Internal Tide And Nonlinear Internal Wave In The Northern South China Sea: Observation And Numerical Simulation

This paper investigates phinomina of internal tide and nonlinear internal wave inthe Northern South China Sea (NSCS) based on hydrographic field observation andnumerical simulation. Including: the analysis of direct-obervation current data in theLuzon Strait (LS), and comparition between observation and polarization relations forlinear internal wave; the geostriphical distribution and seasonal characteristic of thebackground environment and mechanism parameters of nonlinear internal wave in theNSCS, the influence of mesoscale eddy on internal solitary wave propagation, thenumerical simulation for depression to elevation conversation of large-amplitudeinternal solitary wave in the shelf of the NSCS, and the numeicial simulation of thegeneration for internal wave on the double ridges in the LS.1. The analysis of observational current in the LSDue to rare observation for current in the LS, the moring observation was carriedout. The characteristics of current are investigated by using statistic analysis andEmpirical Orthogonal Function. Spectral analysis and energy estimation show that thediurnals and semidiurnals carry most of the energy of internal tides. Near-inertialpeaks are only present in the baroclinic component. The behavior of typical tidalfrequencies and the near-inertial frequency is basically consistent with linear internalwave theory, which predicts E+(ω)/E-(ω)=(ω-f)~2/(ω+f)~2at depths above66m, whilenot all prominent tidal components coincide well with the relation of the linearinternal wave field at other depths. The surface tides and internal tides are both ofmixed type. The K1and O1tides have comparable cross-and along-shelf components,while the M2and S2tides propagate toward the shelf in the northern South China Seaas wave beams. The M2and S2tides appear to have structures dominated by the firstmode, while the K1and O1tides resemble second-mode structures. The minor tomajor axis ratios are close to expected values of f/ω in the thermocline.2. Geostriphical distribution and seasonal characteristic of the backgroundenvironment and mechanism parameters of nonlinear internal wave in theNSCSThe phenomena of internal waves in the SCS show remarkable seasonal andinterannual variance from satellite image. On the basis reanalysis data, the stratification characteristics, and the geographical and monthly variability of theeKdV equation coefficients, are analyzed. Seasonal pycnocline with maximalbuoyancy frequency at about20m depth, begins to appear in February, is strongest inJun and Jul., weakens in Aug., starts to dissipate in Oct. Another deeper pycnoclineappears with maximal buoyancy frequency at about80m depth during Aug. and Nov.,and maximal buoyancy frequency moves to120m depth in winter. The seasonalpycnocline is very distinct from Apr. to Sep.. The double pycnocline is obvious fromAugust to Oct.. In winter, only the second pycnocline exists. Maxima buoyancyfrequency in deeper basin is higher than that in shallow shelf area from Jan. to Mar.and from Oct. to Dec., but the configuration is opposite during May and Sep.. Thedepth of maxima buoyancy frequency varies with season. It is deepest in winter, andis shallowest in June and July. It is shown that the variations of the long wave phasespeed, the dispersion parameter and amplitude factor are mainly related to topographycharacteristics without obvious seasonal variation. The quadratic nonlinear parameteris very sensitive to variations of the vertical stratification, and the cubic nonlinearparameter depends on water depth and stratification condition. Holding higheroccurrence in summer in the NSCS and the scarce existence of nonlinear internalwave on east of the LS is interpreted by using theses coefficient characteristic.3. Mesoscale eddy effects on the internal solitary wave propagationThe mesoscale eddy and internal wave both are phenomena commonly observedin oceans. This paper aims to investigate how the presence of a mesoscale eddy in theocean affects waveform deformation of the internal solitary wave propagation. Anocean eddy is produced by a quasi-geostrophic model in f-plane, and theone-dimensional nonlinear eKdV equation is used to simulate an internal solitarywave passing through the mesoscale eddy field. The results suggest that the modestructures of the linear internal wave are modified due to the presence of themesoscale eddy field. A cyclonic eddy and an anticyclonic eddy do differentinfluences on background environment of internal solitary wave propagation. Theexistence of a mesoscale eddy field has almost no prominent impact on thepropagation of a small-amplitude internal solitary wave only based on the first modevertical structure, but the mesoscale eddy background field exerts a considerableinfluence on solitary wave propagation if considering high-mode vertical structures.Furthermore, whether an internal solitary wave first passes through anticyclonic eddyor cyclonic eddy, the deformation of wave profiles is different. Many observations of solitary internal waves in the real oceans suggest formation of the waves, excepttopography effect, this study shows that the mesoscale eddy background field is alsoan considerable factor which influences the internal solitary wave propagation anddeformation.4. The numerical simulation for polarity covertion of interal solitary wave in theself of the NSCSThe phenomia of polarity covertion of internal solitary wave oftern are capturedby satellite images in the shelf of the NSCS. The physical process that internal wavetransfers their polrity is investigated by the numerical simulation. When the depressssolitary wave propates from deeper water area to shallower water area, the rear of thewave begin to speepen owing to effect of shallower topography, some soliton split outfrom the rear of the wave, the elevation solitary wave appears slowly, with theshallower water depth, the amplitude of the elevation solitary wave is growing, andthe distance between the front wave and behind wave is also increasing, showing thepersperion property of internal wave.5. The numerical simulation for generation of internal wave on the doubleridges in the LSThe satellite images show the LS is an important original area for internal wavein the NSCS. It is difficult to trace the internal wave siginal in the sea surface of theLS from the results of the numerical simulation, this consists with observations fromsatellite images, which also demonstrated that the internal signals mainly present westof120.5°E. The tidal bore is firstly built up to west of the Luzon ridge in the ebbyphase. When the ebby tide current weaks and becomes flood, the soliton splits outfrom the front of the tidal bore. More and more solitons split out with the enhancedflood. An elevation wave is observed to east of the Luzon ridge during both ebby andflood phases, howere, there is no any solion fission from the evevation wave. Thedouble ridges is supercritical with respect to both diurnal and semidiurnal tides, andthe tide excursion parameter is less than1, so the LS is suitable for the generation ofinternal tides. The tidal beam radiates upward and downward and evolutes the internaltidal born, and the solitary waves split out on the premise of nonlinear and dispersioneffects.