECS and the YS, the seasonal variation of biological productivity in the SCS is rather weak. The atmospheric pCO_2 increases from 1982 to 2005, which is consistent with the anthropogenic CO_2 input to the atmosphere. The oceanic pCO_2 increases in responses to the atmospheric pCO_2 that drives air-sea CO_2 flux in the model. The modeled increase rate of oceanic pCO_2 is0.91 μatm/a in the YS, 1.04 μatm/a in the ECS, and 1.66 μatm/a in the SCS, respectively.

Seasonal Prediction of June Rainfall over South China:Model Assessment and Statistical Downscaling

The performances of various dynamical models from the Asia-Pacific Economic Cooperation（APEC） Climate Center（APCC） multi-model ensemble（MME） in predicting station-scale rainfall in South China（SC） in June were evaluated.It was found that the MME mean of model hindcasts can skillfully predict the June rainfall anomaly averaged over the SC domain.This could be related to the MME＇s ability in capturing the observed linkages between SC rainfall and atmospheric large-scale circulation anomalies in the Indo-Pacific region.Further assessment of station-scale June rainfall prediction based on direct model output（DMO） over 97 stations in SC revealed that the MME mean outperforms each individual model.However,poor prediction abilities in some in-land and southeastern SC stations are apparent in the MME mean and in a number of models.In order to improve the performance at those stations with poor DMO prediction skill,a station-based statistical downscaling scheme was constructed and applied to the individual and MME mean hindcast runs.For several models,this scheme can outperform DMO at more than 30 stations,because it can tap into the abilities of the models in capturing the anomalous Indo-Paciric circulation to which SC rainfall is considerably sensitive.Therefore,enhanced rainfall prediction abilities in these models should make them more useful for disaster preparedness and mitigation purposes.

Model study of the upper circulation in the Sulu Sea and its relation to the South China Sea circulation

According to the satellite remote sensing monthly mean sea surface temperature data from 1998 to 2002, it is shown that, the Sulu Sea is dominated by a cold eddy both in summer and winter. A coupled single-layer/two-layer model is employed here to study the summery and wintry characteristics and dynamic mechanism of the upper circulation in the Sulu Sea. According to the numerical experiments, it is found that, due to the local monsoon stress curl, the upper circulation in the Sulu Sea is dominated by a weak anticyclonic eddy in summer and a strong cyclonic eddy in winter. Once there is a large outflow via the Sibutu Passage flowing out of the Sulu Sea in summer, the upper circulation in the Sulu Sea may be dominated by a cyclonic eddy instead of the normal anticyclonic one. Moreover, in summer, the water exchange between the Sulu Sea and South China Sea via the Mindoro and Balabac Straits might have some effect on the separation position and strength of the northward western boundary current next to the Indo-China Peninsula in the southern South China Sea.

A 3-dimensional baroclinic circulation model of the South China Sea

The summer and winter circulations in the South China Sea (SCS) including the surface elevation and water temperature are simulated using the model described by Cai and Li (1996) with the monthly mean wind stress and air temperature field at the 1000 mb level from the European Centre for Medium-Range Weather Forecasts as inputs. The boundary conditions at Bashi Channel and Taiwan Strait are taken from the simulation results of the Kuroshio using the same numerical model with a grid size of 0.5°×0.5° and the results of Cai and Li (1996) as boundary conditions. The computational domain for the present paper is between 100°E and 123°E and between 4.5°N and 27°N. The horizontal resolution is 0.25°×0.25° and the vertical variations of the velocity components are resolved by 6 layers The computed steady flow, temperature and elevation fields are consistent with the corresponding fields observed. In particular, the temperature and elevation fields of the South China Sea Warm Current (SCSWC) have been successfully simulated. The paths of the branch of the Kuroshio entering the South China Sea (SCSBK) through Bashi Channel in winter and summer are discussed It is found that the SCSBK flows southward to the southern SCS from the coast of the Guangdong Province. A portion of the SCSBK returns to the Bashi Channel and subdivides again in deep waters in winter with a branch flows to the south along the coast of the Philippines instead of flowing back to the Pacific In addition, our results confirm the existence of a eastward current to the northeast of Dongsha in summer with the Kuroshio as its source as suggested by Huang et al. Since the value of the eddy viscosity adopted for the simulation of the Kuroshio is on the high side, resulting in a weaker west boundary current in the western Pacific as the boundary conditions for the present simulations, some deviations from the actual situations are expected although the results are in general consistent with observations.

Seasonal variability of the isopycnal surface circulation in the South China Sea derived from a variable-grid global ocean circulation model

In this study, we develop a variable-grid global ocean general circulation model （OGCM） with a fine grid （1/6）° covering the area from 20°S-50°N and from 99°-150°E, and use the model to investigate the isopycnal surface circulation in the South China Sea （SCS）. The simulated results show four layer structures in vertical： the surface and subsurface circulation of the SCS are characterized by the monsoon driven circulation, with basin-scaled cyclonic gyre in winter and anti-cyclonic gyre in summer. The intermediate layer circulation is opposite to the upper layer, showing anti-cyclonic gyre in winter but cyclonic gyre in summer. The circulation in the deep layer is much weaker in spring and summer, with the maximum velocity speed below 0.6 cm/s. In fall and winter, the SCS deep layer circulation shows strong east boundary current along the west coast of Philippine with the velocity speed at 1.5 m/s, which flows southward in fall and northward in winter. The results have also revealed a fourlayer vertical structure of water exchange through the Luzon Strait. The dynamics of the intermediate and deep circulation are attributed to the monsoon driving and the Luzon Strait transport forcing.

