A 2D-numerical modeling of the generation and propagation of internal solitary waves in the Luzon Strait

The wide presence of internal solitary waves （ISWs） in the northern South China Sea （SCS） has been confirmed by both Synthetic Aperture Radar （SAR） images and in situ observations.These ISWs are believed being generated over the varying topography in the Luzon Strait.They typically propagate westwards into the SCS with a diurnal or semidiurnal period.Their generation sites are,however,not yet solidly identified.To obtain a clear picture of the ISWs,we designed numerical experiments to analyze the generation and propagation of the ISWs in the Luzon Strait using a 2-dimensional non-hydrostatic model.The model current is forced by barotropic or baroclinic currents imposed at open boundaries.The experiments show that the tidal current serves as a kind of triggering force for the ISWs over the submarine ridges in the strait.Under the forcing of tidal currents,depressions are formed near the ridges.The ISWs then split from the depressions through a process different from lee-wave generation mechanism.The appearance of the ISWs is influenced by the strength and period of the forcing current： the ISWs are more likely to be generated by a stronger tidal current.That is why the ISWs in the Luzon Strait are frequently observed during spring tide.Compared with diurnal tidal current,the ISWs generated by semidiurnal tidal current with the same amplitude is much more energetic.It is partly because that the wave beams in diurnal frequency have a larger angle with the vertical direction,thus are more likely to be reflected by the topography slope.The impact of the Kuroshio to the ISWs is also analyzed by adding a vertical uniform or shear current at boundaries.A vertically uniform current may generate ISWs directly.On the other hand,a vertically shear current,which is more realistic to represent the Kuroshio branch,seems to have little influence on the generation process and radiating direction of the ISWs in the Luzon Strait.

A detection model of underwater topography with a series of SAR images acquired at different time

underwater topography is one of oceanic features detected by Synthetic Aperture Radar.Underwater topography SAR imaging mechanism shows that tidal current is the important factor for underwater topography SAR imaging.Thus under the same wind field condition,SAR images for the same area acquired at different time include different information of the underwater topography.To utilize synchronously SAR images acquired at different time for the underwater topography SAR detection and improve the precision of detection,based on the detection model of underwater topography with single SAR image and the periodicity of tidal current,a detection model of underwater topography with a series of SAR images acquired at different time is developed by combing with tide and tidal current numerical simulation.To testify the feasibility of the presented model,Taiwan Shoal located at the south outlet of Taiwan Strait is selected as study area and three SAR images are used in the underwater topography detection.The detection results are compared with the field observation data of water depth carried out by R/V Dongfanghong 2,and the errors of the detection are compared with those of the single SAR image.All comparisons show that the detection model presented in the paper improves the precision of underwater topography SAR detection,and the presented model is feasible.

Reconstruction of eddies by assimilating satellite altimeter data into Princeton Ocean Model

An optimal interpolation assimilation model for satellite altimetry data is developed based on Princeton Ocean Model （POM）, which is applied in a quasi-global domain, by the method of isotropic correlation between sea level anomaly （SLA） and sea temperature anomaly. The performance of this assimilation model is validated by the modeled results of SLA and the current patterns. Comparisons between modeling and satellite data show that both the magnitudes and distribution patterns of the sinmlated SLA are improved by assimilation. The most significant improvement is that meso-scale systems, e.g., eddies, are well reconstructed. The evolution of an eddy located in the northwest Pacific Ocean is traced by using the assimilation model. Model results show that during three months the eddy migrated southwestward for about 6 degrees before merging into the Kuroshio. The three dimensional structure of this eddy on 12 August 2001 is further analyzed. The strength of this warm, cyclonic eddy decreases with the increase of depth. The eddy shows different horizontal patterns at different layers, and the SLA and temperature fields agree with each other well. This study suggests that this kind of data assimilation is economic and reliable for eddy reconstruction, and can be used as a promising technique in further studies of ocean eddies as well as other fine circulation structures.

Ocean temperature responses to Typhoon Mstsa in the East China Sea

The MASNUM wave-tide-circulation coupled model, with 21 layers in the vertical and （1/8） °horizontal resolution, was employed to investigate the oceanic responses to Typhoon Mstsa which traversed the East China Sea （ECS） during the period of 4 - 6 August, 2005. Numerical experiment results are analyzed and compared with observation. The responses of the sea surface temperature （SST）, in a focused area of （27° -29°N, 121° - 124°E）, include heating and cooling stages. The heating is mainly due to warm Kuroshio water transportation and downwelling due to the water accumulation. In the cooling stage, the amplitude of the simulated cold wake （ -3℃ ）, located on the right side of this typhoon track, is compared quite well with that of the satellite observed SST data. The wave-induced mixing（Bv） plays a key role for the SST cooling. Bv still plays a leading role, which accounts for 36%, for the ocean temperature drop in the upper ocean of 0 - 40 m, while the upwelling is responsible for 84% of the cooling for the lower layer of 40 - 70 m. The mixed layer depth （MLD） increased quickly from 28 to 50 m in the typhoon period. However, the simulated MLD without the wave-induced vertical mixing, evolution from 13 to 32 m, was seriously underestimated. The surface wave is too important to be ignored for the ocean responses to a typhoon.

Response of Internal Waves to 2005 Typhoon Damrey over the Northwestern Shelf of the South China Sea

Continuous observation of sea water temperature and current was made at Wenchang Station(19°35′N,112°E) in 2005. The data collected indicate vigorous internal waves of both short periods and tidal and near-inertial periods. The temperature and current time series during 18-30 September were examined to describe the upper ocean internal wave field response to Typhoon Damrey(0518) . The strong wind associated with the typhoon,which passed over the sea area about 45 km south of Wenchang Station on 25 September,deepened the mixed layer depth remarkably. It decreased the mixed layer temperature while increasing the deep layer temperature,and intensified the near-inertial and high-frequency fluctuations of temperature and current. Power spectra of temperature and current time series indicate significant deviations from those obtained by using the deep ocean internal wave models characterized by a power law. The frequency spectra were dominated by three energetic bands:around the inertial frequency(7.75× 10-6 Hz) ,tidal frequencies(1.0×10-5 to 2.4×10-5 Hz) ,and between 1.4×10-4 and 8.3×10-4 Hz. Dividing the field data into three phases(before,during and after the typhoon) ,we found that the typhoon enhanced the kinetic energy in nearly all the frequency bands,especially in the surface water. The passage of Damrey made a major contribution to the horizontal kinetic energy of the total surface current variances. The vertical energy density distribution,with its peak value at the surface,was an indication that the energy injected by the strong wind into the surface current could penetrate downward to the thermocline.

A possible generation mechanism of the strong current over the northwestern shelf of the South China Sea

Continuous observation in late April 2005 on the northwestern shelf of the South China Sea reveals vigorous strong currents, the maximum velocity of which exceeds 3.8 m/s. The strong currents occurred around spring tide period, when the internal tide waves were also expected to be vigorous. Analysis shows that the major peaks of the current power spectrum are in low frequency band. Using a numerical method applied to the actual ocean stratification, we find that the amplitude profiles of the strong current are similar to that of the currents induced by some low-mode internal waves （at diurnal or semi-diurnal frequency）. It indicates that the temporal and spatial features of strong currents were possibly induced by low frequency internal waves.