Internal solitary waves on the southwest shelf of Dongsha Island observed from mooring ADCP

Internal solitary waves(ISWs) are frequently observed in the area between Dongsha Island(DI) and Taiwan Island. However, there have been few in-situ observations southwest of DI. To improve our knowledge of ISWs in this area, we observed the ISWs over the continental shelf(115.4°E, 20.3°N) from Aug. 29 to Oct. 10, 2011 with temperature sensors and an acoustic Doppler current profiler(ADCP). The observations showed that the a fully developed ISW produced a current whose maximum westward velocity was 0.92 m/s and maximum northward velocity was 0.47 m/s. During the 41-day observation period the ISWs appeared for three periods with about 7-day gaps between each period. During each day, two types of ISWs were observed. The first type of wave arrived regularly diurnally at the same time each day, with a similar pattern to that of the type- a wave identified by Ramp et al.(2004). The second type arrived about 12 h after the first type and was delayed about 1 hour each day; this wave type was related to the type- b wave. Thus, our observations confirmed that both type- a and type- b waves can reach the area southwest of the DI. Moreover, the waves observed by the mooring propagated toward the directions of 270°–315° clockwise from true north, indicating obvious refraction from uneven topography around DI.

Comparison of wind data from ERA-Interim and buoys in the Yellow and East China Seas

We compared data of sea surface wind from the European Centre for Medium-Range Weather Forecasts Interim Reanalysis(ERA-Interim) with that collected from eight buoys deployed in the Yellow and East China seas.The buoy data covered a period from 2010 to 2011,during which the longest time series without missing data extended for 329 days.Results show that the ERA-Interim wind data agree well with the buoy data.The regression coefficients between the ERA-Interim and observed wind speed and direction are greater than 0.7 and 0.79,respectively.However,the ERA-Interim wind data overestimate wind speed at most of the buoy stations,for which the largest bias is 1.8 m/s.Moreover,it is found from scatter plots of wind direction that about 13%of the ERA-Interim wind data can be classified as bad for wind speeds below6 m/s.Overall,the ERA-Interim data forecast both the wind speed and direction well,although they are not very representative of our observations,especially those where the wind speed is below 6 m/s.

Highly nonlinear internal solitary waves over the continental shelf of the northwestern South China Sea

Large amplitude internal solitary waves(ISWs) often exhibit highly nonlinear effects and may contribute significantly to mixing and energy transporting in the ocean.We observed highly nonlinear ISWs over the continental shelf of the northwestern South China Sea(19°35'N,112°E) in May 2005 during the Wenchang Internal Wave Experiment using in-situ time series data from an array of temperature and salinity sensors,and an acoustic Doppler current profiler(ADCP).We summarized the characteristics of the ISWs and compared them with those of existing internal wave theories.Particular attention has been paid to characterizing solitons in terms of the relationship between shape and amplitude-width.Comparison between theoretical prediction and observation results shows that the high nonlinearity of these waves is better represented by the second-order extended Korteweg-de Vries(KdV) theory than the first-order KdV model.These results indicate that the northwestern South China Sea(SCS) is rich in highly nonlinear ISWs that are an indispensable part of the energy budget of the internal waves in the northern South China Sea.

Depression and elevation internal solitary waves in a two-layer fluid and their forces on cylindrical piles

Both large amplitude depression and elevation internal solitary waves (ISWs) were observed on the continental shelf of the northwest South China Sea (SCS) during the Wenchang Internal Wave Experiment. In this study, we investigate the characteristics of depression and elevation ISWs based on comparisons between observational results and internal wave theories. It is suggested that the large amplitude depression wave is better represented by the extended Korteweg-de Vries (EKdV) theory than by the KdV model, whereas the large amplitude elevation wave is in better agreement with the KdV equation than with the EKdV theory. Wave-induced forces on a supposed small-diameter cylindrical pile by depression and elevation waves are also estimated using the internal wave theory and Morison formula. The wave-induced force by elevation ISWs is rarely reported in the literature. It is found that the force induced by the elevation wave differs significantly from that by the depression wave, and the elevation wave generally produces greater force on the pile in the lower water column than the depression wave. These results show that ISWs in the study area can present a serious threat to ocean engineering structures, and should not be ignored in the design of oil platforms and ocean operations.

