Shuqun Cai

A numerical study of the generation and propagation of internal solitary waves in the Luzon Strait

A new composite model, which consists of a generation model of the internal tides and a regularized long wave propagation model, is presented to study the generation and evolution of internal solitary waves in the sill strait. Internal bores in the sill strait are first simulated by the generation model. and then the internal tidal field outside of the sill region is given as input for the propagation model. Numerical experiments are carried out to Study the imposing tide, depth profile. channel width and shoaling effect, etc., on the generation and evolution of internal solitary waves. It is shown that only when the amplitude of internal tide at the forcing boundary of the propagation model is large enough that a train of internal solitary waves would be induced. The amplitude of the imposing tide in the generation model, shoaling effect, asymmetry of the depth profile and channel width have some effects on the amplitude of the induced internal solitary wave. The imposing tidal flow superimposed on a constant mean background flow has a great damping effect on the induced internal waves, especially on those propagate against the background flow direction. The generation and propagation of internal solitary waves in three possible straits among the Luzon Strait are simulated. and the reasons for the asymmetry of their propagation are also explained

A method to estimate the forces exerted by internal solitons on cylindrical piles

Internal soliton is the large amplitude wave existing in the pycnocline, induced by internal tide in the condition of special bottom topography. During its propagation process, the induced disturbance can bring about strong convergence of sea water and sudden strong current (wave-induced-current), which can cause severe threat to the ocean engineering structures, such as oil drilling platform and pipeline. In this paper, Morisons empirical method, modal separation and regression analyses are introduced to estimate the forces and torques exerted by internal soliton on cylindrical piles. As an example, a limited set of observational data recording a passage of the internal soliton near Dongsha Islands is used to estimate the horizontal velocity and its acceleration in a vertical section for computing the force and torque on a supposed pile, and the estimation results are reasonable. It is shown that, the higher number of the modes retained in the calculation, the better the estimation of velocity profile tits the observational one. A better overall approximation to the real solution can be reached if there are more observational current data acquired in a whole vertical profile

An Overview of Internal Solitary Waves in the South China Sea

Internal solitary waves (ISWs) are the nonlinear large amplitude waves existing in the oceanic pycnocline. They are very active in the northern South China Sea (SCS). With more and more remote sensing data, in situ observational data and the development of the numerical simulation technology, a number of studies on the characteristics, the source site, the generation, and evolution of the ISWs in the SCS are reported. In this paper, earlier studies on the ISWs in the SCS are summarized and some problems worthy of study are put forward.

The numerical study of the South China Sea upper circulation characteristics and its dynamic mechanism, in winter

A coupled single-layer/two-layer model is used to study the South China Sea (SCS) upper circulation characteristics and its dynamic mechanism, in winter. The conclusions reached by the research are: (i) the effect of the Kuroshio on the upper circulation to the north of Dongsha Islands seems to be important in the northern SCS, there is a cyclonic gyre with three anticyclonic eddies, two of them to the north and southwest of Dongsha Islands respectively, and one in the northwestern Luzon Island; (ii) it is mainly a wind-driven current which exists to the south of Dongsha Islands-the water from alongshore of western Guangdong is driven by the northeast monsoon, then flows southward along the western SCS boundary, thus forming a rather strong western boundary current; and (iii) in the southern SCS, due to the actions of beta effect and bottom topographic effect of the continental shelf to the south of Beikang Shoals, an anticyclonic eddy is formed periodically over the Nansha Trough, which grows and spreads westward, then dissipates in the western boundary, with around a 50-day period

An analysis of the current deflection around Dongsha Islands in the northern South China Sea

Based on the in situ data and ADCP observation in fall, it is found that a northeastward current at inter-middle level flows on the Northern South China Sea (NSCS) continental shelf. This current flows almost along the isobaths, and it deflects from the isobaths veering toward deep water when flowing over the Dongsha Islands. Geographic currents derived from the climatologic hydrography data (WOA01) and absolute dynamic topography (ADT) data confirm the deflection of the northeastward current on NSCS continent. A fine resolution regional ocean model which can well reproduce the large scale circulation in the NSCS is used to analyze the dynamic about the deflection. The vorticity term balances shows that JEBAR (Joint Effect of Baroclinicity and Relief) drives the water column to depart from the isobaths. To the east of the Dongsha Islands, the isopycnal is almost orthogonal to the isobaths. The joint effect of the topographic and the baroclinic effect supplies negative vorticity and drives the water column to deflect from the isobaths and veer to deeper water. Momentum analysis along the stream line shows that, when the sea water flows around the Dongsha islands, the pressure gradient along the isobath pushes the sea water to accelerate, and then the Coriolis force orthogonal to the isobath increases and overcomes the corresponding pressure gradient, which drives the water deflected from the isobath toward the deep sea. Citation: Wang, D., Q. Wang, W. Zhou, S. Cai, L. Li, and B. Hong (2013), An analysis of the current deflection around Dongsha Islands in the northern South China Sea, J. Geophys. Res. Oceans, 118, 490-501, doi:10.1029/2012JC008429.

