Laboratory experiments were conducted in an internal wave flume ( m), at the National Sun Yat-Sen University, Kaohsiung, Taiwan. A series of fundamental experiments on wave generation, propagation and interaction with obstacles were carried out using stratified two-layer fresh/brine water with a total depth of 50 cm in the flume. Factors governing the experiments included the thickness ratio of the upper and lower layer H1/H2, interface difference

In the last few years, internal waves have been extensively studied by many scholars, mostly focused on the physical property and the effect on ecology and geochemistry. The geological influence, however, was rarely discussed. By EK500 and 3.5 kHz sub-bottom sonar system, it is reported that many sand waves exist in the South China Sea at 600 meter water depth. Internal waves are a very important driving mechanism in the South China Sea. Its movement over the marine bed causes unsteady flow field disturbance. In order to clarify whether the internal wave is the main factor to form sand wave, we conduct a series of numerical simulations. Most studies on the formation of sand waves are mainly in the nearshore area. Due to the difficulty in observation, only very few special discussions consider depth of 500 meters or deeper. First of all, in this thesis, we use the Korteweg de Vries (KdV) equation to derive wave and current in an internal soliton. Then, the flow field is substituted into the Regional Ocean Modeling System (ROMS) numerical model to simulate the three-dimensional movement of internal waves and the associated movement of suspended sediment in order to discuss the mechanism of sand wave formation. Finally, the variation of wavelengths of sand wave is analyzed and compared with in-situ measurement. From the simulation result, the internal wave causes the formation of sand waves. After the passage of dozens of internal waves, a flat sea floor will gradually form sand wave topography. Different depth and slope of the sea bottom will affect the sand wave wavelength also.

The good of this thesis is to apply a numerical model for studying waveform of an internal solitary wave (ISW) on variable seabed topography. The numerical model developed by Lynett and Liu (2002) is adopted for this work but with modification to improve its accuracy, both mathematically and in programming codes. Numerical experiments using the modified model are then performed and the results compared with laboratory experiments of Kuo (2005), in order to validate its accuracy. The mathematical model derived in the present study is based on the assumption that an internal wave is weakly nonlinear and weakly dispersive in an inviscid fluid. The governing equations based on the continuity equation and Euler equations are solved for ISW propagation over variable topography. The input conditions for the numerical experiments include physical parameters related to water depth and geometry of submarine obstacle, such as depth ratio between upper and lower layers (H1/H2), height (hs) and type (triangular ridge and trapezoidal shelf) of obstacles, in addition to the amplitude (ai) of an incident ISW. From the results of numerical experiments, wave amplitude, phase speed, and wave energy of a transmitted ISW are obtained and compared with that of laboratory experiments. (Kuo, 2005) ISW propagation over a single obstacle is affected by a dimensionless parameter called ¡§blockage parameter", £a= (a1+h1)/(h1+h2-hs). Three types of interaction may be classified (weak interaction, moderate interaction, and wave breaking) depending on the value of£a . For an ISW propagating over two consecutive obstacles, the interval between them is significant in reducing its amplitude and energy, as the interval reduces. Moreover, the effect of relative height between two obstacles may also be classified into two types: (i) within the range of weak interaction, energy dissipation is less for a high obstacle first than for it as the second; (ii) within the range of moderate interaction, the energy dissipation is higher for a high obstacle first than for it as the second. Further comparisons have shown that the modified numerical model is in better agreement with the results of laboratory experiments (Kuo, 2005) than the original model of Lynett and Liu (2002). The results obtained from the present numerical experiments for ISW evolution on variable topography is encouraging which could benefit other who may be interested in internal wave propagation for practical applications in oceanography.

57 synthetic aperture radar (SAR) images were collected over the Mid-Atlantic Bight (MAB) during the Shallow Water 2006 experiment (SW06). The dependence of internal wave (IW) signature occurrences and types in SAR images on the wind conditions is studied. A defined signature mode parameter (S sub m ) quantifies the signature of the IW intensity profile in relation to the mean backscatter in the image background to determine different IW types (single positive, single negative and double sign). The statistical results show that moderate wind speeds of 4-7 m/s are favorable for imaging IWs by SAR, whereas very few IW signatures are observed when the wind speed is higher than 10 m/s and lower than 2 m/s. Many S sub m values are larger than 1 (positive signature) even when the angles between the wind direction and IW propagation direction (theta sub Wind-IW) are less than in the MAB, which does not agree with the result of da Silva et al. (2002). An advanced radar imaging model has been run for different wind conditions, radar look directions and IW amplitudes. The model results indicate that the proportion of S sub m values larger than 1, when theta sub Wind-IW < 90 degree , increases with IW amplitudes. In general, relating IW signature types mainly to the wind direction is an oversimplification without considering other factors such as look directions and IW amplitudes. An IW interaction pattern has been studied on the basis of two sequential images from ERS2 and ENVISAT with a time lag of 28 minutes and temperature and current measurements from moorings. Phase velocities of the pattern can be derived by two-dimensional cross correlation of two images or in-situ measurements. In this pattern, the IW packet with a larger amplitude shifts less while the one with a smaller amplitude shifts more due to the interaction. The strong intensity in the interaction zone implies an amplitude increase. The intensity changes in the same IW packet after the interaction implies the energy exchange. All the characteristics agree well with the dynamics of the two-soliton pattern with a negative phase shift, according to Peterson and van Groesen (2000).