Vertical structure and evolution of the Luzon Warm Eddy

Eddies are frequently observed in the northeastern South China Sea(SCS).However,there have been few studies on vertical structure and temporal-spatial evolution of these eddies.We analyzed the seasonal Luzon Warm Eddy(LWE) based on Argo float data and the merged data products of satellite altimeters of Topex/Poseidon,Jason-1 and European Research Satellites.The analysis shows that the LWE extends vertically to more than 500 m water depth,with a higher temperature anomaly of 5°C and lower salinity anomaly of 0.5 near the thermocline.The current speeds of the LWE are stronger in its uppermost 200 m,with a maximum speed of 0.6 m/s.Sometimes the LWE incorporates mixed waters from the Kuroshio Current and the SCS,and thus has higher thermohaline characteristics than local marine waters.Time series of eddy kinematic parameters show that the radii and shape of the LWE vary during propagation,and its eddy kinetic energy follows a normal distribution.In addition,we used the empirical orthogonal function(EOF) here to analyze seasonal characteristics of the LWE.The results suggest that the LWE generally forms in July,intensifies in August and September,separates from the coast of Luzon in October and propagates westward,and weakens in December and disappears in February.The LWE's westward migration is approximately along 19°N latitude from northwest of Luzon to southeast of Hainan,with a mean speed of 6.6 cm/s.

Water exchange and circulation structure near the Luzon Strait in early summer

Using hydrographic data covering large areas of ocean for the period from June 21 to July 5 in 2009,we studied the circulation structure in the Luzon Strait area,examined the routes of water exchange between the South China Sea(SCS) and the Philippine Sea,and estimated the volume transport through Luzon Strait.We found that the Kuroshio axis follows a e-shaped path slightly east of 121uE in the upper layer.With an increase in depth,the Kuroshio axis became gradually farther from the island of Luzon.To study the water exchange between the Philippine Sea and the SCS,identification of inflows and outflows is necessary.We first identified which flows contributed to the water exchange through Luzon Strait,which differs from the approach taken in previous studies.We determined that the obvious water exchange is in the section of 121°E.The westward inflow from the Philippine Sea into the SCS is 6.39 Sv in volume,and mainly in the 100±500 m layer at 19.5°±20°N(accounting for 4.40 Sv),while the outflow from the SCS into the Philippine Sea is concentrated in the upper 100 m at 19°±20°N and upper 400 m at 21°±21.5°N,and below 240 m at 19°±19.5°N,accounting for 1.07,3.02 and 3.43 Sv in volume transport,respectively.

Variations in the eastern Indian Ocean warm pool and its relation to the dipole in the tropical Indian Ocean

Based on the monthly average SST and 850 hPa monthly average wind data,the seasonal,interannual and long-term variations in the eastern Indian Ocean warm pool(EIWP) and its relationship to the Indian Ocean Dipole(IOD),and its response to the wind over the Indian Ocean are analyzed in this study.The results show that the distribution range,boundary and area of the EIWP exhibited obviously seasonal and interannual variations associated with the ENSO cycles.Further analysis suggests that the EIWP had obvious long-term trend in its bound edge and area,which indicated the EIWP migrated westwards by about 14 longitudes for its west edge,southwards by about 5 latitudes for its south edge and increased by 3.52×106 km2 for its area,respectively,from 1950 to 2002.The correlation and composite analyses show that the anomalous westward and northward displacements of the EIWP caused by the easterly wind anomaly and the southerly wind anomaly over the eastern equatorial Indian Ocean played an important and direct role in the formation of the IOD.