Sea level rise projection in the South China Sea from CMIP5 models

Future potential sea level change in the South China Sea （SCS） is estimated by using 24 CMIP5 models under different representative concentration pathway （RCP） scenarios. By the end of the 21st century （2081–2100 relative to 1986–2005）, the multimodel ensemble mean dynamic sea level （DSL） is projected to rise 0.9, 1.6, and 1.1 cm under RCP2.6, RCP4.5, and RCP8.5 scenarios, respectively, resulting in a total sea level rise （SLR） of 40.9, 48.6, and 64.1 cm in the SCS. It indicates that the SCS will experience a substantial SLR over the 21st century, and the rise is only marginal larger than the global mean SLR. During the same period, the steric sea level （SSL） rise is estimated to be 6.7, 10.0, and 15.3 cm under the three scenarios, respectively, which accounts only for 16%, 21% and 24% of the total SLR in this region. The changes of the SSL in the SCS are almost out of phase with those of the DSL for the three scenarios. The central deep basin has a slightly weak DSL rise, but a strong SSL rise during the 21st century, compared with the north and southwest shelves.

Thermodynamic Modeling of Fluid-Bearing Natural Gas Inclusions for Geothermometer and Geobarometer of Overpressured Environments in Qiongdongnan Basin, South China Sea

It is a very difficult problem to directly determine fluid pressure duringhydrocarbon migration and accumulation in sedimentary basins. pVt modeling of coupling hydrocarbonfluid inclusion of its coeval aqueous fluid inclusion provides a powerful tool for establishing therelationship of formation pressure evolution with time. Homogenization temperature of fluidinclusion can routinely be measured under microthermometric microscopy. Crushing technique has beenemployed to obtain the composition of fluid inclusions, and the commercial software VTFLINC easilyand rapidly completes the construction of p-t phase diagram. The minimum trapping pressure ofhydrocarbon fluid inclusion would be then determined in the p-t space. In this paper, three samplesof YC21-1-1 and YC21-1-4 wells at YC21-1 structural closure, Qiongdongnan basin, South China Sea,were selected for the pVt modeling practice, and the formation pressure coefficient (equals to fluidpressure/hydrostatic pressure) changing trend with time has primarily been established. Themodeling results also indicate that the reservoirs of Ling-shui and Yacheng formations in YC21-1structure are within a very high potential system and would have undergone a discharging of thermalfluids through top seal rupture, which depicts that there is a very high risk for natural gasexploration in this area.

APPLICATION OF HYSPLIT MODEL IN DEFINITION OF THE ONSET OF SOUTH CHINA SEA SUMMER MONSOON

Using NCEP reanalysis data and an airflow trajectory model based on the Lagrangian method, theHybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the daily backward trajectorieson the height of 850 hPa above the South China Sea (SCS) area are simulated from April to June. The onsetdate of the SCS summer monsoon from 1948 to 2009 is determined according to the simulated source ofairflow in the monitored area of the SCS. By analyzing the SCS monsoon onset dates over the 62 years, wefound that the number of years in which the SCS monsoon onset is earlier accounts for 13%, and the lateryears 14%, the normal years 73%, of all the 62 years. Analyses with the Lagrangian method, done incomparison with the other two methods which combine wind and potential pseudo-equivalent temperature,were performed to determine the onset dates of the SCS summer monsoon. In some years, the source of thesouthwest airflow in the monitored area of the SCS is in the subtropical region before the onset of the SCSmonsoon, so the airflow from the subtropics can be distinguished with the airflow from the tropics by usingthe Lagrangian method. The simulation by the trajectory model indicated that in some years, after the onsetof SCS summer monsoon, the SCS will be controlled by the southeast wind instead of the southwesterlyusually expected.

MODEL OF UPPER OCEANIC CIRCULATIONS IN THE SOUTH CHINA SEA DURING NORTHEAST MONSOON

The large-scale upper oceanic circulation in the South China Sea (SCS) during the northeast monsoon was investigited using a 2 1/2-layer model inrolving entrainment and detraininent at the interface between the upper mixed layer and the seasonal thermocline. The model allows heat fluxes at the surface and at the interfaee with a reaxation scheme, the temperatures of the two active layers can vary. The model basin is idenical to the SCS lateral boundary with bottom topography of 50 m or more and is regarded as an enclosed basin by neglecting inflow and outflow through the straits, and is forced by the climetological wind stna of 12 calendar months. It was found tha the upper oceanic currents in winter were mainly wind-driven. Most aspects of streams reported by observations were simulated with eddyresolving.

A comparison of the CMIP5 models on the historical simulation of the upper ocean heat content in the South China Sea

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-ESM2 G 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.

Velocity model and time-depth conversion for the northwestern South China Sea deepwater areas

There are rich natural gas resources in the northwestern South China Sea deepwater areas, with poor degree of exploration. Because of the unique tectonic, sedimentary background of the region, velocity model building and time-depth conversion have been an important and difficult problem for a long time. Recent researches in this direction have revealed three major problems for deepwater areas, i.e., the way to determine error correction for drilling velocity, the optimization of velocity modeling, and the understanding and analysis of velocity variations in the slope areas. The present contribution proposes technical solutions to the problems：（1） velocity correction version can be established by analyzing the geology, reservoir, water depths and velocity spectrum characteristics;（2） a unified method can be adopted to analyze the velocity variation patterns in drilled pale structural positions;and （3） across-layer velocity is analyzed to establish the velocity model individually for each of the layers. Such a solution is applicable, as shown in an example from the northwestern South China Sea deepwater areas, in which an improved prediction precision is obtained.