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Internal waves are vertical displacements of stratified water which can propagate a long distance without much energy dissipation. It plays on important role in conveying nutrient from deep ocean to shallower layers, and promoting biological growth. It also affects acoustics, ocean engineering and submarine navigation. Therefore, in the last few years, many scholars have devoted the research of internal waves, especially their generation and transportation in South China Sea. This research discusses the internal wave source, through the Princeton Ocean Model (POM). There are two simulations, one has bigger grid size 3.6km for the whole Luzon Strait in the region, 118¢XE ~123¢XE 18¢XN ~22¢XN, the other has smaller grid of 1.6km, for 118.5¢XE ~123¢XE and 19.5¢XN ~21.5¢XN. The total simulation period is 25 days, The north and south bomdawes use radiation boundary condition, the east side is driven by tidal calculated from WXTide32 model, the west side also uses radiation boundary condition. Three dimensional flow field during May, 2005, is simulated temperature, baroclinic velocity distribution, baroclinic energy flux, Empirical Orthogonal Functio ns (EOF), Fast Fourior transform (FFT), buoyancy frequency and Froude number are all analyzed, The results show that, when tidal current near the Batan island has the stronger baroclinic flow downward to transmit, after through Heng-Chun ridge the baroclinic energy flux to concenter, extrapolated this time possibly produces for internal tide.

Many oceanographers have postulated that internal wave form inversion would take place at the turning point where the thickness of the upper and bottom layer are equal in a stratified two-layer fluid system. This implies that an internal wave of depression may convert into elevation as the wave propagates over a continental margin comprising continental slope and shelf. Laboratory experiments were conducted on the propagation of a depression ISW over a trapezoidal obstacle in a stratified two-layer fresh/brine water system in a steel framed wave tank of 12m long with cross section of 0.7m high by 0.5m wide. The relative difference in water depth between the upper and lower layer and the initial ISW amplitude were the main controlling parameters, among others. The water depth in the stratified two-layer system on the horizontal plateau of the trapezoid obstacle fell into one of the following case: (1) the upper layer larger than lower (H1>¢Ö2'); or (2) equal depth in the upper and lower layer (H1=¢Ö2'); or (3) the upper layer less than lower layer (H1<¢Ö2'). In addition of the depth ratio, the difference in the length of the horizontal plateau and the thickness of the phycnocline above if were also parameters affecting the outcome of the experiments. In these experiments, three different type of the height and length of the trapezoidal obstacle were used, including long (4.8x0.37m), medium (1x0.35m) and short (0.5x0.35m) types. A full account on the characteristics of the ISW evolution observed during this experimental study is presented in this thesis. As an ISW propagated on the fronting slope, were run-down, vortex motion, internal hydraulic jump (IHJ) and run-up were occurred. Once the wave passed the turning point (where the depth of upper and lower layer equal), the wave form became elevation on the plateau above the obstacle. Based on the laboratory data available, the effect on internal wave evolution can be evaluated by the relative fluid thickness (H1/¢Ö2') on the plateau. The outcome can be classified into three categories: (1) H1>¢Ö2', the relative layer thickness on the plateau unfits for depression ISW propagation and waveform behaves like elevation type; (2) H1=¢Ö2', wave boluses containing mixed fluid propagating on the plateau after breaking on the slope; (3) H1<¢Ö2', ISW propagated over trapezoidal obstacle subjected to shoaling and viscosity effect, without change in waveform. As a depression ISW propagated over the variable length of the plateau, another important factor affecting the intensity of the internal hydraulic jump was the water volume drawn from the plateau. In the case of long horizontal plateau, the interaction range was large, and the IHJ was strong. Consequently, the thickness of the increased which caused the IHJ to move upward along the fronting slope. However, the amplitude and phase speed of the resulting internal wave decreased as if propagated further.