Characteristics of nonlinear internal waves observed in the northern South China Sea

The South China Sea(SCS) is one of the most active areas of internal waves.We undertook a program of physical oceanography in the northern South China Sea from June to July of 2009,and conducted a 1-day observation from 15:40 of June 24 to 16:40 of June 25 using a chain of instruments,including temperature sensors,pressure sensors and temperature-pressure meters at a site(117.5°E,21°N) northeast of the Dongsha Islands.We measured fluctuating tidal and subtidal properties with the thermistor-chain and a ship-mounted Acoustic Doppler Current Profiler,and observed a large-amplitude nonlinear internal wave passing the site followed by a number of small ones.To further investigate this phenomenon,we collected the tidal constituents from the TPXO7.1 dataset to evaluate the tidal characteristics at and around the recording site,from which we knew that the amplitude of the nonlinear internal wave was about 120 m and the period about 20 min.The horizontal and vertical velocities induced by the soliton were approximately 2 m/s and 0.5 m/s,respectively.This soliton occurred 2-3 days after a spring tide.

Energetics and temporal variability of internal tides in Luzon Strait:a nonhydrostatic numerical simulation

A fully nonlinear,three-dimensional nonhydrostatic model driven by four principal tidal constituents(M2,S2,K1,and O1) is used to investigate the spatial-temporal characteristics and energetics of internal tides in Luzon Strait(LS).The model results show that,during spring(neap) tides,about 64(47) GW(1 GW=109 W) of barotropic tidal energy is consumed in LS,of which 59.0%(50.5%) is converted to baroclinic tides.About 22(11) GW of the derived baroclinic energy flux subsequently passes from LS,among which 50.9%(54.3%) flows westward into the South China Sea(SCS) and 45.0%(39.7%) eastward into the Pacific Ocean,and the remaining 16(13) GW is lost locally owing to dissipation and convection.It is revealed that generation areas of internal tides vary with the spring and neap tide,indicating different source areas for internal solitary waves in the northern SCS.The region around the Batan Islands is the most important generation region of internal tides during both spring and neap tides.In addition,the baroclinic tidal energy has pronounced seasonal variability.Both the total energy transferred from barotropic tides to baroclinic tides and the baroclinic energy flux flowing out of LS are the highest in summer and lowest in winter.

Climate variability and predictability associated with the Indo-Pacific Oceanic Channel Dynamics in the CCSM4 Coupled System Model

An experiment using the Community Climate System Model(CCSM4), a participant of the Coupled Model Intercomparison Project phase-5(CMIP5), is analyzed to assess the skills of this model in simulating and predicting the climate variabilities associated with the oceanic channel dynamics across the Indo-Pacific Oceans. The results of these analyses suggest that the model is able to reproduce the observed lag correlation between the oceanic anomalies in the southeastern tropical Indian Ocean and those in the cold tongue in the eastern equatorial Pacific Ocean at a time lag of 1 year. This success may be largely attributed to the successful simulation of the interannual variations of the Indonesian Throughflow, which carries the anomalies of the Indian Ocean Dipole(IOD) into the western equatorial Pacific Ocean to produce subsurface temperature anomalies, which in turn propagate to the eastern equatorial Pacific to generate ENSO. This connection is termed the "oceanic channel dynamics" and is shown to be consistent with the observational analyses. However, the model simulates a weaker connection between the IOD and the interannual variability of the Indonesian Throughflow transport than found in the observations. In addition, the model overestimates the westerly wind anomalies in the western-central equatorial Pacific in the year following the IOD, which forces unrealistic upwelling Rossby waves in the western equatorial Pacific and downwelling Kelvin waves in the east. This assessment suggests that the CCSM4 coupled climate system has underestimated the oceanic channel dynamics and overestimated the atmospheric bridge processes.

