Patterns And Dynamic Mechanisms Of The Kuroshio Intruding The South China Sea

The Luzon Strait (LS) is a large gap of the Pacific western boundary. The Kuroshio passthrough the LS from south to north with a bend path westward slightly and its mainstream isgenerally limited in the North Luzon Trough. However it occurs frequently that the Kuroshiointrudes the SCS, which exert its important influence on the marine environment of the SCS,such as the circulation, hydrography and biochemical filed. The subject on the Kuroshiointruding the SCS is still a research focus and predecessors have done a lot of investigations, butit is under debate up to new. Since the bottom topography of the LS is very complex and there isthe interaction between the Kuroshio and the current filed in its neighbor region, especially themesoscale eddies, it is difficult to understand the mechanism of the Kuroshio intruding the SCS.In this study, we use the advantage of the Argos satellite-tracked drifter trajectoriesindicating the lagrangian current to investigate the basic forms and variation regulars of theKuroshio intruding the SCS. On this basis, we focus to understand the following key problems:What is the driving mechanism of the Pacific surface water cross the LS into the SCS?Whether or not there is a connection between the mesoscale eddies on the two sides of the LS?How does the Kuroshio loop current from? We finally advance a simple physical frame on theKuroshio intruding the SCS, and explain their dynamic mechanisms. The main new results inthis study are as follows:1. Statistic analysis results of drifter trajectories show that: The entrance of surface flowacross the Luzon Strait (SFALS) is mainly the southern Balintany Channel (SBLTC) andBabuyan Channel (BBYC). The SFALS occur most early in last of September, and then tendsgradually to stable. In December it reaches the most powerful, and its entrance stretchesnorthward to the southern Bashi Channel. The occurring probability of SFALS decreased rapidlyin February--March, when the drifters enter from south of20oN may cross the LS. It is surprising that the speed of SFALS was1.5times of that before entering the LS as the entrancewas the Balintany Channel, but the speed increase of SFALS is not significant compared to thespeed before entering from the Babuyan Cannel.2. The oceanographic observed facts showed that in the wintertime there is a positive sealevel anomaly（SLA）southwest of the Taiwan Island including the northern LS, and a negativeSLA northwest of the Luzon Island including the southern LS. Obviously, they should match ananticyclonic circulation and a cyclonic circulation respectively. A slope down sea-surfacesouthward and an accompanied westward current should exist between these two regions, aboutscope of119.5°-121.5°E,19.8°-21.3°N. The combination of anticyclonic and cycloniccirculations is similar to a pump, which brings a westward velocity increase to the Kuroshio,special for the kuroshio water entering from the Balintany Channel, and drives a part of thekuroshio water change its original path to enter westward the SCS. Therefore, we believe that inwinter the generation of the SFALS is the result of season readjust of sea level and circulation inthe northeastern SCS rather than the forced by the perturbations, such as mesoscale eddies，eastof the LS. The occurring condition of the SFALS is solved by a set of movement equations,where the friction drag and the background current filed, the Kuroshio, are considered. Thetheoretical results indicate that whether the Pacific Surface Water can cross the LS into the SCSdepends upon the SLA gradient in the middle LS and the entrance positions of the PacificSurface Water.3. Observational data of the Argos satellite-tracked drifters deployed in the Luzon Strait (LS)in autumn and winter of January of2003,2004and2005are used to analyze the Kuroshiocurrent pattern passing through the LS. The result show that the paths of Kuroshio surfacecurrent in autumn and winter can be classified three types: northward type, westward type andLoop Current—Shedding Eddy type, and the letter two types were intrusion into the SouthChina Sea (SCS). The statistical analysis indicates that the occurring probability of LoopCurrent—Shedding Eddy type was0.23. The Kuroshio Loop Current (KLC), with the maximumlatitudinal scale reached210km, occured southwest of the Taiwan Island, west of the HengchunSea Ridge only. KLC was only a part of current separated from the Kuroshio rather than thewhole Kuroshio cruised into the SCS meanderingly, which differs from the Loop Current in the Gulf of Mexico. The westward current velocity was greater than eastward current velocity in theKLC, which is one of the reasons of its developing westward. The KLC may evolve into theShed Eddy or disappear on the spot. The Shed Eddy moved westward with a speed of about10cm/s.4. Statistical results from the satellite altimeter sea level data revealed that the Kuroshiostream belt is a weak active region of the mesoscale eddy, while the area west of the HengchenSubmarine Ridge (HSR) is one of the strong active regions of mesoscale eddy. The Argossatellite-tracked drifter trajectory diagrams showed that the Kuroshio Loop Current (KLC) andits shed eddy (SE) occurred only west of the HSR. It suggests that the two oceanographicphenomena are induced by instability of the Kuroshio stream on the mesoscale disturbance.5. Theoretical study results on the formation of the KLC:To better understand this mechanism in theory, from the governing equations of disturbanceinstability in coordinate frame we derived the governing equations in phase space and therestrained conditions of solution existence. For the simplified structures of Kuroshio streamand bottom topography, an analytical solution of dispersion relation and growth rate wasconstructed for mesoscale disturbance. The theoretical results are as follows.(1)There are the intrinsic oscillations with nearing inertia frequency in the Kuroshio area,which are related to the horizontal gradients of Kuroshio velocity and density fields. Therelative vorticities ((?)V/(?)x)are positive on the west wing and negative on the east wing of inthe Kuroshio area, which causes two monochromatic waves with nearing frequencies, and theysuperimpose to form a wave packet. The theoretical estimates for the wave packet are closed toobservational results.(2) The Kuroshio plays a role of direction dynamic filer. For the oscillation period of wavepacket, the disturbance wave propagating westward occurring in the subtropical pacific, thedominant wave with period about90days will be intercepted by the Kuroshio, themultifold-frequency waves with the periods T45daysonly may propagate into the SCSthrough the Kuroshio region.(3) The disturbance instability mainly depends on the bottom topography as well is relatedto the horizontal shear of Kuroshio fields. As the disturbance waves pass across the west boundary of LS, their states are different at the northern and southern sections. The disturbancewave excited probably anew at the northern section because the HSR is shallower than600mand deepen bottom westward, and forced a part of the Kuroshio meandering to develop into theKLC. There is not the exciting condition to disturbance wave at the southern section, becauseits submarine ridge has a depth more than that of the wave disturbance.In this paper, we analyze the encounters of the eddies occurring in the west pacific with theLuzon Island Arc, and suggest that the squeezing of anticyclonic eddy through the Bashi channelwill birth some new eddies propagating westward, which may be the mesoscale disturbanceorigin with a45-day period in the southwest of Taiwan Island.