Many oceanographers have conducted field experiments on internal waves in the South China Sea using SAR imagery, ADCP and CTD. The results arising from these field studies are mostly in terms of wave amplitudes and flow velocities. Despite schematic diagram depicting the orbits of water particle motion has been accepted for more than decades, evidence has not been available from field observations or laboratory experiments. Laboratory experiments on water-particle motion were conducted on the propagation of elevation and depression ISW in a stratified two-layer fresh/brine fluid system in a steel-framed wave tank of 12 m long with cross section of 0.7 m high by 0.5 m wide. Numerical modeling was also performed using in put data identical to laboratory experiments. Based on our results of the numerical and laboratory experiments, the velocity field displays significant vortex while an internal solitary wave (ISW) propagates on a flat bottom. The strong vortex appears in the region of wave crest or trough. The track of fluid particle velocity in the upper layer is asymmetric and is moving in the opposite direction to that in the lower layer. The maximum horizontal velocity occurs at the crest of an elevation ISW and at the trough of a depression ISW. However, no horizontal flow is found on the interface of the still water level, and no vertical velocity at the wave peak. The vertical and horizontal velocities are antecedent with the water depth. For an elevation ISW, the maximum horizontal velocity appears in the lower layer, and vice versa for a depression ISW. The direction of the horizontal velocity in the upper layer is opposite to that in the lower layer. This study presents the results of numerical calculations and laboratory observations of the particles originally resting on a specific level and their movements within an ISW. The finding generated from this research would benefit others on the verification of field results or analytical theory for fluid particle motions associated with ISW.

Surface gravity waves and internal waves are two of the most common natural phenomena in the ocean. While oceanographers believe that internal waves have greater influence over the surface waves, if is not clear to what extent that the former have affected the latter. As an internal wave propagating in the ocean, short period flow could be induced on the free surface layer. Moreover, as internal waves propagating over a submarine ridge, internal breaking accompanying by large vortex may have occurred, which may also affect the properties of the surface waves. To prove the relationship between them, basic mathematical equations have been derived, but had never been proven in the laboratory experiments or field observations. In this thesis, the results of a series of laboratory experiments conducted at the National Sun Yen-sen University are employed to study the waveform evolution and change to the physical parameters of the surface waves, resulting from the generation of internal waves induced on a stratified fluid, as both propagate together above a plane bottom or across single ridge. These experiments were carried out in a stratified two-layer fresh/brine water system (upper layer with fresh water density 996 kg / m3; bottom layer brine water with 1030 kg / m3) in a steel framed wave tank of 12m long with cross-section of 0.7 m high by 0.5 m wide. A plunging-type wave maker was used to produce the designated surface waves, from which the internal waves were induced at the interface. Based on the experimental results in the fluid system with uniform density, wave height and period of the surface wave were first calibrated. It was found that the amplitude of a surface wave decreased first due to the breaking of the internal wave on the apex of a submerged ridge and then increased due to wave regeneration at the back of the ridge, when the surface wave propagated over single ridge. Beyond the ridge, the peak period with maximum energy associated with the transmitted wave remained almost the same with that of the incident waves. In a stratified fluid system, wave height of the surface waves and internal waves did not suffer much change but the peak period of a surface wave increased as an internal wave just across the apex of the obstacle, under a condition referred to as weaken interaction between the waves and the obstacle. For the intense wave breaking condition at the interface, wave height of the internal waves decreased and the period of surface waves or internal waves shortened. However, wave height of the surface wave above the apex of the obstacle increased due to the intense wave breaking. The results obtained from the present laboratory experiments on the interaction between a surface wave and the induced internal wave could benefit others interested in surface and internal wave interaction for practical applications in oceanography or numerical modeling.

The Dungeness crab fishery is the most economically important on the West Coast; however, it has experienced dramatic fluctuations in annual catch. Previous research has shown the annual catch of megalopae is correlated with the commercial catch. The catch of megalopae is correlated with the phase of the Pacific Decadal Oscillation (PDO), the day of the year of the spring transition and the amount of upwelling following the spring transition. Further, the daily catch of megalopae is correlated with the internal tide. We developed individual based models of Dungeness crab dispersal, which we validated with results from a light trap. We demonstrated that the retention of larvae in the California Current is enhanced during negative phase PDOs. Further, we suggest that larvae migrate to or almost to the bottom each day. Specifically, megalopae exhibit a twilight vertical migration off of the continental shelf and remain in the neuston on the continental shelf. This concentrates megalopae at the continental shelf break. We also observed megalopae in situ and demonstrated that they swim in the neuston with the surface current at speeds of ~ 10 cm s-1. Using these results and data from a mooring, we demonstrated that this behavior would increase the distance internal waves would transport larvae. We analyzed mooring data and suggest that catch of megalopae is greater when the thermocline is deep and weak and there is less horizontal shear. We hypothesize this allows internal waves to remain coherent longer on the continental shelf. We show that the spring transition coincides with a shallowing of the thermocline, which would ultimately lead to the development of internal waves of depression rather than elevation. We hypothesize that the change in surface flow, based on whether the wave is one of elevation or depression, explains why most megalopae are caught following the spring transition. In general, these findings help us better understand the dispersal of Dungeness crabs. We suggest the dispersal patterns support Michael Sinclair’s member vagrant hypothesis. Further, we suggest these findings apply to many of the continental shelf species in the California Current. This dissertation includes both published and unpublished co-authored materials.