Seasonal characteristics of internal tides and their responses to background currents in the Luzon Strait

The spatial-temporal characteristics of internal tides （ITs） in the southwest Luzon Strait are examined, based on 9-month mooring current records from autumn 2008 to summer 2009. The results of spectral analysis show that the ITs in diurnal and semidiurnal frequencies are prominent at the mooring site, especially for the clockwise rotary component. The diurnal ITs are mostly dominated by the first mode except for that in spring when the second mode is relatively predominant. The semidiurnal ITs display a variable multimodal structure. Moreover, an apparent difference is detected in the kinetic energy of diurnal ITs. The energy is strongest in winter, and followed by that in summer, whereas the value is smallest in spring and autumn. It is suggested that the incoherent motions are responsible for the significant seasonal variations of diurnal ITs, reflecting interaction between diurnal ITs and the varying background conditions. However, the semidiumal ITs are independent of seasonal change, whose energy is smaller and only one-third of the diurnal energy in winter. Nevertheless, the abnormal variations of semidiurnal ITs are also related to the variable background conditions. The incoherent semidiurnal constituent accounts for about 37% of the total semidiurnal tidal kinetic energy, but the diurnal tidal motions contain fewer incoherent component （22.2%）.

Modeling vertical heat transport in the wave affected surface layer of the ocean

In considering the vertical heat transport problems in the upper ocean, the flat upper boundary approximation for the free surface and the horizontal homogenous hypothesis are usually applied. However, due to the existence of the wave motion, the application of this approximation may result in some errors to the solar irradiation since it decays quickly in respect to the actual thickness of the water layer below the surface; on the other hand, due to the fluctuation of the water layer depth, it is improper to neglect the effects of the horizontal advection and turbulent diffusion since they also contribute to the vertical heat transport. A new model is constructed in this study to reflect these effects. The corresponding numerical simulations show that the wave motion may remarkably accelerate the vertical heat transferring process and the variation of the temperature in the wave affected layer appears in an oscillating manner.

Numerical studies on the degeneration of internal waves induced by an initial tilted pycnocline

The pycnocline in a closed domain is tilted by external wind forcing and tends to restore to a level posi- tion when the wind falls. An internal seiche oscillation exhibits if the forcing is weak, otherwise internal surge and internal solitary waves emerge, which serve as a link to cascade energy to small-scale processes. A two-dimensional non-hydrostatic code with a turbulence closure model is constructed to extend previous laboratory studies. The model could reproduce all the key phenomena observed in the corresponding labo- ratory experiments. The model results further serve as a comprehensive and reliable data set for an in-depth understanding of the related dynamical process. The comparative analyses indicate that nonlinear term favors the generation of internal surge and subsequent internal solitary waves, and the linear model predicts the general trend reasonably well. The vertical boundary can approximately reflect all the incoming waves, while the slope boundary serves as an area for small-scale internal wave breaking and energy dissipation. The temporal evolutions of domain integrated kinetic and potential energy are also analyzed, and the results indicate that about 20% of the initial available potential energy is lost during the first internal wave breaking process. Some numerical tactics such as grid topology and model initialization are also briefly discussed.

Numerical Simulation of Waves Generated by Seafloor Movements

Waves generated by vertical seafloor movements are simulated by use of a fully nonlinear two-dimensional numerical wave tank. In the souree region, the seafloor lifts to a designated height by a generation function. The numerical tests show that the linear theory is only valid for estimating the wave behaviors induced by the seafloor movements with a small amplitude, and the fully nonlinear numerical model should be adopted in the simulation of the wave generation by the large amplitude seafloor movements. Without the background surface waves, many numerical tests on the stable maximum elevations η0^max are carried out by beth the linear theory and the fully nonlinear model. The results of two models are compared and analyzed. For the fully nonlinear model, the influences of the amplitudes and the horizontal lengths on η^max are stronger than that of the characteristic duration times. Furthermore, results reveal that there are significant differences be- tween the linear theory and the fully nonlinear model. When the influences of the background surface waves are considered, the corresponding numerical analyses reveal that with the fully nonlinear model the η0^max near-linearly varies with the wave amplitudes of the surface waves, and the η0^max has significant dependences on the wave lengths and the wave phases of the surface waves. In addition, the differences between the linear theory and the fully nonlinear model are still obvious, and these differences are significantly affected by the wave parameters of the background surface waves, such as the wave amplitude, the wave length and the wave phase.