A propagation model for the internal solitary waves in the northern South China Sea

A two-dimensional, regularized long-wave equation model is developed to study the dynamic mechanisms of the propagation and evolution of the internal solitary waves (ISWs) in the northern South China Sea (SCS). It is shown that the bottom topography would cause the polarity reversal of ISWs, the change of the local wave crestline shape, and some diminution in wave amplitude; even if the ISWs are induced at the small sill channel along the Luzon Strait, they could propagate westward with their crestlines covering a large area in the latitudinal direction in the northern SCS. When there are two trains of ISWs propagating from the same source site with a time lag but different amplitudes of initial solitons, the latter train of ISWs with a larger amplitude may catch then swallow the former one with a smaller amplitude, and the wave amplitude of the merged ISW train decreases while the wave number increases. When there are two trains of ISWs propagating from the different source sites at the same time with the same amplitude of initial solitons, the crestlines of the two ISW trains may meet and a new leading soliton is induced at the connection point. Once the ISW trains collide with the island, before the island, a weak ISW train is reflected; behind the island, the former crestlines of the ISW train are torn by the island into two new trains, which may reconnect after passing around the island. The propagation direction, the wave amplitude, and the reconnection point of the new merged ISW train behind the island depend on the relative orientation of the original soliton source site to the island.

Mesoscale eddies cases study at Xisha waters in the South China Sea in 2009/2010

Analyzing the observed currents at Xisha (110.3899 degrees E, 17.1038 degrees N) during May 2009 to May 2010, it is found that the kinetic energy has significant mesoscale variability, and each peak responds to large positive/negative ocean surface current curl caused by mesoscale eddies. Compared the kinetic energy with the wind stress work and the pressure work, it is also found that the barotropic pressure work which is mainly contributed by the sea surface height (SSH) corresponding to the mesoscale eddies behaves like the kinetic energy. The contribution of the mesoscale eddies to the kinetic energy can be up to 90% sometimes and reach deep level every time. Using the satellite altimeter data, the paths of mesoscale eddies contributing to the kinetic energy variability are traced back. In the winter half of the year, the mesoscale eddies propagating along the northern South China Sea shelf or across the basin from the west of the Philippines toward Xisha arrive at Xisha, influencing the kinetic energy. In the summer half of the year, the mesoscale eddies are mainly from the south, which were shed from the Vietnam coast current. And the cause for eddy shedding may be related to the relaxation of the Ekman transport anomalies.

Simulations of Internal Solitary Wave Interactions with Mesoscale Eddies in the Northeastern South China Sea

AbstractWith the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern South China Sea (SCS), this study found one type of distorted phenomenon of internal solitary wave (ISW) with the long front caused by the oceanic mesoscale eddy. Motivated by these satellite observations, the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear MITgcm to investigate the perturbation of ISWs by an isolated cyclonic or anticyclonic eddy. The results show that the ISW front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides. The ISW energy along the front is focused onto (scattered from) the wave fragment where a concave (convex) pattern is formed, and the previously accumulated energy in the focusing region is gradually released after the ISW propagates away from the eddies. The ISW amplitude is modulated greatly by the eddies due to the energy redistribution along the front. Se...

Diurnal wind and nonlinear interaction between inertial and tidal currents in the South China Sea during the passage of Typhoon Conson

Diurnal wind (DW) and nonlinear interaction between inertial and tidal currents near the Xisha Islands of the South China Sea (SCS) during the passage of Typhoon Conson (2010) are investigated using observational data and a damped slab model. It is found that the DWs, which are dominated by clockwise wind components, are prominent at our observational site. The DWs increase after the passage of the typhoon from 1 to about 4 m/s, which may be due to the decrease of the sea surface temperature caused by the passage of the typhoon. Kinetic energy spectra and bicoherence methods reveal nonlinear interactions between the inertial currents and the 2MK3 tidal constituent at our observational site. The slab damped model reproduces the inertial currents successfully induced by the total observed winds, and it is shown that the inertial currents induced by DWs are positively proportional to the DWs speed. Even though the observed inertial currents are distinct, the proportion of inertial currents induced by DWs to those induced by the total observed winds is just 0.7%/4% before/after the passage of typhoon. This shows that the inertial currents induced by the DWs are unimportant near the Xisha Islands during the typhoon season.

Three-dimensional numerical model study of the residual current in the South China Sea

A three-dimensional baroclinic shelf sea model is employed to simulate the tidal and non-tidal residual current in the South China Sea. The four most significant constituents, M-2, S-2, K-1 and O-1, are included in the experiments with tidal effect. At most stations, the computed harmonic constants agree well with the observed ones. The circulations of the South China Sea in summer (August) and winter (December) are mainly discussed. It is shown that the barotropic tidal residual current is too weak to affect the South China Sea circulation, whilst the contribution of the baroclinic tidal residual current to the South China Sea circulation would be important in the continental shelf sea areas, especially in the Gulf of Thailand and Gulf of Tonkin. In the deep-sea areas, the upper barotropic or baroclinic tidal residual current is relatively very weak, however, the speed order of the deep baroclinic tidal residual current can be the same as that of the mean current without tidal effect. Moreover. the baroclinic tidal residual current seems to be related to the different seasonal stratification of ocean