Subthermocline anticyclonic gyre east of Mindanao and its relationship with the Mindanao Undercurrent

The quasi-permanent anticyclonic gyre(ACG) east of Mindanao is a dominant feature of the subthermocline circulation in the southern Philippine Sea, and it is believed closely associated with the continuous northward alongshore flow of the Mindanao Undercurrent(MUC). In this study, the structure and variability of this ACG were investigated using the 1950–2012 output of the Oceanic General Circulation Model for the Earth Simulator(OFES), which can reproduce well the structure of the climatological intermediate-layer circulation and satellite-observed sea level variations in the southern Philippine Sea. Between 26.8–27.3 σθ, the ACG covers a large area from the Mindanao coast to 131°E and from 3°N to 10°N. Its anticyclonic flow structure is unrelated to the surface Halmahera Eddy. The eddy-resolving simulation of the OFES revealed that the ACG consists of two components. The southern ACG(SACG) is centered at ~6°N, while the northern ACG(NACG) is centered at ~10°N. Seasonal and interannual variations of the ACG are linked to the variations of the northward MUC transport along the Mindanao coast, and the role of the SACG is more important than the NACG. Stronger(weaker) ACGs lead to greater(smaller) MUC transport. On the interannual timescale, the SACG shows a spectrum peak at 4–8 years, while the NACG has enhanced power within the 3–5-year band. A lead–lag correlation analysis indicates that interannual variations of the ACGs and the MUC transport are partly associated with the El Ni?o-Southern Oscillation. Possible causes for the ACG variability are discussed.

Strongly nonlinear long interfacial waves in uniform basic flows and their dispersion relationship

In this paper, long interfacial waves of finite amplitude in uniform basic flows are considered with the assumption that the aspect ratio between wavelength and water depth is small. A new model is derived using the velocities at arbitrary distances from the still water level as the velocity variables instead of the commonly used depth-averaged velocities. This significantly improves the dispersion properties and makes them applicable to a wider range of water depths. Since its derivation requires no assumption on wave amplitude, the model thus can be used to describe waves with arbitrary amplitude.

Determination of fractional energy loss of waves in nearshore waters using an improved high-order Boussinesq-type model

Fractional energy losses of waves due to wave breaking when passing over a submerged bar are studied systematically using a modified numerical code that is based on the high-order Boussinesq-type equations.The model is first tested by the additional experimental data,and the model's capability of simulating the wave transformation over both gentle slope and steep slope is demonstrated.Then,the model's breaking index is replaced and tested.The new breaking index,which is optimized from the several breaking indices,is not sensitive to the spatial grid length and includes the bottom slopes.Numerical tests show that the modified model with the new breaking index is more stable and efficient for the shallow-water wave breaking.Finally,the modified model is used to study the fractional energy losses for the regular waves propagating and breaking over a submerged bar.Our results have revealed that how the nonlinearity and the dispersion of the incident waves as well as the dimensionless bar height(normalized by water depth) dominate the fractional energy losses.It is also found that the bar slope(limited to gentle slopes that less than 1:10) and the dimensionless bar length(normalized by incident wave length) have negligible effects on the fractional energy losses.

The influence of background waves on internal solitary waves after the transit of Typhoon Neast in the northern South China Sea

On the basis of the time series observations from a temperature chain and an acoustic Doppler current profiler on the continental shelf of the northern South China Sea, a sequence of internal solitary waves （ISWs） and background waves （BWs, including internal tides and near-inertial waves） on the continental shelf were captured simultaneously after the transit of Typhoon Neast in October 2011. These measurementsprovided a unique opportunity to explore the influence of BWs on the ISWs. The BWs appeared a conversion on the current strength and vertical mode structure during the observational period. The BWs were dominated by weak and mode-one waves before October 2 and then turned to strong and high-mode waves after that time. Meanwhile, the ISWs displayed different wave structures before and after October 2, which was closely related to BWs＇ changes. According to the current profiles of BWs, the high-mode wave structure with strong current could significantly strengthen the vertical shear of ISWs in the near-surface layer and promote the breaking of ISWs, and thus it may play an important role in affecting the background current condition.

The variability of eddy ki/netic energy in the South China Sea deduced from satellite altimeter data

We used fifteen years (1993-2007) of altimetric data,combined from different missions (ERS-1/2,TOPEX/Poseidon,Jason-1,and Envisat),to analyze the variability of the eddy kinetic energy (EKE) in the South China Sea (SCS).We found that the EKE ranged from 64 cm2/s2 to 1 390 cm2/s2 with a mean value of 314 cm2/s2.The highest EKE center was observed to the east of Vietnam (with a mean value of 509 cm2/s2) and the second highest EKE region was located to the southwest of Taiwan Island (with a mean value of 319 cm2/s2).We also found that the EKE structure is the consequence of the superposition of different variability components.First,interannual variability is important in the SCS.Spectral analysis of the EKE interannual signal (IA-EKE) shows that the main periodicities of the IA-EKE to the east of Vietnam,to the southwest of Taiwan Island,and in the SCS are 3.75,1.87,and 3.75 years,respectively.It is to the south of Taiwan Island that the IA-EKE signal has the most obvious impact on EKE variability.In addition,the IA-EKE exhibit different trends in different regions.An obvious positive trend is observed along the east coast of Vietnam,while a negative trend is found to the southwest of Taiwan Island and in the east basin of Vietnam.Correlation analysis shows that the IA-EKE has an obvious negative correlation with the SSTA in Nio3 (5°S-5°N,90°W-150°W).El Nio-Southern Oscillation (ENSO) affects the IA-EKE variability in the SCS through an atmospheric bridge-wind stress curl over the SCS.Second,the seasonal cycle is the most obvious timescale affecting EKE variability.The locations of the most remarkable EKE seasonal variabilities in the SCS are to the east of Vietnam,to the southwest of Taiwan,and to the west of Philippines.To the east of Vietnam,the seasonal cycle is the dominant mechanism controlling EKE variability,which is attributed primarily to the annual cycle there of wind stress curl.In this area,the maximum EKE is observed in autumn.To the southwest of Taiwan Island,the EKE is enlarged by the stronger SCS circulation,which is caused by the intrusion branch from the Kuroshio in winter.Finally,intra-annual and mesoscale variability,although less important than the former,cannot be neglected.The most obvious intra-annual and mesoscale variability,which may be the result of baroclinic instability of the background flow,are observed to the southwest of Taiwan Island.Sporadic events can have an important effect on EKE variability.

Path transition of the western boundary current with a gap due to mesoscale eddies: a 1.5-layer, wind-driven experiment

Using a 1.5 layer nonlinear shallow-water reduced-gravity model, we executed numerical simulations to investigate the possibility of a western boundary current (WBC) path transition due to mesoscale eddies based on the background of the Kuroshio intrusion into the South China Sea (SCS) from the Luzon Strait. Because the WBC existed different current states with respect to different wind stress control parameters, we chose three steady WBC states (loop current, eddy shedding and leaping) as the background flow field and simulated the path transition of the WBC due to mesoscale eddies. Our simulations indicated that either an anticyclonic or cyclonic eddy can lead to path transition of the WBC with different modes. The simulation results also show that the mesoscale eddies can lead to path transition of the WBC from loop and eddy shedding state to leaping state because of the hysteresis effect. The leaping state is relatively stable compared with the mesoscale eddies. Moreover, an anticyclonic eddy is more effective in producing the WBC path transition for the path transition than a cyclonic eddy. Our results may help to explain some phenomena observed regarding the path transition of the Kuroshio due to the mesoscale eddies at the Luzon Strait.